Proper way to calibrate a check fixture

J

Jimbo1

#1
Need some help. We have a first piece attribute check fixture that needs calibrated. It has several details that check certain features with respect to the datums.
Would laying the fixture out to CAD on a CMM be sufficient or is there more to it. I've looked everywhere and can't find the answer to this.:truce:


Thanks in advance.
 
Elsmar Forum Sponsor
#2
Welcome to the Cove. This is the right place to get plenty of help.

You can indeed lay out a check fixture on a cmm. That's a very common thing to do. It's not really a calibration, more appropriately a 'verification' by comparison to the cmm's characteristics of linearlity, precision etc.

I'm sure Hershal or someone will add a bit more detail to this....

Andy
 

BradM

Staff member
Admin
#3
Hello, Jimbo! Welcome to the Cove!

First, why do you require calibration? Did someone tell you it had to be calibrated? Has it ever been calibrated before?

In general, calibration is verifying an instrument/tool/other (unknown accuracy) against a N.I.S.T. tracable standard (known accuracy). So, for a calibration, this device should be checked against a calibrated standard of better accuracy.

I'm having a hard time visualizing the fixture, so bear with me. Is this a two dimensional tool, or a 3-D device of some type that the instrument fits into? Or is it some type of electrical fixture?

As far as calibration, what you would be verifying? Would you measure a distance, a hole diameter, or some level of volts/resistance?

You say there are several criteria verified against the datum. have you quantified these criterion and the acceptable tolerance? If this is dimensional, you might need a drawing for the facility to perform the calibration.

Sorry for all the questions. But, hopefully you got some idea of what calibration is. Please repost with a little more information when you can, so we can provide the best advise possible.
 
J

Jimbo1

#4
Andy and Brad thanks for your help.

I am not the one who will calibrate or lay the fixture out. It is to be calibrated yearly and at this point that's all that I know for certain.

Brad, this is a 3-D gage that a welded assembly is placed onto. The primary, secondary and tertiary datums are engaged one at a time until all translation and rotation is eliminated from the assembly.

There are features such as stab pins and blocks on the gage that are used to check for true position of holes and location, orientation of other features. I'm assuming that these pins and blocks are to be verified or calibrated to make certain they're still checking the assemblies properly to CAD or the print. I hope this gives you a better mental picture.

Again, Thank you for your help.

Jimbo
 
S

sathis

#5
Normally , it is done based on the drawing of the fixture .
which contains Co ordinates X,Y,Z and origin.
We did using 3 Axis CMM.
By the way our part is an Exhaust assembly for cars.

:bigwave:

Sathis
 

BradM

Staff member
Admin
#6
Like Sathis stated, I have seen verifications of tools like this made by taking the drawing, establishing some critical parameters, and having them verified. I say critical parameters because checks like these can get very expensive and time consuming. Andy's suggestion was a good one. Do you have access to a CMM? How does it's uncertainty compare to that of the device under interest?

Another thing may be to, say, pick some components of the system and verify them. Assure no wear in certain areas, verify pin gauge sizes, take some dimensional measurements x, y, z, etc.

Just be aware of the tolerance required for your fixture dimensions. Your requirement will determine your different options for performing this task (and what kind of budget you will need).
 
A

andygr

#7
I see this as a couple of elements.
First is the validation of the function of the fixture relitive to the part design requirements. The standard rules of not using all the part tolerance and keeping datums the same ect need to be applied and reviewed. This can be done by review of the check fixture print you will use to create the fixture to the part design print.

Next you make the fixture(s) to their print.
To validate the fixtures you then have them inspected to the fixture print ( calibrated, as you will record the actual "as found" values). You will check all the features of the fixture print this initial time.

Last after some time ( set based on wear and risk) you will recheck the check fixtures ( calibrate them). Now keep in mind that not all features of the fixtures will see wear and so they can be verified by a visual check of general condition ( but would probaly need dimensional check on some infrequant basis). The features that are subject to wear or drift you will need to remeasure these features at each calibration and based on actual.
Hope this helps
 

Jim Wynne

Staff member
Admin
#8
I see this as a couple of elements.
First is the validation of the function of the fixture relitive to the part design requirements. The standard rules of not using all the part tolerance and keeping datums the same ect need to be applied and reviewed. This can be done by review of the check fixture print you will use to create the fixture to the part design print.

Next you make the fixture(s) to their print.
To validate the fixtures you then have them inspected to the fixture print ( calibrated, as you will record the actual "as found" values). You will check all the features of the fixture print this initial time.

Last after some time ( set based on wear and risk) you will recheck the check fixtures ( calibrate them). Now keep in mind that not all features of the fixtures will see wear and so they can be verified by a visual check of general condition ( but would probaly need dimensional check on some infrequant basis). The features that are subject to wear or drift you will need to remeasure these features at each calibration and based on actual.
Hope this helps
This is good advice. Another thing that needs to be done is some sort of MSA that proves that the device will always identify "bad" parts and never reject good ones. This means that you will need nonconforming parts, which might have to be deliberately fabricated in nonconforming condition for each of the features or attributes that the device is intended to verify. You should be able to construct a written theory of operation for the fixture, that explains what it does, and how it does it, with respect to specifications.
 
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