Mill Boring bar cut a taper... how did this happen?

How old is your boring head, and how is the bar secured? Heads do wear with age. Often times if the tool isn't secured tight enough (usually 2 set screws) either the tool will move in the head, or will cause enough vibration that the position of the slide will actually change.

Try making a few more cuts paying close attention to the scale when starting a pass and look again before retracting it for a second pass. If the scale hasn't moved you could have a problem with the bar moving in the head, or actually flexing. I know larger heads can withstand cuts as deep as .040" per pass. However I believe that's way too much for a 2" head. The maximum depth of cut I would make is .020", (.010" is more reasonable with cheap Chinese made sets) with several passes on the finish cut at a maximum depth of .005".

I would also use the shortest, thickest bar available to minimize the chance of deflection.

As for the mill, if there is runout it will be magnified the farther the quill is extended. To minimize the runout I would retract the quill as far as possible, and raise the table to bore the hole. It will take a little practice to get a mirror finish, but it can be done.

I also use the bars pictured in John O's post. Often times they come from the factory poorly profiled. I have seen them have little or no cutting edge, tapered in the wrong direction with the cutting edge on the top, and on one set the steel backing bar was extending beyond the carbide. There are some US and German made sets that are much higher quality. They do however cost considerably more.
 
It was a few years ago. I found the reason some where on the internet but can't find it back now.
 
projectnut said:
How old is your boring head, and how is the bar secured? Heads do wear with age. Often times if the tool isn't secured tight enough (usually 2 set screws) either the tool will move in the head, or will cause enough vibration that the position of the slide will actually change.

Try making a few more cuts paying close attention to the scale when starting a pass and look again before retracting it for a second pass. If the scale hasn't moved you could have a problem with the bar moving in the head, or actually flexing. I know larger heads can withstand cuts as deep as .040" per pass. However I believe that's way too much for a 2" head. The maximum depth of cut I would make is .020", (.010" is more reasonable with cheap Chinese made sets) with several passes on the finish cut at a maximum depth of .005".

I would also use the shortest, thickest bar available to minimize the chance of deflection.

As for the mill, if there is runout it will be magnified the farther the quill is extended. To minimize the runout I would retract the quill as far as possible, and raise the table to bore the hole. It will take a little practice to get a mirror finish, but it can be done.

I also use the bars pictured in John O's post. Often times they come from the factory poorly profiled. I have seen them have little or no cutting edge, tapered in the wrong direction with the cutting edge on the top, and on one set the steel backing bar was extending beyond the carbide. There are some US and German made sets that are much higher quality. They do however cost considerably more.
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Its pretty old? Its a Criterion that I bought off a retiring machinist. I also got the bars off of him, they seem like high quality (not chinesium). I will admit I left a smaller bar in than I could have used, so maybe that is it.
 
John O said:
I used boring bars like these, they cut a taper. Switched to indexable and taper was gone
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If the indexable tool cuts parallel, you can probably skip the dti test. The feed axis is perpendicular. The problem is deflection of the tool. Maybe not bending of the shank, but deflection of the entire tool. This is often caused by incorrect sharpening causing the tool to cut more on the side than the front. Compare the grinds on the brazed bars with the indexable tool. There might be a clearance issue on the front of the tool.
 
ErichKeane said:
I was getting a surprising amount of vibration in my cut, so maybe something worked its way lose...
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So that is the first problem to address.
Machine problem? Boring bar dull? Using cutting oil?(cut brass dry or it may chatter) Boring head gib loose?etc.
 
I'm going to guess that you might have excessive spindle wear. It sometimes shows up as bell mouthing of a bore. The deeper you go, the more constrained the bar is and the taper is accordingly reduced at the bottom. You won't detect this when measuring static run out because its a dynamic thing but you might be able to see it with some careful measuring and comparing that with the specs of the spindle bore.

It can also happen if you had a chip or debris in the spindle taper when you inserted the shank of the boring head. It might be a good idea to clean the taper well and see if you can reproduce the bell mouthing. If you are sure the spindle is clean and the shank of the tool is clean and you still have bell mouthing then you may have a spindle wear issue.
 
At first glance, .050" seems like a lot of taper, but to give us some perspective, how thick was the piece that you bored? As others have pointed out, the deeper the hole, the more the quill must extend, thereby giving the bar more leverage to deflect the spindle.

Tom
 
higgite said:
At first glance, .050" seems like a lot of taper, but to give us some perspective, how thick was the piece that you bored? As others have pointed out, the deeper the hole, the more the quill must extend, thereby giving the bar more leverage to deflect the spindle.

Tom
Click to expand...
Probably about 3/4 "?
 
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