Mini lathe threading problems

J

jschmidling

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Gave up trying to use the treading die in another thread and am now having problems cutting threads with the lathe.

It is set up for 20 tpi and almost any depth at all creates a banging sound in the back somewhere.

Got all the covers off and my wife with a flashlight can only see what seems to be the pulley or at least the belt bouncing around. I can't watch to verify because of the other things I have to hang on to or watch.

What is pretty obvious it that the motor seems to be mounted on a bit of an angle with the pulley end higher than the other end. The motor is solidly mount with no movement trying to push it around. Guess is has always been this way and not sure it's worth "fixing".

However, I have cut threads with this thing before with no such problems.

Any ideas?

Jack
 
Hazarding a guess, some hardware came off or is very loose. Might look in the chip tray or on the floor for a fastener or nut. If you haven't done it recently, adjust all your gibs. Also adjust the carriage plates that hold the carriage to the ways.

Your motor being mounted that way is not normal. It should be level and the pulley should be vertical. The motor being off kilter will not help you get the most of of your mini-lathe.

Supplying us with pictures always helps. We love pictures here and they really do help to converge on the problem.
 
I agree with Wobbly something must have broken or come off. Need to dig deeper back there and see what's up
A lot of folks consider these lathes "kits"; it's possible you might have to do a bit of re-engineering to put things right
 
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The motor mounting scheme for these machines is not the greatest. It's all too easy to get the motor/pulley cocked over at an angle. I own a 7x12 so I have some personal experience with that very problem.

However, if the motor is solidly mounted (i.e., it hasn't shifted) you may have something else going on, like a frayed belt. You can check that by unplugging the machine and then exposing the motor+pulley. Turn the spindle by hand to see how the belt looks. A frayed belt can cause odd symptoms -- on mine, the lathe made no noises until I started cutting something. And it initially wasn't obvious that it was the belt. I thought it was a bad transmission gear, based on where the noise was coming from. I was in the process of stripping my lathe down to remove the headstock before I discovered the belt. Some time lost there, but I took the opportunity to shim the headstock to align it better to the bed.

The fact that the motor isn't correctly aligned could have led to premature belt failure, so you will need to fix that problem before getting back to making chips.
 
Thanks for the ideas.

I will never know if the motor shifted or it always was that way but I was able negotiate the primitive mounting arrangement and get it straight and tight. However, I am not clear on how much tension is proper for this type of belt.

I also checked to see if the belt was worn and it seems fine.

I am now able to cut a 20 tpi thread on a half inch bar without that thumping but still have problem on 1/4" stock.

The sound is more like crunching than cutting and is very erratic. I cut off the attempt and then chucked it up so that only 1 inch protruded from the chuck.

This helped some but still a lot of crunching. Within a few passes of finishing, it jammed and the next pass messed it all up. I assume the piece slipped in the chuck when it jammed.

Seems like it may be hopeless to thread 1/4" rod.

For the record, I am using hand ground HS bits that look more or less like the pics in the manual.

Is is likely just a tool bit problem? Should I invest in a commercial known entity? Any suggestions?

Jack
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I had one of those machines a while back, but never did any real work on it. Gave it to a startup maker group so they would have something to start with. Wasn't a real lathe, but it was something to start with. . . I acquired the machine to use at work. It was easy to hand carry, and using the machine shop at the mill took a week or more for some piddlin' job, all over paper work. Simpler to do the job myself, and I taught a little at the same time. The lathe spent a goodly portion of its' time in my locker. Just keep in mind, I left that job nearly 20 years back.

I never noticed the motor being out of line, the work I was doing would have run if it was connected with a rubber band. But belt alignment is a pretty big issue. As is belt pressure. The general recommended practice is a small amount of deflection with a given pressure. The details are so old I simply don't remember, I do it by TLAR, "that looks about right". For a small machine, I would hazard a guess at about 1/4 to 3/8 inch with a very few pounds pressure at the middle of the span.

This is all speculation on my part. . . The machine I had came from Horrible Fright and was never intended as a serious machine. Just for side jobs at work. If it ever gave trouble, time was on my side. I just set the project aside until I could make things right. I have a couple of machines at home that do any serious work. As I understand HF, most of the machinery they sell is essentially in kit form. It is all there (M/L) but needs to be disassembled and aligned before doing anything productive. That applies to most any small lathe from any supplier as well.

There is a lever on the back of the headstock that engages a (sort of) back gear. The crunching sound may be that lever not fully engaged, or over engaged, or just poorly aligned. Something rubbing where it should be sliding past????? You speak of cutting threads on half inch stock being (fairly) smooth, but on quarter inch stock some "crunching". The only difference there is the chuck jaws being tighter, closer to the center. Perhaps something inside the chuck is loose or out of line. Or there may be some end play in the spindle and tightening the chuck shifted it along its' axis. Or perhaps something so simple as having the cross slide closer to the center of rotation. . .

In that respect, I often cut very small screw threads, often at or below Nr 2-56. (0.086 dia) I don't remember doing any that size at work but the limitations for small sizes should not be related to machine size or capability. Cutting screw threads takes a little more care than plain shaping. On a small machine at 20 TPI, cuts need to be at or below 0.005 with a sharp tool. I like slower spindle speeds as much because I'm old now and don't think as fast as I used to. For a younger man just beginning, the same thought applies. It takes practice to do some things fast. . .

.
 
Just a curious passer-by. How deep are your thread making passes? Personal experience with HSS requires the clearance angles prevent rubbing and the cutting point being at the vertical center of the work. 1" overhang exceeds the recommended 2 times the diameter thus requiring a very deft touch. Is it possible that heat build-up on work that small weakens it quickly?
Have a good day
Rock breaker
 
I'm guessing your problems with 1/4" stock might be cutting tool related. The tool should be very sharp, with the correct amount of relief and perfectly on center vertically. These requirements become more important as the diameter of the stock reduces.
Always use some type of cutting fluid when threading steel
 
Since it's more of a problem with 1/4" stock than with 1/2", I'm guessing the top of the tool isn't on the center line of the work piece.

Tom
 
Yes, cutting tool related sounds like the problem. To eliminate the drive system, try (using HSS) turning the chuck by hand or with a spindle handle. A few tricks will help a lot. Sharpie on the tool to make sure the clearance is not rubbing. And, use a flat rule or your choice method of making sure that the cutting edge is exactly on center. With 1/4" rather than 1/2", the amount sticking out of the chuck is more critical. And it worse than twice as critical.

I am not a very visual person, but pictures would help. I think another responder mentioned this as well.
 
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