CNC Programming

[qman]

I have a question about CNC Programming and would like any input on this situation. There has been a major discussion on how we have been running some machined aluminum parts that are a major component in our product. We have a tolerance that is +/- .0003 and .0005 very tight. We are clamping the parts in the middle of the part, leaving most of the part hanging out, this is being done on a horizontal machining center with the fixture in the middle of the table. We machine the parts on all sides and the tool hangs out almost as far as it can when machining. The sizes vary from 8" x 5" x 3" deep, to 27" x 32" x 11" deep. We make a wide variety of part in the same way with the same problem.
Here is what the machinists are doing. If the nominal dimension on a through bore say X1.5000, Y0.0000, Z3.0000. They will measure the part on the CMM and if all is not green they will do this make it work. At the entrance point they will put in X1.4992, Y-0.0004 and the exit point will look something like this, X1.5006, Y 0.0007. This is done across the board on all sizes. The interesting thing is that anything that is not measured on the CMM always remains at nominal. The big problem is that assembly is having nothing but problems putting these part together the mating parts hit and cause lock'ups. Assembly has to sand and pretty much custom make these parts, it is very frustrating for them.
We check the housings on the CMM .250 on the entrance side of the bore and rotate the probe and check the exit side. Leaving 2.500" open and not checked. We also use 3000 PSI clamping pressure for roughing and reduce it to 1000 PSI for finishing. We have taken the fixture out and checked the parts at the different pressures and it is very dramatic differences. One more thing. When they rough bore, the numbers are at nominal. When we finish bore that is when the numbers change all over in the program. Also when they machine the two faces they will do a little ramping of the mill so they come out parallel.
I have an understanding of the problem but am having a hard time convincing others. The machinists have been doing it so long they think it is ok. The managers don't understand and blame assembly for sanding, etc. The answer is always that everything is ok. Look at the readouts on the CMM, the numbers are "green". I have done experiments of alternate ways of measuring and found that there are problems with the bores being straight, when I try to change the program to better show the problem. I am told to change it back because no one can figure out how to read the new results. So it is a very steep uphill battle. I am running out of ideas, and everyone is getting frustrated about why nothing works when all of the readouts are "Green" Sometimes I feel it is better talking to a wall.
I have made charts and showing what is going on. We have done studies proving that there is a problem, yet it always comes back to the results are good because it is green.
Does anyone have any ideas, about what I can do, it is becoming frustrating.

[PBMW]

You have an engineering issue.
I can take any part and show you on a CMM that it is good. If the design parameters are being met, and there are assembly problems, then the design parameters are incorrect, or the assembly proceedures are incorrect. One of the two.
It would be the responsibility of engineering to rectify the issue.

[Mitsui Seiki]

Are you boring this hole from two sides?With different coordinates?
If so,no wonder assembly can't make it work.

[wganders]

qman,

when was the last time the machine was calibrated? it seems like the axes are out of alignment.

[Mitsui Seiki]

Can you take a pic of the fixture with the part and post it?

[big_mak]

Dude, 1000psi Clampng is way over kill for some parts when finish machining. We've got parts, that have the nuts clamped finger tight, with out a wrnech for final skim passes to eliminate distortion. From the sounds of things though your machine is out of alignment, or you fixture is not in the position you think it is.

I'd have the machine alignment checked first, then make sure your work coordinates are correct. this is the best we can do with out seeing the part.

Also if your methods on the CMM are all out of whack, you can have results that are not repeatable, as well. I've seen it before. Really.

[WayneHill]

There are too many variables to pinpoint a cause of your problem or problems.

When you say the part is hanging out and the tooling is hanging out, that is a sign of vibration.

Changing clamping forces moves things around also. These problems are compounding each other.

Rough bore, finish bore, final finish bore again with a spring pass to remove tool pressure.

[big_mak]

qman,

where are you? What's going on with this problem?