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

E

ErichKeane

Making scrap at ludicrous speed.
H-M Lifetime Diamond Member
Joined
Aug 3, 2017
Messages
2,437
So, I had a problem last night that I ended up solving another way (just ran an endmill down it), but I found myself very surprised by the result! I had a piece of brass that I was boring out with a boring head. I got my top dimension about right (~.800), however when I pulled it out of the mill, the bottom was 50 thou smaller(~.750)!

I cannot for the life of me figure out how this could have happened! I was cutting fairly small passes, and in brass, and it seemed to be cutting the whole way. I wasn't particularly careful about placing the material (it needed to be somewhat straight, but even that was subjective), but I would think that would just result in a tilted hole.

I can only think the boring bar could deflect, but I don't see a reason why it would deflect further as the spindle came down (I used spindle feed to cut it). Any mis-alignment or problem with my mill I would think would just 'tilt' the hole instead of cutting a taper, so I've got no idea!

Thoughts?
 
The boring bars tend to bend the deeper the hole goes, so you should always run the boring head thru a few times on the same setting, especially on a finish cut.
 
That much taper makes me wonder if the boring head setting moved during the cut. When the head was retracted out of the work did you notice how much clearance there was at the top?
 
I'd run it backwards up through the hole while running, and it didn't cut on the way back up, so I didn't think about that being the problem.

Tozguy said:
That much taper makes me wonder if the boring head setting moved during the cut. When the head was retracted out of the work did you notice how much clearance there was at the top?
Click to expand...

Hmm... thats a good thought, I hadn't though of that. I was taking multiple passes and it didn't seem like it was cutting less than it should on each successive one, but perhaps?
 
What condition is the quill in? If it is worn the cutting action will push the quill away. The further out that the quill is, the farther it will push away resulting in a tapered hole. This is one reason to take several "free cuts" as 4ssss suggested.
 
Its bit of a run-out mill, an old burke millrite that saw high school duty. I replaced the bearings a few years ago, and the spindle itself seems to fit nice (and I've never had issues with it extended in the past...), but perhaps that is it? I was getting a surprising amount of vibration in my cut, so maybe something worked its way lose...
 
Sweep the bore with a dial test indicator attached to the spindle. That will tell the difference right away.
 
ericc said:
Sweep the bore with a dial test indicator attached to the spindle. That will tell the difference right away.
Click to expand...
I'm having trouble visualizing what you mean. Of the thing I cut? It is long out of the vice :)
 
I used boring bars like these, they cut a taper. Switched to indexable and taper was gone
1582302684485.png
 
You must log in or register to reply here.
Back
Top