Tolerance Problem - How to consistently measure a mass production item

J
#21
JRKH, and bobdoering,
On another note, heat does affect these parts greatly. For one trial run when the customer was at our facility, I actually stored the spacers in my office over night and set the air conditioner at about 50°. It helped the first time we measured the parts, but after they sat for awhile, my advantage disappeared.
But, the customer is not going to measure these things in a controlled environment, so that is not an option for us.
I completely disagree with the bolded statement. It appears that you will need to educate your customer just as I had to do with mine.
Once you sort through all of the issues relating to getting consistant measurements and have determined the correct methods, the customer may need to have this explained to them.
This may require several attempts but, if necessary, it must be done.

As an example:
You will likely determine that, because of the coefficent of expansion of the material that certain conditions must be maintained in order to get consistant readings. Such conditions may include leaving parts in a room for temp and humidity for 24 to 48 hours, use of gloves to prevent transfering heat or dirt to the parts, use of a specific dedicated guage etc.
When the customer rejects parts you have shown to be correct, you will have to demonstrate that the coefficient of expansion makes accurate readings impossible unless teh controlled conditions are met.

This is what happened to me. We went round and round with our customer about the teflon parts I mentioned in my earlier post. Finally I had them send a part over (they were local) and the owner came by the next day. I demonstrated that the part they had rejected fit perfectly onto the inspection jig we had made. I then told him and our owner to come back in an hour. They went out to lunch and I placed the teflon part in the back of my pick-up truck with the cap closed up (it was July) while keeping the jig in the inspection lab.
When they came back I said come with me, picked up the jig and we went to my truck. I opened the tailgate, laid the jig down and the part would not fit. It was almost 1/4 inch longer. It would not fit on the jig.
FINALLY the customer (and our owner too) understood the very basic principle that materials expand and contract with temperature changes - sometimes a lot. The tighter the tolerance, the more sensitive the temeperature variation.

It may well be that in order to keep this contract, there will have to be a protocol between you and the customer as to exactly how and under what conditions these parts will be checked.

Peace
James
 
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S

Sturmkind

#22
The parts may, or may not, meet the perpendicularity tolerance. The comments about measuring device resolution are bang-on target. The only valid way to check perpendicularity is with a high precision rotary table device like a Talysurf (www.taylor-hobson.com) or Mahr Formtester (www.mahr.com).

The indicator that was pictured does not appear to have the needed resolution of 0.000200" but both of the companies above have solutions that do resolve to that degree. Chances are they would be willing to come to your location and demonstrate them on your parts.

I worked for a manufacturer that maintained and was capable at such tolerances. To help our customer (and new hires) understand the effect of temperature we place a precision probe (depth gage) between 2 gage block stacks in a closed room and recorded the readings at 1 hour intervals -they remained constant to 0.000040". Then we turned on a fan......the indicator needle buried itself at maximum range of -0.00025" in under 1 minute. Hope this helps!
 

bobdoering

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#23
The parts may, or may not, meet the perpendicularity tolerance.
The OP had a parallelism tolerance, not perpendicularity - although your suggestion for measuring perpendicularity is a good one.

Excellent idea to show people the effect of temperature on tight tolerances! I may add that demo to my metrology class!

20 millionths gage resolution is the worst case - it really needs to be much better so that they can do SPC (which would be appropriate once they have the measuring system figured out to minimize the sampling and control the process).
 
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