Threading question

[acondit]

I was trying to turn a new crossfeed leadscrew for my 9x20 lathe. I was trying to use 9/16 drill rod. Everything went fine until I started to thread the 7/16x20 LH threads. I just succeeded in making a mess. It was like it was tearing the metal rather than cutting it. I have threaded other metal in the past, sometimes with good result and sometimes with similar results.

Questions:
Is drill rod a suitable metal for threading? If not what should I use?
If yes, what are the most likely things I am doing wrong?

Would 12L14 be a good choice for a leadscrew?
Would 4140 be a good choice for a leadscrew?

Thanks in advance,
Alan

[WayneHill]

Drill rod can be threaded.

http://www.bedair.org/Crossfeed/crossfeed1.html

[Mcgyver]

Is this an acme thread? <edit ok, probably not acme at 7/16 20 ) you describe the rest of the set up – tool geometry, speed, traveling steady etc? Keep in mind when grinding thread cutting tools for left hand cutting that the helix angle is reversed and you need to grind the side clearances in the opposite direction.

drill rod's not the best choice imo – a tool steel's machinability is lower and as its unlikely you’re going to harden and grind it, you're not getting the advantage of using tool steel.

not sure what 4041 is? could it be 4140 which is a chrome moly steel? Chrome-moly steels can be heat treated or you can buy the pre-hardened; if your machine is up to it the pre-hardened chrome molys’ might be the best choice. You end up with a surface that is harder than mild steel but without the need to heat treat (an possibly warp, crack) and grind afterwards

You want a feed screw to wear well which you get by using a steel that can be hardened. However there are problems with this; these steels have lower machinability, threading is already a wide cut, the lathe is small and the piece is spindly.

If the machine isn’t up to cutting pre-hardened chrome moly, the free cutting steel isn’t a bad choice. Many will probably be horrified with that as its going to be the worst wearing one, but I’ve gone this route before based on the logic it I can get a the job done with a good finish …. If i keep it oiled, and the demands of a hobby lathe are light it is a better outcome than no screw or a crappy finish in a tough tearing material that will chew up the nut

[acondit]

Mcgyver,

I corrected my first post, you are right I meant 4140. I am using a 12x36 to do the threading, a 1/2" HSS bit ground to a 60 degree point, compound set to 29.5 degrees, 60 rpm speed. I can't use the traveling steady because it would run into the live center at the end of the run, or without the live center it would run off of the threaded piece before the end of the threading.

The lead nut will be brass so wouldn't that be softer than the 12L14 at any rate?

Alan

[Geof]

Use the leaded and while you are set up make a couple of spares. Mcgyver's logic is impeccable.

[Mcgyver]

thanks Geof you are very good at saying in 1 line what takes me 20!

when i first wrote all that I was thinking that it was acme which is a wide cut (lots of cutting force) and that being in a tough material it was too much for the lathe. if you are using a bigger lathe, and its a V thread, you can ignore a lot of what i said.

I still think the problem could be in the grind. Check the side clearance on each side - what might work perfectly for RH, won't for LH - for RH you need no clearance on the right side of the tool, for LH you do - greater than the helix angle.

you can still us the traveling steady, which i suggest you do for a long spindly part like a feedscrew. Reverse the lathe spindle direction and turn the tool upside down, now you're cutting a LH thread from tailstock to headstock!

I'd use something tougher than brass, ie bronze. just make sure the tap is sharp. leaded freecutting is not the best material for the job, but it makes the job eminently doable and may still give a lifetime of service in a hobby lathe.....then again if have access to the bigger lathe and its only a v thread, you could try pre-hardened chrome moly.

[acondit]

Geof and Mcgyver,

Thanks to both of you. I ordered some 12L14, enough for 4 of them. I will probably do the first nut out of the brass since I already have it, but I will keep my eye out for a piece of bronze. I will probably keep my out out for some hardened 4140 as well.

Thanks again,
Alan

[acondit]

Just another question,

What about making the nut out of cast iron? It works great for slides, would it work for a nut as well?

Alan

[9566317]

I'm not sure but Aluminum bronze would probably give you the best
of luck and good life.I use it a lot for thread roll pins and am amazed at the life I get out of it.100,000 3/8 16 parts and no adjustment yet!!In 1018 yet!!
Leaded steel C12L14 will give you an excelent finish but will not give much
life.Use lots of oil and do not abuse and will last a reasonable time though.
Your life with these softer materials will depend a lot on your finishes on the threads.The beter the finish the longer it will last.
Have fun and keep it simple.

[Geof]

I will disagree with both Mcgyver and 9566317; stick with brass for your nut it is much easier to work with and the life on leaded will be good. Aluminum bronze is certainly one tough hard wearing material but a real b**** to machine.