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View Full Version : SB 10L Toothed Spindle Drive Belt



gzig5
08-18-2011, 12:11 PM
I hope to finish up the rebuild on my 10L this Fall. One of the things I need to look at is the spindle drive. As a bit of background, I got the lathe without a cabinet, so I need to build one. It is a 4' hard bed, large dial, D1-4 spindle with a taper attachment. I have the underdrive pulley assembly but will not use it. I plan on doing a direct drive with a PM servo motor and I already have all the electronics. I would like to use a toothed belt like an HTD or XL series so I won't need a second encoder directly off the spindle shaft to compensate for belt slip when threading with the servo driven leadscrew. I would make or modify the spindle pulley for the toothed belt.

I am worried that a toothed belt could induce vibration that would affect the surface finish. All the small lathes I am familiar with use a flat or V belt. I am sure that the larger CNC lathes must use a toothed belt with their higher horsepower, but they have a lot more mass in to soak up any vibrations and they are using tapered or roller bearings not a plain bearing. So, am I over-thinking this? I'd appreciate any pro's and con's.

gzig5
08-18-2011, 07:45 PM
Crickets chirping??

flathead4
08-18-2011, 08:24 PM
There was a discussion over on the PM board a few years ago. Vibration and resulting pattern in the work is discussed. Here is the link:

http://www.practicalmachinist.com/vb/south-bend-lathes/sb-9a-timing-belt-drive-156844/

Tom

tdmidget
08-18-2011, 08:36 PM
The Okumas and Nakamuras I have run had Vee belts. Separate servo motors with out an encoder won't work because the input to the lead screw will have to be compensated for belt stretch and slippage on the spindle.
When you get done you will have a 50 year out of date lathe with a bunch of cobbled up cnc stuff worth less than a decent South Bend of similar vintage.

Evan
08-18-2011, 08:53 PM
Belt slip is your friend in a crash and you will have crashes. Use a spindle encoder and either a Poly Vee belt or a standard V belt. I use a poly vee belt (same as car engine belt) on my SB9 running on the regular cone pulleys and it can stall the motor. In a crash it will slip just enough to soak up most of the momentum in the drive system.

gzig5
08-22-2011, 01:08 PM
The Okumas and Nakamuras I have run had Vee belts. Separate servo motors with out an encoder won't work because the input to the lead screw will have to be compensated for belt stretch and slippage on the spindle.
When you get done you will have a 50 year out of date lathe with a bunch of cobbled up cnc stuff worth less than a decent South Bend of similar vintage.

I fully understand that and it was why I was looking at a positive drive belt, and thus my concerns about surface finish. Even if I planned on selling it, which I don't, I don't see how a lathe that will cut any thread standard or metric without changing gears and has infinite speed and feed control would be worth less than one that doesn't? What would you suggest?

Evan,
I hadn't considered the benefit of the belt slipping in a crash. It's not a big deal to put a 2000 line encoder on and I've got several to choose from.

dneufell
08-22-2011, 05:37 PM
gzig5... "When you get done you will have a 50 year out of date lathe with a bunch of cobbled up cnc stuff worth less than a decent South Bend of similar vintage. "

crazy talk....go for it....you are right with being able to make any type of thead and a whole lot more!! :)
I did it and bought a brand new cnc lathe with the money i made with it! (full size cnc lathes use v belts) .....Dean

gzig5
08-23-2011, 01:38 PM
Well if the big boys all use v-belts, I'll just stick with a flat serpentine belt and eliminate any potential for issues. I have no intention of making this SB 10L into a CNC lathe, I just want it to be more versatile than it currently is. I have the guts of a barely used linear servo motor that has some nice THK slides/trucks and a good bit of length to it. When I get caught up on my other projects in about ten years, I'll put a lathe head on it it with a couple balls screws and make a proper CNC lathe out of it.