threading problems

[Fixnair]

Good afternoon folks! I haven't been around for awhile but I've been moving my household and shop from California to Oklahoma. It;s taken me a couple of years but I'm finally making chips again. I have a South Bend 13 X 40 lathe from WW II. I know there is a lot of wear in the machine but the machine was free to me.
I've done a lot of turning, facing and boring since I've owned the machine and I feel I'm ready to learn how to thread. This is where the machine will not cooperate with me. It will not track properly. I'm trying to cut 1/2" - 20 threads and the first pass looks OK and measures OK with a pitch gauge but when I go for a second pass I'll engage the 1/2 nuts on the #3 & it will just rip the existing threads to bits.
I know the gears in the single lever quick change gearbox are worn out. The 1/2 nut is new though. the gear train between the head stock and the quick change are a little sloppy also. the lead screw has some wear too, how much I don't know.
I guess it's time to rebuild the QC gear box. Can you guys turn me on to where I can purchase new gears? Bushings and shafting I can make with the exception of the splined shafts. Also is there any way to determine wether the QC gears are at fault or the power train gear set between the head stock and the QC box?

Thanks in advance for any help you experts may pass on

 

[T Bredehoft]

Don't be too fast to blame the worn out machine.
I'll engage the 1/2 nuts on the #3

I always use the same number, but with an even numbered thread you should be able to use any number.

Are you advancing the tool with the cross feed or the compound? do you retract the tool at the end with the cross feed? do you have a stop set on the compound? or do you return to the same number every time?

 

[Fixnair]

I'll answer in the order you asked; Yes, I'm advancing the compound feed, Yes I'm backing the cross feed out to return, I do not have a stop on the compound, Being the first time I've tried to cut threads I engaged the 1/2 nut on the #3 each time.

 

[JR49]

Thanks in advance for any help you experts may pass on

I Am certainly no expert (very far from it), but I figure I can get the ball rolling for you. You need to give the experts a lot more info so that they can help. T Bredehoft asked some very good questions, but there are more. Basically, you need to describe EVERYTHING you do from the time you put the work in the chuck, until the time when the tool tears through the thread you cut on the first pass. I guarantee they will find the problem, once you do that. Good luck, JR49

 

[cathead]

Have you tried cutting some threads leaving the half nuts engaged?

 

[Glenn Brooks]

Also not an expert on threading, but, here is a possible test of the machine. Try threading on a setting that isn't worn out much in the QCGB. Say, 36 TPI or higher on. Any diameter. 1" would be fine. If you find a gear setting that isn't badly worn, and the machine still wipes the thread, then either the lead screw/half nuts are really buggerrd or perhaps something in your process isn't as consistent as it should be.

 

[markba633csi]

Could one gear be missing a keyseat and be slipping? I can't think of anything else. Can you back out the tool, reverse the lathe keeping the halfnuts engaged, and try another pass? If it still wipes the threads something must be slipping. You are using the backgears, aren't you?
Mark S.

 

[T Bredehoft]

One more ask.
Is your compound set at 29 1/2º or at 60 1/2º? Not all compounds are numbered the same. The compound should be not as much as 45 º to the work.

 

[Charles Spencer]

Thanks in advance for any help you experts may pass on

Well, that certainly isn't me. I did have a similar problem for a bit on my SB 9 though. It turned out that my threading dial wasn't tight enough against the lead screw, causing it to slip.

 

[Reeltor]

I don't see how a worn gear in the QCBG could cause the wiping of the previous thread as you describe. Can anyone enlighten me on how this would occur? I am thinking that the first pass using the "worn" gear in the QCGB would cut the thread and all subsequent passes would follow the same path.

From the info you provided it sounds like you are doing everything correctly; do you have a copy of SouthBend's How to Run a Lathe? While I haven't gone looking for it on this site, I'd bet a nickel that you can find a copy. Years ago I watched a guy cut threads on his SB, on his machine the thread dial was on a pivot. He pulled the thread-dial away from the lead screw when he wasn't threading and pushed it in to make full contact when he wanted to cut threads. If the thread dial wasn't fully seated onto the lead screw, the dial would turn but it wasn't accurate or repeatable. Is your SB setup this way?

Mike