Tuesday, February 28, 2017

Lathe: Speed Control

Apparently most of the newer 7x12 mini lathes are now coming with a KBIC-240D speed controller stock. Unfortunately, mine was not one of them. While the power of my lathe is adequate, I wouldn't mind a little more torque, especially at low RPMs.

The lathe came stock with a PWM controller, and for the same reasons as with my mill, I decided to stick with a PWM controller for the lathe upgrade. Since installing the KBWS-25D in the mill went so well, I decided to do the same thing with the lathe. I eventually picked up a well used KBWS-25D on eBay for a great price. While the mill has a 350W motor, the lathe only has a 250W motor, so I installed a 9849 HP resistor on the KBWS.

NOTE: The older version of the KBWS-25D come with a very tall capacitor, while the newer versions come with a shorter one. The shorter one definitely makes mounting easier, especially on the mill.

My eBay find.

The tall capacitor kept me from simply installing it in the control box, so I marked out the KBWS chassis on the exterior of the box and cut it out with a Dremel and some files. Then I used UHWM strips to space the KBWS out a little more and clean up the hole. The KBWS then slides in from the outside of the control box, and is bolted into place. If I was using a SCR controller from KB (like the KBIC or KBLC) then I could have just bolted it directly into the control box since they lack the tall capacitor of the KBWS. A hole was cut in the clear plastic protective sheet to provide clearance for the capacitor and the armature fuse holder was mounted to the side of the box.

Cuts and holes marked.

Hole cut and drilled. It's not clean since I did it with my Dremel and the spacers will clean it up nicely.
Bolted in place along with the spacers.

Plastic cover and fuse holder mounted.

 Like in my mill, my lathe's wires are numbered which made things easier, though different lathe's might have different numbers or no numbers at all. For me, wire 1 (AC hot) went to L1, wire 2 (AC neutral) went to L2, wire 3 (motor +) went to A+, and wire 4 (motor -) went to A-. The potentiometer wires are in reverse order so P1 goes to P3, P2 to P2, and P3 to P1. I removed any extra wires I wasn't using to keep things clean.

Labeled connections on the KBWS.

Since the mill doesn't come with a pre-installed run/off switch like the mill, I needed to add one to allow me to set a speed and then turn the spindle on and off independently. A mini toggle switch from Amazon worked nicely, and fit in the existing hole above the direction switch, and connected to the inhibit connectors on the KBWS.

The IR trim pot controls how much compensating voltage is given to the motor to maintain RPM, so I goosed it upward a little.

Like with the mill, I really like this mod. The board is more reliable, gives me much more control and adjustability, and allows me to set an RPM and then turn the spindle on and off. Considering if you bide your time you can find these boards for not too much, I think it's a great upgrade and definitely helps with the usability of the lathe.


  1. How much did you pay for the controller

    1. You can buy one new on Amazon for $133.