Refurbishing a Woodturning Lathe

Published 22 November, 2016
Woodturning Lathe

It is possible to buy old and cheap woodturning lathes. You can just clean and brush them, but it‘s not that hard to give it some new features: Emergency stop, frequency control with ramps, start/stop push button and main switch.

Of course it starts with the cleaning. Some parts of the metal had to be welded because they were too rusted. The drills for the switches and mountings were made.

After this, the lower part, not the upper (it was well enough), was painted with a hammered finish.

First, the machine was separated in all parts to see, what is still usable. All cords and switches had insulation issues and were thrown in the waste (not the one at home!). The motor is the only electric component that remains.

As written in the beginning, I cleaned all the metal parts. The ones which should get a new finish were cleaned with knotted brushes for the angle grinder, the others with a damp cloth and then with a little bit oil to protect against rust (there are unpainted parts). The bottom part was welded (some stands were broken in cause of rust) and prepared for the switches, wires and the frequency converter. The two interlocks of the case doors were crimped on the back of the sheet metals and had to be opened carefully with the bench vice, a chisel and pliers.

Motor Type Plate

Time to make a plan for the electronics: The belt drive is an uncomfortable speed setting. I got a used and therefore cheap 750W frequency converter (the power must be high enough for the motor) and my benefit is, that I only need a normal power plug, not a tree-phase one. With this part it is possible to set a variable speed with a potentiometer, add a soft start and a break function.

I wanted to have an emergency stop. I preferred an emergency stop, not an emergency shutdown (switch off) because the motor is still running for a while after it is disconnected from the power. At best it should break when you push the emergency stop button. To have a little bit redundancy, the machine should stop, even if there is a defect in the control circuit. A contactor is my additional safety component and the service to switch off the whole device.

A lockable mains switch has only a design purpose (of course, it has got full functionality), because the machine itself has got a normal AC power plug and the switch is therefore not really needed.

In the end, I need drills for two push buttons, the emergency stop, a mains switch, a potentiometer, and mountings for the converter, a top-hat rail and a strain relief.

The painted parts were painted with hammer effect enamel. Sadly, it was not completely in the same color as the other parts.

If you work with a frequency converter you have to take a lot of care about EMI. In the manual should be instructions about the installation, the wiring (you have to use shielded wires to the motor) and if it is not included you have to add an EMI filter.

The converter has got an intermediate circuit where the rectified current is stored in capacitors. If the motor gets the break command, it charges the intermediate circuit too, because it works as a generator. If you switch of the main power now, the main contactor switches of und set the converter to the break mode. While breaking, the motor charges the intermediate circuit and keeps the control unit alive, even though the power is switched off. For this reason, the frequency converter is able to break down the motor. That’s a possible and redundant way to make an emergency stop.

Circuit Diagram

The switches have built in LEDs, so I just connect the green one behind the mains switch and the red one behind the contactor.

The three pins of the potentiometer are connected to the frequency converter too. You normally can adjust the “gain” of it in the software configurations.

Now you have to set all the parameters of the converter. Maximum current, start and break ramps, minimum and maximum speed and so on. If the speed range is big enough, you never have to use the belt drive again. For me it worked best just to go through all parameters from the first to the last one.

At last all parts were screwed together and the right setting of the belt drive is tried out (slow enough at the lowest speed and fast enough at the highest one). I added a piece of perforated plate around the belt drive for additional safety – it was possible to get with your fingers inside it.

The woodturning lathe is working well and looks quite nice. The handling is great, a slower start and a quicker break should give a little bit of extra safety and comfort.