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[4]

Hard threading anyone?

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SmokeWalker

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#1
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.
 

benmychree

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#2
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.
 

Eddyde

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#3
I would start at the slowest speed the lathe will go, then increase speed as able.
 

tertiaryjim

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#5
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.
 

SmokeWalker

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#6
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?
 

mmcmdl

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#7
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 !
 

Eddyde

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#8
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!
 

tertiaryjim

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#9
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.
 

benmychree

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#10
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.
 

benmychree

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#11
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.
I had a Monarch toolroom lathe with a set thread stop and reverse to the lead screw, and one of my friends has a Pratt & Whitney with the same feature, and they are wonderful! they will thread a coarse thread right into a thread relief, they just stop dead, then you withdraw the tool using the cross feed that has a dead stop, for long threads, the half nuts can be opened to return to the starting point for reengagement, then the lead screw is re engaged, and it's off to the next cut. Short and coarse threads, the lead screw is merely reversed and runs back to the starting point, where it is again kicked off by a set stop.
 

P. Waller

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#12
I do hard turning several times per year using ceramic hard turning inserts, works well when the part is 40-50 RC.

Above that it is a crap shoot, have used the same tools for turning the shanks of HSS taps with decent results. You will likely go through many tools and parts before you get it working. Traditionally thread grinding is the way to go.
 

mmcmdl

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#13
So I'm guessing this clutch set-up is similar to Hardinge HLVH or a Colchester ? Sounds like it .
 

mmcmdl

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#14
Another way is invert the tool , run in reverse and thread out . Won't have to depend on that clutch if you don't trust it .
 

SmokeWalker

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Okay. I decided not to do it. I spoke to another fellow who's done hard turning and threading, and what he told me is that there is a good chance that without thread grinding, I'll end up in a position where it's really challenging to complete the thread form; a tool sharp enough to complete it might break, and the only tool strong enough to do it will have too much nose radius to form a complete thread. I think I could still make it work for my application, but I'm going to try it out of aluminum (!) and see if I can't get away with it.
 

Ray C

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Okay. I decided not to do it. I spoke to another fellow who's done hard turning and threading, and what he told me is that there is a good chance that without thread grinding, I'll end up in a position where it's really challenging to complete the thread form; a tool sharp enough to complete it might break, and the only tool strong enough to do it will have too much nose radius to form a complete thread. I think I could still make it work for my application, but I'm going to try it out of aluminum (!) and see if I can't get away with it.
I've cut threads in metal at/around the Rockwell 42 range with no problem at all using standard carbide threading inserts. What are the dimensions of the workpiece and most important, precisely how hard is it? If you don't know the actual Rockwell, do you know if it is just store-bought prehard material? If so, it will cut like butter because that's only 28-32 Rc -which is not really hard at all. If you scrape it with a file, does the metal scratch or does it laugh at the file?

We need more details...

Ray
 

SmokeWalker

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#17
Hey everybody. The project is over, and I didn't end up having to deal with the hard threading. I ended up making a part myself out of aluminum and it worked fine!
I've cut threads in metal at/around the Rockwell 42 range with no problem at all using standard carbide threading inserts. What are the dimensions of the workpiece and most important, precisely how hard is it? If you don't know the actual Rockwell, do you know if it is just store-bought prehard material? If so, it will cut like butter because that's only 28-32 Rc -which is not really hard at all. If you scrape it with a file, does the metal scratch or does it laugh at the file?

We need more details...

Ray
 

mksj

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#18
FYI, I also use an electronic stopping system, but even though it will stop to 1/10,000" at just about any spindle speed, I still do not thread at high speeds. If you look at the recommended SFM for hardened steel using carbide inserts it is somewhere in the 100-150 SFM range under ideal conditions. With a 2.5" diameter at 400 RPM you would be around 260 SFM. Fine for aluminum and softer materials, but not super alloys and hardened steel (45-60HRC) per the carbide insert manufactures recommendations. There are specific inserts for hardened steel, I am using lay down BMA type. I usually shoot for 50% of the manufactures SFM, based on decreased lathe rigidity and also non optimal coolant (brush).

sfm.jpg
 
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