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Small diameter/deep/blind hole boring

Kyle M

Jun 18, 2022
Hello everyone, I'm new to this forum and in need of some advice. Specifically from the Swiss guys.

I've got a copper alloy part that is .500" OD 1.800" Long with a .2148" through bore.

Here's where it gets challenging. The ID tolerance is +/- .0002" and the OD to ID concentricity tolerance is .0004"... Yeah crazy.

These are being machined on a Swiss lathe (Star SR20). Currently drilling with a carbide drill, leaving .012". Then with a solid carbide boring bar, cutting 2 passes @ .005"DOC. On the main spindle, so this is a blind hole (tried boring on the sub to have a through hole but cant hold concentricity). With .002" remaining we send them out to be honed to the final diameter.

We may eventually switch to reaming to final size but honestly holding the diameter tolerance isn't the problem. The problem is concentricity. Either the boring bars I've tried are deflecting too much and causing the backside to go out of tolerance. And/Or I've got chips building up in the blind hole that are causing the problem. The front side of the part usually has .00015" runout. When I check the back side it's .0007"-.001"

Can anyone recommend a different strategy or tooling that can help? Thanks in advance.


Feb 24, 2018
Greenfield, Mass
I've never seen a bore bar that lost concentricity at the back of the hole. Long bores with tiny tools might have taper issues, but I believe your concentricity issue is something else.

Are you sure your bore bar is cleaning up the drilled hole? Your drill is tiny, working in what I ass-u-me is a pretty gummy and soft material (copper, not sure what alloy it is). Tiny drills can do some pretty amazing acrobatics, especially if it is HSS (it can bend more than carbide before snapping).

Is your bar stock ground perfectly straight and round? Swiss machines are very picky about what you put in them for bar stock, and any problems with the stock can and will show up in the work piece, and even more noticable with such tight tolerances. I would also make sure you have a properly adjusted guide bushing and a good collet, if you haven't already checked.

Additionally, if you are working with an exotic alloy, it may be doing funky things when it is machined and cut off, due to stresses in the material that come out when you machine and cut it off the bar. Other material -related issue could be a shitty heat of material. If you've run this job successfully in the past, and are running into problems all of a sudden, could just be a batch with a little extra or not quite as much of alloying elements