Mini lathe threading problems

[WobblyHand]

Think we all are jumping ahead too much. Some basic pictures and measurements would be very useful. Like what is the TIR measured at the chuck of a dowel pin. If good, or reasonable, we can rule some things out. If bad, remove chuck and measure spindle. Make all the adjustments as @homebrewed mentioned, which is similar to my first post. Lets rule out all the easy stuff, before assuming the sky has fallen.

 

[jschmidling]

I didn't go look at your other thread. My main computer keeps dying on me, so I am on a Raspberry Pi. Is the piece too large to be chucked? Or rectangular and you have a 3 jaw? Something else?

I think I have a handle on the problems of threading now but it seems a monumental waste of time just to get a few threads on the end of a 1/4" shaft when it should be a 5 min job with a die.

The problem with using the die is that it requires so much pressure and torque that the 3 jaw chuck can't hold the 1/4" rod and it just slips and never starts cutting the thread.

I have a die holder that fits into the tail stock but it is useless because the chuck will not not hold the part. This worked well on my South Bend but is useless with the chuck on the mini.

Is there any trick to improve on the grip of these small chucks?

js

 

[WobblyHand]

I think I have a handle on the problems of threading now but it seems a monumental waste of time just to get a few threads on the end of a 1/4" shaft when it should be a 5 min job with a die.

The problem with using the die is that it requires so much pressure and torque that the 3 jaw chuck can't hold the 1/4" rod and it just slips and never starts cutting the thread.

I have a die holder that fits into the tail stock but it is useless because the chuck will not not hold the part. This worked well on my South Bend but is useless with the chuck on the mini.

Is there any trick to improve on the grip of these small chucks?

js

Seem to be converging on one (or more?) of the issues. Something is wrong. It sounds like the rod is too large for the die to cut. What is the diameter of the rod? Measure it with a micrometer. Max major diameter of 1/4-20 is 0.2489, min major diameter is 0.2408". Your rod should be between those diameters. It would be best if it was 1/2 way between (about 0.244 or 0.245"). Values for imperial threads can be found at: https://www.engineersedge.com/screw_threads_chart.htm
If the rod is greater than the maximum major diameter, your die will have a terrible time. That means very high cutting forces. You might try turning the rod to the nominal value and see if die threading is easier.

Also concerned about the chuck not holding very well. That doesn't sound good. Can you take a close up picture of the chuck with the rod in it, all tightened up? Just the jaws and the rod? Does the chuck tighten down to "zero" easily? Some chucks need the other sections tightened for a better fit, if there are 3 sockets to tighten, try tightening them all.

 

[ericc]

That is the weirdest problem I have seen. The lathe threads well far away from the chuck but poorly close to the chuck. The only time I have seen this happen is when the ways are trashed near the chuck. Usually, these operations all work better near to the chuck due to the increased rigidity.

If the chuck cannot hold the rod, the rod is either below its capacity or the chuck is worn out. If the chuck is worn out, replace it. A 3 jaw of that size should be able to hold a 1/4" rod for threading. I have worked on small diameter stock by using a drill chuck held in a 4 jaw chuck and dialed in on the stock. There are no rigidity problems, since the majority of flex would be in the stock.

 

[WobblyHand]

That is the weirdest problem I have seen. The lathe threads well far away from the chuck but poorly close to the chuck. The only time I have seen this happen is when the ways are trashed near the chuck. Usually, these operations all work better near to the chuck due to the increased rigidity.

If the chuck cannot hold the rod, the rod is either below its capacity or the chuck is worn out. If the chuck is worn out, replace it. A 3 jaw of that size should be able to hold a 1/4" rod for threading. I have worked on small diameter stock by using a drill chuck held in a 4 jaw chuck and dialed in on the stock. There are no rigidity problems, since the majority of flex would be in the stock.

We haven't seen any close up pictures of the rod in the chuck, so I am not sure what is really going on. Could be as simple as set up error, or the chuck is bad, or incorrectly installed, or there is too much spindle runout. No TIR measurements nor photos of measuring TIR have been posted to my knowledge, so it's hard to know what is going on. All we are left with is speculation, which is not a good way to proceed. I do know that my mini-lathe could grip a 1/4" rod with no problems. For all I know, the chuck isn't being tightened enough, or it is full of swarf preventing it from being tightened adequately. Can't tell without pictures.

We are trying to help, but it is hard because the OP isn't providing the requested information. So instead of solving the problem right away, it takes days and days and days...