Notes for Connecting the CNC4PC model C6 Spindle Speed Control Interface
to the Sieg FC250J/110V DC Motor Speed Controller Board Found on 
Sieg Model C2 7x12 Mini Lathes from China.

This modification requires a minimal amount of wiring changes to the lathe which may still be operated manually or by CNC. The FC250J/110V DC motor speed controller board is found in many late model versions of this lathe having the big red power button type switch on brands including Grizzly, Harbor Freight and others. 

WARNING! The motor controller board in these lathes is connected directly to the AC mains. Any external "ground" connections must be isolated from the lathe's AC mains ground (the ground that is connected to house or shop wiring through the third prong on the AC power plug). Extreme caution should be taken when making this modification because contact with the AC mains can cause injury or even death. 

The changes made to the lathe wiring are:

1. The Direction Relay is added in series with the existing Direction Switch (SW3). The existing direction switch should always be set to the Forward position when operating in CNC mode. In Manual mode, it may be used to change the spindle direction as usual.

2. A DPDT switch (SW4) is added to serve as an input mode selector. The modes are CNC operation and Manual operation. In the Manual operation position, the wiper of the existing manual speed control potentiometer is switched onto the P2 terminal of the motor speed controller board and the existing switch on the potentiometer (SW2) supplies AC power to the motor controller only when the potentiometer is turned on. In the CNC operation position, the output control voltage from the C6 interface is selected instead of the wiper of the potentiometer. Also, the other pole of SW4 performs the same function as the switch (SW2) on the potentiometer, supplying the AC power to the motor controller board.

3. The ground from the C6 is connected to terminal P3 on the motor controller board. This ground must be isolated (or "floated") from the AC ground which is connected to the lathe chassis, the mains ground and the computer port ground because the P3 terminal is connected directly to the mains through the bridge rectifier on the motor controller board. Therefore, the power supply for the C6 board must be isolated from all other grounds and the jumpers must be set on the C6 boards for optoisolation to keep the computer's ground isolated from the C6 ground.

Notes on Operation:

1. Never change the position of the Direction Switch (SW3) or the Manual/CNC Selection Switch (SW4) while the lathe spindle is under power and/or moving. The spindle must be stopped before changing it's direction. Abrupt direction changes can damage the motor controller board or cause mechanical damage to lathe parts or, at best, blow the fuse in the lathe.

2. The CNC software must be configured to disallow the change of state of the Direction Relay (and thereby abruptly reversing the DC voltage applied to the motor) when the spindle is under power and/or moving. To accomplish this, be certain that the software or g-code inserts a delay that is long enough to allow the spindle to come to a complete stop when direction change is desired. In Mach3, for example, there are dwell settings available for this purpose and if the spindle is running as a result of an M3 command, issuing the M4 command will first command the spindle to stop (M5), then wait for the programmed dwell time and then finally issue the M4 command. It can also be set to do the same thing when changing direction the other way from M4 to M3. This can also be achieved by programming dwells directly in G-code in a part program, but if the automatic control of spindle direction change can be implemented in the software it absolutely should be for the added protection of the equipment. 

3. The diagram was drawn with Mach3 software in mind and shows one of the computer's parallel port pins assigned to the Step output of the spindle motor set up in the software. This pin is connected to the Step input of the C6 board. The associated "Dir" (Direction) signal is not required and does not need to be assigned to any output pin of the parallel port. 

In Mach3, the spindle settings should be set as follows:

Config Menu>Ports and Pins>Motor Outputs(Tab): Spindle enabled, Dir pin number assigned to available port pin.

Config Menu>Ports and Pins>Spindle Setup(Tab): Motor Control section: check boxes "Use Spindle Motor Output" and "Step/Dir Motor".

Config Menu>Ports and Pins>Spindle Setup(Tab): Special Functions section: check boxes "Use Spindle Feedback in Sync Mode", "Closed Loop Spindle Control" and "Spindle Speed Averaging".
This assumes you have a one pulse per revolution spindle sensor input to the parallel port and assigned as such under the Input Signals tab, Index signal input configuration.

Config Menu>Ports and Pins>Spindle Setup(Tab): Relay Control section: uncheck the "Disable Spindle Relays" box and assign the Clockwise (M3) Output number, then go to Config Menu>Ports and Pins>Output Signals (Tab) and assign a port pin number to the Output number. Note that you can also assign an output function to one of the Flood/Mist commands (M7 or M8) and use it to drive Relay 1 on the C6 board to control one of these functions.

Config menu>Ports and Pins>Spindle Setup tab: in the General Parameters section (where spin-up and spin-down is set) DO NOT check the box "Immediate relay off before delay". When checked, the direction relay will switch if you give an M3 or M4 command while the spindle is still turning and hasn't had time to spin down, that is, the relay is activated or deactivated immediately at the start of the spin-down and abruptly switches the motor leads -- not good if the spindle is still turning! With the boxed unchecked, the spin-down occurs first, then the relay switches, then the spindle starts up in the opposite direction and it is all very smooth. Just be sure that the spindle along with whatever mass of the part it is holding has time to come to a stop with the spin-down delay settings for both directions.

Refer to the CNC4PC C6 Manual and the Mach3 Turn Manual (both available online from the CNC4PC and Mach3 websites) for additional information on these and other settings.