What is the maximum hardness steel that I can expect to single-point thread?

[MattiJ]

On my quest to make my own ER32 collet chuck I was wondering what is the maximum hardness steel (4140 crmo) that I can expect to thread without huge problems?
Thread is M40x1.5 so reasonably small cut of depth.

I have generic laydown carbide inserts, supposedly Mitsubishi VP15TF grade.

Is there any benefit to have some sort of leading taper so that threading insert is not entering to full width cutting at once?

[RB211]

Thread mill on a CNC would do it

[Magicniner]

Thread Mill in a horizontal spindle mounted to the cross slide can also be useful for difficult materials ;-)

[MattiJ]

I’m trying to avoid thread milling or grinding thus the question

[MasterMaker]

Using carbide you could basically scrape-cut the tread using a crank-handle to turn the spindle, it would take a bit of time(lots of incremental passes) but you could probably thread hss with this method...

[CCWKen]

You should be able to thread into the HR50s range without too much effort. You're not plunging full depth when threading and the leading angle determines thread form so don't change anything on the insert.

[JoeLee]

On my quest to make my own ER32 collet chuck I was wondering what is the maximum hardness steel (4140 crmo) that I can expect to thread without huge problems?
Thread is M40x1.5 so reasonably small cut of depth.

I have generic laydown carbide inserts, supposedly Mitsubishi VP15TF grade.

Is there any benefit to have some sort of leading taper so that threading insert is not entering to full width cutting at once?

You could try the flank threading method.
Using your compound cross slide you cut one side of the thread per pass, then index to the other side of the thread. I use this method when I have to cut larger / deeper threads where full tool engagement causes chatter.

JL..............

[old mart]

I recon you could do it by taking no more than 0.002" deep cuts to start with and reducing to 0.001" at half depth. Use tapping oil as lubricant. A test first on a piece of the same steel would be prudent as there is only one chance, or make the workpiece longer and cut a thread on the end that will be discarded.
Some research would be required to find a tip of the right grade if yours don't give a satisfactory finish.

I presume that you will buy a nut, make sure that you get one that has the correct female thread in it first. I think there are three types of nut available and the threads may not be interchangeable.

I have put spanner flats on the shanks of er25 r8 fitting collet holders with solid carbide endmills no bother, and I think they are just soft enough to be filed.

[CCWKen]

We're all assuming you know how to cut threads.

I forgot to mention that if you're using 4140 "pre-hard", it ain't that hard. You can thread that with HSS.

[MattiJ]

We're all assuming you know how to cut threads.

I forgot to mention that if you're using 4140 "pre-hard", it ain't that hard. You can thread that with HSS.

Prehard 4140 is really nice to work with. I was thinking of hardening the body to full hard ie HRC 55 or so for the 42crmo4 if I can just cut the threads without going through full box of inserts.

[CCWKen]

That should work but as already mentioned, you'll be taking light scratch cuts. Are you sending it out for hardening or doing that yourself? An uneven hardness can turn a good thread effort to bad in a hurry. Pay particular attention to even heating and soaking. HT oven is your best friend.

My general rule of thumb is; If you can scratch it with a file, you can thread it without special attention.

[Toolguy]

You can go with 48 - 50 Rc. That will be plenty hard for wear purposes and about the upper limit of what will thread easily. You can do harder, but have more problems. If it were mine, I would be in the 40 - 45 Rc range. That is still hard and very tough.

[old mart]

I'm with Toolguy on the hardness issue, if you can back off on the maximum hardness a little, tool life will be fine unless it is used in a production environment.

[MattiJ]

That should work but as already mentioned, you'll be taking light scratch cuts. Are you sending it out for hardening or doing that yourself? An uneven hardness can turn a good thread effort to bad in a hurry. Pay particular attention to even heating and soaking. HT oven is your best friend.

My general rule of thumb is; If you can scratch it with a file, you can thread it without special attention.

It's going to be backyard hack and tempering in a oven after the pizza.
Plan was to start with 42mm od 22mm id cylindrical chunk, dip to oil bath in axial direction and hope for the best.

10 mm too thick for the small calibration ovens that I have at work, could do the heat treatment with unnecessary accuracy

[old mart]

Are you saying that the workpiece is too big to fit in the oven at work? If it fits, you may be good to go.

To harden the steel, you heat it red hot and quench in water or oil, then you reheat to the required temperature for whatever hardness and let it cool. It can be left in the oven for an extended time to ensure that it is evenly heated and then the oven is turned off, or the workpiece is removed and allowed to cool naturally. There are charts for tempering steel you can google for.