Hard threading anyone?

SmokeWalker

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I'm in a position where I might need to do a 8 TPI thread in some hardened steel. I don't know exactly how hard the steel is yet.

I've got some import C6 carbide that I've ground to the appropriate thread form with about 2-5º of back rake.

I've never done this before. Is this a good idea? Can this work?

I can run the 2.5" OD part at about 400 RPM.
 
I doubt that you would be able to engage/disengage the half nuts at that speed, and you would have to use a relief at the end of the thread; If the material is on the order of 4140 heat treated, it can be easily threaded with HSS, be sure to use an appropriate flat on the point of the tool in either case. 400 rpm would be way too fast, that comes out to 600 FPM.
 
I agree with the others.
Get your RPM under 100 and see how that works. Hardened material must be run slower and you may need time to disengage.
You can always crank er up.
When you use back rake on a threading tool it reduces the 60deg thread angle just a smidge.
Those factory inserts had someone doing the trig. I do remember a member posting that he used some back-rake on 60deg tools without a problem
but he mostly had loose fits.
If you use side rake it probably wont cut or dress the back side of the thread without some galling so the compound has to be set at 30deg.
Many lightly hardened materials, like 4140 pre-hard will cut and thread nice with good tools and setup.
 
Don't forget the cutting oil.
Will it make much of a difference? I figured with all that hardness, I'd need coolant more than I'd need oil. What kind of oil should I use? I have way, spindle, WD 40, milk, juice, Coca-Cola and bacon fat.
I doubt that you would be able to engage/disengage the half nuts at that speed, and you would have to use a relief at the end of the thread; If the material is on the order of 4140 heat treated, it can be easily threaded with HSS, be sure to use an appropriate flat on the point of the tool in either case. 400 rpm would be way too fast, that comes out to 600 FPM.
I would start at the slowest speed the lathe will go, then increase speed as able.

Is there a particular reason to stay slower besides the general "holy s**t, don't destroy the lathe" issues associated with threading? I gave up on that ages ago when I built an auto-stop gizmo, so I do all my threading at turning speeds now. It's hilariously useful. Still, I'll definitely have a runout-groove. Just in case.

I agree with the others.
Get your RPM under 100 and see how that works. Hardened material must be run slower and you may need time to disengage.
You can always crank er up.
When you use back rake on a threading tool it reduces the 60deg thread angle just a smidge.
Those factory inserts had someone doing the trig. I do remember a member posting that he used some back-rake on 60deg tools without a problem but he mostly had loose fits.
If you use side rake it probably wont cut or dress the back side of the thread without some galling so the compound has to be set at 30deg.
Many lightly hardened materials, like 4140 pre-hard will cut and thread nice with good tools and setup.

Ok. Nobody told me to turn off the lights and close the door, so unless I'm being blindly optimistic, it sounds like this is a possibility at least. I might have to feed the tool in with .001 passes, but at least it's feasible. Yeah?
 
You never stated what size thread you were chasing . If it is 2.5" about 100 rpm may be appropriate . If it's a 1/2" thread and the stock is 2.5" then crank it up . Not sure what you made clutch wise to kick it out but I sure would be wary of it . ( be ready for a manual dis-engage ) . Set your compound at 29.5 degrees and use whatever oil you have . You can most likely thread up to 55 Rockwell with the correct grade of carbide , so have at it and lets hear the results !
 
Is there a particular reason to stay slower besides the general "holy s**t, don't destroy the lathe" issues associated with threading?

Yeah, that's was what I hoped you'd avoid... Didn't know you had a snazzy tricked out lathe!
 
Other members who have built the auto stop have made them sound great and much faster.
Will have to build one myself when I get a couple years of other projects done.
That 2.5 inch dia. will limit your rpm some but the hardness might be the determining factor.
The oil or coolant will depend on the material and your cutting speed.
If its an oil hardening material you may not want to use cutting oil and the interface must be kept cooler. IE: slower cutting speed and lighter feed as you already mentioned.
You got a adventure if ya get something wrong. A nice part if ya get it right.
 
The variable that makes all this discussion of little value is that the question of how hard is it? has not been answered, and needs to be answered before any meaningful discussion can pertain to the job at hand. As I previously posted, if the material is 4140 heat treated, which would be in the low 30s Rc; there is no problem to thread it and carbide would not be necessary, cobalt HSS being the better choice due to the tendency of cheap import carbide to easily chip.
 
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