JET 12 X 40 threading problem

eugene13

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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

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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.
 

SLK001

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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.
 

eugene13

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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.
 

eugene13

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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

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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.
 

Mitch Alsup

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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 ?
 

eugene13

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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.
 

Jubil

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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.
 

eugene13

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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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