Interrupted Turning

That should say "lathe bit," not "insert" in that question.

I don't have an answer for you, but it would probably help if you added some details about the material to be cut and its size and shape as far as it affects the "interruptedness" of the cut.

The first HSS bit I grab for quick jobs in aluminum, brass, mild and harder steel and stainless is ground like a threading tool, but closer to 90 degrees than 60 so the point holds up longer. The cutting edge slants downhill toward the point as a result of freehand grinding, not intent, but it's handy to remove the nub left on the stock from a cutoff bit and not too sensitive to being a little off center height, and quick to resharpen. It stood up to turning stainless squares into circles for the races of a solar panel thrust bearing.

I don't think it's the ideal shape for any particular task but it adapts well enough to a range of them. I originally ground it to turn down straight pilot sections on stainless screws for my antenna. The root-diameter pilots align inner and outer holes in telescoping mast joints.

I should mention that I use Multifix tool holders that rotate in 9 degree steps, which is very convenient to compensate for the imprecise angles of hand-ground bits. The Aloris system has a better selection of insert toolholders.

--jsw

The angle is very close to the 80 degrees of a Trigon or Diamond insert, which is good for both straight turning and facing a shoulder.

--jsw

Well, I have two things on the shelf that get used from time to time. Several long pieces of left over steel solid jack shaft opener shafts with a key way full length.

Several pieces of hex 360 brass rod used for various custom parts.

So ultimately its the equialent of one point of a triagon with negative rake?

It has some positive top rake sideways away from the cutting edge, not enough that I recall having a problem with it on brass. The goal was an easy-to-resharpen shape for roughing cuts on unidentified scrap metal but it also seems to have less impact on interrupted cuts like the stainless squares.

Well, I just did it, and it was not bad. I went with a very conservative right hand turning tool. About 5 degrees back angle on the leading cutting edge and somewhere just a little less than 90 degrees, but not much, included angle. Slight under cut on leading face and front face. 7 Degrees maybe. About the same as the bit already came. Just rotated to match the right hand included angle. I didn't get to fancy with the corner radius. Somewhere between 1/32 and 1/16 radius. I gave it a very slight back rake on top away from the cutting edge. When it looked ok I just stopped, and honed the edges.

I can definitely see why "real machinists" like a bigger grinder. On my little 6" with its half wore out wheels the faces of the bit have a very noticeable scallop. (Yes I squared it first.)

Then on some mild steel jack shaft with a keyway cut full length (more likely extruded that way) I gave it a try. I ran about .05 depth of cut, then .1 depth of cut, then .145 depth of cut. At .05 it cut almost like cutting regular shaft. At .1 it made some noise, but if I didn't know what it was I might have only noticed a difference in pitch. At .14 (completely removing the keyway in one pass) it made some noise and it was pulling the lathe bit into the work piece. Overall I was really impressed, although I suspect it was as much the heavier lathe I was using as the cutter. Still it did the job, and the lathe bit still looks good. The feed was pretty small. .0014 I think. I also played with RPM at 115, 190, and 300. They all seemed to cut the same, but I didn't get straw turning to blue colored chips on the lighter DOCs until I bumped it to 300. I suspect I could crank the RPM up some more at the lightest DOC of .05.

The lathe bit looks virtually just like it did after I honed it. I'll look at it with the loupe in a few minutes, but it looks good. It will certainly last more than long enough for basic common interrupted type cuts. Best part is I can regrind this in just a couple minutes anytime I need to.

That's good for me. 300 RPM and .014 feed is where I leave the lathe most of the time.

Now for my next cut hand grind. A vertical shear so I can cut better finishes before I polish on those generic mild steels.

P.S. I might add before I tackled the interrupted cut I tested the lathe bit on a 1" piece of 304 stainless. It was really really good. I think I've been selling myself short by not hand grinding more HSS bits.

Glad to help.

I grind blank bits nearly to shape with an angle grinder before finishing on the pedestal grinder, or the surface grinder for threading bits and gear cutters.

You can grind a concave corner rounding bit with a conical stone held vertically relative to the final position on the lathe.

--jsw

I've noticed that what really matters is keeping the wear flat below the cutting edge less than about 1/32" high, in agreement with the experimental edge geometry results in the Cincinnati Milling Machine book from Lindsay. Top rake angle (or none) and having the bit below center don't make too much difference. I no longer bother to measure angles.

--jsw