Cutting Theads

[KenS]

Give me a short course in cutting threads in steels that are smooth enough to lick.

Any and all info appreciated. Cutters, speeds, lubes, etc. I've got the gearing stuff in the lathe gearbox, and that's easy enough to figure out.

I'd be using an Aloris toolholder on a SB 10" lathe.

[SGW]

The basic procedure is well described in South Bend's "How to Run a Lathe" book (available through Lindsay Publications), L.H. Sparey's "The Amateur's Lathe," and other places. That will explain about setting your compound at 29 1/2 degrees, feeding with the compound so nearly all the cutting is done on one side, etc.

A major key to good threads is to have a SHARP threading tool. Then use an approprite lubricant. I use a high-sulphur cutting oil, slathered on. A lot depends on the kind of steel you're cutting, too. It's much easier to get good threads in Ledloy than in drill rod, for instance.

Once you've got the threads cut, you can clean them up a bit. first run a fine three-corner file along the thread in slowest backgear, if they're really rough. Then take a soft pine stick, dip the end in about #320 lapping compound, and press the end against the thread with the lathe running a couple hundred rpm. The soft pine will conform to the thread and track along it. Do that for a while, and it will polish up the thread pretty well.

[halfnut]

Ken,

Agree with SGW, I'll add something.

You mentioned Aloris toolholder.

I love an Aloris toolpost setup, but their treading tools stink. I don't know what grade of steel they are but I just don't have much luck with them. Anybody else have this opinion?

I just grind one up out of High Speed and hone edges, and just break tip with small radius. If you are cutting really large pitches you might grind a bit more radius or just hone on a flat.

[KenS]

It's threads for rifle barrels that interest me most, and in stainless steel. I've used 60 degree carbide bits for threading but seem to always get a rough finish. I use the compound to load the left side of the cut, moving in .003" with the crossfeed and adding .002" more with the crossfeed set at 29 1/2 degrees or as close as my eyes can see the darned thing. I don't have a counter, but I do have a VFD now (thanks!) and can slow the machine to a crawl without using the backgear. Everything SHOULD work, I think, but I'm not getting the fine result that I see from other people so I'm missing something. Probably something simple, given my history.

[halfnut]

Moving with both compound and cross slide?

The whole idea of setting compound at 29 is to advance compound and bring cross slide back to 0 every time, well, you might take couple thous at last with last cuts.

The idea is to cut with one side of the cutter, and just shaving a little off opposite side. So just feed in with compound. Take some free passes at last.

You might also try a high speed tool ground up with correct clearances, remember that leading edge needs more clearance than back because of the lead of the thread.

Carbide is wonderful stuff but you need to run it hard and fast to get good finishes.

Oil, you need to use some kind of good cutting oil, I have used lots of black oil, sulphurized cutting oil. Also use this stuff called Bio-Tap. I think it is lard oil and Tap-magic mixed. Any oil is better than none.

Oh, keep the black oil off brass, it works OK but sometimes discolors it.

Good luck, get a chunk of scrap and practice, stainless cuts a pretty thread.

[KenS]

Thanks.
I use a thread cutting oil that has much sulfur. I started using the compound after reading the book, but it seems like I had better results by just feeding in with the cross and letting the drive direction dictate the lead edge used. I can reverse electrically or with the tumble, but am I wrong in thinking that a gear reverse would disrupt the cut just made? Maybe it's time to go sleep, eh.

[SGW]

You may want to try a high-speed steel toolbit in place of the carbide. A lot of people seem to have the idea that "carbide is the answer to everything," but it's not, especially in a home shop.

Carbide requires a VERY rigid setup to be at its best, and a lot of power. You can put a much keener edge, requiring less power to drive, on HSS than you can on carbide, and for nearly all applications it's far more suitable for home shop use.

Grind up a HSS toolbit with the proper relief angles, finish it by hand on an India or Arkansas stone so it is wicked sharp, and see how you do.

[halfnut]

Ken,

By using compound you cut with one side of tool instead of both, the chip has to have somewhere to go. If you are cutting with both sides the chips run together, and finish will be degraded, curl that chip out.

Don't touch that reversing gear unless you have a Hendy or a Hardinge
. If you do you will get to learn a new trick, picking up a thread.

Are you using Aloris threading tool? I think they are a JOKE, poorest piece of toolsteel I have ever had displeasure of using, I love an Aloris toolpost but not the threading tool.

If you do have Aloris threading tool make sure that you have it adjusted right, angle wise, that is what the 2 screws in side are for. Set to Helix angle in the direction you are threading.

Are you cutting standard pitch threads, if you are learn how to use threading dial. If you are cutting metric you will have to reverse spindle every pass.

Quick general lesson on dials, Even threads every line, odd # threads every other line, 1/2 threads every 2 inches of dial travel, 1/4 threads every 4 inches of dial travel.

And a multiple of you lead screw, you don't have to use dial. So if you have a 8tpi leadscrew you can cut a 8-16-24 etc without using dial.

Get another piece of scrap and keep practicing, you'll get the hang of it.

[KenS]

OK, I think I'vee got it now. I was reversing so that the feed would take me back to the start without disturbing anything. I figured that if I did that the tool would always be in the groove on each successive pass. BUT, the tool was chewing on the way back too. Now I'm making the initial setup, remembering the setting of the crossfeed, cut then back the bit away. Go back and place the point exactly where it was at start. Add .005 with the compound. Just like the SB book says.

Viola! Lickable threads!

I'm using a 60 degree bit in a #2 Aloris holder. Don't have their threader--and won't now. I did try a Williams threading tool but it flexed so that I could see it move during a cut.

Thanks much, for giving my head a kickstart.

[SGW]

Reversing is certainly a way not to lose the gearing relationship (I did it for years before I finally bought a threading dial), but it sounds as though you weren't withdrawing the tool before reversing. You do need to back out the tool, otherwise the backlash in the leadscrew and halfnuts will get you.