Cross slide leadscrew replacement

[kb58]

If I adjust the split nut on my Webb TSL-800 so that backlash is "about right" mid-travel, it binds as I move away from there. That tells me that the nut isn't the problem, it's that the leadscrew is worn in the center travel area. At first I figured "how hard can it be", just order replace the leadscrew, but that was before finding that it's a really odd 7/8-8 left hand Acme thread. It's odd enough that it's been really challenging to find anyone who carries it. Google-foo turned up a few names. Huyett has it on their site but say they don't carry it (okay, then remove it from your site already...). Green Bay Machinery was contacted, but no word back yet. Roton and Nook sell leadscrews, but not that size.
I would rather get the correct size leadscrew instead of adapting a different size and making a custom nut, as it seems like more work than necessary. If the right size can't be found, however, it means either living with the backlash as-is, or taking the plunge into another project. (Also, there's the issue of needing to machine bits of a lathe that's been partially disassembled. Hah.) Yet another choice is cutting my own leadscrew, but I don't have a full set of change gears - which if I ever build it, the electronic leadscrew would solve, but I digress.

 

[mikey]

There is a good chance that there is wear in both the nut and the leadscrew, although the leadscrew will typically wear more than the nut. The ideal solution is to make a new nut and leadscrew and fit one to the other so as to minimize backlash from the get go. Cutting tools for both are readily available or you can grind your own, although most thread pitch templates are for the general purpose type of thread vs the centralizing fits.

If you can find some commercial threaded rod that works suitably with your nut then the easiest solution is to cut the worn section off your leadscrew and fit a new section onto the remnant. Silver solder works for this, although I prefer a press fit and pin arrangement. I had to do this for my Emco lathe that has a Witworth thread and it works fine.

 

[pontiac428]

Cost may be a factor, but one of the manufacturers you've found should be able to produce a custom lead screw for you. Might not be too painful, maybe $50 on top of the price of an off-the-shelf part based on what I've seen in the past.

 

[benmychree]

I would disassemble the screw and make a drawing of it, reassemble it, and make a new screw; the tool to cut the thread is ground to fit a standard acme gage, then I would make the nut to fit the screw. If I made one, I would use "stressproof" steel.
You do not say how much backlash is in the screw/nut assembly; one may just live with it, backlash does not much matter in the operation of a manual lathe, so long as it is not excessive, not more than maybe 1/5 turn.

 

[kb58]

Went out and measured it. If mid-range backlash is adjusted to 0.030" (more than I would prefer), it's about 0.010" as the cross slide moves towards its extremes. You have a fair point in that it's still perfectly usable, but I'm still going to look for some Acme rod though, as it's just enough backlash to be noticeable (I realize it's a very subjective thing).

 

[kb58]

Green Bay Manufacturing does indeed have the 7/8-8 LH Acme lead screw in 4140, about $65 shipped for a 24" section. Mine is ordered.

 

[kb58]

The replacement cross feed screw arrived from Green Bay. Disassembly of the existing cross slide screw showed the expected wear, with the middle portion visibly worn far more than the ends. The current thinking is to machine down the replacement screw and press/pin it to the front portion of the existing assembly, instead of machining the whole thing, or maybe I'll change my mind. The OEM bronze split nuts are heavily worn as well, and can be rocked back and forth on even the new shaft. I’ve never purchased bronze and was surprised by the price, a 1.5 x 1.5 x 6″ piece is $60 (from McMaster).

The good news is that I know who has a tap for the oddball 7/8-8 LH needed to make new nuts, and maybe I can sell him half the section of bronze. Anyway, that, the electronic lead screw, and wooden gear clock will have to wait until after the upcoming hillclimb

 

[kb58]

All done, thanks in part to the bronze and tap lent to me by "brother Mark." Rather than reprint the process and pictures here, you can see it all at my Midlana car site. This pic here shows just how worn the old Acme screw was:

 

[rbahr]

Thanks for writing this up! Interesting car!

 

[zamboni2354]

Great write up! I haven't checked mine yet but wouldn't be surprised if I end up needing to do this as well.

Jeff in long Beach.