Advice For Making Leadscrew

[Jester966]

I have a 10" Standard Modern Utilathe. The crossfeed and compound are both metric, and I would like to replace them with imperial screws & dials as that is what I am used to. (The longitudinal screw and threading gears are already imperial).

The sketch below is what the crossfeed screw looks like. The screw diameter is actually 1/2" but the pitch is 2.5mm.

I believe the imperial version of these lathes use 8tpi (.125" per revolution) screws, but I am going to go with 10. I have ordered a 1/2-10 LH ACME rod from McMaster Carr. I plan on turning the left section separately and mating it with a section of the threaded rod. My question is, what is the best way to mate the two parts while keeping them perfectly in line?

I was thinking I would drill/ream the right end of the left section, and turn a "tang" on the screw for a light press fit, as seen below. Either epoxy them together or weld the seem.

Any thoughts or suggestions??

The compound screw is short enough that I figured I would just turn it all from one piece.

Thanks

 

[derf]

That's the way I'd do it, but I would pin them instead of welding.

 

[dirty tools]

I dough epoxy will hold due to the torque load which can be applied.
You could cross drill and pin the joint. That way you could dissemble for any reason.

 

[4GSR]

Yeah, I would pin them too. Please don't weld!!!
I have seen where the lathe manufactures, as on my Sheldon lathe, made the leadscrew a close fit to the piece with the gear on it. Then took a roll pin and pinned it. I would have used a tapered pin myself. Once installed, cut it off, put it in the lathe and turn the pin flush with the rest of the part. Put two pins in it if you don't feel comfortable with one pin.
Ken

 

[mikey]

I just did this for my Emco lathe. I used a 0.0002" press fit and a 3/32" roll pin. Came out with under 0.002" run out over the entire cross slide lead screw, just over 12" long.

 

[Mark_f]

that is basically the way to do a lead screw repair, but leave a press fit, heat the outer part in an oven and freeze the screw in a freezer , put them together and when cool they are permanent. put a small pin in for assurance. Many south bend screws have been repaired this way.

 

[Bob Korves]

Another thing to consider is a DRO, which will give you instant imperial and running values, as well as leaving your lathe as built, and saving a whole lot of work. Decent DROs are now available for around $300, and are easy to install compared to what you are attempting. Also, making accurate lead screws is in no way a trivial undertaking. Those screws are the dimensional accuracy foundation of your machine. They also would not be hardened. Downside, it would still be a metric lathe and imperial thread cutting would still be more difficult, and if your existing screws are worn, they still will be worn afterwards. Adam Booth, Abom79 on YouTube, bought a nice K & T horizontal mill that is metric. He is a professional machinist. His attitude was, hey, it has a DRO and I don't even have to look at the dials...

Disclaimer: I have no DROs on my machines, which are all imperial...

 

[4GSR]

Yeah, don't press fit or shrink fit the two pieces. There's no need in it.

You are not going to have cutting forces involved that will shear off a nice healthy pin, which will be in double shear. So save yourself some time. Somebody in the next generation will thank you for that! Or cuss you for the shrink fit you left them!

Ken

 

[chips&more]

A +1 on what 4gsr (Ken) said. Just make the two parts fit good and snug together and then taper pin it. And paying very close attention to concentricity when machining the assembly. And avoid excessive hammering when installing the taper pin and or rough handling while putting all of it together. All of this this could change the trueness of the assembled shaft and lead to unforeseen problems…Dave.

 

[Jester966]

Thanks for all the feedback, I'll pin it for sure. I had thought of this too, but for some reason thought the other options would be better (can't remember why though).

Also, making accurate lead screws is in no way a trivial undertaking. Those screws are the dimensional accuracy foundation of your machine. They also would not be hardened. Downside, it would still be a metric lathe and imperial thread cutting would still be more difficult, and if your existing screws are worn, they still will be worn afterwards.

Not sure If I was unclear but I bought the threaded rod, I am not cutting the thread myself. McMaster Carr calls it "precision" threaded rod and the specs are .009" over 12" accuracy or better - is this standard for "precision" screws?? I would have thought it should be better, but realistically I don't think that would be an issue for the crossfeed.

Another thing to consider is a DRO

I had considered this too, but decided I'd rather just "fix" the lathe and use the dials. The lathe already has an imperial lead screw, gear set, and threading chart. I believe Standard Modern may have "retrofitted" a lot of imperial lathes to metric for use in Canadian schools, and this is one of them.

Yeah, don't press fit or shrink fit the two pieces. There's no need in it.

I had thought a very light press fit would be good to ensure concentricity and no movement between the parts in operation. I would not do anymore than a tenth or so as I would not want to bend anything. Does this sound reasonable?