A well-preserved EXE miniature surface grinder with an interesting set of modifications to the control system. The owner writes: The large box on the front controls the motion with the help of the main drive inverter (out of view underneath the back of the bench). First there's a limit switch on the infeed, if this trips it disables the inverter which shuts everything else down. The box contains 3 relays - one to control the traverse forwards and backwards based on the limit switches on top the intermediate carriage. These are tripped by the sliding stops (not original as this function was missing). The second relay supply the power for the motor - either 12v for slow speed or 24v for high (neither are fast by commercial standards, but intended to emulate what you might achieve by hand, but without the pain!) - via the motion control relay. The final relay turns on 240v output for a
powered rotary table for grinding round things - piston rings perhaps. The switches as follows: first controls inverter enable via the rear limit (one NO contact). Second selects low volt relay or high volt relay (one NC and one NO contact); both need the inverter internal relay to be on (programme only to turn on when the motor is up to full speed). The third switch selects high or low voltage output to the motor (two NC and two NO contacts). The fourth is a three position switch selecting clockwise or counter-clockwise table rotation as well as a centre off (six NO contacts). The box also contains a centre tapped toroidal transformer which feeds (via 3A fuses) to a bridge rectifier for the 24v and a full wave rectifier for the 12v. The transformer is permanently on (subject to the mains supply in common with the inverter) to supply voltage to the relays which have 24v coils.
Beyond the electronics you'll notice the rather spindly rods around the limit switches. These actuate a ratchet attached to the infeed. Slight variation is possible by moving the position of the pivot on the limit switch arm. Fully out gives 10 thou (slightly variable!), right in cuts this to 5 thou. The ratchet wheel (tried to take a photo, but there wasn't enough room to get the camera to focus on a decent shot) was made on the machine itself with a wheel dressed at an angle and a simple spin index. The ratchet has check pieces to stop the pawl falling off. The gears are home made (20 DP). The motor gear is steel, the idler (needed to enable disengagement) is brass and the final on is Tufnol mounted on an aluminium boss.
This is a Mk. 2 version - the Mk. 1 used an old car wiper motor and the idler provided a second stage of reduction. It was too slow and the motor packed up (not designed to run backwards!). This prompted the move to 24v and the Parvalux motor. I decided to incorporate the rotary capability into the same controls, having previously used the table independently, at the time of this upgrade.
Only very minor modifications were made to the machine to fit this assembly.
I'm not sure much more 'automation' is possible without resorting to a PLC and steppers, though this would give more infeed flexibility and open the possibility of vertical control. I doubt I'll ever bother!