CMM Handbook

The CMM Handbook Revision 5

2010-11-08T19:29:00.004-05:00

CMM Quarterly announces the new release of The CMM Handbook Revision 5. This version comes with a CD with explanations of specific topics. Below are listed some of the topics covered in The CMM Handbook. This spiral bound handbook is a must have for all CMM programmers and operators.

Common Conversion Factors
Environment Settings
Coordinate-Measuring Machine (CMM) Probes
Touch Trigger Probes

Resistive
Strain-gauge
Piezo

Scanning Probes
Passive Sensors

Simplicity
Dynamic Response

Active Sensors

Complexity
Dynamic Response

Accuracy
CMM Stylus Selection
Stylus Variables

Stylus Sphericity (roundness)
Stylus Bending
Thermal stability
Coefficient Expansion

DMIS Probe Command Set
Measurement Uncertainty
Measurement Uncertainty and your CMM System

MPE = Maximum Permissible Error
MPEE - Maximum Permissible Error for length measurement
MPEP - Maximum Permissible Error for probing
ISO 10360-2 Where do E and P apply?
MPERONt(MZCI) - Maximum Permissible Error for form measurement (roundness)
MPETHP and MPEτ - Maximum Permissible Error for scanning probing

Projection Planes and Plane Normal Vectors
Understanding the 6 Degrees of Freedom

Using The Correct CMM Alignment Principles

Words To Be Familiar With

Alignment
Datum
Origin
Coordinate System
3-2-1 Setup
Coordinate Systems

What is a Coordinate System?
How Do I Create A Coordinate System?
Myths And Truths About Establishing Coordinate Systems
Cartesian Coordinate System
Polar Coordinate System
Spherical Coordinate System
Datum Definition When the Datum Is a Cylinder
Basic Alignment Strategies
Sample Alignments
Plane/ Line/ Line
Plane/ Line/ Point
Plane/ Circle/ Line
Plane/ Circle/ Circle
Advanced Alignment Strategies
Best-Fit Alignment
RPS Alignment
CAD Alignment

Feature Types

Circle
Cylinder
Line
2d Line
3D Line Spatial Vector
Cone
Partial Arcs
Measurement Methods
Measurement Strategies
Diameters

Filtering of Scanned Data

Data from Scanning
Filtering – What is it?
Filter Methods
Filtering Types

Direction Vectors

What Are Direction Vectors .
How To Calculate Direction Vectors
Direction Vectors for Common Angles

Gage R&R

Terminology of R&R
Variability
How to perform a Gage R&R
Gage R&R Method
The 10% Rule
Calculating Gage R&R
What If the Gage R&R Is Not Good?

DMIS Commands

General Setup Commands
Machine Settings
Operator Prompts
Sensor Select

Datum Alignment Commands

Three Step Method
One Step Method
Measurement Features
Work Plane

Trigonometry Calculations

Angle Measurement
The concept of angle
Right Hand Triangles
Oblique Triangles
How to Calculate Chord Length

Bolt Hole Circle Calculations
Create Your Own Bolt Hole Calculator in Excel

Getting Started
The Formulas

Geometric Dimensioning & Tolerancing

G,D&T Resources
G,D&T printed or web materials
G,D&T Training Companies
Datum Feature Constraints
Profile
Changes to G,D&T
New and Refined Modifiers

CAD

Data Exchange
IGES and Step
Issues When Programming From CAD
Surface Normals

CAD Modeling

The Basics
Reverse Engineering
CMM Scanning Options

Higher Level Language Examples

Common HLL Usages
Jump To
If-Then-Else Statement
Relational Operators
Call External Program
Write a program to calibrate a disc, cylinder, or shaft styli .
Save results and increment serial numbers
Write a program to query if a fixture alignment needs run .
Write a program to add date and time stamping
Write a program to add operator input

CMM Glossary

This spiral bound book sells for $115.00 plus shipping ($5.00 US, $10.00 International)

The CMM Handbook

2009-07-13T16:08:00.006-04:00

Announcing the most comprehensive CMM Handbook. The CMM Handbook is now available. This book will put the best CMM resource material at your fingertips. Never before has this data been collected and placed in one resource book.

This book is the resource book you want in your library of books.

Click on the picture to see a sampling of the table of contents.

This book is available for $115.00 U.S. and includes shipping in the U.S. add $10.00 for international orders to cover shipping.

To purchase The CMM Handbook click on the Buy Now Button on the upper right hand panel.

DMIS Programming

2009-06-22T20:08:00.000-04:00

Getting Started: Creating a program shell with a setup section.

This article builds on the previous overview and gives you some examples of program structure along with suggestions regarding some ‘must have’ settings to get your program underway.

Most DMIS based CMM’s provide icon based tools to create DMIS programs, however a DMIS programmer does not need CMM software to create a DMIS program. DMIS programs can be created from any text editor and the program can usually be opened by or imported into the CMM software later. It should be noted however that a text editor works well when you are measuring primitive features or making simple changes using engineering drawings but will not suffice if you need data from a CAD model in order to measure complex surfaces or simulate your measurement routines. These types of tasks are best left to a professional CMM system.

A DMIS program that does nothing:

DMISMN/’EXAMPLE OF A PROGRAM SHELL’,4.0
$$
$$
$$ this program does nothing
$$
$$
ENDFIL

Click here to read the entire article

DMIS programming ... Overview.

2009-06-15T20:06:00.000-04:00

This is the first in a series of articles describing the process of DMIS programming.

This is the first in a series of articles describing the process of DMIS programming. In future articles I will be covering specific areas of a typical DMIS program and providing actual program listings of those areas. I will try to cover the most important aspects of each area, giving the reader tips and tricks learned along the way. This first article is designed to provide an overview of the basic structure of a program and highlight the areas that any user should consider including in their program.

DMIS programs should be designed to be as portable as possible so that they can be used on different DMIS compatible CMMs. This means that all aspects of a program’s operation must be defined and the user should not assume that the CMM will take responsibility for any setting. A typical DMIS program starts with a DMIS main or module statement (DMISMN or DMISMD) and ends with an end file statement (ENDFIL). The commands contained between these statements will be driven by the demands of the inspection process. Most DMIS programs should include the following areas:

Click here to read the entire article

PC-DMIS

2009-06-08T13:00:00.000-04:00

Forms

With the introduction of new versions of software, there will be leaders and there will be followers. The leaders take the reigns and forge ahead using the “beta” version of the software, wallowing in the fact that they have the new version and they are at the forefront of their profession....

The followers wait for the leaders’ feedback on what they have encountered with the new software; good or bad.
PC-DMIS is well known for releasing new and improved versions at least once per year. With the introduction of the newest version 4.0 and above, the group at WILCOX has changed the playing field once again. The reporting methodology has been overhauled, HYPER-REPORTS have been removed from the software, FORMS have been added as an option, and a multitude of vision and articulated arm algorithms have been added or improved. Future articles will be covering some of these topics and more.

Today’s topic will look at the usage of the FORM as an alternative to hyper-reports in a program. Specifically, the process for embedding a set up photo into a FORM and linking it in a program will be reviewed. Keep in mind that all the options that a FORM may be appropriate for will not be covered in this article.

When the FORM window is opened for the first time, one would think they are looking at the HYPER-REPORT window.

PC-DMIS

2009-05-30T20:01:00.001-04:00

Probe Qualification

Probably the most important aspect of measuring parts on a CMM is getting the probe, or probes, to be in relationship to one another. As it happens, this procedure is one of the most frequently asked questions on the internet forums and least understood. Qualification, as this process of relating a probe setup to the CMM is called, is the topic of discussion today. We will start by qualifying a single probe setup. To begin, the PROBE UTILITIES window must be accessed. To do this, go to INSERTHARDWARE DEFINITIONPROBE. Once the PROBE UTILITIES window has been accessed, a name has to be given to the probe that is going to be created.

Click here to read the entire article

PC-DMIS Training

2009-05-28T19:58:00.001-04:00

Using CAD models in 4.1 and above; importation and transformation.

The software developers at Wilcox have been hard at work to improve PC-DMIS for the last few years. With their hard work they have added many new features that may not be known to the average programmer. This flexibility is making the usage of the software easier and easier for the seasoned programmer. At one time the programmer would have to hunt down an engineer that had some knowledge of CAD software if a “tweak” was need to an .iges or .step model. As of the PC-DMIS 4.1+ version(s), the CMM programmer can easily manipulate the models from within the software.

As an example, many times a program will have been created in standard units and the next CAD model given to the CMM programmer will have been exported from its native software in metric units. This change may have been intentional or an oversight, but either way, it would have a required a rewrite of the program or the very least for someone that had access to the native CAD software to export in the proper units. All of this manipulation can cause errors. PC-DMIS now allows the CMM programmer the ability to import .iges and .step files regardless of the units used for its’ creation.

Another improvement in PC-DMIS is the ability to “transform” the CAD data once the .iges or .step file has been imported. Similar to the units of measurement issue, sometimes a CAD model can be imported and the origin of the part may not be where the blueprint states it should be. In many cases there are a number of authors that work on a CAD model. Each one of those authors opens, saves, and closes the CAD model numerous times in its’ life and at some point the origin is either moved or re-oriented upon a new model being created.

Click Here to read the entire article

CMM Training Survey

2009-05-15T09:32:00.000-04:00

Please click on the link below to answer a 6 question survey about CMM Training. We are exploring using a over the web training software for live training.

Click Here to take survey

Thank you for answering the survey

Converting Diameter Profile Tolerances

2009-05-15T02:00:00.000-04:00

Converting Diameter Profile Tolerances to Size and Position Tolerances

Manufacturers of cast, molded, and pressed products often work with designs that are given a profile tolerance “All Over” the surface of the part. The “All Over” tolerance is an efficient way to control these formed contours but many in manufacturing and metrology like to examine simple geometric features like cored or pierced diameters, often blanketed by the “All Over” specification, in terms of size and location for process control instead of examining numerous surface profile points. They cannot do so equivalently however because a profile tolerance doesn’t distinguish between size and position it just sets an inner and outer boundary for both. Those boundaries reflect a zero tolerance for location when the diameter is at its max or min size and a maximum tolerance for location when the diameter is at its nominal size.

Let us look at a nominal diameter size of Ø7.54mm and a profile tolerance of 1mm that wrapped around that nominal. The minimum boundary would be Ø6.54 and the maximum Ø8.54. If one tried to proportion the profile tolerance between size and position and chose a position tolerance Ø0.65 RFS then the maximum diameter size would be limited to Ø7.89 and the minimum Ø7.19 since either of these sizes with the full Ø0.65 position deviation would reach the inner and outer profile boundaries. The problem with this strategy is that a measured feature size above the chosen maximum or below the chosen minimum with a position deviation less than Ø0.65 would be out-of-tolerance for size but still be within the boundaries for profile. If one chose a variable tolerance value like Ø0 MMC and the feature was a hole… the maximum size would be limited to Ø8.54 with zero position tolerance and the minimum size would be limited to Ø7.54 because a Ø7.54 diameter would have Ø1.0 position tolerance. In fact any value chosen for a variable tolerance value and specified MMC or LMC would have the nominal as one size limit and the constant value of position tolerance + or – from the boundaries as the other limit. Neither of these strategies reflects the freedom that the profile tolerance gives for size and location to vary within the boundaries.

Click here to read entire article

CMM Training

2009-05-14T20:34:00.007-04:00

Datum and Feature Tolerance Modifiers

2009-05-11T02:00:00.000-04:00

Applying Datum and Feature Tolerance Modifiers to Product Designs

Material condition modifiers, (M) Maximum Material Condition, (L) Least Material Condition, and (S) Regardless of Feature Size are used to define how geometric feature tolerances interact with feature sizes in product specifications. There are physical relationships among mating parts that reveal which modifier should be selected and there are practical reasons that drive alternate choices of modifier selection. This paper explains common drivers for modifier selection, how modified tolerances are evaluated and promote a functional strategy for modifier selection.

How do Tolerance Modifiers work?

Geometric tolerances constrain the boundaries that a surface, axis, center plane or point of a feature may occupy and the modifiers (M), (L), & (S) define how the size of the feature can contribute additional tolerance. For example, if a set of fastener clearance holes on one part mate with threaded bolts and tapped holes on another part, a clearance hole at its maximum size may be permitted to deviate more from its specified location while providing clearance to the threaded fastener. Likewise a hole that is at its minimum size will provide no additional clearance to the relationship. The least material condition modifier (L) does exactly the opposite. For example, an oil passage hole that mates with a gasket and another passage on a mating part may be permitted to deviate more from its specified location if it is at its smallest size without violating the gasket edge boundaries. A hole at its largest size would therefore be permitted no additional location tolerance. Finally the regardless of feature size modifier (S) denies the application of additional tolerance due to feature size.

How are they chosen?

Click Here to Read the Entire Article

Using Charts to Define Data

2009-05-07T05:00:00.000-04:00

The Meaning of Data Part II

In the last installment we discussed the meaning of data. I have data but what does it mean and how can I convey this data to the customer or machinist to give them the information they need.

This article will explore some basic concepts of data and data collection. At Chiron America, the machine tool builder in Charlotte, NC, they use a system that works very well to bridge the gap between CMM inspection data and the machinist. Along with the main division in Germany, Chiron-Werke, they have developed a system on how to best use the data coming off the CMM.
SPC

Statistical Process Control, or SPC, is a valuable tool when used correctly. SPC in some companies means Sort Parts Continually, until you make the data read what you what. It has had its abuses but when used properly it can be a great asset. In this article we will look at a process and show the basics of SPC and CMM data. This is only a glimpse and will be very basic in its scope but you will get the idea.

Process

Whether you realize it or not each part coming off a machine center has a process. The things that go into making that part are the building blocks of that process. If one of these things breaks down or needs attention then the whole process is affected and it will directly affect the quality of that part or a series of parts coming off that same machine.
Chiron America uses a Zeiss CMM with Calypso software. Each inspection result is written directly to the company’s internal server both as a PDF, for permanent record storage but also a .chr and .tab file which can be pulled directly into Excel. This allows Chiron to be totally paperless. A SPC program has been developed in Excel to give each machinist the ability to analyze the data for their machine based on the CMM results.
One aspect of the SPC program is the ability to graph run charts which give a telling picture of the actual machine process. This is what we will examine in this article.

Terms To Be Familiar With

CP = How much of the specification limits are being used.

CPK = How close your process is to the target value.

You might a good CP but have a low CPK meaning the process meets the specified limits but is far from the targeted value.

True Position Charts

Let’s look at some charts and see what data is revealed in them. In figure 1 we see a true position run chart. At first glance we see the data is at the top of the chart above the UTOL, or upper tolerance limit. Look at the bottom left corner of the chart and you will see the CPK is a -3.26. These parts are out of tolerance but this chart reveals something else. Look at the range between the parts, it is 0.045 microns.

Figure 1. Click Image to enlarge

This is acceptable and within the tolerance of this process so while these individual parts will be scrapped the process can be targeted. However this one chart can’t reveal what exactly needs to be done other than the obvious that one of the axis that make up the true position needs to be better targeted.

Figure 2. Click image to enlarge

Figure 2 shows the X axis that make up the above true position. The data is distributed along the median of the tolerance. In this chart that is the MW line. Both the CP and CPK look very good for this individual axis. This feature is fine and we can move to the next axis.

Figure 3. Click image to enlarge

This is the Z axis and it shows the axis is out of tolerance but capable. Look at the CP this shows we have a good process but we need to target Z better toward the mean and it will bring in the true position. These charts were created with a machinist in mind so the KOR, or correction value, is shown on the far lower right of the graph. If the machinist moves Z in the negative direction 0.311 he will be at the mean value for this feature. This will correct the CPK value also.

Out of Process

Figure 4 shows a process that is out of control. Obviously we would want to re-clean and recheck parts 21, 22 and 24 to make sure we have the proper data.

Figure 4. Click image to enlarge

If we get the same results then this process needs serious attention. With a CP of 0.87 we can not simply make a targeting move but look at the last four parts. They seem to have an acceptable range. This creates some questions. Did the process settle down? Was the tooling inserts replaced prior to machine and now have settled in? Is the part moving in the fixture? Is there sufficient clamping pressure in the fixture? Do we need to run a larger sample to get the proper overall view of this process?

With so many variables involved in the machining process, such as fixtures, tooling, coolant consistency, hydraulic pressures, etc.. it will take time to get the answers to these questions. As a Quality Inspector you may not be involved in the individual tasks of finding the solution but you need to be equipped with the tools to give a reasonable answer as to what data is coming off your CMM.

Conclusion

Starting out with the right SPC program is key in giving you an overall view of your process. There are plenty of programs out there and you need to look at reporting functions of all of them to rightly discern which one will work for your situation. This is just one tool you can use to improve the quality of the parts coming across your CMM.

The Meaning of Data

2009-05-01T05:00:00.000-04:00

So, I Have Data, what Does it Mean?

One of the biggest struggles in the manufacturing environment is interpreting data.
There has always been a disconnect between data collection and data interpretation. Data coming from the coordinate measuring machine (CMM) and can be misinterpreted or presented in a way that does not show the whole picture. Having data that reveals what is really going on in the machine process begins with having a CMM programmer who knows what data that needs to be generated in order to make the machinist’s job easier. Knowing what and how to inspect these characteristics extends beyond just measuring to a blueprint. True, all parts must be to blueprint but what does a profile value on the CMM report tell the machinist?

Let’s look at profile as an example. Profile callouts are becoming more common on blueprints today, but there are some things to consider when checking profile. Profile considers the deviation of a nominal and actual measurement along the normal vector of a surface. If the profile callout is .2mm the profile must lie in a +/-.1mm tolerance band. See Figure 1

Figure 1

Here are a few things to consider:

There are several issues that come in to play when applying a profile tolerance as shown below. You should report any supporting tolerance elements that would give a clearer picture of what is happening. Below is the same part with machining variations where profile can be acceptable but there are other factors that are out of spec.

Example 1: Profile is acceptable but Flatness is out of spec. Figure 2

Figure 2

Example 2: Profile is acceptable but Z depth is out of spec. Figure 3

Figure 3

Example 3: Profile is acceptable but parallelism is out of spec.

Figure 4

When interpreting data from the CMM it helps if there has been a discussion between the machinist and the engineer and the CMM programmer prior to writing the CMM program, such as a startup meeting. This will help define what data the machinist needs to make offset moves or changes in his program. It will also reveal how the part sits in the machine fixture so the CMM programmer can duplicate the machine axis if at all possible.

CMM Programmers

The CMM programmer must look beyond the blueprint and consider the machinist in writing his program. It pays to know how your software looks at certain functions and have a clear understanding of how these functions relate to your CMM report. This will open the broader picture of what is truly going on in the machine process

Datum Definition When the Datum Is a Cylinder

2009-04-27T05:00:00.000-04:00

A datum is defined in ISO 5459:1981 [2] as:
A theoretically exact geometric reference (such as axes, planes, straight lines, etc.) to which toleranced features are related. Datums may by based on one or more datum features of a part..

When measuring a part with a datum feature requirement that requires you to use a cylinder as the spatial orientation datum (base plane datum) there are several considerations you must keep in mind. The surface of the datum cylinder will never be ‘true’. When measuring a part surface form, waviness, and roundness deviations will cause some variation in the calculation of the cylinder thus changing the orientation of the cylinder. The orientation (See Figure 1 a) is the angular deviation of 3d line through the center of the cylinder. It will be imperative that you take as many points when measuring a cylinder on a part that are equally distributed around each circle segment of the cylinder to create a more consistent best-fit cylinder.

A datum simulator is the best method to get consistent measurement values.

When creating a simulated datum the specs state:

ISO .A real surface of adequately precise form (such as a surface plate, a bearing, or a mandrel, etc.) contacting the datum feature(s) and used to establish the datum(s).
NOTE: Simulated datum features are used as the practical embodiment of the datums during
manufacture and inspection..

ANSI Y14.5: When a diameter is designated as a datum feature, the datum axis is derived from placing the part in a datum feature simulator. This simulator can be a pin gage (internal) or a ring gage (external). The datum feature is the surface of the part. The gage, the pin or ring gage is the simulated datum and is the datum axis.

Figure 1: Datum is the axis of a cylinder. Click on image to enlarge.

If you measure the physical hole or the datum simulator an issue will arise. See Figure 1. This shows the largest inscribed cylinder and its axis for a hole. The orientation of the cylinder depends on the surface condition and form of the hole and might therefore not produce an unambiguous orientation. For that case the standard requires an equalization of the angles a in all directions of space. The same will be for the smallest circumscribed cylinder respectively and consequently its axis for a shaft. Due to the geometry of a cylinder it lies on two planes of orientation. These planes are the center axis of the cylinder and will be averaged during the spatial orientation process of establishing a datum structure on your CMM.

In addition to the single axes it is necessary to define common axes as well. When the part has two different sized diameters but share the same axis both diameters must be used to create the datum axis. For establishing a datum being the common axis of two cylinders (see figure 2) the following definition is stated in ISO 5459:1981 [2]: .In the example (.), the datum is the common axis formed by the two smallest circumscribed coaxial cylinders…

Figure 2: Datum axis with two coaxial cylinders. . Click on image to enlarge.

When measuring features as in Figure 3 the datum axis line must be created between Datum A and Datum B to measure the runout of the journal. In Figure 3 both datum A and datum B have an angular orientation roughly in the same axis but we measure in a real world and if the orientation between the two datums pointed in the opposite direction then we are faced with a dilemma. At what point along the datum cylinder do we make our measurement?

Figure 3: Establishing a common axis between two different size diameters. Click on image to enlarge

In this case we need to measure two circles one on each datum and connect those with a 3d line to get the datum center. While this will give us the orientation of the part based on an axis center line it still does not give us the true datum axis line like a datum simulator would but usually a datum simulator is not practical in a production environment. Slipping a pin gage in a hole or placing the part in a ring gage to make sure in both cases I am really contacting the ‘working’ part on the datum feature is not practical. Having parts with good form and proper surface condition will reduce your measurement variation.

As I pointed out in previous articles (The Meaning of Data Parts 1 & 2) in CMM Quarterly knowing the data your CMM is producing and what are the expected results goes a long way in building confidence in the results you give to production.

Mark Boucher

All figures are from ISO 5459:1981

Training Material for PC-DMIS Users

2009-04-24T08:28:00.001-04:00

For PC-DMIS users James Mannes provides detailed insight into PC-DMIS

Click Here

CAD Modeling

2009-04-23T05:00:00.000-04:00

The Basics

By Mark Boucher, CMM Quarterly

I want to cover some basics about CAD models that might help us understand what is happening with some model features when you program from a CAD model. By understanding surfaces we can better evaluate any anomalies we may encounter when we import a model into our CAD base CMM software.

There are several model types and we will cover two of the most common ones you would come across today, solid models and surface models. To be more accurate, they are parametric models and freeform surface models.

Parametric Models

Parametric models are created from features that are defined by parameters, or dimensions. These dimensions can be changed and the feature moves with the change. Prior to parametric modeling if a change was made then the feature was recreated, extruded, trimmed, etc…, in the new position and the old feature was deleted. Parametric modeling maintains the relationship of part creation, assembly, to output of the blueprint and a change anywhere along that process will update the model at every level.

Parametric models are referred to as a solid model, as opposed to a wireframe model. A wireframe model is made up of lines that represent the part but have no surfaces on them and makes 3d viewing somewhat tedious.

Parametric modeling revolutionized the CAD industry and allowed more affordable CAD software to become available to anyone. You can now pick up parametric CAD programming software up at your local Best Buy right off the shelf.

Freeform Surface Models

The second model type I want to cover is the surface model. With surface models curves are used to define the surface area and surfaces are applied between the curves then they are trimmed and merged, to make a solid. The problem with this method is to make sure all the voids between the surfaces are filled in. The surface definition changes as the need requires. Let’s say, you have a plane that requires basically four lines to define the boundary of the plane. A chamfer merging into a radius requires a greater amount of defining to create this type of feature. While creating the surfaces you may end up with a small void as you try to fill in the feature. Point placement from your CMM program will be dependant on where it sees the plane boundaries and a void will not be inclusive in this plane so the boundaries are redefined not giving you a true representation of the surface. In parametric modeling these types of transitions are automatically resolved.

Some CMM CAD based software have a ‘healing’ or ‘repair’ functionality that will mend some of these errors. It is always advisable to use healing when using this type of model. If your software does not include this functionality there are third party softwares that do the job for you.

Surfaces

Creation of surfaces begins with a spline, aka curve. Splines are single lines that make up the shape of the surface. Imagine points that make up the shape of your surface and spline will fit through these points. These splines are then used to create the surface through a method known as ‘swept’ (using the curves as a guide rail) or meshed (lofted) through. A ‘swept’ surface follows the shape of the curve line. If you had a helical curve the swept surface will follow that helical shape as it extrudes the surface.

If your engineering department does any sort of reverse engineering they will ask for a series of curve files that they can import into their CAD system. The curve files are then used to ‘mesh’ or ‘loft’ the surfaces. The density or frequency of the curve lines along the surface will depend on the complexity of the surface being scanned. For flat planes only several are needed but scanning the chamfer to radius transition we discussed before would require a greater amount of curves to define the feature.

Another method is direct creation of the surface with manipulation of the surface control points. Points are created along the curves that can be grabbed and drawn in any direction to create a new surface shape. This inherently will create new surfaces to fit the new configuration.

It is important to note that the majority of CMM software in the market today do not have true CAD functionality and thus do not have the ability to manipulate surfaces as described above but it is important to know what is happening during model creation.

For more information go to www.cmmquarterly.com

Cone

2009-04-20T05:00:00.001-04:00

A cone is a 3d geometric shape that tapers smoothly from a flat, round base to a point called the apex or vertex. The axis of a cone is the straight line, passing through the apex.
The cone is a solid body that has a rotational symmetry about the axis line. Since a cone is 3d and possesses an axis line it can be used for a spatial alignment on the CMM. If you were to hold a weighted string and begin to rotate the string in a arc the result would be a cone. The amount of arc determines the diameter and angle of the cone.

A cone may be a right handed angle or oblique angle element. A cone is made up of a radius/ diameter, angle of the cone, the apex, and the axis line.

A cone may also be intersected with a plane to produce a circle that lies on the plane. It may also produce a point at the intersection of the axis line and the plane.

Issues When Programming From CAD

2009-04-15T05:00:00.001-04:00

Surface Normals

There are several issues that arise when bringing a CAD model into your CAD based programming software. One of these issues has to do with surface normals (surface vectors). You can bring in a model and the entire model or a portion of that model is visible, but dark (Figure1), or certain sections are not visible at all. This problem arises from the surface vectors pointing in the opposite direction than your CAD system views them. You are looking at the back side of the surface. You must reverse the surface normal (Figure 2). If your software has this capability, you are looking for something similar to ‘reverse surface normals’. This will flip the surface so the front faces in the correct orientation for your software to view the surface.

Figure 1

Figure 2

All surfaces have a front and a back side; a CAD program must know which is which. How is this done? The model must somehow include information to specify the front of a surface. This is done by surface normals. This is a line perpendicular to the front surface and beginning on that surface pointing away from the surface. Meaning it exists only on the side of the surface that is its front. The CAD system must have this information to shade the model properly. Those that use a CAD system need this to drive the probe normal to the surface.

Direction vectors have been covered extensively by Richard Clark, his three part series was featured in CMM Quarterly. Suffice it to say that these surface normals are what give you the direction vectors from CAD models when programming. If you do not use a CAD model to program then you must calculate the normal vector. Contact info@cmmquarterly.com for a Direction Vector Calculator created by Richard Clark.

When picking a feature off a CAD model the software will extract the normal vector from the CAD surface. As mentioned above you may have the ability to flip surface normals or you may have the ‘view surfaces from both sides’ option. Care must be given to this selection because the surface will be visible but the vector may point in the opposite direction you need to probe the part, your probing direction vector. Just know when viewing the vector after feature selection, that it is correct.

Spherical Coordinate System

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A spherical coordinate system is made up of three components; Radius (r), Phi (w), and Theta (u)

Radius is the radial distance from the origin to the point

Phi is the angle along the given work plane, in this example the XY plane.

Theta is the angle from the work plane, i.e. XY plane, to the point up toward Z

Cylinder

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A cylinder has a total of three surfaces: a top, bottom, and middle. The top and bottom, which are circles, are easy to visualize. The third is actually the curved wall of the cylinder. It is a surface that has measurable volume.

A cylinder is one of the most basic curvilinear geometric shapes. Each cylinder has an axis; this is a straight line that passes through the upper and lower surface that makes up the cylinder. The cylinder is bound by two planes perpendicular to the axis. The height of the bounding planes is determined by two fixed points along the axis line, in this case 83 mm.

The surface that lies between them is the cylinder. While a circle is 2d a cylinder is 3d having a height(h), radius (r), and volume. Like a circle, a cylinder has a diameter, radius, a chord, a tangent line, and a secant line.

When a cylinder is intersected with a plane a 2d circle is the result.
If a cylinder was sliced opened then laid flat it would become:

A rectangle

A cylinder is a surface. It has volume. If you think of a circle extruded down along an straight line normal to the surface the circle lies on you would have a cylinder.

Circle

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What is it?

Circles are simply closed curves. Where points along the curve converge to close the curve and the results are a circle.

Parts of a Circle:

Chord:
A chord of a curve is a geometric line segment whose endpoints both lie on the curve
Diameter:
A diameter is a chord passing through the center. The length of a diameter is twice the radius. A diameter is the largest chord in a circle.
Secant Line:
A secant line of a curve is a line that (locally) intersects two points on the curve.
Tangent Line:
The tangent line through a point P on a circle is perpendicular to the diameter passing through P

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Data Exchange

IGES and Step

You’re the CMM / Vision machine programmer and you’ve been given a CAD model that’s missing a surface. The engineer swears it was there when he sent it to you and sure enough its there on his computer. Now what do you do. This kind of thing happens all the time; data turns up missing on the CAD model you are to use when programming the part.

In the late 80’s for Boeing we were only allowed to IGES out simple geometry, points and lines, to use in our CMM programming. Later as Step began to emerge this requirement was relaxed but not after doing a lot of testing. Model traceability was the big issue and making sure that models sent out to sub-tiers were not modified or altered in any fashion.

What Happens During Model Translation

IGES and Step translators are known as Neutral-format translators. Meaning they may be used as common link software to go from one CAD system to another, i.e. CATIA to Solidworks or Solidworks to your CAD based CMM programming software.

These translators are massive in scope because they support a variety of different fields from architectural to mechanical and others. This creates its own series of issues and variables that are introduced into your translation process. During this exchange from one native CAD format to IGES or Step then back into another native CAD format data can be lost this is because not all systems support the same subsets. Each individual CAD software OEM can choose to support the subsets he wants to achieve the type of software he is looking to manufacture. During the translation process if an unsupported entity is found it is dropped or lost in the translation. CAD based CMM softwares are by rule not a true CAD system although they may use the same CAD kernel that are used by well known CAD systems i.e. ACIS kernel not all the subsets will be supported.

What You See Is Not Always What You Get

After a model is brought into your CAD based CMM software sometimes errors will appear. It might appear that two surfaces are joined when in fact they are not causing a problem when you try to put points on a surface. You could have points out in mid-air off the part but the points display a plane. This might be caused by an untrimmed surface that was translated but not visible in your system.

Modeling kernels will force some data to connect or at least treat surfaces as if they are connected when in fact they are not. Data exchange will reveal this type of condition.

IGES or Step

IGES was the standard used by North America for decades but is losing ground to Step. Step has become the preferred translator for many reasons. Solid models are translated as a whole better in Step. Solid models translated in IGES are converted to surfaces in most CAD based software although IGES is working to better support solids. This does several things, it increases the file size and can slow down your software, and based on what software you have it may not know how to handle B-splines and other native CAD entities. With Step the solid model is kept as a solid therefore reducing the amount of translation errors although some may still exist.

How To Avoid Translation Issues

This will take cooperation between you and your customer or engineering department. Typically engineers will only use the defaults in the software provided translator. This is not necessarily wrong but it must be tested to make sure you’re getting the best model available. Then again it may just come down to your CMM software and the subsets it supports. You may need to hunt for some happy median.

Get a 3d model from your customer or engineering department in both IGES and Step and begin to test the way your CMM software reacts to the model data exchange. Don’t be fooled by what visually shows up on your monitor. You need to write a program and see if the model is acceptable. In most cases, the Step model will perform better.

The ACIS kernel will allow for dimensions to be sent along with the data in the same file this might create a problem with non ACIS kernels. You may need to notify engineering so it can be turned off when data is sent to your system. The intent of this feature is so that you can see the dimensions on the screen while you program your part. Pro-CMM had this feature, so long as you programmed the model in Pro E. It is being explored across more CMM software platforms if they use an ACIS kernel.

Conclusion

When choosing a file format to standardize on I would choose Step. This will return the most consistent model accuracy and will give you less headaches. Make sure to keep up with translation revisions. These translators are updated continually so make sure this is in your software maintenance agreement to stay current. They can be pricey but how much will it be worth when you reduce the amount of time wasted on trying to get good models.

There are some fine CAD translation softwares available today, one of those is TransMagic. For information on TransMagic CAD translation contact mboucher@newcastlemeasurement.com

Line

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There are two line types available when programming a CMM. They are 2d line and 3d line. A 2D line is a physically measured line, which has an origin, a vector, and a length.

If you consider a 2D line mathematically, it is a plane with zero width. The line has a vector that is perpendicular to the plane the line lays on rather than in the direction of the line.
The important thing to remember is that a 2D line is 2D and not 3D. It has a start point, end point, and a vector direction.

A 3D line has a start point, an end point and a vector which define its orientation. A 3D line can be particularly useful when defining the axis of a cylinder. The axis is created by recalling two or more circles into a 3D line. The 3D line that is created is our cylinder axis.

Spatial 3D Line

A 3d line can be viewed through two perpendicular planes. When used for an alignment the 3d line makes planes normal to the CMM axis. For example, creating a line by using circle centers will allow you to create a 3d line. This replicates the 3d axis line of a cylinder that can be used to create a spatial alignment.

Right Hand Triangle

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Datum Feature Constraints

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In our on going series of covering the CMM basics we need to cover the all too familiar DFR, or the Datum Reference Frame. We see this DFR associated with a feature on the part but what does it mean? and how does it change my approach to programming this part? We will attempt to answer these questions. This won’t be an attempt to address the overall make up of the DFR but we will look at the datum order and how that affects our programming.

The Datum Order

In the above example, reading in the order that they are listed, we see that C is the primary datum. Datum A is secondary. The datum order plays a key role in revealing what the engineer is looking for. This shows the features of constraint and what priority each one has. Here is the simple part we will be working with. We will concentrate on the 10mm diameter.

Again, this is a simple part but the principle applies to every feature that carries a DFR. Let’s look at Datum A it was established off Datums C, B, and D. The 10mm hole is called back to datums C and A. So why is C listed before A? And why does it matter? Datum A is a cylinder through the part and as a cylinder it has a 3d axis line. Meaning it can be used to create a new spatial orientation, or base plane. Your software may use a different name but the effect is the same. If you reoriented on Datum A if would take Datum A’s perpendicularity and form into the orientation and thereby effecting the 10mm diameter location. The engineer wants Datum C to be the primary datum. It might be a mounting surface or an assembly face, whatever the reason the engineer wants Datum C to be the surface the spatial orientation is on. Datum A is then used as the X, Y origin from which to locate the 10mm diameter. With only two datums listed what about the axial alignment? As shown above Datum C is the spatial rotation, Datum A is the origin, the axial rotation is left to the rotation that set up Datum A since no realignment is called for.

Datum A listed first

If Datum A was listed first then you would use Datum A to reorientate the part. Datum C would control the Z depth. In this case Datum A might have a fastener running through it and Datum C hangs out in space in the assembly and plays no crucial role other than to control depth.

Conclusion

The datum order is important to understand. It lists the order of datum constraints. You must first constrain the primary then the secondary datums, and tertiary if shown. Care needs to be given to ensure the part is inspected correctly and using the correct datum structure will guarantee you’ve written the program to meet the engineer’s needs.

Direction Vectors

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One of the most important factors when programming a part on a coordinate measuring machine (CMM) is knowing what direction vectors are.

What Are Direction Vectors

Direction Vectors are the vector paths that are normal to any surface you are measuring. Each feature has a direction vector that the probe will follow to probe normal to that surface. To determine the vector direction is very simple when using a CAD model to program. The CAD based software will automatically extract the vector and write the code for the probe. There are instances, such as having to program manually, No CAD model exists, etc… where you will need to know what vectors and how vectors work. The direction vectors are the cosine of the angle needed to drive normal to the surface.

How To Calculate Direction Vectors

First we must look at the vector reporting in the software. Vectors are reported out as

I – this represents the rotational from X axis

J – this represents the rotational from Y axis

K – this represents the rotational from Z axis

This is a hard fast rule no matter what work plane you are in.

As stated direction vectors are the cosine of the surface normal angle.

Let’s look at an example:

If we are in the XY plane and the angle from the X axis is 45° which has a cosine of 0.70711, the angle from the Y axis is 45°, which has a cosine of 0.70711; the Z axis is 0°, which has a cosine of 0. Our vector direction for the point:

I - 0.70711

J - 0.70711

K – 0.000

What if the angle from X is 20° and the angle from the Y axis is ___.

We don’t any movement in Z.

90° total angle - 20° given on the B/P from X = 70°

Enter 20° on your calculator and press the COS button this is I (X)Enter 70° on your calculator and press the COS button this is J (Y)Z has a vector direction of zero.

I - 0.93969

J - 0.34202

K – 0.0000

Direction Vectors will never exceed the value of 1.0000 in your program and goes from negative 1.000 to positive 1.000 and will appear in program as 0.93969,0.34202,0.0000. I is listed first, then J, and finally Z. A negative sign will reverse the drive direction of the probe.

The Importance Of Direction Vectors

Failing to drive on a normal vector to the surface will cause cosine probing error.

The cosine error is the cosine between where you told it to touch the part and where it actually made contact. The larger the probe ball size the greater the cosine error.

As you can see that driving normal to the measurement surface prevents cosine probing error. With CAD based programming the vectors are extracted from the model surface and therefore already inserted into the measurement strategy.
For more information on Direction Vectors see the reference below.

Reference Director Vectors Parts 1-3 By Richard Clark http://www.cmmquarterly.com/

Polar Coordinate System

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A spherical coordinate system is made up of three components; Radius (r), Angle (a), and Height (h)

Radius is the radial distance from the origin to the point

Angle is the angle along the given work plane, in this example the XY plane.

Height is the height from the work plane, i.e. XY plane, to the point up toward Z

Cartesian Coordinate

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The Cartesian coordinate is the most basic of coordinate types. In the Cartesian system each feature has a linear straight line distance for the origin.

The point below has a linear X value, a linear Y value, and a linear Z value.

Example of a diameter on the XY plane

Coordinate Systems

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What is a Coordinate System?

One of the first things that you must do in creating a part program is to create a coordinate system on the part. This locates all subsequent measurements back to this location. How do I know where to create this coordinate system on my part? The location of the coordinate system needs to be the same as on the blueprint. The blueprint will indicate where the engineer wants the measurements to be taken from. This is usually determined by how the part is assembled or its relationship to the other parts in the assembly.

A coordinate system is the ‘you are here’ indicator on a map and how far it is to the next feature is the linear dimension. Linear dimensioning is the method of locating a feature. In order to know the exact number to this feature you must have a start point and that is the coordinate system.

How Do I Establish Create A Coordinate System?

In order to create a coordinate system you must constrain the ‘6 degrees of Freedom’. This was covered in detail in a previous CMM Quarterly; however, in order to cover the coordinate system thoroughly we must discuss some of the aspects of constrains in this article.

Some CMM software use different terminology but the basics are the same. First, you must create a spatial orientation, spatial rotation, or a base plane. You must use a plane or any feature type that has a 3d axial line through it to create this base plane. This will constrain the first 3 degrees of freedom. This limits the part in its movement on the CMM and makes the part normal to CMM. You would use a plane, cylinder, cone, or a sphere. These 3d features have a theoretical line that runs through the feature and is normal to that feature. Your CMM software will align that 3d line normal to a CMM axis.

In this picture a plane was probed on the top surface and used to create the spatial orientation. This forces the part orientation normal to the Z axis of the CMM. We also set the Z origin to this plane. As you can see it prohibits the part from moving up and down in Z but it can still move side to side in X and Y and rotate around the Z axis. This has constrained 3 degrees of freedom.

1) Movement up and down in Z.
2) Rotation around X.
3) Rotation around Y.

The next step is to use an axis alignment. This will make one edge of the part normal to a CMM axis. You may use either the X or the Y axis to align the part. The terminology in your system might be Axis Alignment, Planar Rotation, or some other term but the process is the same.
What features can I use to create the axial alignment? You can use a line, 2 circles that are aligned (offset circles can be used but we will not cover that in this article), a plane, a cylinder, etc…. You may use any feature that is a line or has a line running through it.

Here we probed a line on the Y axis and used this line for our axial alignment. This squares the part to the CMM Y axis. We also set the X origin on this line. This constrains 2 degrees of freedom. We can no longer rotate around the Z axis and can not move back and forth in the X axis. We now have only one degree of freedom left. We can still move the part back and forth in the Y axis.

You may now use a point or a line on the remaining side of the part and set the Y origin to that feature. This fully constrains the part.

We now have a coordinate system on the part. The coordinate system indicator shows the location of the origin of the part. This should match the starting location using the same features on your blueprint.

What if I use a CAD based system?

The alignment routine is exactly the same. You would just use the model system to generate the features for your program.

This type of alignment is known as a 3, 2, 1 alignment, indicating the method of constraining the 6 degrees of freedom.

Myths And Truths About Establishing Coordinate Systems

I’ve been told I can set the part on the CMM anyway I want to.
True with some exceptions.
If you set your part at some skewed angle to the CMM axis’ the software will still align it mathematically to be normal to its own X, Y, Z axis’. As you have seen above the software will square the part to be normal to the CMM. So yes, this statement has some truth to it but some care must be given to how the part is to be probed. If you set your part on a compound angle and you are probing manually or with a joystick you may encounter some probing error. Each probe point should be probed normal to the feature. If a point ‘slides’ or glances off the surface at an angle that is not normal to the surface then you will have some cosine error.

I have to use this 3, 2, 1 alignment every time.
False.
This is by far the most common alignment you will come across but is not the only alignment. For parts that have free form surfaces and no defined prismatic features you will need to use an iterative alignment. This is accomplished by probing several points on the surfaces of the part and letting the software mathematically align the part to the nominal values of the points. This may take several iterations, each time getting closer and closer to the nominal value.

I can measure my alignment manually and run the program in DCC.
True.
Well, you can certainly do this but the best course of action would be adding a DCC alignment. After you measure your part manually you should include a repeat of the alignment in DCC making sure you are driving normal to the feature. This will better align the part and measure the alignment in the same measurement parameters that the remaining features will be measured with.

If I want to quickly check a feature I don’t need an alignment.
False.
Let’s take a gage ring as an example. If you wanted to check the diameter you could lay it on the CMM and probe the diameter and probably get a good reading because the top and bottom surfaces of a gage ring are typically ground. At a minimum you should probe the top surface and use this as your spatial orientation before checking the diameter. This will square the top to the CMM axis and allow you to check the diameter normal to the top plane.

For more information on coordinate maeasuring machine be sure an visit http://www.cmmquarterly.com/

Welcome to CMM Handbook - The Blog

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CMM Handbook is designed to help the coordinate measuring machine programmer with general understanding of how to program a CMM. Techniques and advice will be given along with knowledgeable articles.

Your comments are welcome.