Hss Tool Threading O2 Drill Rod

[ProfessorGuy]

I have been trying for weeks to make a steel screw. I'm trying for one about 6mm long and with about a 1mm shaft. I am using O2 drill rod for the screws, and I can easily cut the profile and even the thread relief. But when it comes to the threading, no luck so far. I've been filling a little box with my ruined attempts.

My lathe uses handwheels for threading (no motor), and I use a 4x jeweler's loupe as I thread, so I have excellent control over the process. I just don't know what the heck I'm doing.

I started with a 60 deg. carbide tool, but the tip of that tool shattered within the first 5 mm of cutting. But I don't want to use carbide, I'd rather use steel.

The tools I've ground so far are from 1/4" HSS blanks. On the first one, the tip became rounded off on the first pass (about 3mm) after taking a 0.04mm cut. I ground a new one, then tried to harden it by bringing it to cherry red, then quenching it in black oil. Then I tried taking only 0.02mm cuts (just dusting). The tip of the tool crushed down, with the very tip (just the last 0.01mm) slipping down off the edge--ruined!

Made yet another tool. The screw simply bent out of the way of the tool, which cut nothing. Each subsequent pass just pushed the screw further out of the way until it bent and was ruined.

What's the secret of cutting a tiny steel screw with steel? I know the old timers were able to cut steel--they weren't using carbide in the 1880's. How did they do it?

 

[gi_984]

I've never tried threading such a small diameter shaft on the lathe under power. Perhaps you should try turning down your drill rod to the major diameter and then use a threading die in a handheld wrench to cut the threads while still chucked up in the lathe. I use the tailstock to get the die aligned at the start of the thread. After you get the thread started one to two turns you can back the tailstock off. After hand threading use a cut off tool as usual. I've threaded the ends of several larger shafts like this.
Drill rod is pretty hard stuff. Perhaps a different type of material would be easier. I've used O-1 and 1045 for bolts before.

 

[RJSakowski]

The tools I've ground so far are from 1/4" HSS blanks. On the first one, the tip became rounded off on the first pass (about 3mm) after taking a 0.04mm cut. I ground a new one, then tried to harden it by bringing it to cherry red, then quenching it in black oil.

HSS steel is so called because you can heat it to a fairly high temperature before you soften it making it ideal for high speed work. The annealing temperature is actually around a cherry red incandescence, about 1600F. To harden HHS, you have to heat it to a much higher temperature, 2200F. The tempering temperature is around 1000F. What you actually did in your heat treating was to soften the steel.

 

[mikey]

I have some suggestions:
1. You need tailstock support - period.
2. Use a 1/8 cobalt blank and grind the tool with 15 degrees of side relief, then use a diamond stone to put a very tiny flat on the tip. What I bet is happening now is you are fracturing the tip of your tool and increasing the cutting forces so the tool won't cut. Hone the tool to a fine finish before using. Optionally, you can try a P1N parting tool and grind the tip of that as a threading tool, same angles as above.
3. Stainless will work harden and push away from the tool. I suggest taking 0.002" cuts for the first 2-3 passes and then lighten up a bit.
4. Use sulphur based cutting fluid.

Edit: for some reason I had it in my mind this was stainless - my mistake. Ignore #3.

 

[T Bredehoft]

You might try annealing your O2 steel before threading it. It might have been hardened. Suspend it from a magnet, over a some dry sand or better yet, wood ashes. Heat it with a torch (probably will need acetylene and Oxygen) until it falls from the magnet. Wait until its cold. Then try threading it.

 

[brino]

Hey Professer,

I am sorry I don't have anything new to offer, any suggestions I had were already said above.

But I did want to add that I am already watching this thread. I have never tried something so small, but I am keenly interested in how it can be done.
Please keep posting your findings (what helps _and_ what doesn't).
I'd like to learn about this.

Thanks!
-brino

 

[mikey]

I've been thinking about this and you might have better luck asking this one on one of the horology forums or the Yahoo Sherline group. There is a guy named Jerry Keiffer who hangs out on the Sherline group that does a lot of fine model work and he makes screws an order of magnitude smaller than you're doing here. If anyone can help you, he would be the one.

 

[ProfessorGuy]

Thanks for the suggestions. I have a couple of comments and questions:

No magnets are allowed in my shop--watches don't like them. This limits the tooling and storage systems I can use, but I like using less modern methods. Can I use the "sharpie" method of annealing O2: that is, can I heat it until a sharpie mark on the surface disappears (as you would on silver or copper)?

I polish the tip of my tool to a mirror finish (or as close as I can come) because I need the last 0.01mm of it to do the very shallow thread cutting. I thread under magnification and could easily tell if the tip were damaged. At one point I had a beautifully-shaped, undamaged tool that wouldn't cut, but rather pushed the work out of the way. Huh?

How can I use my tailstock? Once a chuck is within a few mm's of the headstock, there's no room for the 1/4" tool, much less for the toolholder or the cross slide which carries the tool. And how do you start the threads if the tip of the screw is chucked?

I just applied to join the Yahoo Sherline group. Thanks for the heads up.

 

[mikey]

Prof, I was under the impression that the O-class of tool steels was sold in the annealed state. I don't know how to make it softer still. I've heard of the Sharpie thing but only as it applies to silver. If I recall correctly, and it has been a few years, the Sharpie disappears at the temperature silver anneals at. I don't know that this is the same annealing temp as O2 but I suspect it is not. Could you not just have one magnet that you could leave outside the shop where you do your torch work? If not, send it to a heat treat facility and have them do it for you. Another option is to use another kind of steel - is O2 necessary?

Okay, if the tip of the tool is okay then could the tool not be on the exact spindle centerline? A slender work piece like you have would require this.

You can use the tailstock if you have an extended tip live center. Of course, they don't sell one so you'll have to make it or maybe make an extended dead center that will accurately fit the tailstock. At hand-turning speeds a dead center would work fine, I think.

Anyway, here's my solution:



I used this to make a screw for an archery arrow rest that used 4-40 threads. Granted, not as small as yours but it worked fine and it was in 303 stainless, not O1 or O2. I ground a P1N blade (o.038" wide) to screw cut it and it walked right through that thread. The good thing about using a parting tool is that it is very stiff vertically and can be extended without adversely deflecting under the forces of a hand-turned piece when taking very tiny cuts. You might consider it. It is a good idea to have a very solid parting tool holder when doing this, though. If you don't have one, make one.

As for threading a screw when the tip is chucked, I have no idea unless you cut it in reverse, which the Sherline can do.

Regarding polishing, I use a Translucent Arkansas to polish when working with this scale of thread. Sharp takes on a different meaning when working with small diameters. The same applies to tool geometry - cutting forces on very small work pieces become big deals. I assume you already know this but others may not.

I've pretty much shot my wad with regard to tiny stuff. Ask on the Sherline Yahoo group - many skilled guys who know how to work on a Sherline lathe there.

 

[ProfessorGuy]

I am using O2 drill rod because I have it on hand. It is not a requirement at all!

Is there an easier steel to work with that would hold a thread at these tiny dimensions? Obviously the amount of strength required in the finished piece is miniscule and the weakest of materials would probably work. And I would like to harden the screw after construction anyway (and heat blue it as well).

What steel would you recommend for screws of this size?