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Odd thread pitch question

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GrayTech

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#1
Hi all, hoping for a miracle...
Yesterday I broke my lathe, I stripped the half nuts while taking a heavy cut on my craftsman 109. Yes entirely my fault! And I am suitably pi$$ed at myself!!!!
I want to make a new halfnut assembly but this lathe has a weird leadscew. It's 1/2"-16 acme thread. I've scoured the web for a nut this size, then for a tap, but no success. I managed to find a 1/2"-16 tap but it's not acme.
Question 1: if I make a new nut with this standard tap will it work or screw up my leadscrew?
Thanks in advance.
Many more questions to follow I'm sure. I'm relatively new at this.
 

benmychree

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#2
Are you sure that is a acme thread? I recently saw one of those, and thought that it looked like a vee thread.
 

tertiaryjim

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#3
A regular thread is 60deg but the acme is 29deg with a flatter, wider top. They won't match.
If you can use another lathe or get a friend to do it you could turn a tap and mill the flutes.
Another option, there are some that used a plastic which was heated and molded to the lead screw.
Try searching this site " half nut repair"
 

dtsh

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#4
Your threadform and pitch have to match pretty closely, if the tap isn't an ACME and your screw is, they won't be compatible.

It's very hard to see the threadform in the suplied picture, can you get some pictures of the leadscrew and perhaps the damaged half nuts?
 

mksj

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#5
You may be able to cut your own using a Micro100 carbide tool like this https://www.amazon.com/Micro-100-IAT-750-16-Threading-Projection/dp/B00Q8LVDE2/ref=sr_1_fkmr0_4?s=industrial&ie=UTF8&qid=1526569302&sr=1-4-fkmr0&keywords=acme+tap+1/2+16

There are a number of vendors that may be able to cut you that pitch in bronze, some will do custom thread/pitch. Also one can get a custom tap made, or possible cut your own out of steel and use it in bronze stock.
https://www.roton.com/products/acme-lead-screws-nuts/engineering-data/
http://www.westportcorp.com/acme-taps-special-left-or-right-hand.html
http://www.besly.com/
http://www.greenbaymfgco.com/ACME-nuts.php
 

GrayTech

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#6
Thanks for the ideas so far.
Yes it's an acme thread, it has flat crests and valleys, see pic 1.
A SAE 16 thread guage fits in there pretty good with less than 1/4mm backlash (Pic2 )(sorry about the mm thing, I grew up in a metric country and still think that way about minute measurements) but it's obviously not the right pitch/angle.
I have very limited funds available at the moment so having an acme tap made is probably out of reach.
If I make a nut from brass with the standard 1/2-16 tap it should wear the brass nut rather than the steel leadscrew, but how well will it engage the threads.
Alternatively replace leadscrew with standard 1/2-16, but can it take the forces involved. I need to stay with 16tpi to keep my changegear threading ability.
Also wondering if the standard tap could be modified by filing or grinding the valleys in the tap...
Wracking my brain for a solution here.
 

tertiaryjim

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#7
Found the post. " delrin cross feed nut "
Some good information there.
Dont know how to add a link to it.
 

GrayTech

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#8
Found the post. " delrin cross feed nut "
Some good information there.
Dont know how to add a link to it.
Thanks. I'm not sure plastic would work on leadscrew half nuts, especially since mine are offset and don't clamp together.

I found a 1/2-16 tap which I will try to modify/grind. I figure if I can get the valleys on the tap ground as close to correct as possible then the crests of the produced nut's internal threads should be close enough.
 

extropic

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#9
The best picture of the thread looks like a "square" thread to me. The flanks appear to be 90º to the shaft rather than 29º. What do you see in person?
 

P. Waller

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#10
As mentioned above.

Square thread, often used when making lead screws for machine tools, difficult to machine but more efficient, do not confuse them with trapezoidal threads such as acme
 

GrayTech

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#11
Good eye xtropic and P.Waller! Square thread it is.
Soooo.... Back to the drawing board...
 

pdentrem

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#12
I can make you that tap to cut bronze if you wish. Send me a PM
Pierre
 

Cadillac STS

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#14
Last edited:

pdentrem

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#15
Unfortunately this is a Dunlap Lathe sold by Sears.
 

magicniner

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#16
Thanks. I'm not sure plastic would work on leadscrew half nuts, especially since mine are offset and don't clamp together.
It will work, but you need the ingenuity to make, trim and mount the plastic into your lathe's mechanism.
 

Bi11Hudson

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#17
A little late, I know. But that's me, a day late and a dollar short.

I butt in here to bring up an old technique used in Gingery's first book on the lathe. He uses a mold around the leadscrew and casts a babbit half nut set, or three. It is involved what all is necessary. Candle smoke on a greasless screw is the least of the set-ups. But done with a lot of hard work and a chunk of babbit and a fire. Could just as well use lead or tin(shorter life) or any other low melting point metal such as bismuth, et al. As I recall, the extra pair I acquired for my 12" Craftsman was of babbit. They're still on the shelf, sitting around just in case.

I have some stuff for my models that melts below the boiling point of water. A bismuth alloy that I don't know how long it would stand up. But a starting point, once you find the book. I had a set but gave it to a maker's group a while back. Could maybe run it down but I haven't been active for a couple of years. And Gingery is retired, or maybe even deceased. Who knows? Some of his books are still around, at www.yotb.com or somesuch. Your Old Time Bookstore is the English name for the place. A maybe worst case solution, but I'm proud that my memory still works that good. Just wish the rest of me was in that shape.
Bill Hudson​
Edit: The link didn't work. Best look it up with a search engine. BH
 

GrayTech

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#18
Thanks for the ideas Bi11Hudson, I have a tap made for me by pdentrem. Part way through the repair, but short on time.

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