"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.