Easy Threading Job. Or?

[Dr. Rob]

Looking for ideas on a threading job that came in last week from one of the larger industrial suppliers of fasteners. A one-off order. Looking for the fastest, easiest, smoothest way to get done quick.

These are 200 x of cylindrical steel nuts, threaded internally M6 x 15 mm long (thru) that need to be given an external thread of M10, all the way. Diameter is now 9.9 mm. No critical tolerances to worry about, just a functional M10.

No prob, right? Just make an external M6 spigot to hold the nuts on while in the lathe chuck, and use a tailstock die holder under power.

Wrong. The torque from the die just turns everthing in the chuck. So, made it out of hex instead. No good. The torque from the die just torqued the M6 spigot right off.

Oookay- maybe single-point cut the thread, and just use the die to clean up? Nope- Tried cutting the thread all in one go, and the result was not good. (infeed 1.5 mm) So, two cuts then. Nope, still no good. Three cuts?

No, this is getting out of hand- this is a 200 x job. Three cuts per, means six hundred stops, goes, revs and infeeds and die chases. NO WAY. NO. Will NOT do. I want this done in the easiest fastest way.

Thought about using those three-pointed (well, 2½-pointed then) carbide inserts, thought about making a 60* multi-cutter to drive in the toolpost grinder, or an old tap, thought about using an old die cut in half as a one-pass tool...

Seriously, this is the esiest job I've had in years. Question is, what is the most effective way to go about it?

[John Stevenson]

Rob, Scrap what you have, buy a length of M10 allthread and drill and tap the M6, then part off.
Or part off to length and hold in a split threaded collet or bush and D&T the M6.

John S.

[Dr. Rob]

John!! Welcome back! Been thinking of you recently. We gotta talk gears later.

Was onto the allthread thing too, but it would mean (probably) having to deburr each piece twice. (each end). So, maybe your second alternative there. But, that's a fair amount of centering, drilling and tapping and parting. Was hoping for a two or three-move sequence.

On the other hand, I just came across a small CNC lathe for not too much money. Maybe this job is a good excuse to get into CNC!

[Dr. Rob]

On second thought, John, maybe you're right. If I go buy one of the combined drill & chipfeeding taps, I could get a pretty good pace going. Assuming they were parted off beforehand, then.

[Tony]

Dr.R --
since you've already listed just about any option i could think of off hand.. and johnS came up with a pretty good candidate, i'll just throw this out there..

since the ext thread isnt "critical" and only needs to be functional.. perhaps you could get away with cutting them all, 10 or 20 pcs at a time. thread them onto an M6 bolt (hex head) -- even some allthread with a nut tackwelded as a head -- run each one on pretty quick, so they 'jam up' against eachother.

take a very light skim cut with sharp HSS tool.. maybe even a few passes with a good file, to even out any bumps.. and also tighten up any play.

have at it with a single point.. two passes min, i'd guess. then a few passes with an M10 threadfile for cleanup.

not pretty i'll admit. but i'm sure they would pass for functional.

i've done this with ext. threaded bushings. they were a bit longer, and were brass. but it saved alot of time parting/facing individual pieces.

if the individual cylindrical nuts are provided already to length and have decent face finishes, i'd try to take advantage of that before parting and turning from scratch.

let us know how you work this out.

-knucklehead

[DR]

Regarding single point threading...

The 10 mm is roughly equivalent to a 3/8-16. For single pointing this thread on the CNC we'd probably take about 5 passes.

With that chip load these could most likely be held on a threaded mandrel. Too any passes for a manual operation on 200 pieces, though.

[Peter S]

Dr Rob,
Before you write off the external spigot idea, try using an Unbrako M6 capscrew (12.7 grade, not one of those crappy other brand 8.8 type). Use a bit of hex stock in the chuck, maybe drill the hex 6mm and weld head of capscrew to back end.
May work...

[darryl]

Your idea of the 60 deg. multi-cutter seems ok. I have tried it, and it worked in brass, though the homemade cutter didn't really cut it. It seems that with a rigid enough tool post spindle it should allow you to do the thread cutting in one pass. You would need to be able to adjust the left-right tilt angle of the toolpost spindle to match the angle of the thread as it would be once cut. You should also be able to do several nuts at a time on a good grade threaded steel mandrel, supported at the tailstock end with a live center. What would scrap the idea for me is having to make the cutter. Maybe that's available?

[Dr. Rob]

Cool, darryl. Did you make the cutter yourself, you said? Do you think using a thick slitting saw, ground to 60* might be a good start? How big were your threads?

This trick isn't new. It was (is?) commonly employed for large threads. I just haven't needed it before, and don't have the proper stuff for it, so I thought maybe there was a simplified trick for it. Like a tap running backwards @ just below center height, and with helix angle compensation. I can imagine that when bugs worked out and chips are flying, the work rate is astonishing.

Naw, scratch that last thing; I don't have the synchro stuff anyway.

[This message has been edited by Dr. Rob (edited 10-13-2003).]

[darryl]

Dr Rob, I made the cutter from a 1/16 inch slitting saw, 2 inch diameter. I ground each side to leave 60 degree teeth, but had to grind each side of each tooth for relief angles. It's not very consistent, and took time. I made the toolpost spindle with adjustable tilt to get the helix angle setting, and turned the lathe spindle by hand to bring the cutter across the workpiece, threading gears engaged. I don't recall the actual thread I was trying to create, but I think it was on a diameter of 3/4 in, probably a common coarse thread for that diameter. Aside from the cutter not being very good, I think there wasn't enough rigidity in my setup to control the cutting action. The teeth digging in caused lots of vibration instead of just chewing out chips. I think a stronger setup would have produced good results. In operation, the thread groove is created full depth, one pass. Another problem I had was the toolpost spindle needed torgue behind it, and instead is has speed. That's normal for grinding, but this is essentially a hogging out operation, where slower speed and higher torgue is called for. If you can supply that to a tiltable spindle, rigidly mounted to the crosslide, I'm sure it would work.
Hmm, if there's some way of using the cutter in a mill, and feeding the spindle down whilst rotating the workpiece- let me think on this. Would still need helix angle compensation. This is sounding more like milling a worm gear. Just brainstorming.

[This message has been edited by darryl (edited 10-13-2003).]