# JET 12 X 40 threading problem



## eugene13 (Nov 20, 2020)

I was threading 1 1/4" - 12 today, everything was going along fine and then the tool dug in and broke.  the cross feed and compound are all adjusted up snug, nothing was disconnected on the main drive and power feed.  I replaced the tool and began to look for a reason and discovered that the tool was out of time.  I checked the pin that couples the lead screw to the gearbox, and it was intact.  All the open gears behind the end cover are tight as well.  I had a similar problem a while ago cutting 3/8" - 24 left hand but I never investigated the root cause.  I have cut threads at other pitches with no problems but, not lately.  Since 12 tpi and 24 tpi are in the same column I'm thinking I may have a problem with a sheared key on the coresponding gear.  Any ideas or trouble shooting advice will be appreciated.


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## Nutfarmer (Nov 20, 2020)

My first guess the shear pin on the lead screw ,you already checked. I  had one shear on my Clausing 6300 and I couldn't tell until I tried to drive out the pin. That leaves something that is slipping in the lead screw drive chain or possibly a lot of back lash in the compound screw. Have not come up with anything you haven't considered. Let us know what you find.


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## SLK001 (Nov 21, 2020)

Did you half-nuts open up?  If they're not completely engaged, they can be popped out of engagement by the cutting forces.

If the problem doesn't become obvious, you're going to have to force it to occur again.  Chuck up a scrap piece of steel and take a deep threading cut.  If the problem reoccurs, don't stop the machine, but find out what isn't turning properly.  The cutting forces on the gearing get higher as your TPI drops, so experiment with the lowest TPI you can cut.

You said that you examined the lead screw shear pin and it was there.  It may have been there, but was it sheared?  If you can't see the LS turn with the driving gear, you might have to remove the pin to inspect it.


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## eugene13 (Nov 21, 2020)

I'm sure the lead screw pin is intact, when I tap on one end the other end moves, I forgot to mention that I have a lot of backlash on the lead screw. however I'll try your suggestions and go from there, thanks.


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## eugene13 (Nov 21, 2020)

Here's what I've found; first two pictures, when the gearbox is in the E position (32 to 112 tpi) the lead screw has about 5 degrees of back lash. Last two pictures, when it is in C position (4 to 28 tpi) it has 20 degrees of backlash.  I've never worked on a lathe before but I have a parts list with an exploded diagram, let the adventure begin!


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## SLK001 (Nov 21, 2020)

The backlash isn't your problem.  Most lathes have some amount of backlash, but once loaded, it is constant.  It is common practice to advance the carriage enough to take up the backlash so that it isn't an issue.


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## Mitch Alsup (Nov 21, 2020)

Can I ask how much cut you had dialed in when the tool dug in ? What material was being cut ? and What RPMs you were turning ?


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## eugene13 (Nov 21, 2020)

Mitch Alsup said:


> Can I ask how much cut you had dialed in when the tool dug in ? What material was being cut ? and What RPMs you were turning ?


.001, steel, an old conveyor head shaft probably Nicroloy, and 40 RPM.  The first picture is after the wreck with a new insert installed and the leadscrew engaged.


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## Jubil (Nov 21, 2020)

I had a similar problem a while back. Finally found a gear inside gearbox had sheared a pin. Had a real hard time finding it cause both ends were still in gear and looked to be ok.  When I did find where the problem was, I had more trouble removing broken pieces because gear and shaft were drilled off center. I didn’t realize it was off center and the gear was twisted on the shaft half way around. I finally got the pieces out and that’s when I discovered the off center hole. Must have worked on it 4 or 5 hrs trying to remove old pin without bending or breaking something.
Anyway it’s something to look for. Hope you have better luck than I did.


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## eugene13 (Nov 22, 2020)

All Right, thank you very much, that's what I've been thinking, and a former co-worked agrees, the excess backlash is a symptom of a problem inside the gearbox.  This is why I joined this forum.


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## eugene13 (Nov 29, 2020)

Here's where I'm so far, and every job needs a fluster-cluck.  To get to the gearbox I have to remove the lead screw, feed shaft, and control shaft.  My plan is to leave everything hanging on the carriage and to move it aside enough to attack the gearbox.  Bad news; the work bench is in the way, good news it's easy moved.


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## eugene13 (Dec 1, 2020)

Finally, down to the nitty gritty, tomorrow I tackle the gearbox, Experience is what don't have when you need it, if any one has ever done this before, I will appreciate your input.  During my big cleanup prior to this job I found this rusty adjustable wrench, the other part is a 95 year old Model-t chassis part, I'm going to see what Evaporust can do for them.


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## eugene13 (Dec 5, 2020)

Here's where I am so far, this is the lead screw shaft, the small gear on the right is loose on the shaft, all I have to do now is get the shaft out of the gearbox.  Looks pretty straight forward on the exploded diagram but I can't get it to move, tomorrow's another day.


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## eugene13 (Dec 7, 2020)

I need to remove the upper shaft in the photo.  I can turn the flange but I can't get it to pull out of the gearbox, am I doing something wrong?  
The alternative is to remove the two nuts that set the pre-load on the thrust bearings. (115 in exploded view)  Building it that way doesn't make any sense from a manufacturing point of view, but we all know inscrutable Japanese reverse engineering.  Need help please, thanks


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## Jim F (Dec 7, 2020)

eugene13 said:


> I need to remove the upper shaft in the photo.  I can turn the flange but I can't get it to pull out of the gearbox, am I doing something wrong?
> The alternative is to remove the two nuts that set the pre-load on the thrust bearings. (115 in exploded view)  Building it that way doesn't make any sense from a manufacturing point of view, but we all know inscrutable Japanese reverse engineering.  Need help please, thanks


I am not seeing the part you need to remove in the parts breakdown.


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## eugene13 (Dec 8, 2020)

Jim F said:


> I am not seeing the part you need to remove in the parts breakdown.


I need to remove the entire lead screw shaft to replace the small gear (#118), this is the bottom shaft in the exploded diagram, thanks.


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## Jim F (Dec 8, 2020)

eugene13 said:


> I need to remove the entire lead screw shaft to replace the small gear (#118), this is the bottom shaft in the exploded diagram, thanks.


Yes, those nuts need to be removed.


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## ericc (Dec 8, 2020)

We had a similar problem at the local makerspace with a Jet 1440.  I don't know what the exact problem was, but the gearbox had to be fully disassembled, and it took weeks to get it fixed.  Every time I came in, I noticed a few more gears sitting on the work table, so progress was being made.  Eventually, the lathe got put back together and it worked.  They did not bother fixing the other lathe.  They just said only use the one lathe for threading.

After seeing this, I swore that I would never get a lathe with a gearbox for my shop.  The change gears are slightly more inconvenient, but no mysterious slipping problems.  If the key is there and the teeth mesh, it is going to work.  Just a little messy every time you need to swap gears.


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## Jim F (Dec 8, 2020)

ericc said:


> We had a similar problem at the local makerspace with a Jet 1440.  I don't know what the exact problem was, but the gearbox had to be fully disassembled, and it took weeks to get it fixed.  Every time I came in, I noticed a few more gears sitting on the work table, so progress was being made.  Eventually, the lathe got put back together and it worked.  They did not bother fixing the other lathe.  They just said only use the one lathe for threading.
> 
> After seeing this, I swore that I would never get a lathe with a gearbox for my shop.  *The change gears are slightly more inconvenient, but no mysterious slipping problems.  If the key is there and the teeth mesh, it is going to work.  Just a little messy every time you need to swap gears.*


And most home/hobby guys are not pressed for time to thread something.


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## eugene13 (Dec 8, 2020)

This has turned into an adventure but I am making headway, good thing the mill works, I may have to build a wrench.


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## Jim F (Dec 8, 2020)

Are items #110 the bearings ?


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## Jubil (Dec 9, 2020)

If I’m seeing what I think I’m seeing in the diagram, that gear is installed backward. And possibly not engaging key as it should. But that doesn’t help dismantling it at all. 

Chuck


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## eugene13 (Dec 9, 2020)

Jim F said:


> Are items #110 the bearings ?


Item 110 are ball thrust bearings, good eye Jubil, I'll look into that when I get it apart, thanks.


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## eugene13 (Dec 12, 2020)

As I said, it was a fluster cluck; the radial bearing (78) was stuck on the shaft and the hole that the flange fits into is tapered, I couldn't figure what was holding it in.  After I got everything broke loose it came right out.  The root cause of the problem was the person who assembled the gearbox, evidently they couldn't get the gear on the shaft so they filed down the key and the resulting sloppy fit wallowed out the keyway in the gear, the shaft keyway is OK so now I am waiting on parts, oh well, If ya got time to lean ya got time to clean.


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## Mitch Alsup (Dec 12, 2020)

WoW !! How many of these barrel threads have you cut on this lathe to have so wallowed out that key slot ?


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## eugene13 (Dec 14, 2020)

Mitch Alsup said:


> WoW !! How many of these barrel threads have you cut on this lathe to have so wallowed out that key slot ?


I bought the lathe new twelve years ago and have only threaded with it a few times, the high side threads have always been poor quality but the low side have always worked fine.  Cheap Chinese labor, you get what you pay for.


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## eugene13 (Jan 8, 2021)

I'm back, hollering for help again.  I got my parts but they sent the wrong thing for a retaining ring (snap ring) I need a 15mm external snap ring (MSH-15)  I only need to buy one, thanks.


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## eugene13 (Feb 10, 2021)

Finally got it back together and it still does the same thing, now I'm starting to think operator error


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## SLK001 (Feb 10, 2021)

I reread your first post.  If your tool dug in, then your problems most likely are in your carriage.  Do you have a lot of backlash in your cross or compound leadscrews?

Your cut of 0.001" shouldn't have caused any issues.  My guess, is that your backlash isn't being taken up properly and the work piece is pulling in the rest of the backlash into the cut.


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## eugene13 (Feb 28, 2021)

SLK001 said:


> I reread your first post.  If your tool dug in, then your problems most likely are in your carriage.  Do you have a lot of backlash in your cross or compound leadscrews?
> 
> Your cut of 0.001" shouldn't have caused any issues.  My guess, is that your backlash isn't being taken up properly and the work piece is pulling in the rest of the backlash into the cut.


Thanks, I've checked all that and snugged everything up, I have .010 backlash on the leadscrew. I also opened up the headstock and inspected every gear in the train, I ran about 50 passes and had no malfunction in aluminum.  Replacing the gear made it thread "better", if it does it again threading in steel I will blame it on tool springing or operator error.  thanks again for your help.


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