Convert a lathe to variable speed drive for under $100

Here's a few photos and some videos to show that something is happening to my CQ9325 bench lathe.   It now has VSD (variable speed drive) fitted.

I used a 1.5 HP DC treadmill motor, DC drive controller, and a LED tachometer with Hall effect sensor.

This conversion is great for light to medium turning, but heat rise in non SCR rated DC motors like this is a problem when pushed hard.

Be aware that a serious cooling system is required to keep things under control when revs drop and the motor is applying full torque.

Being a brushed motor, voltage duration to windings increases as speed drops, and this will cause significant heat rise.

The two PC case fans I fitted were not sufficient, and something like a small inline bilge blower for a boat may be required for heavy use.

I got this great blower cheap, but it's way too noisy, so I used it for my backyard furnace,  and the hunt goes on.  Maybe a 12 volt car dash blower would do the job.

The reversing switch is now gone and in it's place is the speed control.

  lathe VSD control

CQ9325 front panel conversion

KB Electronics KBI-240 DC drive controller

Hall sensor for lathe spindle tachometer

DC motor cooling fans and cradle

sleeved motor pulley

Lathe tachometer display

For the VSD conversion you need two main things

A junked/recycled DC treadmill motor of suitable horse power.

treadmill motor

And a secondhand (or new) DC drive controller, such as this KB Electronics KBIC-240

DC drive controller

And a few other bits and pieces including:

- 1 three wire 5K ohm linear potentiometer (type B),
- 1 or 2 PC case cooling fans,
- 1 240 V or greater, three pole toggle switch,
- 1 junked mains wall power pack of 12 volts DC,
- 1 junked XT PC power case or small metal tool box,
- 1 AC power outlet (unswitched),

and if you want to install a tachometer

- 1 LED tachometer display,
- 1 Hall sensor.

Here's some videos of the completed installation

And the lathe in action machining some medium carbon solid steel stock.

The DC controller is using the factory trim pot settings.  

Here's an overview of the KBIC-240 controller.

The maximum motor speed is set at the recommended 60%.  This allows best use of available torque, and provides the greatest motor efficiency.

Anything over 70% will push up motor temperature and increase brush wear.

It is advisable to retain the original lathe motor pulley and drive system  - to provide the DC motor with multiple speed ranges, allowing maximum use of the increased torque. 

Speed ranges of 0 - 2000,  0 - 800, and 0 - 250  RPM provide adequate power and torque.
This link provides excellent information on the torque and power characteristics of DC motors.

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