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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
 
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