9 x 20 half nut problem

[savarin]

I believe I may have a major problem with my lathe (generic 9x20)
This is the second time I have had the half nut threads wear away.
The first time it happened I hadnt had the lathe long and the problem was swarf collecting inside and preventing the nuts from fully closing.
I sorted that out by making a new lever, placing the detents further apart and adjusting the mechanism ensuring the nuts fully clamped onto the lead screw and regularly cleaning out any swarf.
I stripped and reassembled and re adjusted it this time, most of the thread had gone but I needed to finish a job so had a go.
A couple of things I checked.
The saddle travels from end to end and the lead screw doesnt move up, down, in, or out so its in alignment with the worm and tail stock end bearing and the half nuts clamp firmly to the leadscrew. I will also check out the leadscrew for straightness some time but it doesnt wobble in the tail stock end is not bolted down so I'm assuming its pretty straight.
BUT
When I engage the threading lever to clamp the half nuts to the lead screw the lead screw flexes inwards towards the bed. I ensured the remaining threads were fully engaged.
This has caused wear in the worm housing bearing at that position as well so its been going on a long time.
I have thought of placing some shims behind the worm housing to move it inwards towards the lead screw and possibly facing the tail stock end bearing down to move the lead screw inwards the amount it seems to flex.
The threads on the lead screw also appear to be very sharp on their edges and in particular where the keyway is cut and from the colour of the dust seems to be actually cutting the half nuts.
Major questions -
Where can I purchase the half nuts without the 12 to 16 weeks delivery time I'm being quoted in Australia?
Is it feasible to dress the complete thread on the lead screw to remove all the sharp edges? (or worth it)
Are the mods mentioned above worth doing?
Can anyone suggest any other things I could do? (other than purchasing a better lathe)
Thanks

 

[hman]

I just checked Grizzly's website. T the half nuts (P/N P4000741) appears to be available. I'll need to phone them Monday to confirm, and also to verify that they're sold by the set. Wouldn't want to order 1, thinking it was a pair, and just get one of them. I'll also check the lead time. Once I have all the info, I'll send you a PM, and we can figger out how to get them to you in the most expeditious manner.

I have a 9x20 (Grizzly G4000), and have noticed the sharp corners on the leadscrew at the groove. I didn't worry about it, once I'd verified that there were no burrs. Sharp corners themselves shouldn't be that big a deal.

I'll also do the checks you did regarding leadscrew movement, both when the carriage is moved and when the half nuts are engaged, and let you know the results.

 

[savarin]

Thanks John, thats awesome.
Update on the half nuts pushing the leadscrew towards the bed.
The apron is held on with two bolts only, with some experimenting I found that by engaging the half nuts whilst the apron is loosely held then tightening them aligns everything up as it should be. One problem solved.

 

[Downunder Bob]

Im think from memory your generic 9 x 20, as you call it, uses a single shaft for lead screw and feed shaft, with a longitudinal keyway, to drive the feeds. It will always collect swarf, and as it turns around the swarf will act like steel wool on the internal threads of the hlf nuts, rapidly accelerating the wear.

Some of the more modern versions have incorporated a telescopic sleeve covering the shaft in an effort to stop this happening Another method is to set up a brush on the headstock end of the saddle to keep the shaft clean as it rotates.

 

[Cadillac]

Sounds like the leadscrew roughness might be your problem. Along with alignment issues. Might look into getting a new leadscrew if it has sharp or ruff threads. Any pics of leadscrew and worn half nuts

 

[savarin]

I'm convinced your correct Bob, I had thought of the brush idea but never got round to it.
I'm going to experiment with a bicycle innertube held in a couple of clamps to see if it can compress and stretch sufficiently to act as a cover.
Been looking for old clocks for the springs to make a telescopic cover but havnt found any so far.
Cadillac, the actual lead screw is a very good smooth thread, its just the edges are sharp and the part where the keyway is cut out is also sharp.

 

[Cadillac]

When I bought my jet 9x20 It was my first lathe and previously used. I ended up tearing her down assessed and replace any worn parts I thought needed replacing. I had replaced the worm gear and had found it had slop left to right within mount. I ended up putting a brass washer on each side which made the worm snug in housing. I also replaced the drive engagement gear so the apron is nice and tight.
When I reassembled carriage I ended up stoning the entire keyway your talking of. Was ruff like you say. Then with leadscrew mount loose I aligned the shaft to the bed of the lathe. Then I aligned the apron to the bed and screw. With having the half nuts engaged on the screw. Seemed to work very well haven’t had any issues.
One thing I love about my bigger lathe is it has the coil covering on the leadscrew left and right. Would think it can be bought but may not wanna hear the price. Ive also seen where people have installed a deflector on cross slide to divert the chips away from leadscrew kicks them back between the bed rails.

 

[markba633csi]

Perhaps make a new set of half nuts from a different material? Bronze possibly?

 

[Bob Korves]

Another issue on many lathes is that the half nuts are not lubricated often enough. Out of sight, out of mind. That is an issue even on lathes like my Kent 13x40. Neither the feed gear pinion or the half nuts are lubed by lubing the normal lube points. They need to be manually lubed regularly to make them last. Cleaning and lubing the lead screw regularly helps, but not enough. The manual says to manually lubricate them. My lathe came from the factory with spare half nuts and a spare feed screw Edit, gear. Those were the only spare parts that came with the lathe. The factory obviously knows about it...

Good job, savarin, of getting the lead screw better lined up with the half nuts. If the factory does not do it, then we have to.

 

[RJSakowski]

Two thoughts on your rapid wear of the half nuts. First, with the longitudinal slot and sharp edges, it may be working like a tap shaving away the half nut threads. Second, if the half nuts are not completely disengaged when in the power feed mode, the sharp edges of the threads may be shaving the threads. An examination of the wear pattern should tell you which.