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old mart
02-20-2018, 11:58 AM
In another thread, I am discussing working on the apron of a Smart & Brown model A. The leadscrew nut has seen better days, although the leadscrew itself is not bad. It is 1" X 6 ACME, and the nut itself is about 1 3/4" wide in a 130 degree arc. I will have to use the lathe to make a new thread unfortunately, so the apron will have go back on first. If I had the use of another lathe, I would recut the existing threads and then re position the nut to the original centreline.
I intend to produce a thin wall threaded tube, cut a suitable section out of it and soft solder it as an inlay to replace the old thread. The old thread would be removed using the mill and rotary table, or by boring on the lathe.
Doe's this sound ok?

dian
02-20-2018, 12:03 PM
not sure what your doing, but why not just split the nut and make it backlash free?

MattiJ
02-20-2018, 12:21 PM
Sounds good, exactly what I did on my lathe.
Just do it earlier than I did, the thread form was already paper thin and couple of threads had crumbled off.

J Tiers
02-20-2018, 12:23 PM
not sure what your doing, but why not just split the nut and make it backlash free?

Because if the screw is worn, it will just cause lockups 20 thou short of where you want to move to........ when the unworn area hits the nut.

dian
02-20-2018, 12:31 PM
did op say the screw was o.k.? he could also replace it with a ball screw. they are getting cheap. but splitting is free and fast. whats the downside?

old mart
02-20-2018, 01:26 PM
Just to reduce unforeseen confusion, the nut in question is the threading nut which engages the main leadscrew, it slides in and out of engagement for traversing the saddle. On the opposite side of the leadscrew is a bronze slipper which is fixed.

MattiJ
02-20-2018, 02:03 PM
bit of wear:
http://i.imgur.com/RLv2Lflh.jpg

new "insert"
http://i.imgur.com/GRfvyk8h.jpg

Bored the old half-nut to size and soft soldered the new half-nut in place.

old mart
02-20-2018, 02:19 PM
Thanks MattiJ, that's exactly my problem, although the S&B nut is not as bad as yours.
I have just ordered an IR16 6 ACME laydown tip which will be fitted to a 16mm threading bar. Also I have ordered a bronze piece of 38 od by 19 id by 50mm long. I may produce a test thread in aluminium, to make sure it fits well as there will be only one chance with the bronze. The bronze will be bored and threaded first, before reducing the outside diameter. The bore size will be 0.833", I may have to relieve the underside of the tip a little.

Norman Bain
02-20-2018, 02:45 PM
MattiJ; dang you got me going ... for a bit I thought old mart had posted a picture.

MattiJ
02-20-2018, 02:48 PM
Expect some struggles with chatter. Coarse internal acme thread is prone to chatter when single-pointing and bronze is chatter-happy in my experience.
Had to go pretty slow and IIRC I finished one thread flank at a time.

I ended up to temporarily solder the half-nut to some lump of brass so its easier to mount to 4-jaw chuck:

http://i.imgur.com/in8JHmph.jpg

and my simple but effective boring bar:
http://i.imgur.com/RwZDiJxh.jpg

Alan Smith
02-21-2018, 09:33 AM
Like the boring bar idea Matti!

Duffy
02-21-2018, 10:56 AM
Would this not be a candidate for a Delrin replacement using the "Evanut" technique? See MANY previous posts on the subject.

old mart
02-21-2018, 02:44 PM
Plastic threads would not last long in this application as they are not engaged with the leadscrew permanently, and to enable disengagement, they cannot have much more than 1/3 of the thread anyway. Even if half of the thread was used, it would have to move at least 1/3 of the diameter to disengage.

J Tiers
02-21-2018, 03:16 PM
And, even if made as shown by the metal pieces, they would likely get torn up by the act of engaging, as one is never really perfect in reacting when the line comes around.

You want something a bit less easily damaged, I think. The stuff is tough, but this is actually a rather nasty application, in some ways.

old mart
02-21-2018, 03:51 PM
The bonus of making an inlay and soft soldering it in place is that you have a spare threaded section for future use.