Smithy lead screws/nuts

Spottswood3 said:
Thanks for the info. I thought about doing the same thing with my Smithy, but wanted to explore all the "easier" options first.
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You're welcome, and welcome to the forum! The is the 10 tpi version of the tap I used. You'll need a bigger tap wrench than what comes with most tap and die sets up to 1/2" and be prepared that it takes a LOT of force to tap that hole. I used aluminum bronze from Speedy Metals because I couldn't find an economical piece of bearing bronze. The aluminum bronze was very tough. I couldn't see spending more than this on a tap that I might only use once or twice in a lifetime.

 
Thanks David:

I found a 3/4 - 10 LH Tap on E-bay which has not arrived yet. I got a piece of Brass from Speedy and was planning to make the half nut out of that but I'll probably make one out of aluminum first to see if my machining skills are good enough. I want to see how close the 3/4-10 thread is to my lead screw thread before I try to make a new lead screw out of ACME threaded rod. I'm not in any hurry.
 
As long as the pitch of your screw is 10 TPI not much else matters. If your hand wheel is marked to give .100 inch of travel per revolution, that means the pitch is supposed to be 10 tpi. Diameter is not very important as long as it's reasonably close. That only matters to match the strength. The other consideration is that you will have enough wall thickness in the leadscrew nut if the replacement is larger than the original. Mine is fairly thin at the bottom of the nut but I made the nut as large as would fit in the channel without touching anywhere that it shouldn't. I think I wrote that my original was not a standard size. It is now and about .020" larger than the original. From the operator's perspective there is no difference other than losing an entire turn of backlash. I knurled two of the 1" round brass nuts tonight. It takes a lot of pressure but there were no difficulties.
 
Thanks, I really like this forum. It may be a while, but I'll let you know how it turns out.
 
I figured I should chime in, I recently made this part to the original specs in brass. It works perfect and the machine works like new. The lead screw on my 1220 is 10 tpi acme which was verified by Smithy when I emailed them, and I used an acme thread gauge to check it. I’m not sure if Smithy changed the thread profile at some point in production. The thread major diameter is slightly larger than 3/4” so using a standard acme tap wont work.

I used a 4 jaw chuck to bore a hole in the brass stock that I had already milled to size. Single point acme threaded the hole, turn the stud profile, then machined half the nut off. I’d be happy to go into detail on how I made this part, there was a bit of math involved in figuring out the hole diameter prior to threading. I would also be willing to remake this part if anyone wanted to purchase it.

Here are a few photos of the part and the process. Mac1911 had a previous post about molding the thread profile with JB weld onto the old worn out part, i used this method to limp my lathe alone so I could thread this part.

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This is great, thanks. I have already ordered a 3/4-10 TPI LH Acme Tap and it should be in soon so I want to compare it to the lead screw threads on my machine. Mine is about 25 years old and they may have had a slightly different thread size back then but I put my calipers on it and came up with .785" which is definitely oversized from a standard 3/4" acme thread as you mentioned. I would be interested in purchasing one or even 2 of the half nuts from you and I might also suggest you talk to SMITHY about supplying for other customers because they told me that there are a number of other hobbyists who have this same dilemma. Please let me know if you are still willing to make a couple for me.

I was planning to approach machining the half nut in the same manner as you, except I think that 2 nuts could be machined out of the same stock and just slit it into 2 halves with a fine slitting saw. The thickness of the saw should not be a big problem as long as it is the thinnest available, such as a jeweler's saw. The inside half of the diameter of the half nut does not need to be exactly 1/2 of the lead screw, in fact mine appears to be slightly less than half already.
 
My lead screw major diameter measured the same as well at 0.785". I'd be happy to make you a couple of these, I was thinking about selling some on ebay but I wasn't sure of how much interest there would be in fixing these machines. I'll shoot you a PM and we can work out the details.

I think the original part is saw cut, it makes sense to economically produce it this way. I have a cheep harbor freight band saw that I use for cutting stock material, it's not the most accurate. Now that I've made a couple I may try out the saw and clean it up with a facing mill if needed. My original part measured 0.72" from back of nut to the face of the nut like in the CAD photo below. It's less than a half nut to clearance for easy engagement with the lead screw. I'm not sure what the centering mark is for on the stud, I didn't see any reason to make that feature with the work so close to the chuck.

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I think your dimensions are spot on. The distance from the face of the half nut to the bottom of the diameter of the the thread is definitely less than .785/2 = .3925" and you can see that in your drawing . Unfortunately I won't be back in town to put a scale on it until next week and I'll verify the dimensions. I think I can do it in CAD and I'll send you the file. I would wat to purchase 2 of them and, as I suggested you ought to contact SMITHY and let them know or your E-bay idea is good as well.
 
Spottswood3 said:
I think your dimensions are spot on. The distance from the face of the half nut to the bottom of the diameter of the the thread is definitely less than .785/2 = .3925" and you can see that in your drawing . Unfortunately I won't be back in town to put a scale on it until next week and I'll verify the dimensions. I think I can do it in CAD and I'll send you the file. I would wat to purchase 2 of them and, as I suggested you ought to contact SMITHY and let them know or your E-bay idea is good as well.
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I checked and I have enough material to make 4 more, which I'll get working on here in the next couple of weeks. When I'm done we can work out the purchase details. I do have a CAD model that I created for the part, which I can share with you so you can verify dimensions.
 
Sounds great ! Thanks. I will be back in town next week and will verify the dimensions. Do you want me to mail you a check for a security deposit ?
 
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