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Forum topic by paratrooper34 posted 01-18-2011 03:38 AM 1067 views 0 times favorited 9 replies Add to Favorites Watch
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915 posts in 2919 days

01-18-2011 03:38 AM

Hi Everyone!

I picked up an old Craftsman lathe. Not sure what model, but it is pretty old. Anyway, it has an externally mounted motor and it is operated by an on/off switch. The motor is connected to the headstock piece with a v-belt. The motor and the headstock both have 4 different sized pulleys for changing speeds on the lathe. I really don’t want to have the hassle of changing the belt to the different sizes for speed change and was thinking there has to be a way that I can make this a variable speed by putting some type of potentiometer in the circuit. So that is my question: have any of you done this with an old machine and what type of switch did you use? I was thinking a dimmer switch for a house light circuit might work, but not sure. The motor is a 110V type, 1/3 hp 1750 rpm. Any help would be greatly appreciated.

-- Mike

9 replies so far

View richgreer's profile


4541 posts in 3042 days

#1 posted 01-18-2011 03:52 AM

I considered this idea myself at one time and was told that it was not a good idea. I’m not an expert on electric motors but I understand that some motors can handle a varying circuit. That is usually true of the motors in a ceiling fan, a router and most hand held tools. The motor normally used on a lathe (or table saw, planner, jointer, band saw, etc.) cannot handle a varying circuit.

As an FYI, most high end lathes ($2000+) have an electronically controlled motor speed but those of us with more modest lathes have to change speeds with the belt.

I think you will find that after you do it a few times, changing the pulleys the belt runs on is not a big deal. I can probably do it in less than 15 seconds if I am in a hurry.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View docholladay's profile


1287 posts in 3026 days

#2 posted 01-18-2011 04:11 AM

You probably could do it. A cheap and easy way to test would be to connect one of those items designed to control router speed for non-variable speed routers. However, you may find that you lose power also. To do it well would probably replacing the motor with an appropriate vs motor. That could be expensive.

-- Hey, woodworking ain't brain surgery. Just do something and keep trying till you get it. Doc

View BTKS's profile


1986 posts in 3432 days

#3 posted 01-18-2011 05:19 AM

I could be wrong here but I’ll throw my two cents in the mix. Most of the motors you see controlled directly with the current are universal motors. The external mounted motors on larger tools, the name of them escapes me. I believe they are induction motors. They don’t run the same way and a drop in supply, amps, will cause them to fail early. A dimmer switch is not meant to handle the kind of amps a drive motor pulls. They are fine for a few watts of lights but not a motor load. If you forget to turn the controller back to full power you wont have the needed power to the capacitors to start the motor from a dead stop.
I have an older craftsman lathe like your talking about and the belt changes really aren’t a big deal once your used to doing them. The motor weight what causes the friction.
I improved the performance of mine quite a bit by putting a couple of gallons of water in a closed bucket and hung it from the motor mount. The water absorbed the vibration and kept the motor from bouncing. It didn’t slow down the belt changes because it came up when I lifted the motor.
Hope this helps,

-- "Man's ingenuity has outrun his intelligence" (Joseph Wood Krutch)

View Tim_456's profile


170 posts in 3563 days

#4 posted 01-18-2011 05:31 AM

Check with an electrician but I don’t think this will work and is probably a dangerous thing to do. The lighting dimmer switches aren’t meant to carry current required by motors especially at start up and at best you’ll get a spark and the switch will die, you may even get the switch to light on fire.

View TopamaxSurvivor's profile


18249 posts in 3643 days

#5 posted 01-18-2011 09:01 AM

Lighting dimmers will not change the motor speed. Most of the tools made today are universal motors. There are several types of small single phase motors. Since it is expternally mounted, if you take it to a motor shop they will probably tell you what it takes to vary the speed. Since you bought the old Sears, you probalby do not want to spend the money. The most economical way will probably be to get a small AC drive.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Rob Drown's profile

Rob Drown

787 posts in 3800 days

#6 posted 01-18-2011 09:12 AM

The speed in an A/C motor is controlled by the no. of poles in the motor and the frequency of the power. The no of poles is fixed but you can vary the frequency of the A/C. I think it will be more expensive to get a small frequency controller than to buy a new lathe. As you decrease speed, torque is relatively constant but the power decreases as power is torque times speed.

It has been a while since i did this type of power transmission but i believe this is correct.

-- The expectations of life depend upon diligence; the mechanic that would perfect his work must first sharpen his tools. Confucius, 经过艰苦的努力的梦想可以成真

View TopamaxSurvivor's profile


18249 posts in 3643 days

#7 posted 01-18-2011 10:21 AM

The number of poles definitely determines the speed of an ac motor. He is most likely dealing with a 1750 or 3400 rpm motor and changing the number of poles is not an option. The power and torque of ac motors goes down with the speed for all practical purposes. To get a constant torque motor with variable speed you have to go to a DC motor. The most effective way to control the speed of a single phase induction motor is to use an AC drive that internally uses 3 phases to get a constant voltage supply. This is getting too technical for the purposes here. Any cheap drive that is used to control the lathe will definitely reduce it’s toque considerably. Using the variable pulley is the most effective way to change speeds if you do not want to spend a lot of money to do it. I do not want to debate electrical theory. This is as simple as I can be explain it for the intended audience.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View paratrooper34's profile


915 posts in 2919 days

#8 posted 01-18-2011 12:53 PM

Well, I got alot of input, thanks everyone! Looks like I will stick with the belt chamges. You guys say it is no big deal, so I am going with that. Thanks!

-- Mike

View TopamaxSurvivor's profile


18249 posts in 3643 days

#9 posted 01-18-2011 01:08 PM

Just roll the belt off the edge to where you want to go.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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