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DC motor for my lathe has low torque

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vtsteam:
I've run into a problem at low speeds on my new homemade lathe. Just not enough torque for cleaning up my iron sand castings.

I'm thinking about possibly going to a vfd and and three phase motor of 1 hp. One issue though (besides the cost) is that the small shop only has single phase 120 V, though I guess that there are a few inverter VFD's that will work for that situation.

What do you guys think -- is it worth making the change?

I guess the alternative would be making a jackshaft for the present motor and belting it down 2:1 in hopes that that will give enough torque (toothed belt).

SwarfnStuff:
Hi Steve,
      For what it's worth I just changed from a 360W DC Brushed motor to a 600W Industrial sewing machine motor that came with all controls needed. Sure, it is designed for a foot pedal to operate the speed etc but a bit of messing around and I now have my little 7x12 Seig clone almost unstoppable. Still a bit to do to get the speed control fitted to get easily set speeds from 100 - 3500 RPM.
  What I have suits the 240V here in OZ but am sure it works 110-240V.
Hopefully insert Pic from eBay works.
Regards,
John B

chipenter:
My solar powered lathe runs off of two 12 volt battery's , I have found that power is very voltage dependant at low rpm , I use a e bike controller for speed control and if I use the battery charger the voltage goes up to 28 volts giving more power .

vtsteam:
Thanks John and Jeff, I should have explained that I already have a 90VDC 10 amp treadmill motor running the lathe with a 5 to 1 belt reduction and a variable DC controller. If I remember correctly that motor was label listed for 10,000 RPM. I made the 5 to 1 reduction decision to bring it to 2000 RPM max (which I'm guessing is open speed anyway).

At 100 RPM, however it just doesn't have enough torque when doing iron work of about 3" diameter.

Could that be the fault of the controller? Maybe I need a better one?

Not quite sure how these controllers work. Do they limit the current always and just vary the voltage? In that case If a controller is rated for say 2 amps, does that mean that at a low voltage (to keep the speed low) it will keep the amperage at 2 amps?

In other words, would a 10 amp controller supply more torque at low speed than a 5 amp controller? Or would the 5 amp controller simply cut out, no matter what the speed if you work started to draw  5 amps?

I guess I may be thinking wrongly about the amperage rating of a controller --- I imagined it was just a cutoff type of rating, not a normal continuous current supply.

awemawson:
Steve,

If the main issue is getting 'under' the hard crust and sandy inclusions have you considered a 'tool post grinder'. The cutting forces are much reduced, as is the damage to your tooling, but cover your ways well against the dust.

I don't usually like the idea of grinding in the lathe but I had once to thread an HSS tool for a screw in collet chuck and was amazed how well a 'jury rigged' tool post grinder did, and my grinding disk' was just an abrasive angle grinder disk dressed to 55 degrees!

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