Why out-of-round holes?

[BobM3]

We're finding some bores in our parts aren't very round (as measured with a 3 point bore gage). They are 1.125 Dia x .375 dp in aluminum. The bore diameter varies by .005. I'm surprised this could happen as the bores are made with a boring head on a mill. Has anyone seen this before? How could they be so out-of-round with a boring operation?

 

[CSLufkin]

Only times I have seen it was when the part was moving (which usually looks horrible, finish wise) or when material hardness was not consistant.

 

[mrainey]

Are you clamping the part too hard or in the wrong place?

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[unclesyd]

How much is the tool bit hung out of the boring head?
What size is the bit?
What are the feed rates?
Or any other information about the tooling.

 

[Kapitan]

Can you put a picture on here of the part and set-up?
[ul]
[li]How much material thickness is there around the bore?[/li]
[li]Is the part held in a vice?[/li]
[li]Does out-of-round mean the hole is elliptical, or does it have three lobes or what?[/li]
[/ul]

 

[ajack1]

Apart from what others have said, especially clamping is this done early in the manufacture of the part, namely large pieces of material are moved after the bore is finished?

It could be the aluminium is not that stable and “moves” after the bore is finished, maybe a finishing cut on the bore as the last operation (if practical) would solve the problem?

 

[BobM3]

The parts are made on the outside so I don't have specifics on how they are made. From what everyone has said, it sounds like it is possible for the holes to be out-of-round by as much as they are. Thanks for all the inputs!

 

[MadMango]

Anything is possible, but are the parts acceptable?

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[BobM3]

No, they are not. We press oil seals into the holes and the bad holes produce leaks between the bore and the seal OD.

 

[ajack1]

Have you spoken to your supplier, why are they having problems?

 

[vzeos]

It may be possible that the milling machine spindle has some lateral play and when you use a single point of contact tool, such as a boring head, the un-equilibrates forces on the tool bit might cause the spindle to oscillate thus creating an out of round hole. Try roughing the hole with the boring head, then use a rose reamer to bring the hole to size.

 

[ncordova]

Rgaseng, correct and important, for most precision they have to use a reamer. The Boring head is not so precise.

 

[ajack1]

Not sure I agree with the last two posts, in fact I am sure and I don’t.

If a spindle has enough play in it to cause a finishing cut on a bore to be .005” put of round then it should be in a scrap yard.

Boring is far more accurate than reaming, if a spindle has that much, the reamed hole will be a real mess.

 

[mrainey]

"Not sure I agree with the last two posts, in fact I am sure and I don’t."


Neither do I.


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[moldcore]

.005 out of round is impossible if the hole was bored with 100% cleanup. Could it possibly be that the hole was roughed out too big, i.e. with an end mill or drill? If it was a blind hole then it was probably roughed with an end mill perhaps at too high a feed rate for the machine to hold a round profile.

 

[RRBD]

Boring is way more accurate than reaming. You state that the holes are bored, are you sure? Are all parts consistant, as in out of round the same way? This problem will be a simple fix, like clamping pressure,or "bores" are being interpolated in a mill with bad axis compensation. If its a spindle problem, believe me they would know it by the finish. First thing to do is fix up the print, my suspicion is that its not up to snuff or the parts would be back at the vendors shop getting remade on their dime. Remember, Accuracy dictates process, always. If the print is good, let them worry about it.

Scott

 

[BobM3]

Mystery solved! The shop that does these tried to cut corners on the last run and milled the bores to the finished dimension (actually not quite - they weren't to print) instead of roughing them and then finishing them with a boring head as they had done for years. Thanks for all the info. I was armed with knowledge when I called him.

 

[unclesyd]

Thanks for the follow up on the successful resolution of your problem.

This is a good lesson in resolving problems if something has been working and quits all of a sudden. The first question always is "Has anything changed?" and push for answers.