converting an old lathe to vfd drive

[TheGary]

I have an old south bend heavy 10 lathe. It is very limited for speeds especially mine since someone did away with the motor pulleys to change it to low speed. It has a small 110 volt motor, I think it is only 1/3 hp so I am thinking about getting a 1.5 or 2 hp 3 phase motor and put a 110 volt input vfd on it so it will have almost infinite speed changes. I know very little about vfd's so if some of you have some suggestions let them fly but please explain why so I can learn too. The primary concern is money. I do not want to invest a fortune in this old lathe. There are many better ones on the market but I already have this one so I would like to keep the drive conversion under $300 . It looks doable if I get some parts off ebay.

[Rambozo]

The motor needs to be inverter duty to handle the full range of a VFD. Many will go to 200Hz or higher and that can breakdown the insulation in older motors. You can always limit your frequency to 60Hz then any good quality motor will work. The torque speed curve depends on the number of poles in your motor and the current capacity of the VFD. For something like a lathe, you don't need a lot of bells and whistles. A programmable ramp up and ramp down will lower the current you need to start, and be gentle on the equipment. You probably don't want any breaking as that can unscrew a threaded lathe chuck, like an old SB. Ebay is a great resource for things like that. I have bought several lightly used or NOS name brand VFDs for pennies on the dollar. But there are also a flood of new low cost Chinese VFDs that might be another option. You might want to go with a VFD that is rated to a little more HP than what you plan to use, if you go that route. You should make a list of what options you would like to have. Some have displays that can be programmed to display RPM even taking a reduction system into account, while others just display the frequency or a 0-100% count. If you can power the VFD with 240 single phase, your input amperage will be much less than what it will take running on 120. Also be aware that some VFDs that are 3 phase input can also run on single phase but with a lower HP rating. Do you have a flat belt or a V belt spindle?

[wade23454]

There seems to be quite a few folks using treadmill motors for lathes. That will be the route I will go when I get a lathe.

[its_34]

I recently bought a KB Electronics unit for a 3ph grinder. They have a few models with enclosures that make the install quick and easy. Some have rotary pots for quick speed adjustments too.

[TheGary]

All good info. I like the idea of a tread mill motor. I will investigate that. My lathe is a flat belt to the spindal and a v belt between the motor and the flat belt jack shaft. I am open to all ideas the cheaper the better. From the little I know about vfd's it looks like I could use a 1725 rpm motor and at the higher speeds it would run at 90 htz to run overspeed. Or I could use a 3450 rpm motor and would limit it to 60 htz. My concern there would be torque at low speeds. I think I would be running low speeds 80% of the time. My lathe does have back gear but I would like to get down to 150 or 200 rpm without using it. Then in back gear I could get down into 10 or 15 rpm for some larger dia. work. I am wanting to compensate low rpm torque by using a 1.5 or 2 hp motor as at low rpm it would only have frational hp. Most of the SB lathes this size are running a 1 hp motor. I have a 1hp capacitor start 1725 rpm motor that has been on the shelf waiting to be used on this lathe but I think it would still limit my rpm choices a lot. Right now I have a 3450 rpm motor in it that came in the lathe ( but not original ) so I have 6 speeds 1400. 700 , 545, back gear 250 , 145 , 55 . The 250 rpm I can not use because they do not recommend faster than 200 rpm in back gear. If I put in the 1725 rpm motor my fastest speed would be about 700 rpm. I think I would need at least 1200 rpm sometimes. But the lower speeds would be much better than I have now. I would like to keep 110 volt input because I do not want to run a 220 line and this way if I change my shop around it could plug into any outlet I have. It might be possible for me to put a 2 step pulley on the motor to increase rpm with the motor I have but then it would take me 10 min. to change it over every time and that would be a hassel that I would probably be too lazy to do most of the time since most of the time when I am making something on the lathe I am in a hurry.

[TheGary]

After looking into the use of a treadmill motor for the lathe , I have ruled it out. I could do it cheaper than with a vfd but the motors are open to dust and chips and I do not think it will hold up as well as using a 3 phase motor and a vfd. If I had one laying around so it was free a treadmill conversion would probably be a good option but I don't have one and do not have time to scrounge one.