Tuesday, December 11, 2012

Y-axis servo arrives! Damaged!

Mid-August 2012:

Did some digging on eBay to find myself a servo that is comparable to what is already installed on the mill, my primary concerns being torque and voltage. Like the Z and X servos I intend to drive the ballscrew directly (as in, no mechanical reduction). Be aware that if you go servo shopping on eBay there is an OVERWHELMING number and variety of servos on there. My first inclination was to find something approximately the same size as what is already on the mill, but I have since been told (warned?) that improvements in servo tech over the last 33 years since this mill was made have reduced the size and increased the torque of many of these motors, even in the same "style" as what I have on my mill (2- and 4- brush, brushed permanent magnet DC).

So I found this gem for $149 + $13 SH, says it will work in my voltage range (anticipating 70-80 volts, since the power supply is still not assembled yet), has encoder, 1500 oz-in of peak torque and I'm sold. Buy now, enter paypal password, eagerly await tracking number.

Then it shows up :(


As you can see, the end of the pin that holds the encoder wheel on is smashed from poor packaging, the pin itself is bent and encoder pins are all bent up. Another view:


I grumbled to the seller and got $25 back on it, hoping to repair the shaft and straighten the encoder pins and see what happens. I didn't take any photos of the shaft repair, but the encoder drive pin is actually just a piece of 3/16" round solid that was (poorly) machined down to 1/8" to fit the wheel center collet. I expected it to be pressed into the motor, but it was only glued in with some cheap epoxy. I applied some heat to the end of the pin and was able to pull it out with a pair of vice grips to take measurements and make a new one on the lathe, glue in place and reattach the encoder. Minor setback (hopefully) averted. We'll find out for sure when we attach a drive to that encoder.

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