Old Lathe Threading Woes........what I Learned

Randy803

Randy803

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I had a Grizzley 12" modern lathe for a decade and could cut the most beautiful threads. I would really whirl the machine when I was cutting the threads, like 400+ RPM and I could do it all day long.

Well after a life event where I had to replace my entire shop and all my equipment I ended up with a 1944 Southbend 16" lathe. The lathe is in wonderful condition and I restored anything that was needed right up front.

This lathe does not have a thread dial and you cannot add one due to the lead screw having a groove all the way down it length. Trust me I tried, but it gets out of sync when it rotates over the groove. This however is not a big deal I can cut threads without a thread dial.

So I started cutting threads and they were horrible. The machine would make 5 perfect passes and then it would split the thread in the middle. I know all the fundamentals of threading, alignment and height so this wasn't the issue.

I just happened to find an old book called "How to run a Lathe" by Southbend. It recommended running the lathe in the slowest back gear for threading. So I gave it a shot, hell I had never even had a back gear on my other lathes. Man it turned SLOW but cut (formed) the prettiest thread I have ever seen.

The moral of the story is these old machines CANNOT cut threads at high speeds like modern lathes. Also a good sulfur based cutting flood is essential when cutting threads on these old lathes at slow speeds. The results however are worth the effort, finely formed threads with smooth shiny contours. No shearing, tearing or ragged cuts.

I am glad I figured this out, I was about to go out and buy another Chinese modern lathe, honestly though I prefer the nostalgia of using a 75 year old lathe. There is just something abut working on it knowing many men have used it over the years, it has a story of it's own (Sorta like my Ex-Wife).
 
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When I picked up my SB Heavy 10 I had no previous experience on a lathe, much less threading. Fast forward 2 years now. I have progressed to the point where I can cut threads almost effortlessly with this machine. It DOES have a back gear and I use it for threading. Also has the dial which really helped me learn to thread without wasting a lot of bar stock in the learning process.

I remember trying to thread at the middle speed of the lathe - almost a disaster/crash a couple of times. Then I came to the conclusion back gears are a blessing.
 
I had a 1960's Clausing lathe that had a keyway in the lead screw that drove a gear in the apron for power feed. The thread dial had an enveloping gear design that wrapped nearly 120° around the lead screw, the keyway posed no problem even when worn.
 
My SB has a keyed leadscrew/power feed rod and it has an independent threading dial and its never skipped.
 
Andre said:
My SB has a keyed leadscrew/power feed rod and it has an independent threading dial and its never skipped.
Click to expand...

Same here---the lead screw key way should not have any bearing on a threading dial. Must be something else going on with the thread dial.
 
As Wreck Wreck mentioned, you can make a thread dial that will work with a slotted lead screw. My Hercus has the same arrangement - the gear on the bottom of the shaft is wide enough to bridge the slot in the screw. You would probably have to make a hob to cut the gear so that it wraps around. It would be an interesting project. The gear could even be made of plastic.
 
My Sheldon has a keyway slot/lead screw, no issues with the thread dial. But, yes, these old machines cut beautiful threads when in backgear! At my age, I could not imagine hitting the half-nut if going any faster than the slowest spindle speed (30 RPM) in backgear.
 
In 1947 the thread dial assembly for a 16" lathe was catalog No 813. You should be able to find one.

Vlad
 
You can always leave the half nuts engaged like you would if cutting metric threads. At least until you get the problem solved. I always disengage my chasing dial when I'm not using it.
 
I also have a South Bend heavy 10L (my first lathe), but still struggle to make proper threads. I'm pretty certain it's me and not the lathe... The difficulty I have is making sense of depth of cut. At an angle, the cutter isn't cutting as deep as the dial says and I understand this (it's at an angle after all...). Is there a quick rule of thumb for gauging depth of cut by the compound dial, or should one simply cut, measure, cut, repeat...?
 
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