Hi all, :)

I am currently having a debate with a CMM programmer on my staff. He insists on using the fixture co ordinate system when measuring parts. I completely disagree. :argue: In my experience, which is about 7 years worth, you always fixture a part in a reliable, repeatable way and according to the print datum’s, you build your part csy from those datum’s. Independent of the fixture.

The only time I have ever seen a fixture coordinate system used, was on a machine that was dedicated to a couple of specific parts, where the fixture never moved. Then fixture csy was only used by the machine for a rough idea as to where the parts are positioned. Once the program completed the fixture csy it would then measure the part csy and continue with the rest of the measurements.

Anyone have any feed back or have experienced this before, Id appreciate your comments.

Kevin :thanx:

Hello Kevin,

Allow me to respond with caution here. The fixture can be viewed as a work-holding device to ensure that the part does not move while measurements are being taken. Thus ensuring repeatable measurements. The datum features indicated by the "datum feature symbol" on the drawing specify the features that are used to establish a datum reference frame. So when programming the CMM, the expection is (per the drawing callouts), that the datum features are used. Not the fixture.

Now, if the fixture is used to verify certain geometric characteristics, the fixture may be used (or may be called) a "functional gage." But that's another story.

Consult the drawings, consult ASME Y14.5M-1994, look at the CMM programming manuals, be open-minded, and I'm sure you'll come up with a solution. Keep us posted.

I trust other Covers here will chime in.

Stijloor.

What is "csy"?

My guess is it is 'co-ordinate system'

Hey Guys,

Sorry, csy is an abbreviation for co ordinate system. :agree1:

Stijloor, I agree that sometimes you have to measure from the fixture, in order to get some heights or something of that sort but my issue was that the cmm programmer was using the fixture to build his coordinate system instead of the datum of the part.

Kevin

Hey Guys,

Sorry, csy is an abbreviation for co ordinate system. :agree1:

Stijloor, I agree that sometimes you have to measure from the fixture, in order to get some heights or something of that sort but my issue was that the cmm programmer was using the fixture to build his coordinate system instead of the datum of the part.

Kevin

Hi Kevin,

Is your issue now resolved? Please let us know.

Stijloor.

not really. Like I said, I agree that sometimes you need to use a plane of the fixture in your part coordinate system for a height or something of that sorts, but this programmer uses the whole fixture to build his coordinate systems...I just dont agree with that. The datums on the drawings are there for a reason, I just feel that the fixture should be used for holding purposes only and the part itself needs to have the proper coordinate system. I find that he is getting a lot of pieces out of spec and I feel that the reason is poor coordinate systems. We are currently using pc dimis, I have used many different softwares in my past, but not pc dimis. I am going for training on this software soon, hopefully I will discuss this issue with the trainer as well, Its just nice to have everyones view.

Kevin

The most common time you'd use a fixture to build your coordinate system is when your fixture is designed to function as the mating part and the other features are based off the mating part(such as using two alignment pins on the mating part) or situations where accessing the datum itself is impossible/too difficult for the marginal improvements.

I've found it's pretty rare to need to do.

I agree, thats about the only time and its very rare.

It realy is a mater of if you can live with the veraition introdced from the tool. If it is below 1/4 of the resolution required there would be no effect and you can pick another battle.
When using fixtures I have referance points to do a quick location verificationa nd then always take the datums from the part per print. This is the only way to ge the most accurate and repeatable measurements.
:2cents:

My point exactly. the points taken from the fixture are for general location only. The datums from the drawing are what builds the coordinate system for the part.

I preffer part csy, as we measure 3D parts,(ex. mobile phone covers)
- the fixture is newer perfect
- you(operators) will newer place the parts on fixture the same
- tight tolerance
It may lead to longer measuring program, I suppose your programmer want's them shorter.
Check here as well:
http://www.cmmtalk.com/welcome.htm
http://www.pcdmisforum.com/index.php
http://www.pcdmis.com/
On the other hand you can easily stop discussions by a GR&R. Just give him 5 parts, he is the only operator, and let him measure the parts with program built up on fixture csy, then make a new program based on part csy.
Good luck
György

I also find this methodology very odd. I program Mitutoyo CMM's using PC DMIS language. And, OGP Smartscopes - which have their own independant background language ; the programmer then just has to establish Coordinate systems based on part geometry. We use several fixtures to stage parts that run at different points throughout the year in conjunction with a custom base plate that we built. I have never established geometry off of the fixture and would not recommend it to someone that I was training either. Unless you have an edge or cut that you just cannot get to without compromising the stage, why would you? You are not measuring any aspect of the fixture - you are checking the part right? You can also establish different Coordinate systems within the same routine, so, if you were worried about a location / crash, why not set up the program to begin with positioning based on your fixture and then blow it away with a part Coordinate system immediately after? I do this often - it is very effective, no crashes and the actual part is getting measured not the fixture. :agree1:

Hi all, :)

I am currently having a debate with a CMM programmer on my staff. He insists on using the fixture co ordinate system when measuring parts. I completely disagree. :argue: In my experience, which is about 7 years worth, you always fixture a part in a reliable, repeatable way and according to the print datum’s, you build your part csy from those datum’s. Independent of the fixture.

The only time I have ever seen a fixture coordinate system used, was on a machine that was dedicated to a couple of specific parts, where the fixture never moved. Then fixture csy was only used by the machine for a rough idea as to where the parts are positioned. Once the program completed the fixture csy it would then measure the part csy and continue with the rest of the measurements.

Anyone have any feed back or have experienced this before, Id appreciate your comments.

Kevin :thanx:

It depends on the nature of the fixture. In the automotive world, datum points are usually based on "car position," with the extreme points on the actual finished vehicle (fore and aft, top of the car, side-to-side) being the zero points. Drawings are based on this coordinate system, and so are fixtures. In other words, parts are dimensioned based on their final position on the car, rather than independently, and in isolation. This means that datum points on the fixture must be used, and will remain consistent every time the part is measured.

Unless the parts in question (and the fixtures) are based on such a fixed coordinate system, it makes no sense to use the fixture as a reference point. In such cases the only purpose served by the fixture is to hold the part for measurement in a consistent manner, such that everyone who uses the program and measures the part does so in the same fashion.

From what I've seen, when using in car coordinate systems, the fixture will have three tooling balls on the fixture from which to build the coordinate system as well. So far we havn't been told that is the case.

Hawat, we just use a regular fixture, no tooling balls.