Help please? Threading on a 618

[machinistnoob]

Hello all. My 618 has been up and running for a bit now. It's doing quite well and I've been making lots of chips.
I have been trying to turn some threads and I'm having trouble. The tool tip won't follow the same path twice. I'm being very conscious with the threading dial. Even without moving the compound after my scratch pass. It just does another scratch pass about 5-10 thou off the last pass. I can't figure out why.
Here's what I know:
- everything on the cross slide and tool post is tightened and set as it should be.
- gears appear to be properly meshed. No excessive play or tightness and gears are in good shape and have all teeth
- split nut clean cleaned and no unusual wear. Mechanism also clean and everything operates as it should.
- lead screw clean, in good shape and in good order. Both mounting points of lead screw are secure.
- threading dial clean and in good condition, including the gear at lead screw.

...and it still won't follow the same path twice.

Any input from you fine gentlemen, would be most appreciated.

M.Noob

 

[Jim F]

Leave the half-nuts engaged.

 

[Makintrax73]

Work spinning or moving in the chuck?

Are you using the same number on the thread dial each pass? Depending on the tpi of the lead screw and number of tpi of the cut threads you can use different numbers but for the sake of being sure try the same number every pass and make sure it is the same, not a partial turn off.

 

[mattthemuppet2]

with the half nuts closed can you move the carriage back and forth at all using the carriage hand wheel? If either thrust bearing/ washer at the headstock end of the leadscrew is loose it will do what you describe. That's a good first place to look.

After that, maybe a description of how you're going about your threading would be useful to help diagnose the issue.

 

[benmychree]

When you do the scratch pass, is the number of threads correct? Wondering if you may have set up an incorrect change gear.

 

[Makintrax73]

with the half nuts closed can you move the carriage back and forth at all using the carriage hand wheel? If either thrust bearing/ washer at the headstock end of the leadscrew is loose it will do what you describe. That's a good first place to look.

After that, maybe a description of how you're going about your threading would be useful to help diagnose the issue.

I think it only matters if it moves in between cuts? I just cut 3/8-16 on my Logan yesterday. For grins I checked ... with the half nut engaged the carriage has .040" end play. But it always stops +/- .001" of the same spot either direction

 

[WobblyHand]

Hello all. My 618 has been up and running for a bit now. It's doing quite well and I've been making lots of chips.
I have been trying to turn some threads and I'm having trouble. The tool tip won't follow the same path twice. I'm being very conscious with the threading dial. Even without moving the compound after my scratch pass. It just does another scratch pass about 5-10 thou off the last pass. I can't figure out why.
Here's what I know:
- everything on the cross slide and tool post is tightened and set as it should be.
- gears appear to be properly meshed. No excessive play or tightness and gears are in good shape and have all teeth
- split nut clean cleaned and no unusual wear. Mechanism also clean and everything operates as it should.
- lead screw clean, in good shape and in good order. Both mounting points of lead screw are secure.
- threading dial clean and in good condition, including the gear at lead screw.

...and it still won't follow the same path twice.

Any input from you fine gentlemen, would be most appreciated.

M.Noob

We like pictures here. Can you show us pictures of your setup, both some closeups and maybe a few steps away to get the bigger picture? We'd like to see the compound and your tooling setup. Can you tell us how and where you are engaging the halfnuts? If you engage just before the mark, then just after the mark, this can give poor results. It doesn't matter if it is slightly before or after, but it is essential it is the same place every time. Consistency is what we are striving for.

Ideally, your dial lines up exactly with the engagement point, but it may not. This doesn't really matter, as long as you consistently engage in the same place every time. The more you tell us, the quicker we can help you sort this out.

 

[machinistnoob]

Wow! You guys are on top of it. I will catch up and address some of the suggestions soon. However, I have approached most of these.

And what pics would you like?

 

[WobblyHand]

Wow! You guys are on top of it. I will catch up and address some of the suggestions soon. However, I have approached most of these.

And what pics would you like?

Show your compound and tooling set up as if you were cutting threads. So we can see everything is set up properly, including angles. A couple of steps back and show more of the lathe. Don't be afraid to take some details or close ups if you so desire. The tip of the cutter with respect to the work piece, things like that. They usually help. You can attach quite a few pictures per post, so don't worry about that. We are a friendly bunch and want to be able to help you solve your issues so you can go back to making chips. The pictures can help stimulate discussion - they really help.

Do you have a thread gauge? If not, do you have a screw with the same TPI so you can lay it on top of the scratch pass and check you got the gearing correct? We don't (here) know if everything is set up correctly, so we ask these questions so we can establish what we know for sure (by measurement or comparison) vs what we are still guessing about. That's the general process we use to try to help.

 

[markba633csi]

If the work isn't slipping in the chuck (which I doubt) then the problem is either at the carriage or at the leadscrew drive or mounts