Minimum Lathe Speed: How Slow Should It Go?

[John Hasler]

I'm planning to convert my Logan 400 to DC drive and I'm looking for suggestions as to what the minimum useful speed might be. I already know that I'd like to go slower than the 60RPM the present drive system allows.

 

[Grumpy Gator]

Low speed on my Sidney is 13 RPM. And I use that speed a lot.
***Just Saying********Gator*********

 

[mattthemuppet2]

what speed DC motor? Treadmill or proper industrial DC motor? The treadmill motor on my lathe makes useful torque down to about 25-30% of motor rpm, so around 1000-1250rpm. With backgears and countershaft, I can get that down to ~50rpm which is pretty useful for 4-5in chunks of steel on a 6in lathe. If a 1750-2000rpm DC motor, 25-30rpm should be doable, depending on the pulley set up you have.

 

[great white]

I've converted my atlas to a Baldor DC drive with a cycletrol controller.

Works great down to under 10 rpm.

I haven't needed that slow yet. I can't imaging why I would need that slow unless it would be useful for threading up to a collar or similar. I'm also just a beginner so there are probably uses I don't know about yet.

I went DC since the original motor was a single phase 1\8 hp which was just too weak for my uses. I figured if i was going to replace it, might as well go variable speed to cut oit some ofmthe messing aroind with belt changes. I'm usually somewhere around 400-500 for working on aluminum...

 

[John Hasler]

what speed DC motor? Treadmill or proper industrial DC motor? The treadmill motor on my lathe makes useful torque down to about 25-30% of motor rpm, so around 1000-1250rpm. With backgears and countershaft, I can get that down to ~50rpm which is pretty useful for 4-5in chunks of steel on a 6in lathe. If a 1750-2000rpm DC motor, 25-30rpm should be doable, depending on the pulley set up you have.

5500RPM GE treadmill motor, but with a fairly decent closed-loop controller. I'll build a new one if it proves inadequate after I modify it. I'm trying to decide if I should keep the jackshaft and step pulley. Qualitative bench experiments indicate that I should get acceptable performance from the motor down to at least 500RPM.

[Edit] Though treadmill motors are not high quality (and the "treadmill" rating is about double the real HP) your lack of low-speed torque is probably due to the controller. Even the closed-loop ones always have a current limiting circuit designed to keep the motor current under the namplate rating to keep UL happy. Since torque is proportional to current the result is obvious. You can modify this circuit to raise the limit to something close to the maximum rating of the switching device. If you do so you may want to improve the heatsinking.

 

[george wilson]

If you try facing off your face plate with HSS tools,you will find that 10 RPM is not too slow. Cast iron quickly dulls HSS. My big Promaster from MSC (Had many flaws!) at work would go 11 RPM up to 2000. It had a very useful speed range.

Even at that low speed,I never tried anything but carbide for truing the large face plate.

 

[mattthemuppet2]

good point John, it's just a standard MC controller so nothing particularly fancy. It was free though. If I were you I would still keep the jackshaft and and step pulley to vary spindle speed range, particularly as you're going to need to gear the motor down to keep the full motor speed range without overspeeding the spindle (I'm guessing 5500rpm is a touch fast for a Logan?!). Unless you're going to add a supplemental or standalone fan to the motor, I would worry about motor cooling if you're taking heavy cuts at low motor speed (ie. below 25-30%), so a countershaft to spindle step pulley will give you some different options even without back gear.

Other than that it's suck it and see time I don't think you'll be disappointed though, I love mine - being able to vary speed facing a large backplate is just awesome!

 

[John Hasler]

good point John, it's just a standard MC controller so nothing particularly fancy. It was free though. If I were you I would still keep the jackshaft and and step pulley to vary spindle speed range, particularly as you're going to need to gear the motor down to keep the full motor speed range without overspeeding the spindle (I'm guessing 5500rpm is a touch fast for a Logan?!)

The spindle drive belt would bring the top speed down to around 2000RPM which is as fast as I want to go. As shipped the top speed was about 1400.

Unless you're going to add a supplemental or standalone fan to the motor, I would worry about motor cooling if you're taking heavy cuts at low motor speed (ie. below 25-30%),

Being completely open frame and mounted right behind the headstock the motor has to be shielded so it's going to get external forced cooling through a duct. This plus constructing a motor mount is most of the project.

so a countershaft to spindle step pulley will give you some different options even without back gear.

I've got a backgear, of course. However from the other comments it would appear that I will find use for the "creeper gear" that keeping the jackshaft will provide.

Thanks, everyone, for the input.

 

[mattthemuppet2]

neato Dickybird over on HSM has a neat trick where he heat shrinks 2l soda bottles (PET?) over a form to create fan duct. Just be aware, if you aren't already, that there'll be a hoooge amount of back pressure trying to force air through the motor, so either get a fan to suit or mod the duct so that some air flows over the outside of the motor. 12V PC fans offer a lot of options and 120mm ones can flow a lot of air, plus if you use an eBay tach you could run the fan and tach off the same 12V power supply.

For the motor mount, mine was pretty ghetto, just a block of wood with the motor offset so that I could swing it up and down to adjust belt tension. Screwed to the bench with a couple of rubber washers on each bolt to isolate any vibration.

Don't forget the choke coil for the motor, unless your set up doesn't need it. Made a world of difference on my relatively crude set up.

You can never have enough speed options. Even DC motors have a sweetspot (sweetrange?) so more options = more likely you'll be in it, whatever you're doing.

 

[John Hasler]

neato Dickybird over on HSM has a neat trick where he heat shrinks 2l soda bottles (PET?) over a form to create fan duct. Just be aware, if you aren't already, that there'll be a hoooge amount of back pressure trying to force air through the motor, so either get a fan to suit or mod the duct so that some air flows over the outside of the motor. 12V PC fans offer a lot of options and 120mm ones can flow a lot of air, plus if you use an eBay tach you could run the fan and tach off the same 12V power supply.

I'll be using a centrifugal fan salvaged from a piece of Western Electric microwave equipment. I'll eventually build a tach.

Don't forget the choke coil for the motor

Part of the controller.