Controller for mini-lathe motors using a microswitch
by Mike Cox
A microswitch can be used to turn off the leadscrew when the carriage reaches a certain position. On my lathe I have small dc motors driving the cross-slide and the leadscrew. These are very useful for facing and turning to a shoulder. The microswitch can also be used to turn off the main lathe motor. This is very useful for screw-cutting.
The circuit below is the controller for the auxiliary motors on the crosslide and leadscrew and it also controls the main motor. The function is selected by a 4 pole 3 way rotary switch. Position 1 selects the cross-slide, position 2 selects the leadscrew auxiliary motor and position 3 selects the main lathe motor.
The component marked PWM is a commercially available motor speed regulator designed for controlling small dc motors. It is a small panel mounted unit. (see www.mfacomodrills.com). To the right is Sw1. This is a reversing switch (double pole, double throw centre off switch) that alters the polarity of the output from the PWM and thus alters the direction of the motor. M1 is the leadscrew motor. This has a diode in series with it so that when the limit switch is open then the motor will only run in reverse. When the limit switch is closed it can run in either direction. This is so that when the limit switch is triggered open it is still possible to reverse the motor in order to back the carriage away from the limit switch. The motor M2 is the cross-slide motor and this has no limit switch and will run in either direction.
Everything to the left of the PWM is concerned with controlling the main lathe motor. It is a simple timer circuit based on the charging of the capacitor C1. When the limit switch opens the base of Tr2 is immediately taken high and the transistor conducts sending a pulse to the relay2 via the 470 uF capacitor in the collector circuit. This pulse lasts for about 1 second. C1 slowly charges and eventually the voltage is sufficient to turn on Tr2 and this activates relay 1. The time delay between the limit switch opening and relay 1 activating is 2-3 seconds. To summarise when the limit switch opens relay 2 immediately opens for about 1 second and then closes, and the relay 1 opens after 3 seconds. These timings are to some extent dependent on the relay characteristics so it is best to breadboard this part of the circuit first, before construction, to check the timings. Increasing C1 will increase the time delay before relay 1 opens and increasing C2 will increase the pulse period of relay 2. The transistors should high gain components (hfe min 100). I used 2N 3709 transistors.
The relays have a 12 volt coil and a resistance of 300 ohms.
The diagram below shows how the relays are connected into the main motor circuit.
The auxiliary controller is the circuit above and the solid line indicate that it is controlling the relay contacts.
When the limit switch opens, relay 2 briefly opens for about 1 second. This triggers the no volt relays in the main lathe controller and the ac power remains disconnected. One or two seconds later relay 1 opens and the diode is now in circuit allowing current to flow in only one direction, the direction that moves the carriage away from the limit switch. The reason for this complex switching arrangement is because small relays do not like switch high voltage, high current dc. This arrangement removes power from the ac and dc circuit before relay 1 activates ie no current is flowing in the dc circuit when the switching takes place.
In operation for screw cutting the procedure is as follows:
1. Make the first pass at cutting the thread. When the carriage hits the limit switch the lathe will turn off.
2. Set the forward reverse switch to reverse and turn the lathe speed control to zero.
3. Increase the lathe speed and the carriage will back away from the limit switch.
4. When the tool is clear of the thread turn off the lathe and change to forward direction and make the second pass.
5. Repeat the above sequence of events until the thread us cut.
6. Note that in step 2, if the lathe is not put in reverse then the tool will not move due to the blocking action of the diode. This is a safety feature that prevents the carriage ramming into the microswitch and damaging it, if the switch is in the wrong position.
Stripboard circuit layout - x indicates a break in the copper strip
Warning: Installation of this controller requires connections to the mains power supply and to the high voltage dc line to the motor. The voltages involved are potentially lethal. Every care should be taken during construction and installation to ensure safe working practices are followed.