Adding an ammeter
Ian Foster, an active member of the Yahoo 7 x 12 minilathe group, has fitted an ammeter in series with the main lathe motor lead to measure the motor current. The current is directly related to the power being consumed by the motor. This then gives an indication of how much the motor is being loaded. I was rather sceptical about the benefits of adding an ammeter when I started this modification but driven solely by Ian's enthusiasm for the modification I decided to do it. I am a complete convert. Next to what is happening at the tool tip the ammeter is the most looked at part of the lathe during turning. Prior to fitting the ammeter I had several times blown the fuse on the lathe controller due to overloading motor. Since fitting the ammeter I have never blown a fuse. It is especially useful when turning large diameter workpieces since it is so easy to have an overload situation.
This modification requires making connections to the high voltage dc output from the controller. The voltages from the controller are potentially lethal. It is important, therefore, that the wiring and connections are made according to best practice, all cable entry points are made through cable glands and that proper attention is given to earthing where appropriate. If you are not confident and competent in these matters than do not do this modification yourself.
This modification requires making connections to the high voltage dc output from the controller. The voltages from the controller are potentially lethal. It is important, therefore, that the wiring and connections are made according to best practice, all cable entry points are made through cable glands and that proper attention is given to earthing where appropriate. If you are not confident and competent in these matters than do not do this modification yourself.
The drawing above shows the wiring diagram of the lathe motor and the controller. The ammeter should be connected before the reversing switch so that the current through the ammeter is always in the same direction irrespective of the motor direction. The connection is easily done by removing the connector from terminal 1 on the control board and inserting the ammeter between terminal 1 and the removed connector. The insertion point is marked with a cross on the above diagram.
The header photo shows the ammeter mounted on the lathe. The ammeter reads 0-5 amps. In retrospect a 0-3 amp meter would have been better. The front face of the meter is approximately 60 x 40 mm and it is mounted in a small ABS box with dimensions 79(L) x 61(W) x 40(H) mm. The meter and box were purchased from ESR Electronic components (www.esr.co.uk). The order code for the 0-5A meter is 124-133 and the box 400-550.
This photo shows the back of the box and the cable entry via a cable gland. Note that the meter is mounted in the body of the box with the lid at the back. The lid is secured to the box by 4 screws that screw into insulated pillars in the corners of the box. The lid screws pass through the metal bracket that attaches the box to the headstock, then through the plastic lid and into the box. By mounting in this way there is no grounded metal work exposed inside the box.
This picture shows the cable entry to the lathe controller box via a cable gland. This photo was taken before fitting the leadscrew autostop. The cable entry point has since been moved so that it is in the bottom of the controller box rather than out the side as shown here.
My lathe is rated 300 watts at 240 volts giving a supply side current of 1.25 amps at the rated load. In use I normally try to adjust the depth of cut and feed rate to ensure that the measured current does not exceed 1 amp during normal continuous turning or drilling. On intermittent turning brief peaks of up to 2 amps are permissible and the lathe seems quite happy. Moving from a high gear to a low gear produces a lowering of the motor current as would be expected. During screw cutting the ammeter provides a visual indication of the increasing load on the motor as the depth of the thread increases and this can be used to guide the depth of cut increase at each pass.