[Bendak]

Hi all,
I am an undergraduate student at WPI needing to cutting a certain oddly-shaped hole as part of an independent study project. The material being cut is normalized 4340 chromoly steel. The hole starts off straight for about 0.916" which I can cut with a regular drill bit. The problem lies in having to cut the two tapered portions of the hole, the first being a 1° per side taper and the second a 3° per side taper. Seeing as how the starting and ending diameters of such holes are non-standard and lie past an already 0.259” diameter hole, I have not been able to find any tapered end mills which meet such requirements. I tried using some old boring bars found on Ideal Surplus from Bokum Tool (BSC5), but was unable to get the bar to bite without significant deflection, leaving the hole undersized. If needed I may resort to attempting to machine my own custom tapered D-Bit as a last resort. Any ideas on how best to machine this part?
Hole Geometry:

Thanks,
Jack Ruddat
WPI 2021

[Jim Dawson]

The boring bar you chose is a good choice for the job. Is it sharp, razor sharp? Sometimes lighter cuts and less spindle speed is helpful. 4340 cuts pretty nicely, but can work harden a bit if you allow the tool to rub. I would probably bore that at about 80 SFM, and around 0.001 IPR feed, and a 0.003 DOC

[Bendak]

The radius of the tip is 0.003", so would 0.006" be a good depth of cut in your estimation? My original parameters were 0.004 DOC, 300 SFM, and 0.001 IPR feed.

[Jim Dawson]

I would try the same parameters as you were using with the exception of slowing down the spindle. 0.006 might be a bit much for that tool. Take a good look at the tool tip so make sure it's still sharp, use a magnifier.

[Bendak]

I would try the same parameters as you were using with the exception of slowing down the spindle. 0.006 might be a bit much for that tool. Take a good look at the tool tip so make sure it's still sharp, use a magnifier.

I've managed to lose the tip on three of my five boring bars in the past. I don't think BUE was the issue because I had plenty of coolant and very conservative parameters. If the surface speed was too fast then I suppose that might account for the wear? I guess I could always resort to boring from the other end using a back boring bar such as the SDM 668 492 (solid carbide) from GenSwiss as I would only need 1" of stick-out instead of 2" (8:1 instead of 12.8:1 length to shank diameter ratio).

[Jim Dawson]

Too fast surface speed would account for tool wear.