Lead screw or thread dial not right?

[woodchucker]

I don’t believe so, because it happens every time. For a 24 TPI thread I should be able to engage anywhere with the 1/2 nut, not even looking at the dial. As I’m just starting, I pick a number, typically 2 or 4. If I stay with that number, everything is fine. If I choose 4 or 2 respectively, meaning the opposite of what I started at, I get what appears to be a quarter thread.
Threading in reverse, away from the chuck, I don’t get this. The threads are dead on, every time, with 16, 24, and 28. (the only threads I’ve tried so far.)

I’m wondering if maybe one side of the 1/2 nut isn’t messed up from the fall….???

no, you can't just pick anywhere with the 1/2 nut, you need to choose a mark on the dial.

 

[timmeh]

I may have missed something but bent leadscrew is mentioned, how is the thread dial gear/worm wheel? Would only require a little shaved off one flank of a few teeth to throw off accuracy in one(?) spot and/or direction.
Sure I saw mention of crash/topple/faceplant, or have I skimmed too quickly?

 

[Ischgl99]

wondering if maybe one side of the 1/2 nut isn’t messed up from the fall….???

That’s what I was getting at with my comment, I’m wondering if there might be some more damage that you didn’t notice that might be causing this.

 

[OTmachine]

I don’t believe so, because it happens every time. For a 24 TPI thread I should be able to engage anywhere with the 1/2 nut, not even looking at the dial.

I believe that is wrong. For an even number of threads, you can engage on ANY DIVISION. Not anywhere. Have to look at the dial.

 

[Parlo]

Did your machine come with other gears for the thread dial? Perhaps the wrong gear is being used which will work most of the time but not for the pitch you need. Another thing that I find useful is to back out the carriage a bit further from the chuck so you can rest your hand on the carriage handwheel to take out any backlash before cutting.

 

[Jake M]

In a message to @Dabbler - I sent this earlier:

I'm actually wondering if the 1/2 nut was damaged on one side when it fell on it's face. The leadscrew was bent pretty badly as well. The lathe was laying on the controls for the Apron and the cross slide. When it fell over, it hit the apron handles first, then the cross slide, then came to rest on both, and sat that way for almost a week.

I don't think the half nuts could do this. You have two half nuts, which close to become one nut, and drive the carriage. There is one thread in there. One single start thread. The same is true of the leadscrew. One thread. They only close in one position. If you had a bad half nut that was capable of partial or full engagement, it would do it on any number, any line, or any place between the lines and numbers. (Which I suspect you have one engagement point between each number and line, for a total of 16 possible engagement points. All of those would be affected intermittently. You would also necessarily get a "short throw" of the half nut lever during a partial engagement.

I would verify that there is no damage to the thread dial gear (which I suspect you are right, but I don't know for sure- I suspect that you should be able to throw away the thread dial and still cut a 24 tpi thread, and so is probably not the problem), and I would also make sure you have measured the pitch of the actual thread you are cutting. A thread gauge, a 10-24 screw, a 5/16 fine thread bolt. Whatever you've got, you're not checking the thread for technical accuracy, rather you only need to make sure it's not one pitch finer or coarser than what it should be. Just to be 100 percent sure about what's going on. Something doesn't add up, so you are (among other things) at the point where you need to verify and prove the absolute basics that should be able to be taken for granted.

 

[woodchucker]

I'm thinking you get an experienced machinist over (someone from here) who is local to help you sort this out.

 

[matthewsx]

Have you looked for one used? I see them going cheap sometimes on Craigslist.

Others here have pointed out stuff I didn’t notice. This one might get the job done, but if you can spend a little more and get something g better it might be worthwhile taking your time and buying quality used.

Here’s what they’re talking about for good pullers, I didn’t know these existed until a customer gifted me three of them. I left the biggest one behind in Michigan since I never want to have to pull anything that big.


For perspective, that’s the drawbar for my Seneca Falls lathe next to it

John

 

[jareese]

A quick check: chuck up some bar stock. Choose a very coarse thread, say 10tpi. Make a clear scratch pass (about .005") in the forward direction more than 2". Don't change anything, except dial in another .002" or so. put lathe in reverse, and scratch (presumably over top) of your previous scratch pass.

These passes should be identical.

Before you go "WTF" over this - This is like trying to solve a computer problem, and the first question is "Is the computer turned on?"

Ok @Dabbler and @WobblyHand I did the test. Results not what I wanted to see... 10 TPI, under power forward, manual in reverse...

 

[jareese]

Have you looked for one used? I see them going cheap sometimes on Craigslist.

Others here have pointed out stuff I didn’t notice. This one might get the job done, but if you can spend a little more and get something g better it might be worthwhile taking your time and buying quality used.

Here’s what they’re talking about for good pullers, I didn’t know these existed until a customer gifted me three of them. I left the biggest one behind in Michigan since I never want to have to pull anything that big.

View attachment 456050
For perspective, that’s the drawbar for my Seneca Falls lathe next to it

John

Ummmmm.... Wrong thread??