Jet lathe threading problem

[ericc]

I was at the Techshop last night trying to thread a 1/2" rod. I wasn't able to get it to work correctly. The thread looked mangled, and it seemed to run over itself. The lathe there looks like a 1340, but I did not see a number.

The problem is that when the gearbox is set up for threading with the leadscrew actuated, the threading dial doesn't turn. If the half nut lever is jiggled, it will eventually turn a little, but if it is not always engaged, you will lose the point for starting the thread. Anyway, the thread dial stops when the half nut lever is engaged, but it is also stopped most of the time that the lever is unengaged. The second lathe was not set up, but its threading dial worked perfectly (turns when lever not engaged, stops when engaged). The manual does not say anything about activating the threading dial. Some lathes require you to tilt a gear in. There is just an imposing box around the threading dial, and the employee there couldn't get in to it. He said it was probably broken.

Is there anything obvious (like a thread dial actuator lever) that we are missing? The manual leaves a lot to be desired. It seems to be tantalizingly incomplete. :thinking:

 

[Don B]

I'm not familiar with this machine but is there any reason why you don't just leave the split nut engaged and go from forward to reverse, it is normal for the dial to stop when the split nut is engaged but it should rotate when not, with it slipping like that you will be hard pressed to complete a thread unless you do the forward/reverse thing.

 

[Ulma Doctor]

at a guess,
it appears the gear that actuates the thread dial may be damaged or worn out and not contacting the leadscrew properly.

i'd give that a check. you won't be able to turn accurate threads without the thread dial operating correctly unless you were to leave the half nuts engaged and operate the lathe in forward to cut the thread, then reverse the lathe at the end of the cut to the start position to restart the thread cutting each pass.

 

[12bolts]

Threading dials are often adjustable for engagement of the gears on smaller lathes. It may just be a matter of ensuring the gears are in mesh.
Alternatively the gear on the bottom of the thread dial that meshes with the leadscrew may be damaged. It should be relatively easy to check and correct.
But yes, as above, the dial should turn when the carriage is moved by the hand wheel, or when the leadscrew is turning and the carriage is stationary, but when you engage the half nuts the dial stops even though the carriage is moving

Cheers Phil

 

[ericc]

Hi. Thank you for all the useful suggestions and observations. This is pretty much exactly what I wanted to hear. I will figure out how to do the job without disengaging the half nuts. This lathe is good for this technique since it has the cam lock chuck attachment as well as a functioning foot brake.

Meanwhile, I have been surfing around trying to find some more information. I found a parts manual for the Jet lathe, and the threading dial mechanism is very simple. No adjustment facility. Everything is pinned, and there is only one screw fastening the box to the lathe and 4 small screws holding the front cover on. So, the observation is pretty accurate about something being damaged. The employee who examined the lathe said it also seems that the leadscrew is bent, adding to the problem. This threading job is going to be tougher for a beginner (me) than I previously thought. :notthis:

 

[pdentrem]

You might want to review Mr Pete collection of youtube videos. They are a great resource for everybody, noob or not.
Pierre

 

[12bolts]

If the leadscrew is bent that will also add to you woes for threading and surfacing.

Cheers Phil

 

[ericc]

Hi. Thank you for all the tips. The one about leaving the leadscrew engaged was especially helpful. Well, I went over there and tried it out again, and it did not work. The lathe cut the wrong sized thread, then suddenly, snap! something jumped, and the thread was eaten up by losing engagement. It appeared that the chuck was still spinning, but the lead screw had stopped turning. Eventually, I was able to determine that I could stop the leadscrew from turning just by pushing on the carriage :whiteflag:

The employee there looked at the lathe and said there was definitely something wrong. He told me not to run the leadscrew in reverse since the gears would grind. That lathe was not meant for threading, and he said that maybe they had to put a sign on it to keep the newbies from trying to thread on it :angry:

He recommended that I use the second lathe. Everything worked. The leadscrew and half nuts worked. The threading worked. And I nailed the thread the first time. Good.

Thanks again. Just to make sure, I watched the Mr Pete videos again. Good instruction. And remember, it is the poor craftsman who blames his tools. )

 

[tmarks11]

\The employee there looked at the lathe and said... That lathe was not meant for threading, and he said that maybe they had to put a sign on it to keep the newbies from trying to thread on it

Let me interprete Babilulus Stupidius for you:

The shear pin on the lead screw broke (possibly because some prior use crashed the carriage while threading), and the employee doesn't know what that is, doesn't know how to fix it, and maybe should get a job at McDonalds.

Hmm, was that a little harsh? But "Not meant for threading " is way different than "it used to thread until somebody broke it". I wonder what said employee thinks you use half nuts for if you aren't threading?

 

[compressorguy]

Let me interprete Babilulus Stupidius for you:

The shear pin on the lead screw broke (possibly because some prior use crashed the carriage while threading), and the employee doesn't know what that is, doesn't know how to fix it, and maybe should get a job at McDonalds.

Hmm, was that a little harsh? But "Not meant for threading " is way different than "it used to thread until somebody broke it". I wonder what said employee thinks you use half nuts for if you aren't threading?

I would say you were being very kind. What kind of moron works in a facility with machine tools and doesn't know that any lathe with a lead screw can thread?!!! Probably why the lathe got broken in the first place, no one qualified to oversee the shop!

Gary H.