Metalworking

7x12 Mini Lathe conversion to CNC. Updated July 8, 2014.
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Mini Lathe

This is our 7x12 mini lathe purchased from Homier before CNC conversion. We don't use it much but does come in handy for making parts for bearings and turning down the ends of acme screws. The quick change tool holder by TS Engineering Inc. was purchased from Little Machine Shop when they were in town for the NAMES model engineering show. They gave us a free tool holder as a show special.

5" Chuck

Another mini lathe upgrade, a 5" 3-jaw chuck purchased from Little Machine Shop We ordered it with the adapter plate and mounting studs. It took about 5 minutes to remove the 3" and install the considerably larger sized 5" chuck.

I found this short THK KR26 linear slide on eBay for really cheap, under $40. With only about 2.5" of travel, it was too short for a gantry mill Z axis but perfect size for the mini lathe cross slide. I placed it on the cross slide to see how well it will fit. The travel of the THK slide is actually more than the original lathe slide.

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I took off the cross slide horizontal section and acme lead screw/handles and drilled 4 holes to mount the THK linear slide. I then made a aluminum mounting block for the quick change holder, which is now solidly mounted on the lathe. Previously with the orginal lathe cross slide, there was alot of play in the dovetail mount with lots of backlash in the acme screw. These THK linear slides are solidly built, very minimal backlash (.0008") and can hand handle large loads (1700lbs).

Bought a 10mm diameter THK ground ball screw complete with bearing blocks for $115 on ebay. When I received it, it looked unused and perfect condition. You can buy cheaper chinese ball screws on eBay but they can't compare with the quality from THK. Removed the carriage acme lead screw and all the unnecessary parts that attached to it. I made a L bracket stepper motor and bearing block mount out of aluminum. I used the original acme leadscew mounting block holes to attach the stepper mount.

A simple plate was made to mount the ballnut to the lathe carriage. That was all the machining needed to convert the lathe to CNC. Pretty simple......here are some more pictures of the converted lathe.

These two gecko210 stepper motor drives were also purchased from ebay for about $40 each, they are mounted on a aluminum heat sink. I then found a dead computer power supply and mounted the drivers inside. Wired up them up to two 300oz-in nema 23 stepper motors that was given to me. Stepper motor power supply is a DIY 36volt output from a transformer salvaged from a old broken desk lamp. I hook up the original power supply fan to +5volts to help cool the heat sink.

 

A short video showing the converted lathe cutting aluminum. Since being new at cnc lathe cutting, I set the roughing pass at 5ipm and the finished pass at 1ipm. The lathe does cut much faster but just playing it safe with the feed/speed for now. Backlash on both axis is very minimal, almost none, due to the high grade THK ground ball screws. Rapid speed is set at 100ipm. I had tested it at 200ipm but thought that was ridiculously fast for such a small lathe.

Mach3 Turn running on a old P4 desktop using parallel port step/direction output.

First time threading on my CNC lathe using Mach3 threading wizard. 10-24 thread. Spindle RPM speed sensor is a Hall Effect wired up to the back of the spindle.

Not to bad at all, the nut was a little tight but it does screw on. A slightly deeper pass would be better.

I bought some Sandvik TCMT 21.50 inserts from eBay. These are made of cermet. They cut aluminum and steel effortlessly. I thought my Valenite carbide insert, the one on the left, cut really well but these are so much better. I got a bunch of them for $2 each, a great deal. I made some tool holders for them.

Just playing around and made a replacement handle.

Made a replacement part for a pretty old John Deer tractor engine for a friend out of brass. He will tap the inside thread himself. It was nice to be able to do this on a short notice.

 

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