My part was rejected by CMM

I made a simple wear ring out of Ni-Resist material on a CNC lathe, 12.000/11.999 O.D. and 11.498/11.497 I.D. and 2.0 long, the ring went .005 out of round when parted off but that is within the allowable spec, the ring is going in a pump made for a nuclear plant so it go's through a complete inspection when the buyer get's it, so it get's rejected by a CMM machine because they claim the O.D. is .002 oversize, they return the ring and upon re-inspection I think it's fine, I can average out the size to 11.9995 and 11.4975 and the wall thickness is .2511 on both ends of the ring measuring with a calibrated ball mic., so if they claim the O.D. is oversize the wall thickness would have to be .001 thicker!!! We don't want to accuse our buyer of not knowing how to inspect a simple ring. Anyway to fight this result without upsetting the buyer?

Cuda said:

I made a simple wear ring out of Ni-Resist material on a CNC lathe, 12.000/11.999 O.D. and 11.498/11.497 I.D. and 2.0 long, the ring went .005 out of round when parted off but that is within the allowable spec, the ring is going in a pump made for a nuclear plant so it go's through a complete inspection when the buyer get's it, so it get's rejected by a CMM machine because they claim the O.D. is .002 oversize, they return the ring and upon re-inspection I think it's fine, I can average out the size to 11.9995 and 11.4975 and the wall thickness is .2511 on both ends of the ring measuring with a calibrated ball mic., so if they claim the O.D. is oversize the wall thickness would have to be .001 thicker!!! We don't want to accuse our buyer of not knowing how to inspect a simple ring. Anyway to fight this result without upsetting the buyer?

Click to expand...

Your customer isn't measuring as you do. That's often a problem right away. As to "a simple ring" with the customer, diameters and tolerances you mention then I don't think many would regard that ring as simple. On 12" you have an OD and ID tolerance of 0.001".

Why does it go "out of round" when parting? Too much "pressure"? Have you "cushioned" the fall?

The bottom line for me, with the information you give, is that the customer is right.

Are there any geometric callouts that effect the usable size?

Unless there is something that you haven't included in your post that "gives" you a wider tolerance, I'd have to agree with Gordon that the customer is correct.

I will go so far as to say that if there is some engineering "folklore" regarding this part that THEY know about but haven't bothered to put in on the print, they should be willing to eat some...but not all....of the cost of rework/remake.

Mike

(edit) And there is no such thing as a "simple" ring when the total tolerance is .0083% of the OD of said ring

Customers always right. Try to make one without the "out of round feature" it is easier to check that way. is there a temperature thing going on between you and the buyers plant?

Face the facts. You said "yes" to what you thought was a simple ring and "forgot" it was for a nuclear plant. On top of that you don't have suitable measuring equipment.

Cuda
I think you are going to be making it over. If it's a good customer I would take my lumps and just do it. I would maybe say "Sorry we are going to make another one but this is how I checked it" That way he won't think you sent him out a bad part on purpose and maybe he will give some thought to accepting it. I doubt they will it's a Nuke application. Not really a simple job, that large a Dia. and such a thin wall I would expect it to spring some. Come up with another way parting it off is not going to work for this one. Is the stock expensive ?? Good luck man.

Another note.
Once I had my shop going and we had the machines to put out the work I focused on our ability to check the work we were doing. Investing here has saved me major $$ and given me the confidence to take on just about anything in our size range. A good CMM and optical comparator are huge time savers. At the start before investing in these tools some of our work would take hours to inspect and depending on the complexity of the part and method used to inspect it, you could get conflicting results on close tolerance hard to check features. Many times we just had to do our best to check it and trust our set up. I have better inspection capability than most of my customers. That alone has taken the shop to another level.
The need to check the parts we produce is often over looked in small shops due to the cost of the tools. It's tough to spend 20 grand or more on something that dose not make chips and sits unused half the day. But it has allowed me to take on difficult work with confidence and produce it at a profit.

Ron

What's the "allowable spec" that allows a ring with +-.0005 tolerance on the OD and ID to be .005 out of round?
Oh ya, the customer is always right.
I hate CMM's, seem to spend way to much time telling the CMM guy how to measure a part.

Conflicting temperatures are your enemy inspecting a part like this.

Maybe ship it for late afternoon delivery, so they won't get to it until the following morning...
Chip

Have it back and bend the out of round out of it

Cuda said:

We don't want to accuse our buyer of not knowing how to inspect a simple ring. Anyway to fight this result without upsetting the buyer?

Click to expand...

Short answer.... No.

I'd think you may have to make it oversize, part off, and then do your final cuts holding the part axially against a face plate.

As has been said, temp is also important. IF I did my math right, assuming a coefficient of linear thermal expansion of 8.5, and a 12.000 OD, I get a .00051 diameter change with 5 degree temperature change.

Change in Radius of Thin Circular Ring with Temperature
Thermal Expansion Metals


What kind of quantity do you have to do?

You say the part is oversize on the OD ,does that mean they are saying the ID is OK ,as you say it is relatively easy to measure the wall thickness and they don't seem to be complaining about the out of round.

Is it possible that you have an issue with rounding errors ,another possibility is that you are making your mike read what you want it to,that is easily done.

I would be very careful to make sure you that you are definately right before you challenge the insection ,that could mean getting a third party with the right equipment.

Is there anything to stop you polishing the OD with some emery? ,it would be a bummer to polish a couple of thou. off and have them reject it for being too small.

Gordon B. Clarke said:

Face the facts. You said "yes" to what you thought was a simple ring and "forgot" it was for a nuclear plant. On top of that you don't have suitable measuring equipment.

Click to expand...

I did not "forget" it was for a nuclear plant and we DO have good equipment, we have a 80K length measuring machine for calibrating standards and other things. And I know how to measure things.

Gordon B. Clarke said:

Y
Why does it go "out of round" when parting? Too much "pressure"? Have you "cushioned" the fall?

Click to expand...

Part was made from a thick wall spin cast material, so chuck pressure isn't an issue and part was caught and did not fall. I've made thousands of rings in the same manner, even on the same stick of material you may have one ring stay round within .001 and the next may go out .010, the the next .001, it's just due to material stress.

bukwerm said:

I'd think you may have to make it oversize, part off, and then do your final cuts holding the part axially against a face plate.

Click to expand...

Been there, done that before and it's too time consuming to make money on this type of part.

sable said:

You say the part is oversize on the OD ,does that mean they are saying the ID is OK ,as you say it is relatively easy to measure the wall thickness and they don't seem to be complaining about the out of round.

Click to expand...

They said the O.D. was big and the I.D. was OK, but by doing simple math on the wall thickness this doesn't add correctly as the ring would have to be .001 thicker than it is.

Are you able to have the material stress relieved? Having that much inconsistency from one part to the next is not good, it either speaks to inconsistent stresses in the material or inconsistent machining process, from what I see you saying its not your machining practices so that leaves the material.

I routinely cryogenically freeze material after heat treat and even if the part does not require heat treat for stability. Being that this is a nuclear part you may not have that possibility but it is worth looking in to.

Joe

are you using a calibrated 12" gage block to calibrate your micrometer before measuring? maybe use an indicator to comparison check the ring to the 12 inch gage block on your surface table.

Do you pre part before finishing to relieve holding pressure stresses?

Cuda said:

12.000/11.999 O.D. and 11.498/11.497 I.D.

Click to expand...

One question that's not been asked (or I missed it):

What is the measurement accuracy and resolution of the instruments being used for this measurement, both yours and the customer's.

An instrument that reads in half-tenth increments is useless for this.

You need accuracy and resolution of ±0.0001" or better to have confidence in the values obtained.

Also, what is the ambient temperature in the work area as compared with the temp at which the instruments were calibrated?

- Leigh