A South Bend question:

Pete Wass

Pete Wass

Well-known member
I have a South Bend 10L made in 1943, It is a bench top lathe with a drive under the bench. The driven shaft has a two step pulley on it. The motor has a 2.5" pulley on it which drives the smaller pulley surface on the driven shaft. This brings up some questions as I have seen for sale on Ebay two step vbelt pulleys for South Bend lathes, both for the 9-10 and the heavy 10, with a larger pullet in addition to the smaller or 2.5" pulley.

The first question is if my motor has the smaller pulley on it shouldn't it be driving the larger pulley on the driven shaft? If it is suppose to drive the larger pulley, won't this make a difference in threading accuracy?

I assume with the two pulley situation one swaps the driving belt back and fourth to change the operating speed of the Lathe in total , No?

I could buy another pulley and add it beside the small one on the shaft if that is something I should do. My "Operating a Lathe" book shows the two pulleys but does not give any instructions about them. HELP PLEASE!
 
Pete,

Usually a two pulley input allows you to change the entire lathes operating speed, as you correctly surmise. Some lathes have two different sets of speeds shown on their selection plates to indicate these different resulting spindle speeds. Threading ratios are entirely in the hands of the change gears or gearbox settings you have set up and changing the input belt speed won't affect that ratio.

I didn't like the rather coarse feed found on my latest lathe and made a new stud gear with half as many teeth to slow all the ratios down a bit - halved my feed rates, but I have to remember that when I want to cut 16 threads per inch I need to select 8 tpi on the gear position selector chart.

If you are wondering about the spindle rpm when on a large or small pulley, just go to the lowest spindle speed and count the resulting rpm's. If the physical count doesn't match the rpm charts on your lathe, then you might wish to change pulleys to correct this (or at least understand that your rpms are off by some percentage from what is indicated).

Scott
 
Thank You Scott,

Unfortunately I do not have a chart to consult, I don't think. What about the situaton I have where the small pulley on the motor is driving the small pulley on the driven shaft? Wouldn't it make sence that the small motor pulley should be driving the large pulley and the large motor pulley driving the small driven pulley? It doesn't appear to me that when a two pulley gang is installed on the motor there would be any choice because there isn't the latitude to have a belt running at a 45* angle.
 
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Your lathe may have the rpm chart on the end of the gear box. The two pulleys from the motor give an option for a high speed range. If you don't adjust the change gears, too, the work piece will turn so fast that you'll burn up lathe bits. Ask me how I know!
 
Pete,

Without seeing the situation it is a bit tough to know what to advise you. Normally the small motor pulley goes to the larger input drive pulley to do most metal work (lower spindle speed results). You may be turning things pretty fast if you are on the small input pulley. It's been a while since I had a belt driven system on the lathe beyond the basic two pulley input. On the South Bend you need to be in back gear mode to obtain the lower speeds, but I am really rusty on that machine.

Somewhere on the lathe there is usually a speed chart, a threading chart, and some pictoral information about what gears/belts need to be positioned to obtain certain speeds and feeds. If your lathe has had these plates removed then you need to do some research on the machinist websites to get a copy. If you have the patience and math skills you can determine all the speeds by careful analysis of the gears and pulleys, but this is really reinventing the wheel and the information should be available somewhere with less effort.

I expect you can find the info needed, but as you are getting acquainted with a new lathe, it is useful to turn some stocks in various sizes and materials and see what the effects of different speeds are: as you may know, blue chips usually indicate that you are running a bit too fast, but that may be OK when using carbide... go slower until the machine feels right. Go faster if the finish is rough or it takes too long to turn a part down. Play around - grind different tool bit shapes and rakes and try different ideas - you will soon get a seat of the pants feel for basic speeds for different materials and diameters (affects surface speed). You may find a specific speed where materials turn a bit smoother - note this speed, as it may be a vibrational sweet spot on a light lathe. You can only find this by experimenting. You really only need to know the actual speeds so that you can replicate them when you find what works.

Scott
 
for pete wass

Call me @ the # i gave you. I have charts for sb lathe speeds for differant size pulleys. bob 716-983-4778
 
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