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9 x 20 half nut problem

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savarin

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#1
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

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#2
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

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#3
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

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#4
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

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#5
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

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#6
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

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#7
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

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#8
Perhaps make a new set of half nuts from a different material? Bronze possibly?
 

Bob Korves

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#9
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.
 
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RJSakowski

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#10
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.
 

savarin

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#11
Looking at the brass coloured oil it seems that yes its cutting very finely.
It will be a long job but I think very worthwhile
 

C-Bag

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#12
My 9x20 had the half nuts not set up correctly so that's why they wore quickly. Don't know if it was previous owner or factory. I replaced them with Grizzley units. Like Savarin in order to get the correct alignment had to set the half nuts and then tighten the saddle. I also rigged an old tooth brush to clean the lead screw but wasn't really happy with the mount. I recently came up with something better. Still a tooth brush but old brushes off our SoniCare brushes that have really strong magnets in the base. This really simplifies mounting after modding the brush. This seems to knock all the swarf off the lead screw. I also put a bellows between the carriage and under the chuck and it overhangs the side eliminating the amount of swarf that can even get slung onto the lead screw.
image.jpeg
image.jpeg
 

BROCKWOOD

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#13
I retrofitted mine with the telescopic sleeves.

Before

20160315 ROTARY TABLE 009.jpg


After

20170920 033.jpg
 

stupoty

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#14
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.
my old 9x20 only has the half nuts for power feed so that put a lot of extra ware on them.

Stu
 

savarin

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#15
Heres a pic of the worn half nut, it looks ok in the pic but theres virtually no real thread left.
worn-half-nut.jpg

Whilst awaiting the new ones I thought I should clean and adjust as much as I could.
Todays job was to check out the cross slide, it had a tiny rock between two corners.
A bit of scraping got it sitting flat with full contact for the full length.
Next I will be drilling and tapping for two more gib screws.
Now another question, Would this be a good idea and worth doing to improve cross slide oiling?
Grinding out hollows on the black lines and feeding them with a ball oiler, if so how many oilers per side?
and Is there a better pattern?
cross-slide-oil-plan.jpg
 

Cadillac

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#16
Savarin that is a good way to get oil on the slides. I would recommend not so many zigzags. I did this exact mod about a year ago on my jet 9x20. I ended up doing my saddle which for the V groove I put the Oiler directly above the v to get both sides. Then for the flat part I did what your doing. I put three perpendicular lines almost the width of slide. Then connected each groove with a long angled groove that went from top to bottom of groove. Like this |\|\|. Only as long as slide. I put oilers at each end also. The trick is to not break through the sides so the oil has a void to stay in. The mod worked great and the fact that their were no provisions for oil. Oiling the flat rail made a big difference for me with saddle movement. It now glides as it should. What’s nice is after filling ports you can see how the wipers contain and spread the oil from the inside. Not mixing oil with chips moving slides hoping it gets under there.
Oh and you want to try and cover the whole area of the slide with the groove. Meaning any spot that has no groove running over it will wear different so it might wear a ledge on sliding surface. Like putting a line parallel with slide. The opposite part would never wear causing a ledge and problems in the future. That’s why the angle going the width of slide is a better option just watch your ends.

EDA286D2-4F1B-4FD3-9B05-F8554C21206C.jpeg 1DC72E6E-BD6E-444A-85A4-9FBF58159A62.jpeg
 

RJSakowski

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#17
my old 9x20 only has the half nuts for power feed so that put a lot of extra ware on them.

Stu
Yes, the same as my Grizzly 602 and my Atlas/Craftsman 6 x 18.

The Grizzly 9 x 19 (G4000) and IIRC, the Enco and HF as well, do have the separate feed system. Savarin had mentioned the keyway that is characteristic of the separate feed so presumably the half nuts are only engaged for threading. Which is why is would be puzzling to see so much wear on the half nuts.

If the half nuts didn't completely disengage when the power feed was being used, they would be sliding over those sharp lead screw threads which would be wearing them down.
 

stupoty

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#18
Yes, the same as my Grizzly 602 and my Atlas/Craftsman 6 x 18.

The Grizzly 9 x 19 (G4000) and IIRC, the Enco and HF as well, do have the separate feed system. Savarin had mentioned the keyway that is characteristic of the separate feed so presumably the half nuts are only engaged for threading. Which is why is would be puzzling to see so much wear on the half nuts.

If the half nuts didn't completely disengage when the power feed was being used, they would be sliding over those sharp lead screw threads which would be wearing them down.

Ow yes now you say that I did indeed miss that in his post :)

Stu
 

Cadillac

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#19
As RJ said I’d be curious if the thread is worn the whole circumference or just at the opening of half nuts. That would tell a lot.
 

C-Bag

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#20
Seems like some kind of drip cup that drips oil on either the half nut or lead screw would be a good idea. As little single point threading as I do that half nut should last forever as its only engaged for threading. It also seems like a good idea to get the proper die for the lead screw and chase the threads. Maybe take a look at the power feed to see if it's somehow causing that slot in the lead screw to develope a burr while in use. That's one thing I've noticed with this 9x20, lots of attention to all the details because of the variable QC. Like the half nuts not being properly adjusted in the first place. Just never know what's going to bite you in the butt until it's too late.
 

savarin

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#21
I did a lot of fettling when I first recieved this lathe but as it was the first lathe I had ever used there was everything I didnt know or have experience with.
I adjusted and checked everything I could after heaps of research on line.
Mostly I used logic to work out how to do it and Cletus's books.
The first set of half nuts were damaged in a crash, totally my fault.
I rebuilt the worm carrier with brass inserts that stopped the key dropping out from excessive end play on the worm.
drilled larger holes in the half nut dove tails so they could be aligned fully and removed all the play.
Made sure they opened and closed fully.
Everything worked well and I have cut heaps of threads with no problems.
I think the main problem was having removed the saddle a couple of times since and never checking how they clamped before tightening the saddle bolts is what caused the problem this time.
I've run a quick experiment regarding the bicycle innertube cover idea and it looks like a good possibility. I will report back if I get it working.
There must also be a better way of preventing swarf from entering the rear of the saddle.
Oh how I love these type of problem solving exercises:bang head:
 

BROCKWOOD

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#22
Seems Half Nut not disengaging enough might be a good thing to look into.

I crashed my cross slide early on. Since rebuilding it, I cannot recall if the auto feed is reversible when the chuck is not reversed. Did I mess up & lose that function or just wish I had it? Grizzly G0773 must be very similar to all of yours.
 

hman

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#23
My 9x20 had the half nuts not set up correctly so that's why they wore quickly. Don't know if it was previous owner or factory. I replaced them with Grizzley units. Like Savarin in order to get the correct alignment had to set the half nuts and then tighten the saddle. I also rigged an old tooth brush to clean the lead screw but wasn't really happy with the mount. I recently came up with something better. Still a tooth brush but old brushes off our SoniCare brushes that have really strong magnets in the base. This really simplifies mounting after modding the brush. This seems to knock all the swarf off the lead screw. I also put a bellows between the carriage and under the chuck and it overhangs the side eliminating the amount of swarf that can even get slung onto the lead screw. View attachment 279234 View attachment 279235
C-bag, I thought this was a fantastic idea. Just had a bit of concern about losing some leftward travel, as the toothbrush in this position would run into the QCGB ahead of the carriage. I did a bit of looking at my own 9x20 and came up with this alternative:

Step 1 - I had some brass brushes, as shown in the first photo. The orange handle brushes came from Home Depot, the grey handles form someplace else, long forgotten. I cut the handle off one and plucked out the tufts sticking out on the diagonal. Found a place on the underside of the carriage where the head would fit, though the bristles were "too long" and bent against the leadscrew. So I milled a bit off the back of the head. This let the bristles reach all the way into the leadscrew threads without too much bending. Added two threaded holes.
kHPIM5555.jpg


Step 2 - Found some 2" x 1/16" aluminum angle and cut off a chunk about 1" wide. Narrowed down the part under the brush, added two holes for brush mounting and two more for mounting on the carriage. Shortened the mounting "leg."
kHPIM5556.jpg


Step 3 - Mounted the brush assembly on the leading face of the carriage, below the leadscrew (after first having checked that there were no carriage parts in the way, either during threading or power feeding). The low profile of the brush mount allows full leftward carriage travel.
kHPIM5557.jpg


Step 4 - Unless power feeding or threading, I've always disengaged the QCGB by lowering the selector all the way down to disengage the gears. My theory is that this saves wear and tear on the leadscrew, as well as reducing noise. I turned the leadscrew to place the drive notch at the bottom, marked the top of the coupling with some paint. This will be the "default" leadscrew position and will let the brush clean the power feed groove whenever the carriage is run back and forth. When the leadscrew is turning, the brush will clean the threads.
kHPIM5559.jpg
 
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Downunder Bob

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#24
C-bag, I thought this was a fantastic idea. Just had a bit of concern about losing some leftward travel, as the toothbrush in this position would run into the QCGB ahead of the carriage. I did a bit of looking at my own 9x20 and came up with this alternative:

Step 1 - I had some brass brushes, as shown in the first photo. The orange handle brushes came from Home Depot, the grey handles form someplace else, long forgotten. I cut the handle off one and plucked out the tufts sticking out on the diagonal. Found a place on the underside of the carriage where the head would fit, though the bristles were "too long" and bent against the leadscrew. So I milled a bit off the back of the head. This let the bristles reach all the way into the leadscrew threads without too much bending. Added two threaded holes.
View attachment 279306

Step 2 - Found some 2" x 1/16" aluminum angle and cut off a chunk about 1" wide. Narrowed down the part under the brush, added two holes for brush mounting and two more for mounting on the carriage. Shortened the mounting "leg."
View attachment 279307

Step 3 - Mounted the brush assembly on the leading face of the carriage, below the leadscrew (after first having checked that there were no carriage parts in the way, either during threading or power feeding). The low profile of the brush mount allows full leftward carriage travel.
View attachment 279308

Step 4 - Unless power feeding or threading, I've always disengaged the QCGB by lowering the selector all the way down. My theory is that this saves wear and tear on the leadscrew, as well as reducing noise. I turned the leadscrew to place the drive notch at the bottom, marked the top of the coupling with some paint. This will be the "default" leadscrew position and will let the brush clean the power feed groove whenever the carriage is run back and forth. When the leadscrew is turning, the brush will clean the threads.
View attachment 279309
That looks like a very well executed mod, should work a treat. I like the concept of marking the coupling so that the power feed groove is always parked in the down position.
 

gzoerner

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#25
John,

That's a super improvement. I'll be doing it on my G4000. Thanks for sharing.

Glen
 

savarin

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#26
I like that brush idea, so simple so brilliant.
Grooved the cross slide and de-burred yesterday.
But, I think I may have made a bit of a boo, I forgot the cross slide doenst fully cover the saddle so the oil grooves at each end will be exposed allowing the oil to squirt out when pumped in. Hopefully its still better than no oil and will help.
The oilers are in and two more gibb screws are in.
Starting on the saddle today.
The "V" way showed a wear stripe on each side almost the full length but the flat only showed wear on one end about 15mm long and a narrow strip at the other end about 3mm wide and 50mm long.
I've started scraping to obtain full length contact and its getting better, there is also a casting defect half way along running almost the full width across the flat.
 

hman

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#27
Regarding the cross slide ... did you groove the slide or the saddle? It's best to groove the shorter of the two (ie., the slide). The slide being on top, gravity is also your friend - less chance for crud to fall into the grooves than if they're on the bottom. But as you've said, it's gotta be better than no oil!
 

savarin

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#28
The slide is grooved.
In thinking about this further I think it will be a method of flushing the oil ways of crud. Move to one end and pump then move to the other end and pump. Or pump when there is a small overlap at each end which will flush from both ends replacing the old oil with fresh.
Just trying to find a positive here :laughing:
 

savarin

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#29
I cannot believe how much I had to scrape the saddle to fit the bed.
Added two ball oilers to the "V" groove and two to the flat way, just have to add an oil groove to the flat.

It felt very smooth when pushed from end to end (without the gibbs)
Got to admit its getting better
cue for a song
 

Cadillac

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#30
I had watched a video on utube I think Stefan G where he put grooves in the compound slide. He talked a lot about finding the limits of the travel of compound so that the groove didn’t go past the saddle. Which as youve found releases the pressure of oil. Either way oiling is way better than not. She will sing like a dream. Good job.
 
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