Threading help on 9a

driftpin · Dec 26, 2014

I've been pulling my hair out trying to thread a rod 3/8-16 on this lathe. I have the left tumbler on the gear box set on B and the right gear select lever in the first hole (the 2nd hole closest to left).

Here is a picture of my gear box (set for a different thread).

Is there any thing else that I need to do to thread? I have my compound set at 29-1/2 and in back gears.

Here is a picture of the configuration of gears on my lathe. It is cutting thread with like a 48 pitch.

lazz · Dec 26, 2014

Back gears is a good thing...
Get everything set up the way you think it should be.
Either use a magic marker in place of the cutting tool or make a supper light cut on the rod you wanna thread.
Get your thread gage and see what is going on.

Learning to thread is one of those things that gets better after a lot of practice. Learning the in's and out's takes practice. And practice means making chips. I got a 10' hunk of 1/2" PVC pipe. Cut it into 1 foot pieces and threaded 2" at a time till I got sick of cleaning it up. But I did get the general idea of how to thread .
If you get a hunk of 1/2 PCV pipe schedule 40 you can practice threading 3/4 X 10 for very little.

kitno455 · Dec 26, 2014

Are you using the half-nut lever, or the clutch?

allan

driftpin · Dec 26, 2014

My half but lever won't work so I'm just using the clutch but I seem to have plenty of time to catch the correct number on the treading dial, it just seems like it's cutting the wrong pitch. PVC is a good idea. I bought a bunch of that acetal rod to practice with. PVC would've been cheaper.

Lost Again · Dec 26, 2014

You need to use the half nut system for threading. Do you have a pic of the front of your lathe?

jocat54 · Dec 26, 2014

driftpin said:
My half but lever won't work so I'm just using the clutch but I seem to have plenty of time to catch the correct number on the treading dial, it just seems like it's cutting the wrong pitch. PVC is a good idea. I bought a bunch of that acetal rod to practice with. PVC would've been cheaper.

I don't think you will be able to use the clutch.
If you have the power feed selector engaged the half nuts won't engage. The feed selector has to be in the middle hole (not engaged) There is a rod inside that prevents the half nuts from engaging if the power feed is engaged.

Real easy to pull the apron off and check things out, just unbolt the apron from the carriage and take the right hand lead screw bushing off and the apron will just slide off the end (be sure and support the lead screw)

driftpin · Dec 27, 2014

Well that would explain why I'm having problems. The lever on the apron is stuck it the top hole position. I've tried moving it while it was off, while it was running but the lever only moves about a half inch and I don't want to force it. I think I need to drop the apron down anyhow because the guy forgot to install the carriage lock when he put it back together and it seems too tight to put it back in with the apron in place.

malexander · Dec 27, 2014

driftpin said:
Well that would explain why I'm having problems. The lever on the apron is stuck it the top hole position. I've tried moving it while it was off, while it was running but the lever only moves about a half inch and I don't want to force it. I think I need to drop the apron down anyhow because the guy forgot to install the carriage lock when he put it back together and it seems too tight to put it back in with the apron in place.

Ah, so the apron's been apart? Probably was reassembled incorrectly.

I've done that several times when "rebuilding" something.

johnoder · Dec 27, 2014

You guys need to have more mechanical snap.

The clutch would never work simply because it cannot repeat like the half nuts - never mind that it has an entirely differing gear ratio when powering the carriage along

Here is this again if you ever want to look in it:

http://campkahler.com/files/How_to_Run_a_Lathe_SB_1of2.pdf
http://campkahler.com/files/How_to_Run_a_Lathe_SB_2of2.pdf

iwananew10K · Dec 27, 2014

as mentioned you cannot thread with the clutch.

I`ll go out on a limb and suggest the halfnut interlock is not disengaging - IIRC your lathe was "rebuilt" before you bought it??

during said "rebuild" the seller likely let paint slop into the interlock pin hole and has it bound up so it can`t retract.

you can try to squirt some solvent into the hole, make sure the halfnut lever is fully down, and wiggle the selector and it just might break free.

it is a good idea though to just remove the apron and give it a good cleaning- probably full of nastiness anyway.

packrat2 · Dec 27, 2014

any and all lathes that I have had all had a lot of gunk in the apron. I am sure yours does too.

packardbill · Dec 27, 2014

There is approximately a 3 to1 (.341,actually) reduction in the worm gear drive for the clutch. Then there is an additional .375 reduction from there, when power cross feeding (This cross feed multiplier figure is present in the center of your feed chart.
Example: 8 tpi would be .125"/rev.
feed spec (when using clutch) on chart for 8 tpi is .0427"/rev. ... (.125 x .341=.0426) close enough.
Then for crossfeed .0427 x .375=.016"/rev
So, if you're cutting approx. triple the thread you desire, the light should go on. (oops, use the half-nut lever, not the clutch) PB

abarnsley · Dec 27, 2014

Not having a 9 A..

Are you sure the end gears are properly positioned??

It looks like the smaller gear on QC input shaft, should be in contact with idler...

This would speed up leadscrew, = cutting coarser thread..

iwananew10K · Dec 27, 2014

the end gearing looks right, the 20T stud gear is on the reverser and the 40T is in the storage position- don`t know what`s going on with that big pulley??

this is a great thing for any new SB owner to have along with the HTRAL document provided by John Oder-

very informative and well illustrated book to completely service your lathe- along with a few commonly needed felts(although you may not need them all)

Rebuild Manual Kit for 9? South Bend Lathe Model A | eBay

martik777 · Dec 28, 2014

Not what the OP intended, but one can cut threads without 1/2 nuts on the 9A, by selecting the closest feed rate to the required pitch and leaving the clutch engaged fwd and rev.

ie: a 32tpi is .03125" pitch which is very close to position A4 (.0310" feed rate)
A 1.5mm can be done with the 40t stud and pos A5 .0594

johnoder · Dec 28, 2014

Take any of the QC positions and you can calculate approximately the relationship between using the half nuts and using the clutch for long feed. Say 224 and .0015 in the corner there - take the reciprocal of 224 which is .004464 and divide it by .0015 and get 2.976

packardbill said:
There is approximately a 3 to1 (.341,actually) reduction in the worm gear drive for the clutch. Then there is an additional .375 reduction from there, when power cross feeding (This cross feed multiplier figure is present in the center of your feed chart.
Example: 8 tpi would be .125"/rev.
feed spec (when using clutch) on chart for 8 tpi is .0427"/rev. ... (.125 x .341=.0426) close enough.
Then for crossfeed .0427 x .375=.016"/rev
So, if you're cutting approx. triple the thread you desire, the light should go on. (oops, use the half-nut lever, not the clutch) PB

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