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New cross slide screw and Delrin nut for Rivertt 1020S

rustytool

Stainless
Joined
Oct 27, 2010
Location
Moscow,Idaho,USA
DSCN0355.jpgDSCN0353.jpgLast year I made a new cross slide screw and nut for the Rivett.
The original screw was 1/2-10. After measuring, I realized that if I eliminated the backlash adjusting nut I could increase the screw to 5/8-10, which McMaster Carr has in stock.
That would make single pointing the nut much easier and internal acme bars for that size are available up to 2" long.
I made the nut 1-7/8" long and snuck up on it so when finished there is little to no backlash.
It's been working like a dream for a year and I can't detect any wear. Greg.
 

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I wondered how well a Delrin nut would work on a lathe this size.

Over on the home machinist there was a thread call "Evan's nut" ? Evan cut a piece of Delrin in half than clamped it in a vise around the unworn end a old lead screw. He heated the lead screw while slowly tighting the vise. That nut was for a smaller HP lathe.

Hal
 
I missed this previously. Nice work! I may end up copying you.

If I did the math right and assuming 8000psi shear strength for the delrin, that nut should handle just shy of 10800lbf.

The is probably academic, but the stiffness of the nut probably matters more than its ultimate strength. As in, low stiffness will allow more deflection during heavy cuts. The tensile stiffness of delrin is 400ksi*, so the nut will have substantially lower stiffness than the steel screw. But stiff enough is stiff enough, and it sounds like this modification worked.

*there's probably a way to use this to calculate a shear modulus, but I don't remember it.
 
I missed this previously. Nice work! I may end up copying you.



The is probably academic, but the stiffness of the nut probably matters more than its ultimate strength. As in, low stiffness will allow more deflection during heavy cuts. The tensile stiffness of delrin is 400ksi*, so the nut will have substantially lower stiffness than the steel screw. But stiff enough is stiff enough, and it sounds like this modification worked.

*there's probably a way to use this to calculate a shear modulus, but I don't remember it.
I agree there.
 
Correct me if my thinking is wrong, but isn't one advantage of a Delrin nut, that it won't wear out the screw ? We've all seen those screws with threads so sharp you could shave with them.
 
Remarkably the Delrin nut will absorb any abrasives and become a lap the screw, so wear will be in the screw and not the nut, ask any clock repairer
 
You are absolutely correct. I saw it in watchmaking school.
The steel pinions always wore out before the brass gears.
So I guess the same would happen with a bronze nut and steel screw ?
 
Yeah, I would stay away from delrin nuts. I had the same concern with Doc's thread. Bore a really chattery bore through the old lead nut and cast moglice into it using an unworn part of the screw to form the perfect thread.

Sent from my XT1053 using Tapatalk
 
You are absolutely correct. I saw it in watchmaking school.
The steel pinions always wore out before the brass gears.
So I guess the same would happen with a bronze nut and steel screw ?

:(

Don't suppose it has crossed your mind that the steel pinion is small, has FEW teeth to spread the wear over, whilst the brass wheel has MANY more teeth, each on a longer lever-arm - the greater radius - and is generally carrying progressively less load "downline"?

Absent proper lubrication, a Nickel-Aluminium-Bronze nut certainly CAN wear out a steel screw.

"Bearing" Bronze, not so much. The length of the nut and size of the screw are generally chosen such that each component wears at about the same rate. Note that they are usually replaced as a set, not one at a time.
 
Yeah, I would stay away from delrin nuts. I had the same concern with Doc's thread. Bore a really chattery bore through the old lead nut and cast moglice into it using an unworn part of the screw to form the perfect thread.

Sent from my XT1053 using Tapatalk
Nope.

I"m going to try a delrin nut. If it doesn't work, I'm out about 20 bucks and some time. I have to make the tap regardless, so I'll start with a cheap material first.
 
Nope.

I"m going to try a delrin nut. If it doesn't work, I'm out about 20 bucks and some time. I have to make the tap regardless, so I'll start with a cheap material first.
Nope.

If you use moglice you don't have to use a tap, as I stated previously.

Sent from my XT1053 using Tapatalk
 
I don't want moglice, nor is it an option. The nut in question is 2.4" long and I don't have 2.4" of virgin thread to model from.

I want bronze, but for the cost of admission and learning potential I'm going to use delrin.

Sent from my SM-G930R4 using Tapatalk
 
Nope.

I"m going to try a delrin nut. If it doesn't work, I'm out about 20 bucks and some time. I have to make the tap regardless, so I'll start with a cheap material first.

Plaster of Paris is cheap. Grease the screw, mold it in place. No machine work required.
 
Damnit, no. Lol.

Sent from my SM-G930R4 using Tapatalk

I'm only half joking. We used it for years to make complex-shape holders. Steel plate, steel "fence", Gypsum pour, embed shape, add a coupla coats of bakelite varnish, it lasted a long time.

Delrin... where one has the SPACE, planned to use it from the outset, did the math as part of an original design - can be strong enough and stiff enough.

OTOH ..where one crams it - too thin and too short - into an already too-tight space where bronze once served? And that bronze wore out?

How long d'you suppose the great expectations for Delrin can overcome the physics?

Quite while. If it doesn't get USED hard. Or used MUCH AT ALL.

ELSE ? not so long...
 
Well, my lathe is 73 years old. In that time, the bronze did wear. They 43xx? lead screw did as well. If I can get 1/3 that life from a stressproof screw and delrin nut Ill be damn glad to refit it in 2041, nigh on its century mark. :)

Sent from my SM-G930R4 using Tapatalk
 
Well, my lathe is 73 years old. In that time, the bronze did wear. They 43xx? lead screw did as well. If I can get 1/3 that life from a stressproof screw and delrin nut Ill be damn glad to refit it in 2041, nigh on its century mark. :)

Sent from my SM-G930R4 using Tapatalk

I'm vintage 1945. Family history tracks, I'll still be alive, 2041. If only barely so.

Either way, I ASSURE you I will not give a shit.

:)

I have Bronze - plenty of it - already stashed for the purpose. I suspect it is earning me more increase in value each year than stock in the companies that make the feedstock for Delrin - I own some of those, as well - are doing.

So there...

:D
 








 
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