From The Workshop: Mini Lathe Series Part III - Powerfeed

Lathe Powerfeed

I bought the 17" model, so getting the cross slide from one end of the lathe to the other is kind of a pain. I had read about a powerfeed for the sherline lathes, but Sherline discontinued the unit years ago. Probaly because it was a BBQ rotisserie motor that would overheat with any load, or at least that's what I read online. Youtube has a few DIY powerfeed designs, but none of them felt like full featured products, with the polish one would expect from a production unit. Especially important was a leadscrew disconnect so the handwheel could be turned manually without having to disassemble the system.

Mechanicals

Sherline has a floating shaft + lever/cam system system, but it has some compatibility issues with DRO models and I wanted to be able to fabricate it myself. I thought about different ways to implement the same floating shaft design, but then I realized I could move the disconnector into the body of the powerfeed and make a self contained unit. I went through a few different iterations with spring loaded cams and transfer bars before settling on the exceedingly simple pin-in-a-slot control lever. As a bonus, the slot would act as the stops so I wouldn't need a additional internal stop system.

To make the disconnector move linearly, I opted for a miniature linear bearing system that I would probably not use again. The one I bought uses super tiny M2 or M1.5 screws and all the metal pieces are hardened, so good luck trying to drill and tap any of them up to M3, which was what I had planned on using. As a side project, I had just disassembled about a dozen old hard drives, and I got lucky that some of the screws I took out were the right diameter and thread pitch. If I were to do it again, I would search out a different linear bearing system or try to fabricate my own from HDPE stock.

The drive shaft is 3/8" with a bored hole on one end for the motor shaft (with a setscrew) and an engagement surface on the other. The actual leadscrew engagement flat is only 6-8mm long, so the dissconnector assembly only needs to move 10mm at most to be able to engage and disengage from the leadscrew.

This video shows how the Sherline system works:

It took some planning to get everything to fit inside the project enclosure I wanted, but I managed in the end. I didn't have good dimensioned drawings for the components, so I resorted to physically fitting the parts together and guesstimating where the holes would need to go. Frankly, it could have gone a lot worse and I'm kind of shocked it went as well as it did.

Electronics

Speaking of shocks, I managed to short 12V across myself because the barrel plugs I used are not ground isolated. I'll get to that later, but just remember that anytime you use panel mount components.


To drive the leadscrew, I opted for a 100RPM geared 12V BDC motor hooked up to a DPDT switch in an H-bridge topology, driven by a 12V PWM motor controller. In between, I wired another barrel plug in the Z-in direction for a limit switch to prevent a moment of inattention from running the crossslide into the chuck at warp factor 9. This was the barrel plug that I shocked myself on (and shorted out the PWM driver). Luckily, the PWM driver was okay and I used a nylon washer and some electrical tape to isolate the plug.


I closed everything up and tested it out, and it worked like a champ. Apart from the inescapable fact that it was waaaayyyyy toooooooo slow. I think 200 RPM is the minimum I would go with for the reverse drive speed to make it any faster than using the handwheel.

 If you want to make one, you'll need to figure out the dimensions that will fit your mounting system. If you run the threading attachment, you will need to consider how you plan on interfacing the drive shaft with the half shaft system.

I actually considered making these to sell, but after the first one, I decided I wasn't interested and couldn't charge enough to make it worthwhile. I still need to make the limit switch and swap out the 100RPM motor for the 200RPM motor I have sitting in a box right next to me.

Good luck.

Parts List

• Offset Geared Motor (https://www.amazon.com/gp/product/B072R5QSRG/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1)
• Motor Mount (https://www.amazon.com/gp/product/B07Z9ZB5NC/ref=ppx_yo_dt_b_asin_title_o07_s02?ie=UTF8&psc=1)
•  Motor Controller (https://www.amazon.com/gp/product/B07WGT8K82/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1)
• Linear Bearing (https://www.amazon.com/gp/product/B07KC2W962/ref=ppx_yo_dt_b_asin_title_o07_s03?ie=UTF8&psc=1)
• DPDT Latching Toggle Switch (https://www.amazon.com/gp/product/B00004WLKA/ref=ppx_yo_dt_b_asin_title_o07_s01?ie=UTF8&psc=1)
• Enclosure (https://www.amazon.com/gp/product/B077YWTSV6/ref=ppx_yo_dt_b_asin_title_o07_s02?ie=UTF8&psc=1)
• DC Barrel Plugs (https://www.amazon.com/gp/product/B07WYXVX37/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1)
• 12V Power Supply (stolen from one of my external USB hard drives)
• Riser (ABS sheet stock I had lying around, use anything you can machine to size)
• 3/8" Brass shaft