Ovality issues for aluminium parts

[Viru]

A peculiar situation is occuring while boring parts on a high speed vertical machining centre. The machining fixture holds 4 identical parts (electric motor housings) vertically

The combination tool use to finish ø80 & ø24 on gives a perfect job at the bottom most position but the ovality on ø24 goes on increasing in the horizontal direction from the second job to 4th job upwards.

Did a lot of trials including using single boring bar but to no avail. Could somebody give any directions on the above phenomena

[joecnc1234]

Are you sure you have the distance between parts calculated correctly. If so could be a backlash problem. but usually if the problem gets worse the further out you get it is a compounding dimensional error.

[big_mak]

How are you claming the part?

You may be amazed at how little it takes to clamp a part for finishing bores to diameter, or to achieve flatness for something.

Be really carefull to make sure you are only removeing a minimum of material, and your tools are really sharp.

And for clamping, use a torque wrench to take out the guess work, once you have established a reasonable claming pressure.

I've worked on parts, that have been clamped so lightly in a vise, you could pull them out with two fingers. This means adjusting your cutting techniques to suit the fixturing means, and some delicate parts demand a delicate touch.

Show us a pic of your setup, and we'll better be able to help.

[Viru]

Thanks a lot for the suggestion, what I would like to clarify further is that the bore for Job No 1 comes ok for ovality but the ovality same bore in the other 3 jobs goes on progressively increasing.

[Verfur]

sounds like it could be wear in the drive screw. or possibly screw alignment.

Has the machine undergone any heavy crash and or work with lots of vibes.

even the ways/guides may have wear in the 2nd 3rd and 4th placements.

may want to break out the ol' indicator and give it a test as it sounds like a possition problem.

just my thoughts

[big_mak]

I think it is a clamping issue, if these results are obtained by a boring bar, or drilling tool.

If it was from interpolation, then yes I'd say it was a position problem.

Can you show a pic of the setup?

[Verfur]

in what direction are they big and are they all they same direction??

If the fixturing is the same for all then I would say the problem is not fixturing/clamping.

And even with a boring tool if there is lack in rigidity, it can move .0005 of an inch of slack is .001 out of round and for a bored hole thats bad news.

.0005 inch out of position for circle interp is about norm for most standard production machines.

Just two bits worth

[big_mak]

My concern is that he is most likely holding castings, which means there will be size variation, and if we can't see what is going one fixturing or machine wise we can't really tell what the problem is.

He's got 4 up at once 1pc good and 3 not. I would check to see how things are clamped, then if you can rule this out, you can go and check something else.

Start with the variables that are human input, generally if you can control these, you will have a better chance of controlling the process.

Once you have eliminated the human factor, then you can go and look into more stable inputs such as program, then the machine.

My biggest thing to check here is that the clamping is equal over all four parts. Are you using one bar clamping across all 4 parts? Then maybe you would need to look into the tightening sequence of the bolts and such.

Sooo many factors that can't really be checked by the info that has been given so far.