.140 True Position on a Large Composite Part

[JakG]

Hi everyone,
I am dealing with a small issue with one of our composite parts. For background, this is a composite enclosure that stores the aircraft evacuation system. This enclosure is off the wing, therefore it has awkward geometry it is 61" long and width of 45" tapering down to 20" overall whole length. This part is made via vendor and then we shipped to the customer, unfortunately vendor is only able to meet True Position of .140 on 33 countersunk mounting holes, spread all around the flange of the enclosure. The problem is, our company and the vendor agreed on true position of .020, and to get through FAI process they froze the design for now, however the vendor promised to lower the tolerances as time goes on. My issue is, I find it very hard to believe that even with awkward geometry, this length and proper fixturing, the vendor is not able to meet true position tolerance of .020. Unfortunately I do not know their set up, however I am aware that they are using a CNC mill machine to do this. My question to you is, do you think their fixturing is not adequate enough? Are they not compensating for thermal error, and with drill heating up it can warp the part a little and throw the position off? Due to improper fixturing, if part vibrates when its being machined, can that throw off the position or just size?? Any input or potentially better questions are welcome. Thank you!

 

[3DDave]

We had a case where using a jigged and fixtured part setup on the CNC we kept getting pushed for larger and larger composite tolerances.

This was because the machine operators felt it was not their job to clean chips from those fixtures; just letting them build up in huge drifts.

Hole-to-hole tolerances were 0.005 inch diametral variation but tolerances back to the datum targets were closer to 0.250 diametral variation.

So, it could be that there is a problem with the datum feature selection or sloppy placement on the machine; check hole-to-hole to see if there is any other variation such as thermal expansion or other causes.

 

[JakG]

We had a case where using a jigged and fixtured part setup on the CNC we kept getting pushed for larger and larger composite tolerances.

This was because the machine operators felt it was not their job to clean chips from those fixtures; just letting them build up in huge drifts.

Hole-to-hole tolerances were 0.005 inch diametral variation but tolerances back to the datum targets were closer to 0.250 diametral variation.

So, it could be that there is a problem with the datum feature selection or sloppy placement on the machine; check hole-to-hole to see if there is any other variation such as thermal expansion or other causes.

Thanks for the input Dave,

It seems like any research I do it comes down to fixturing. I spoke with one our machinists and he actually suggested to have them do the "Ball Test" on the CNC machine. Which I may have to see if that was ever done at our vendor.

 

[ghostdigital]

"vendor agreed on true position of .020" likely somebody other than the tech build it folks..

"composite parts" This material moves around and rarely holds form in an unrestrained condition. Tolerances are measured with a component in a free state condition unless otherwise specified.

"I find it very hard to believe that even with awkward geometry, this length and proper fixturing, the vendor is not able to meet true position tolerance of .020. "

I don't, maybe if the engineering drawing had a note like:

Indicated Tolerances may be verified in a restrained condition as shown on page y, zone XXX.

For non rigid parts, I've used dual tolerances, one for Free State and other for restrained (like it is used).

 

[ctopher]

When I used to do composite parts, it was all in the fixturing. The parts needed time to cool first, then modified. If the fixturing was off, not used correctly, worn, or warm parts, the machining could be off.

 

[JakG]

"vendor agreed on true position of .020" likely somebody other than the tech build it folks..

"composite parts" This material moves around and rarely holds form in an unrestrained condition. Tolerances are measured with a component in a free state condition unless otherwise specified.

"I find it very hard to believe that even with awkward geometry, this length and proper fixturing, the vendor is not able to meet true position tolerance of .020. "

I don't, maybe if the engineering drawing had a note like:

Indicated Tolerances may be verified in a restrained condition as shown on page y, zone XXX.

For non rigid parts, I've used dual tolerances, one for Free State and other for restrained (like it is used).

Ghost,

good points regarding having restrained vs unrestrained condition. From what I saw, I do believe they restrain it during inspection, however it may not be to the same force as manufacturing. I will look at this.

 

[JakG]

When I used to do composite parts, it was all in the fixturing. The parts needed time to cool first, then modified. If the fixturing was off, not used correctly, worn, or warm parts, the machining could be off.

When you're mentioning "cool off" are you speaking between each operation? Per x amount of holes? I def see with my part having 33 mounting holes and doing all at same operation, the heat may build up.

 

[ctopher]

I mean cool off after coming out of mold.