CMM Coordinate Systems: Part Coordinate System vs. Fixture Coordinate System

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. 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

Re: CMM coordinate systems: Part csy vs fixture csy

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.

Re: CMM Coordinate Systems: Part csy vs. Fixture csy

What is "csy"?

Re: CMM Coordinate Systems: Part csy vs. Fixture csy

My guess is it is 'co-ordinate system'

Re: CMM Coordinate Systems: Part csy vs. Fixture csy

Hey Guys,

Sorry, csy is an abbreviation for co ordinate system.

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

Re: CMM Coordinate Systems: Part csy vs. Fixture csy

Hi Kevin,

Is your issue now resolved? Please let us know.

Stijloor.

Re: CMM Coordinate Systems: Part csy vs. Fixture csy

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.