CNC Retrofit Kit

Many folks who have a mill or lathe eventually want to enhance their equipment by CNC enabling. This allows the rig to be driven by a computer, instead of being hand operated. This note documents a kit which was procured from an eBay seller who is based in China.

The kit included the following parts: Three stepper motors, two power supplies, a driver board, a fan and a PC parallel port cable.

cnckit.jpg eBay item listing photo

Well, the eBay item was delivered on 26 May 2008. Took a few weeks. A large package eventually arrived. Driver board, PSUs, fan and parallel port cable look to be as advertised.

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Steppers

The steppers are second-hand. That said, they don't look too bad - made in Japan by Vexta - part number is PK286-03A-C9. Datasheet here

Sold in the US for $99 each here

Specified to have a holding torque of 248 oz-inch. Each motor weighs approx 2lbs 4oz. Single shaft which has two holes drilled into it. There is no 'flat' surface on the shaft.

Connectors are fitted to each stepper. These are polarised and mate with the driver board. The motor has six wires, the connector is fitted to four of these, leaving two wires unconnected. So, no effort required to work out which wires on the stepper to connect to which connector on the driver board.

Driver Board

Schematic diagram of the driver board schematic

Datasheet for the TA8435H chip that is used on the driver board driver

The heatsink gets pretty warm pretty quickly. This will need a LOT of forced air cooling.

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A case for the parts

Having a bunch of parts is all very well, but the driver board and PSUs need to be enclosed. An old PC case seemed like a fine place to start. Decided to leave the PC PSU in place - this can drive fans to cool the driver board. made a mounting plate to hold off the driver board from the chassis. Mounted the 24V PSU and the 5V PSU in the case.

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Mach3

This is an excellent piece of CNC control software, developed by the folks at Artsoft. Simple to install on my Win2K laptop. A few tips re setup:

  • Use the 'DriverTest.exe' program to check that your PC is up to the job.
  • For initial setup, disable the 'estop' line (but be sure to get this working later)
  • Ensure that you have the correct step/direction pins specified
  • Switch to the 'Motor Tuning & Setup' screen, then use up/down arrows on keyboard to drive the motors

Great CNC pendant here http://www.randyray.com/TxMicro/