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1/2hp 3ph motor good size for a SB9? VFD recommendations? Max spindle speed?

mattthemuppet

Stainless
Joined
Apr 22, 2016
Location
San Antonio
I thought I'd break this out as a separate thread instead of it being buried in my wide 9 refurb thread.

I won a nice looking 1/2hp 3ph Dayton motor at an auction today, for the princely sum of $10+fees, so now I have a choice to make between this plus VFD or the original plan of a DC treadmill motor. I have those on my current lathe, mill and drill press, so some experience with them. Never used a 3ph motor before.

Is 1/2hp a good size for a wide 9? My gut says yes, but I haven't had dessert yet, so.. Downside of the 3ph motor is that it's a 3450rpm motor, though that's about the same as a treadmill motor, give or take.

Recommendations for a VFD? Sensorless vector drive mode seems like a good idea and there are a few that have that for reasonable money. Needs to be 110V input. Worth worrying about a breaking resistor or is that silly with a threaded on chuck?

Also, what max spindle rpm is sensible for a bronze bearing SB headstock? That'll help me start thinking about pulley ratios.

thanks!
 
a 120v in, 240v out, 1hp vfd isn't expensive. so that's the easy part.

rpm limits are specific to the bearings you've got, how much clearance, how much wear, etc.

i can run my sb9 lathe with its cast iron headstock bearings at 1500 rpm using synthetic low viscosity atf as spindle oil, 6 cst at 100C. .001 to .0015" clearance at headstock.

i did some tests with a 56 pound steel bar with a center of mass 2 feet from headstock spindle, about 40-60 rpm was needed to maintain an oil film. this test corresponds to about 200 pounds downward force at the spindle.

such a test would tell you a lot about what is really going on with your bearings, though you would want to apply upwards force somehow.
 
I'd recommend against any 3450 rpm motor for 9" lathe service. If you don't use it with VFD you'll be fighting speed reduction requirements forever. If you do use it with VFD it won't have sufficient torque at slow speed to do anything.

A 1750 RPM 1/2 HP motor is perfectly adequate even with VFD. Better still would be an 1140 RPM motor if you can find one. South Bend Lathes are slow-speed devices.
 
thanks for the replies the two of you. Yes, I think the 3450rpm part is the killer. I'd need a mahoosive flywheel on the countershaft to drop the countershaft speed to something reasonable, even using a VFD. I think I'll go with the DC motor instead. I typically use mine at ~1/2 speed, varying up or down as needed, plus the one I'll be using is much smaller and will have a lot more torque at lower speeds.
 








 
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