Tolerance Problem - How to consistently measure a mass production item

somashekar

Staff member
Super Moderator
#11
Talking from an other angle, are you in a customer complaint on this part for not meeting dimensional requirement? Tolerencing in parts sometimes exceeds the need and is ignored in some engineering drawings.
 
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russa2

#12
We do have a customer complaint open. Not because of need, but because we just recently aquired this business from a now bankrupt supplier, and the corporate guys want a complete PPAP that meets ALL tolerences.
 

somashekar

Staff member
Super Moderator
#13
We do have a customer complaint open. Not because of need, but because we just recently aquired this business from a now bankrupt supplier, and the corporate guys want a complete PPAP that meets ALL tolerences.
Great .. Can you make an attempt to look into and cause the tolerance to be modified based on the application, matching parts tolerances and your process capability to manufacture and measure. PPAP can follow this successfully.
 

bobdoering

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#14
I have attached a picture of the measuring machine that we currently use, and that the customer uses. Operation is simple.

  • Place part onto center spindle
  • Press palm buttons (not seen in picture)
  • Machine closes, and measurement is indicated.

As far as the chamfer, it is just enough of a chamfer to knock the edge of, so there are plenty of the ground surfaces still present. We have not really seen a consistent burr issue on these parts.

The parallelism has a tolerance of .0002” as well, but that has not been an issue when measured. The capability studies that I have conducted indicate that we can hold the parallelism and perpendicularity with no problems on these parts.

The fixture is made of heat treated steel. But as far as the environment, it is basically a warehouse that stays around 65 degrees consistently.

It appears by looking at the gage you are measuring Overall Length at MMC, with one end of the part as datum. Is that correct?

Cutting the chamfer after grinding, any burr will go into the measured plane. Grinding after the chamfer would mover the burr outward - out of the measured plane. This is not a suggestion to change your process, as there are other issues, but just basic burr chasing issue.

A parallelism with the same tolerance of the OAL means it can use up the whole tolerance with it. That means, for example, .0001" or less can be available for OAL variation if .0001" is used up for parallelism. Why do you feel comfortable with your parallelism measurement, but not your OAL measurement?

I am not sure my looking, but is there any chance the part is cocking during measurement? If it can, it will provide error.

How much pressure is applied? For such tight tolerance, infinitesimal pressure should be used, as to not provide any part flex.

If you measure the same part over and over - without moving it, how much variation do you get from this gage?

What is the gage resolution?

Is the gage calibrated as a system, or is the indicator taken off and calibrated independently?
 
J
#15
Just to toss something else out there, how much is temperature effecting the measurement?
With a tolerance of .0002 it would seem that temp of part/guage etc could be a big factor.

I recall that we use to make a part for a customer that they were constantly complaining was wrong. We even made a fixture on which every part was checked before shipment. They still complained. It turned out that the issue was temperature.
This was in a teflon part and not dealing with such close tolerances but over the 50 or so inches of the part we were finally able to demonstrate that temperature chnges were the culprit.

So - Do you see teh same type of variation in parts that have been left nect to the measuring devise for 24-48 hours to normallize in tempurature?

Just a thought.

Also you say you got this job from a company that went bankrupt. That sounds ominous.:notme:

Peace
James
 

bobdoering

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#16
Just to toss something else out there, how much is temperature effecting the measurement?
With a tolerance of .0002 it would seem that temp of part/gage etc could be a big factor.
Good point. Just the heat of handling the metal part can make a difference at this tolerance. Tubing seems to magnify the problem.
 
R

russa2

#17
Somashekar,
Thanks for the suggestion, but I already tried that route. I conducted several studies where I observed the line operator all day and recorded which spacers would give him the readings that he needs to pass the part. There range is 18-50 ft. lbs., I know a big range. But there is zero consistency in which spacer he chose, just what ever made the rear end fall into that range, and normally several different spacer would fit that criteria. But the responsible engineer is still not willing to loosen the tolerance. As a matter of fact the last time I was there observing, he actually went on the floor with me, and I had to explain their process to him.
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 love all of the discussions that are going on in this forum. As a matter of fact I took bobdoering’s suggestion of cranking up the air on the measuring device to see if it makes a difference. I did not get a chance to run a study today, but I will over the weekend. I did observe thought that with the increased air pressure, the top of the measuring machine flexes quite a bit. I may end up reinforcing it in the long run. I don’t know, but it is another suggestion toward solving this problem part. Thanks.
 

bobdoering

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#18
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 love all of the discussions that are going on in this forum. As a matter of fact I took bobdoering’s suggestion of cranking up the air on the measuring device to see if it makes a difference. I did not get a chance to run a study today, but I will over the weekend. I did observe thought that with the increased air pressure, the top of the measuring machine flexes quite a bit. I may end up reinforcing it in the long run. I don’t know, but it is another suggestion toward solving this problem part. Thanks.
If the customer does not measure these parts in a controlled environment, then they are not comprehending how tight of a tolerance they have required. You are dealing in millionths. You have a 200 millionth tolerance, you need 20 millionths - minimum - resolution just to have any idea where you are at. This is tight. Temperature counts, and ignoring it is just not getting it. This is a bad start.

Also, I suggested lowering - not increasing - the pressure. Any flex - either in the part or the gage - is error. If you see any movement of the part when the measuring head comes down on it, it is variation. It will need to be eliminated.

This is a tough one to work through from a distance.
 
R

russa2

#19
Sorry, I misunderstood the air pressure suggestion. We were at 60 psi, I raised it to 90 psi, and it flexed a lot. I did not see how that would help, but based on how bad our past studies were, it was worth a try.

But, I will try lowering the PSI now to see if it helps also.
 

bobdoering

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#20
Sorry, I misunderstood the air pressure suggestion. We were at 60 psi, I raised it to 90 psi, and it flexed a lot. I did not see how that would help, but based on how bad our past studies were, it was worth a try.

But, I will try lowering the PSI now to see if it helps also.
I am not sure how the device is built, but really the weight of you upper platen alone should be more than enough on an indicator to take your measurement. Having auto return is not such a bad idea, but downward force is not so good for this measurement - less the better. If you were at .250-.450, then pressure to seat the part may not be so bad. But, you can not do this with tight tolerances, because of part and fixture flex.
 
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