"Bob La Londe" wrote in message
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"Jim Wilkins" wrote in message
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"Bob La Londe" wrote in message
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"Jim Wilkins" wrote in message
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"Bob La Londe" wrote in message
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What is the "old knowledge" about interrupted turning. I've
been doing some Google searches, and checking the videos on
YouTube, but most results seem to indicate a need for new high
tech special inserts. Or more accurately be aimed at selling
such inserts and insert tools.

I don't have to do it often, so I don't mind if it takes a while
or if I have to push it a little bit, but I'd kinda like to know
how to do it the "most effectively" with teh tools I have.

I've got a Precision Mathews PM1440-LB 14 x 40 lathe that
claims a 3HP motor. So far this lathe does a pretty good job,
but I'd like to shy away from maxing out the motor if possible.
Feed per rev is pretty flexible, and its got spindle speeds from
70 upto 1255. Its more rigid than the average Chicom lathe, but
its still a Chicom lathe so I'd also like to shy away from
pounding on it to hard.

I've got a few 1/2" x 4" HSS Cobalt bits. I could sacrifice one
to this with no qualms.

How you would you do it if you were going to hand grind an HSS
insert for the job?

P.S. For why guys no tell me about vertical shears. That's an
sweome grind for getting a smooth/er finish.

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.


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.

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