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Finish quality with lathe tool bit


Sep 17, 2004
Ottawa, Ontario, Canada
I am using a Chinese lathe, possibly not so rigid to cut 1.25 mm pitch thread in water hardening tool steel. High speed tool bit with zero rake and about 25 deg clearance angle below cutting edge. The edge surfaces are polished to about 1500x with a ceramic stone. Depth of cut was 0.1 mm, speed slow as the thread runs up to a shoulder.
Any help in improving this finish will be appreciated, thanksInsert thread.jpg.
Give it a little top rake and speed up the cut a bit and give it a squirt of cuttin' erl, maybe that sulfur stuff that plumbers use. That should give you a start. You're coming in at a 30 (29.5*), right ? so don't need to think about the v, you'll only be cutting on the leading edge, if that's why you have no top rake.

And you can run up to a shoulder reasonably quick, just have your hand ready :)
Use a brazed C6 bit instead. And it's not made in that other country. You would not have had to ask your question.
And the thing can be dressed on a diamond stone and last boom boom time.

I just bought 10 ea. I started using them because I didn't want to use up inserts.
Results are just as good. But no good for production when rotating a insert is faster.

right hand - 3367a518 $7.16 (can get closer to a shoulder)
neutral - 3367a441 $4.51 ( a small 1/4" bit comes in handy)

What metal type and rpm?
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Going straight in puts on huge sterss with cutting on bothe sides of tool bit.
/insert, Setting compound ar 29 1/2 or 30 makes threading a simple turning operation with the bit cutting only on on side..
No side and back cutting edge rake puts on even more stress trying to push the marterial into itself...not peal it away.
25 deg clearance angle is more that enough/too much.
With the Compound at 291/2 or 308 the pull away from a shoulder is easily.
If not on the tail center then too much stick out can be a problem.

Threading should be as easy as turning an OD.. if a lathe can turn well then it shuld thread well.

Good to file check part hardness if unsure.
feed rate small on a smaller lathe.

A trick I use it to put a tab of tape on my chuck and watch that tab come to a place, so to make pull out muche easier., watching the small diameter of the part for pullout is more difficult...I can pull out at the exact place doing tha.
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It looks like you are threading dry. Any good cutting oil os needed. Or bacon grease or beef tallow is very good for steel.

25* clearance is quite a bit. The tool tip may have failed quickly, especially if it is a sharp point. Less clearance makes the cutting edge stronger. 5* clearamce will work, and on the trailing edge you only need a few degrees, since the helix angle automatically provides some clearance.
Things mentioned:
Op never said if feeding straight in or at 29 1/2 - 30*.
Using a Chinese lathe,, not telling machine size..
.Did not tell is using tail ,or how mich length sticking out..
Using a zero Rake bit..
Infeed .004.
Speed not given...slow?
Could have stopped threading before getting down to near finish size to save the part.
Used no lube.

I hope the Op learned something from this thread.
Would be nice to come back with a success story, or take up another interest.
When I first started in threading I got worse results than the picture. Some mystery steel still doesn't agree with me.
But stainless steel is always a friend.
Thanks to all who replied. I added tailstock support, altered the tool bit to 5 deg negative rake and 20 deg clearance, applied safe tap cutting oil, .05 mm final cut at about 600 in / min surface speed. As fast as I dared to go. Happy with result following. Thanks for the advice.
You changed 3 things. Which was the key. Maybe all?
I don’t remember if you mentioned the part material, but if non -hard steel a positive rake may be better. When I magnify you best photo it still looks like the cut is compacting the material.

I would try a positive rake, fingernail sharp, infeed @ 29 1/2*
side cutting edge clearance 10* / top rake 10* positive (about).
Grind the bit to match a fish gauge - make a fininsh pass of .001 and a last pass with no infeed... bump the OD with a fine hone before the last pass.

Plastic Deformation is when pressure of pushing on to it self can make a steel harder..this can be caused by a negitive rake too bit/insert. Most carbon steels are subject to this.
Most file soft steels like a positive rake bit/insert.
Hard steel and cast iron often like a negitive bit/insert
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Thanks to all who replied. I added tailstock support, altered the tool bit to 5 deg negative rake and 20 deg clearance, applied safe tap cutting oil, .05 mm final cut at about 600 in / min surface speed. As fast as I dared to go. Happy with result following. Thanks for the advice.
Are you using a partial or full profile cutting bit?
The root width is very narrow but the crest width is wide.
It looks like you are using a partial profile bit and didn't move it to the left before the plunge at 29 1/2.
You can still recover from this if done carefully.

For 1.25mm pitch:
crest = P/8 = .016
root = P/4 = .012
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The Op should try the recommended tool bit geometry for the material.
Should tell if going straight in or at 29 1/2 or 30.
I think the least threading stress is compound/infeed at 30*, and the then using the cross feed for one or two feeds of .001", after a hone bumpon the OD.
Threading should be as smooth as an OD turning pass.
RPM going to a shoulder might be 80 to 300 pulling out by hand. (but can be as fast as a turning operation if having no shoulder, and the cross can be pulled.)
Another advantage of compound at 29-30 is that the pull-out, pulling out the compound, will be pulling back away from the step shoulder

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MB- never heard of pulling back the compound when going to a shoulder- always used the cross slide...nice idea because of the geometry, but in real life?? just askin- suppose you could, but on most machines, I don't think i could get that tool outa there quick enough when there is no relief groove- yes, I do a fair amount of threading with no groove so......
Coming to a small step shoulder the cross-pull is fine, perhaps to a 1/4 inch step.
If you have 100 thow cross dial and need to crank 2 1/2 turns that can be a lot.

Coming to a large step shoulder pulling back and away at 30* is often safer.
*I use both methods .

Using my tab of tape on the chuck and watching the chuck I can stop the thread right on the spot, I can back it up or go more long by a 1/32 and less. If I stop at 12;00, and then change to 11;00 I can stop .005 or.010 shorter and right on the same spot. . But that takes a little practice. looking at the part is not as accurate. I may be the only guy doing this.

Some lathe guys use a dial indicator for a thread cutting travel stiop .

Ref: a 1/2-13 is .077 so stoppng at 12;00 then 11;00 is .006 shorter length.
One can do this with cross or compound pull.
I am a grinder guy, but I love a lathe
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