Thread: Looking for someone not afraid of this part

We're looking for someone to make a pair of nuts for an anti-backlash assembly that goes on a leadscrew with: ~.5"OD, 8 starts, 1" lead, rounded thread profile. It's the leadscrew in this thread:
How would you measure this lead screw?
Material is Delrin AF.

Don't be too put off by the requirement that the threads in each nut be aligned to each other so that they fit together a certain way. If you have a CNC lathe it's just a little math to figure out how far in front of the part to orient the spindle and start the threading.

I wasn't able to determine the exact thread profile, so we'll be supplying the leadscrew to whoever does the job to be used as a thread gauge. We don't need a super tight callout on thread fit; just something that threads on and doesn't have a ton of play. It is an anti-backlash nut, afterall...

Edit: The quantities to quote are 2, 8, and 16. Send me your email and I'll send full prints.

I don't remember the quantity offhand, but I think it was around 5-10 sets. I'll update when I find out.

Originally Posted by maxh

I don't remember the quantity offhand, but I think it was around 5-10 sets. I'll update when I find out.

That is very important, you're looking at a lot of set-up time, the more parts it is amortized over the better.

I'm curious as to how such a part can be machined . I can't see how that profile could be single-pointed on a lathe. At 1/2" OD and 1" lead, it's as much a shaping op as a turning op.

4-axis plunge EDM?

Regards.

Mike

Not having a clue of the internal thread profile, my first guess would be a carbide sphere (?) burr on a live tool with C axis infeed.

???


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Think Snow Eh!
Ox

I would just grind or pay someone else to grind a cutter like below and single point it. It's only delrin; it shouldn't be that difficult to push a cutter through, as long as proper clearances are ground on the cutter. At this point I'd suck it up and do it myself manually, but my friend's lathe only goes down to 4 tpi (1/4" lead). Again, the thread profile doesn't have to be perfect; just in the ball park. And material can be removed until the nuts thread on. The used nuts currently have almost no material left on the threads, so just about anything will be significantly better.

Unfortunately EDM won't work, as delrin's not conductive.

The quantities we need quoted are 2, 8, and 16. If you'd like to quote send me your email address here or via PM and I'll send the full prints.

Well, I'm about to prove how little I know about Delrin... Here goes...

Since the tolerances are a little loose, could you get serviceable parts by heating a section of your existing shaft, and pressing it into a rough-bored Delrin blank while advancing + rotating shaft the appropriate amount? Basically, burn it in.

Leave on shaft segment until cool, then secure and turn needed outer dimensions.

Securing to shaft, and holding shaft accurately are left as an exercise for the reader...


On edit: Or just mold around shaft, then machine.

Chip

1" lead? Wouldn't that cause some relief issues as far as the cutting tool goes? (if you were to single point it) 1" is pretty fast.

Certainly if one were to make a lathe tool for it, the cutting edge would need to angle to the lead and relieved accordingly. I'm not a lathe guy(not cnc anyway), but curious, can you feed a cnc lathe that high? 1" per revolution?

Are there any takers on this yet? I have another thought that I might be tempted to say I'm not afraid

Originally Posted by chip_maker

1" lead? Wouldn't that cause some relief issues as far as the cutting tool goes?

Yes, for sure. Helix angle looks to be ~45*, add a few for clearance, and you're looking at 50*. But, now you have 50* positive rake on the topside, that won't do, so you need to bring the topside back to 30-40* negative.

I do a fair number of funky threaded parts but this one is way out there. .

Regards.

Mike

Take it to a person that cuts rifling in gun barrels. They have a twist in one foot. They pull a cutter through the barrel and cut one land then index for the next one and so on. Just a thought. Ron

No takers yet. True, it's no ordinary thread, but it is just a thread. In plastic. It's not like it's a 5 axis turbine job in nickel super alloy... All that's needed is a regular lathe and a proper tool ground with proper clearances. I think I calculated something around 50 deg. for the helix angle a while back. I didn't show that in my cutter concept a couple posts back, but it should still be straight forward to make.

Finegrain you do good thinking. 57.5 deg. is the lead angle at od of thread. The tool will look nothing like maxh's drawing and if you could cut this then you would have chip problems bigtime. I might be able to cut this with a push tool and a ridgid tap sub.
I am sure that the screw was rolled formed and any nut made for it was molded.
Good luck on anyone tooling that part.
Mike

I didn't catch the part of being Delrin.

Boy - wouldn't that make a mess of a carbide burr?



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Think Snow Eh!
Ox

'Couple thoughts. The "hot tool" approach is interesting and might work. Along the lines of button rifling, could one cut a piece of shaft, grind the end flat to make a cutting edge, then push or drag it through a hole? Probably have to do it in stages like a broach. Finally, since it's just Delrin, make the lathe tool right out of a piece of the shaft. Basically grind away everything that doesn't look like a threading tool with a huge helix angle on the cutting tip. It certainly would not look like the tool proposed by the OP!

Originally Posted by Conrad Hoffman

'Couple thoughts. The "hot tool" approach is interesting and might work. Along the lines of button rifling, could one cut a piece of shaft, grind the end flat to make a cutting edge, then push or drag it through a hole? Probably have to do it in stages like a broach. Finally, since it's just Delrin, make the lathe tool right out of a piece of the shaft. Basically grind away everything that doesn't look like a threading tool with a huge helix angle on the cutting tip. It certainly would not look like the tool proposed by the OP!

I could see making up a dumby internal blank and turning the helix away from the front edge a bit to leave a "pilot" on the front, and then drill the part out down the middle and also drill the "thread" (?) out in the rearward direction to make a sharp edge fwd. Essentially making a cheese grater type tool.

I would still expect to have to feed it in in C axis, or some sort of "rigid tapping" type setup tho.

Seems like that should werk in Delrin. Might be too much werk for 1 set tho. ??? Might make quik werk of several sets tho?



...and of course it might not werk at all....


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Think Snow Eh!
Ox

Originally Posted by stumpwater

Finegrain you do good thinking. 57.5 deg. is the lead angle at od of thread. The tool will look nothing like maxh's drawing and if you could cut this then you would have chip problems bigtime. I might be able to cut this with a push tool and a ridgid tap sub.
I am sure that the screw was rolled formed and any nut made for it was molded.
Good luck on anyone tooling that part.
Mike

I know my cutter concept wasn't very accurate since it didn't show the necessary angle and relief. However, I know for a fact it's quite possible to do this job with a custom ground cutter on a lathe. About 3 years ago one of my coworkers had a local shop do this very job. The lathe guy ground the cutter himself and cut the parts, no problem. Unfortunately, we've run into a wall trying to get the same shop to do it again. The guy who did it the first time's not around any more, the owner doesn't remember the job, and anyways, the shop has recently started competing with us on some parts. So we're looking for someone else to do it this time...

I would consider using a piece of leadscrew to create a tool, but we don't have any spares and I don't know where to obtain one. This job would've been done already if I knew anything about the leadscrew OEM...

BTW, when calculating the helix angle using the OD of .5" you do get ~57.5º. This is the angle off of the axis of the screw, so the angle at which the cutter would need to be tilted forwards is actually only 32.5º. That's really not so extreme; it still seems more like thread cutting than broaching to me.

Edit: I know that proper thread cutting geometry does not have the cutter inclined to the thread helix angle, but rather it stays parallel to the screw axis, with clearance angles adjusted to compensate for helix angle. (Sorry if my terms aren't proper!) I just thought it would be easier in this case to grind a cutter with profile and typical clearance angles, but the whole thing inclined to the helix angle.

Wow guys, If i was worth a damn with my grinder, I'd take a whack at it and start wrecking delrin... it's cheap enough... There must be a lot of work already around!

Good luck Max!

I'm not turning it down because of capacity, I just don't think I could do it, at any price point. I guess I'm "afraid of this part" .

Regards.

Mike