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Making Lathe Feedscrews

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Chris Trice07/08/2011 17:53:37
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What is the collective wisdom as far as materials are concerned for making lathe feedscrews. Is EN8 suitable left as is or hardened, or, regular EN1A but case hardened? Same question in respect of feedscrew nuts or is phosphur bronze a good choice?
Chris Trice07/08/2011 18:21:47
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For the record, regular right hand acme thread. Nuts will be tapped but the feedscrews lathe cut.
 
John Stevenson07/08/2011 22:09:59
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Chris,
I'd be wary of having a screw hardened unless it was just tuftrided.
Normal hardening is viable to warp something like a screw as the stress isn't equal on both sides because of the helix.
 
In fact when leadscrews are cut they usually come off the machine like a banana and have to be straightened.
 
Tuftriding gives a very thin hard coat, only microns thick, more like hard anodizing but at about 450 degrees which doesn't.
 
Bronze is a good choice but lately I have been having good success with Delrin nuts.
They wear at about the same rate at bronze or even better but don't wear the screws.
 
John S.
Chris Trice08/08/2011 00:01:30
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Cheers John. So do you think I should go for EN8 and its extra toughness/wear resistence over free cutting mild but leave as is? We're only talking 1/2" feedscrews 12" long maximum. That question is posed with bronze nuts in mind. Delrin is an interesting idea but is it hard and precise enough for relatively small feedscrew nuts? I know some smaller lathes use Delrin (or similar) gib strips but I've always felt a bit wary. Food for thought and a little experimentation though. I can imagine a Delrin nut having neglible backlash because it tends to close up slightly after being cut so maybe in the right circumstances...
John Stevenson08/08/2011 00:20:21
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Chris,
I did some two part lead nuts a while ago, these are on a lift mechanism that links two conveyors together. The part goes from one conveyor into a lift, raises 1 metre and onto another conveyor, then repeats.
Wear on screws and nuts, two per lift, meant a rebuild every two years.
Last rebuild I added a Delrin nut to the bronze one as an adjuster to try to get another 1/2 life out of them.
 
Last check up the bronze nut is worn, so is the screw but the Delrin nut and where it runs on the screw isn't.
I will get the 1/2 life but next rebuild will be new screws and double Delrin nuts.
The nuts are just tapped from a length of spare screw that has been tapered, fluted and hardened
JohnF08/08/2011 00:23:56
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Hi Chris,
I agree with John, hardening will cause you a lot of problems--better to leave as machined and EN8 is a good choice with Phospher Bronze for the nut. This combination was used extensively on industrial machines before CNC and ball screws and gave many years of service even when unhardened.
 
One point is that cross and compound slide screws on a lathe are usually left hand thus making the tool move away from you as you turn the handle clockwise.
 
Regards John
John Stevenson08/08/2011 00:41:26
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Easiest way to work it out is if the handle and dial moves with the slide a la Myford then the screw is right hand.
If the handle and dial don't move when the slide does, a la Colchester then the screw is left hand.
Chris Trice08/08/2011 00:53:33
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Thanks guys. I'll be out there experimenting this week. I test machined a length of acme feedscrew today using a self ground tool and have to say I'm delighted with the results so far although that was free cutting steel rather than EN8. Using a piece of the actual feedscrew to tap the Delrin makes very good sense,
 
Er... pretty sure Myford feedscrews are right handed. As you turn the handles clockwise they pull the slides towards the bed. The nuts are fixed on the main saddle and the rotating base of the compound slide. I think the leadscrew is left handed as turning the handle clockwise at the tailstock end pushes the saddle towards the headstock. It obviously depends on the lathe make and model and which part of the lathe the nut is fixed to.
 
Thanks again.
John Stevenson08/08/2011 01:01:03
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Myford Super series tailstock is 1/2" x 10 Triple start left hand.
 
Early Myford lead screw is 5/8" x 8 left hand.
Later ones are 3/4" x 8 left hand.
 
Did I mention I bought all the leadscrew taps from Myfords prior to the sale ?

John S.
Chris Trice08/08/2011 02:33:09
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You didn't pick up a spare 1/2" x 2mm pitch acme feedscrew tap by any chance? Having real trouble tracking anything but imperial ones down.
John Stevenson08/08/2011 08:47:33
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Yes got one, thought it funny they went 1/2" x 2 instead of 12 x 2.
same applied on the ML7 they went from 3/8" x 10 to 3/8" x 2 instead of 10 x 2 ?
 
I'll check but I think there was only one of these, obvious I'm not going to sell it but I will tap some nuts out for you if you want.
 
John S.
Roderick Jenkins08/08/2011 08:50:20
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I'm looking to re-furbish an early Simat lathe that I bought as a bucket case, with a view to eventually converting to CNC. I've been looking for metric trapezoidal form taps myself and, frankly, they are phenominally expensive. I've therefore had to modify my ideas and am going for a 3/8" acme for the leadscrew, which is the smallest I can find at reasonable cost (RGD). It won't matter for CNC. There is a good link here http://bbs.homeshopmachinist.net/showthread.php?t=43645 about moulding leadscrew nuts from acetal (Delrin is a DuPont tradename), which I might try. GLR sell acetal rod.
 
cheers,
 
Rod
Clive Hartland08/08/2011 09:07:50
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2929 forum posts
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I have seen some Lead screws where it appeared that the thread had been 'Rolled'.
The thread top having the appearance of a curled over lip.
Interesting if they cut the thread and then 'Rolled' it , this would give a higher Brinell hardness figure for the metal. Making it longer wearing than a normal cut surface.
Also it could be a method of straightening it.
Has anyone else seen this at all?
 
Clive
John Stevenson08/08/2011 09:23:10
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Thread rolling is very common, most bolts are thread rolled, it gives a stronger thread as the grain is in the correct location.
 
You start off with an undersized bar and roll half the depth in, the displaced material makes up the top half.
 
Most ball screws are made this way, only the better ones are ground as a final operation.
 
John McNamara08/08/2011 09:24:22
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Hi All

Thomson make rolled ball screws and ground ball screws the rolled version being cheaper. They are hardened steel.
 
Also rolled lead screws .003’ per foot error
Cheers
John
 

Edited By David Clark 1 on 23/02/2013 13:37:30

Weldsol08/08/2011 09:31:24
74 forum posts
You could talk to HPC Gears as they do a line of leadscrews (fairly good price) also they do specials if needed.
 
Paul
Chris Trice08/08/2011 10:27:39
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Posted by John Stevenson on 08/08/2011 08:47:33:
Yes got one, thought it funny they went 1/2" x 2 instead of 12 x 2.
same applied on the ML7 they went from 3/8" x 10 to 3/8" x 2 instead of 10 x 2 ?
I'll check but I think there was only one of these, obvious I'm not going to sell it but I will tap some nuts out for you if you want.
John S.

Thanks John, I'll probably take you up on that. It would only be half a dozen or so. Makes sense to do a few at one go rather than one at a time. I'll prep them ready to go. Be in touch.

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