Converting a 12x36 lathe to 3-phase power.

[Highpower]

I had posted this some time ago at another forum - and was asked if I would post it here as well. It's my run-of-the-mill import 12x36 lathe that I used as it came from the factory for 10 years before converting. I wish now that I had done it much sooner...
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I just thought I would put up a few pics of my attempt at making a silk purse from a sow's ear.

I bought this lathe (12x36) to be a step up from my little 7x10 mini-lathe. It was nice to be able to work on larger diameter and longer items with the new lathe. But something that I missed compared to the mini-lathe was the restricted number of spindle speeds available to me on the larger lathe. This became a real problem for me when threading to a shoulder because the lowest speed on the lathe was 60 RPM (66 actual). The chuck would free wheel for quite some time after being shut off, and it is not equipped with a spindle brake. I'm afraid I am just not that coordinated to pull off that kind of timing.

So I set about trying to resolve that issue. I purchased a new 2 HP 3-phase motor to replace the factory 1-1/2 HP single phase motor. Bigger is better, right? (LOL) One of the first problems encountered was that the motor shafts were a different size, which meant that the factory drive pulley would not fit the new motor. And of course I could not find a new double pulley in the configuration I needed. But I did find single pulleys of the proper sizes. Time to modify!



After getting the motor mounted it was time to install a VFD to supply the 3-phase power to it. My lathe sits in the middle of the floor so there aren't any near by walls to mount it to. I just attached it to the rear of the factory electrical box and attached some lexan sheets to serve as a shield against getting any swarf/chips inside the VFD. I also added a large braking resistor.



To finish it off I mounted the VFD control unit remotely near the front of the lathe for convenience and also installed a "tachulator" from LMS, which displays spindle RPM and SFM readouts. I made an encoder wheel and mounting block to simplify the installation of the optical sensor on the lathe.



I also added a toggle switch on the right side of the VFD control box, that allows me to switch control of the lathe motor between the VFD panel and the factory control lever on the lathe apron. Flip the switch UP, and I can start/stop the lathe with the VFD push buttons. With the switch DOWN, the normal lathe lever is in control.



After some fine tuning I am now able to run the lathe as slow as 2 (TWO) RPM!
Threading to a shoulder is no longer a problem.

[doubleboost]

Hi
I have a VFD on my Boxford
They make screw cutting a pleasure
One thing to watch is motor temperature when running very slow
John

[Highpower]

Yes, that is a good point John.

If you run the motor too slowly - for too long, they will overheat since the motor cooling fan blade slows down as well. Work on and off for short periods of time in order to let the motor cool down between times, or set up an additional external fan to help cool the motor as it runs.

They do make special inverter duty motors that have a separate motor and fan built in to handle that, but they are somewhat expensive.

Just use common sense and keep an eye on the motor temperature if you are using a standard TEFC motor.

[Hopefuldave]

Highpower Wrote:Yes, that is a good point John.
If you run the motor too slowly - for too long, they will overheat since the motor cooling fan blade slows down as well. Work on and off for short periods of time in order to let the motor cool down between times, or set up an additional external fan to help cool the motor as it runs.

They do make special inverter duty motors that have a separate motor and fan built in to handle that, but they are somewhat expensive.

Just use common sense and keep an eye on the motor temperature if you are using a standard TEFC motor.

Wot he said...

A lot of VFDs have configurable relay outputs - on my ABB inverter I have one configured as Normally Closed, opening when the VFD output frequency goes above 30Hz - that way it switches an external mains-powered fan *OFF* when the internal fan's running fast enough to cool the motor, back *on* when the motor speed drops (including when the motor's at rest or the VFD's powered off for a cool-off!) The fan at the moment is a cheap desk-fan-in-a-plastic-cage that cost 50 pence at a carboot sale hung on some sheet ally brackets, I'm hoping to find a way to fit something a bit more, er, industrial :)

A lot of the DC motors used in smaller lathes could benefit from a separate fan, too - often they get used at low speeds with heavy loads, a recipe for overheating!

Dave H. (the other one)

[DaveH]

Willie,

A very nice and neat job

Particularly like the speed sensing device

Nicely done

DaveH

[Highpower]

Thanks Dave. The "tachulator" is a handy little unit. They are great for milling machines as well. I've even seen them installed on the little 7 x 10 mini-lathes.

http://littlemachineshop.com/products/pr...&category=

[EdK]

Willie,

Did you fasten the two pulleys together some how or are they just butted together and held in place with the set screws? I have an identical situation with my lathe. I've got the 3-phase motor and the VFD but I can not find a two-step cone pulley with the correct diameters and bores. It looks like I'll be doing what you did.

Thanks for posting that.

Ed

[Highpower]

Ed,

All I did was counter bore the end of the larger pulley a bit so that the OD of the smaller pulley would fit inside. Much like the register on a lathe chuck / back plate. I needed to thin the end of the larger pulley anyway in order to get the distance between the two sheaves to match the original one-piece double pulley.



Other than that, the set screws (and key ways) hold them in place. And since the hub of the larger pulley (where the set screws were) was hanging off the end of the shaft in open air, there was no way to lock it down. So I just cut off the hub, and drilled & tapped new holes for the set screws in the center of the sheave. Neither pulley has budged since they were installed.

[stevec]

Not that it's needed but there looks like enough meat there to drill and tap for a couple of machine screws to bolt the sheaves together.

[EdK]

Thanks Willie.