Monday, March 20, 2017

Lathe: QCTP Rotation

I have a 0XA QCTP installed on my mini lathe. Instead of using the included nut to bolt it to my carriage I used the handle off the stock tool post. This allows me to quickly rotate the QCTP into a new position, an option which I use a lot to either provide clearance or give the tool bit a better angle.

Tool post in rotated position.

Saturday, March 18, 2017

Lathe: Gear Reduction Pulleys

I realized I never used the high speed on my mini lathe, and I really needed a lower speed on there than what I had for parting and knurling. In addition, the mini lathe belt is a proprietary part which I'm not a fan of.

There are a lot of sellers on eBay selling ratio reduction pulleys for the mini lathe. They'll drop the low speed to 700 RPM and high speed to 1800 RPM with an increase in torque and lower low speed.

I bought the cheapest one I could find expecting it to need modification to be usable, and it did. I needed to turn the large pulley down to 10mm wide to fit. I then needed to cut a keyway in the motor pulley. However, once I was done they fit nicely and use a standard xl (163xl037) belt which is easily available on Amazon.

Pulleys and belt in place.

In low speed it's now usable down to about 150 RPM which is a really nice change from before, and should really help parting and knurling. I may eventually experiment with the ratio and go up a tooth on the motor.

Wednesday, March 8, 2017

PWM Control Notes

Peak Voltage:
The KB PWM control I use produces a peak voltage of 160VDC which is very rapidly switched on and off to produce a lower average voltage. I was worried about the 110VDC motor being hit with 160VDC, but after research I found if the frequency is high enough, then it doesn't really matter (assuming the peak voltage isn't absurdly high).
"As long as the PWM frequency is fast enough, it's average voltage is what counts. No, the average PWM voltage should not exceed the motor's rated voltage, at least not for long. This is no different that applying a DC voltage to the motor.

Using a high voltage supply and then less than 100% PWM to compensate is a perfectly legitimate way to run a motor, again, as long as the PWM frequency is fast enough. In effect you are creating a switching power supply that converts the high voltage to the lower one used to drive the motor. It may not look that way because the induction of the motor windings are a integral part of this power supply.

- Olin Lathrop

The things which kill a motor are overloaded bearings, bearing run too fast, brush arcing, and damage from overheating. None of those is really a result of voltage, and a high frequency PWM drive will actually allow the motor to run cooler while producing more torque.

Adjusting Max Voltage:
The high frequency a PWM motor control outputs to the motor can leave a digital multimeter confused or inaccurate if you're trying to measure the max volts, while I've found using an analog multimeter will respond accurately to the PWM output.

If you have a digital multimeter which can read duty cycle and you know the peak voltage (160VDC in case of the KBWS using 115VAC), you can multiply the duty cycle by the peak voltage to get the average voltage, which is what the motor should see. 

To set the max volts to 110VDC for the mini lathe and mini mill's motors I first used a analog multimeter to set the max output to 110VDC. Then to double check I calculated the motor RPMs from the spindle RPMs since I knew the gear ratio; the lathe has a 5000 RPM motor while the mill has a 6000 RPM motor.