Putting a thread on hard steel?

[smollett]

Has anyone had any experience putting a thread onto hard steel? Is it as difficult to cut through as I think it will be? I've got the right size die but don't think it will cut very well into hard steel. Have got a lathe but it?s not set up to cut threads and would be a bitch to get sorted out and find the relevant gears for. It?s only a small diameter rod (7mm). Any help anyone could offer would be great!
Thanks
James

 

[nickwilliams]

How hard is 'hard'? Moderately tempered EN8 is a very different animal to fully hardened HSS!

You need to be more generous with the background information about what you are trying to do! If you tell us what you are trying to do and why we can probably help.

Nick.

 

[Dep]

Dies are pretty hard too.

Have you tried it?

Can you 'normalise' the steel by heating it and cooling it gently?

 

[smollett]

Not sure what grade of steel it is. It's for a screwdriver shaft. I'll be having a go at it when I get home in a week or so and was interested in finding out if anyone else had done anything similar or had any tips. I was also thinking of using a Tine off an acrobat scaler or an engine valve cos they are fairly hard and about the right diameter.

Won't normalising the steel make it more ductile and so more prone to bending in use?

 

[nickwilliams]

You should not have too much difficulty cutting a thread on a piece of metal about the same hardness as a screwdriver blade with a new, sharp HSS die. Your biggest problem will be getting it to cut straight. You need to keep the die accurately at right-angles to the bar you are trying to thread or it will precess about the bar and you will end up with a bar which is helical instead of straight.

I'd hold the workpiece in the lathe and align the die using the tailstock (assuming you don't have a tailstock die holder which is the 'right' way to do this). Take it slowly, use lots of oil to lubricate the cut, and back off frequently to remove the swarf since if this gets drawn into the cut you will end up with a very poor thread.

If your die is not HSS (i.e. if it's a cheap carbon steel one) then it might or might not work. Personally, I never buy non-HSS threading tools.

Nick.

 

[Jopo]

Engine valves have a extremely hard skin which will not anneal, with a softer core.
Nick's advice is good for tough as opposed to hard steels.
You can get, from a decent tool shop, what is known as silver steel bar. This is available in English and metric dia about 12" long.
It is a carbon steel which can be easily hardened, with a little care, after thread forming. Heat to a cherry red and plunge into water or a light oil. Polish well until bright then gently heat until the bit you want hard turns to the colour required. This is the hard bit. A light straw is hard and a dark blue is soft. Light straw would be good for say a wood chisel edge but too hard for say a screwdriver - it would shatter.
A colour for a chisel to cut steel is a dark straw to just starting light blue. Once you see the colour you need - plunge immediately into light oil.
If you want overall hardness you need to temper over a steel plate on top of the heat source - usually a gas ring. If only one end is needed to be hard then gently heat from the end that can be soft. If you have polished well you will see the colour change rise up the bar.
What is your application?
You say 7mm dia is about right. Well you might get a 7mm die but I have never come across one and as a non preferred size it would be very expensive - if available. 6mm is a common thread size (.23"). The nearest English size is .25", (7mm is .27") so you have a choice of threads 1/4" BSF, BSW. UNC being the common ones.

Jopo

 

[smollett]

Just checked on the actual diameter and it's 6.4. Looked up in tapping tables and i think I'll put a 1/4" thread on it. I'll use the lathe as Nick suggests. I'm making a capping rod out of it. That steel rod sounds interesting. I may get a length of that. I'm guessing I don't want to make it too hard else it will shatter. It sounds like it could be the way forwards as the end to be threaded doesn't need to be hard and 6.4mm could be a bit small.
Thanks for the advice!
James