Thread Pitch Error

[Wjw159]

Guys I am going nuts. I have been trying to cut threads on my SB9a, and the pitch always comes out wonky.

In this instance, I am trying for 14 TPI but when I do a scratch pass, it doesn't fit the gage. At 7 threads its off about half a thread already and measuring it puts it at 14.5 to 15 TPI. I don't know how I could set up for this if I wanted to.

The odd thing to me is that its off but close so there isn't a gross error somewhere like I put on the 40 tooth instead of the 20 tooth stud. I have the levers in A-8.

Where do you think I should examine next?

 

[SLK001]

Your gear train is most likely wrong.

An experiment you might want to do, is to put a scrap rod in your lathe and start testing (only do scratch passes) all the different threads. It IS possible to have the gears inside the gear box out of order, but more likely it is the gear train leading UP to the box.

 

[Wjw159]

Off of the reversing lever I have 20T meshing to 80T meshing to 56T input to QCGB. I even counted the teeth haha.

I'll try different TPI on different gearbox lever and see if I can narrow in on an issue.

 

[rjs44032]

First check that the gear-train is not damaged. I have heard of sheared key reaking havoc on the leadscrew gear-train. This can happen and not be immediately noticable. It may only slip under certain conditions.

Then I would check the back gearing as SLK001 suggested. First, try the TPI that matches the leadscrew. Should be dead on. Then others as necessary until you find the problem.

If you still can't find a problem, let us know how the gear-train is set up and your lead-screw pitch. I don't have a South-Bend, but I'm sure other's will chime in with proper specs. Hope this helps.

Best Regards,
Bob

 

[Wjw159]

I tried a different thread per SLK001

I did a 20 TPI, gear box in B-3

Getting 11 threads at 33/64 so ~21.3TPI



RJS44032 - When you say try the TPI that matches the lead screw, do you mean setup to cut 8TPI?

 

[rjs44032]

Yes. Typically most lathes have simple gearing to achieve a 1:1 gear ratio at the lead-screw pitch. In that configuration if the gear-train is not damaged and the pitch is off, you should be able to quickly find the offending gearing. Hope this helps.

Edit: Assuming lead screw is 8 TPI, you should have 1.75:1 ratio in the gear train to get 14 TPI. This is most likely done in the QCGB after it is fed 1:1 ratio from the gear train.

Best Regards,
Bob

 

[Wjw159]

I don't have a gage as coarse as 8TPI but I am getting 4 threads at 15/32 so ~8.533 TPI

I guess the next step is to take off QCGB and count teeth in there?

 

[johnoder]

Many years ago my 12 X 30 P&W had its own ideas about what a thread pitch should be. Turned out to be a sheared key in the left end gear train. This would slip a teeny bit for finer pitches and more for coarser pitches

 

[rjs44032]

I don't have a gage as coarse as 8TPI but I am getting 4 threads at 15/32 so ~8.533 TPI

I guess the next step is to take off QCGB and count teeth in there?

If you're set up for 8TPI, then that should eliminate the QCGB from the equation. You just need to verify that the QCGB is producing a 1:1 ratio. You don't need to check the internals unless it isn't 1:1 Hope this helps.

Best Regards,
Bob

 

[Wjw159]

If you're set up for 8TPI, then that should eliminate the QCGB from the equation. You just need to verify that the QCGB is producing a 1:1 ratio. You don't need to check the internals unless it isn't 1:1 Hope this helps.

Best Regards,
Bob

Okay my plan then is to move the 80tooth out of mesh with the 56, Mark the 56 Tooth, mark the lead screw at the bushing. Use the nut on the 56 tooth gear to rotate 10 turns and see if lead screw is still aligned with its mark on the bushing. Is this what you are thinking too?

 

[rjs44032]

Yes. If your lead-screw is 8 tpi, the gear box should turn 1:1 when it's set for 8 TPI. This assumes no sheared keys in the gear train.

If you've established that then the problem has to be in the back gearing or the lead-screw is not 8 TPI.


Best Regards,
Bob

 

[BDRetz]

Thread a couple months ago on a similar problem... flipped gear on the end of the lead screw inside the QCGB.

Strange threading problem

Hope this helps.

Ben

 

[Wjw159]

Looks like the problem is in the gearbox.

Turning the gearbox input one rev the lead screw does not complete one full turn. It was way easier to turn the input gear that I expected since and could turn it by hand. My original plan was to turn 10 times hoping to magnify a small error to be more obvious. It was evident enough at 1 revolution. Ironically at 10 revs it basically lined up again, probably 1 rev short or something. I might have thought I was okay if I didnt check every turn.

Thanks for the help guys.

Taking the gearbox off and looking into that will wait until tonight when the kids go to bed

 

[rjs44032]

Wjw159 when you get back into it, check out the link in Ben's post above. In that case, a spacer and gear was flipped inside the QCGB. Sounds like similar problem.

Ben, thanks for posting that link.

Best Regards,
Bob

 

[sc repair]

This may sound stupid, but are you pulling the right lever. I also had pitch errors, looked in the manual, resehared and it came out to be I was enguaging the apron lever and not the threading lever. When I figured that out I felt pretty stupid. I had threaded before with no problems but out of habit I just went for the lever I normally used.

 

[Wjw159]

I dug into the other thread and the QCGB. The lead screw was on backwards meshing with the second to last gear in the cone instead of the last gear. I turned it around but haven't gotten it back on the lathe yet.

After I turned it around, I checked again for a 1:1 ratio on input and output and didn't get it, so I was confused. It was more like 1:3.

This morning I did the calculations here for what the screwgear speed and leadscrew gear should be:

https://bnordgren.org/files/model-a-gears.pdf

It turns out the SBL 9A does not use a 1:1 in the QCGB but is 1:1 off the spindle or 1:1 off the stud.

The screw gear should be 0.35714 off the spindle and the lead screw gear is 1:1 off the spindle, that means the output is 2.8 times the input, so my visual ~3:1 is probably right.

 

[Wjw159]

Success!

Thanks for all of the help guys. Gearbox back on and cutting threads at the correct pitch. I also confirmed before I cut a thread that at 8TPI the chuck marked with an index line on the headstock and the lead screw marked with an index line on the gearbox are 1:1

The gearbox also sounds a little quieter. The poor mesh with the slightly smaller gear must have been causing some noise.

 

[rjs44032]

Wjw159,

Edit: Missed your previous post. Glad you were successful.

Best Regards,
Bob