Micro lathe Zero backlash* conversion

[Jonathan]

I recently found a Clarke CL250M micro lathe for a good price, so I got it to convert to CNC. This lathe is very similar to the C1 micro lathe from Arc Euro.

Parts bought:

• Lathe: http://www.machinemart.co.uk/shop/pr...-lathe%20metal
• 4 E8 Magneto (angular contact) bearings
• RM1204-600mm ballscrew
• RM1204-300mm ballscrew
• 4-RM1204 ballnuts
• Belville washers to preload nuts for zero backlash and to tension Z-ballscrew and preload X angular contact bearings.
• Two 12T and two 30T HTD pulleys and belts for ballscrews, 36T,18T and 12T for 1:2 and 1:3 ratio for spindle.
• Already had 6.5kW brushless motor, seemed a crime not to use it: http://www.hobbyking.com/hobbyking/s...eq_70_55_.html

Unfortunately I didn't take many pictures to start with, so this is going to be more like a 'look what I've made' in parts than a build.

Not a chance of getting an RM1204 ballnut in the cross slide, let alone two, so I've mounted them behind. To do this I milled a block of steel to extend the saddle for the ballnuts' mount and drilled it:



Next I turned some 3.5" aluminium bar to accommodate a two ballnuts, one in each end:



One ballnut fixed in this side:



The other goes in this side. On 'top' of that goes a steel spacer, then and end cap to accommodate a pair of beleville washers. This cap is bolted through the ballnut to apply a preload of about 300N and rotationally constrain the ballnut:



Cut the aluminium motor mount on my CNC router. Just a few 5/8" thick aluminium plates with a cavity between to accommodate the pulleys so that swarf cannot get in the belt drive or bearings. I used a 12T pulley on the stepper motor and 30T on the ballscrew for 0.008mm resolution with a 200step/revolution stepper motor. If you count using microstepping it's a convenient 1um resolution. Once assembled I put a DTI on it and you can see the needle move for every microstep...so reasonable to assume somewhere between 8um and 1um, either way plenty good enough for this sort of machine. The angular contact bearings are held in a separate part and clamped with a 5mm steel plate also cut on my router. I machined the ballscrew with an M8 thread on the end used to apply preload to the belville washers / magneto bearings.

Z-assembled, but only with one ballnut:



After much deliberation I decided I couldn't get an RM1204 ballnut to fit under the lathe bed without milling some of the bed and raising it. I just milled a couple of mm off the webs in the bed to get the ballnuts to clear, so that shouldn't weaken it to much. Forgot to take a picture of that.

Next I milled the middle protrusion off the bed stand:



Then made some 10mm spacers from ground flat stock to go between the bed stand.
I cut the ballnut mount from 2.5" aluminium bar. Bored, drilled and tapped both ends to accommodate the ballnuts in a similar fashion to the X ballnut. Then some milling and finishing with flycutter. Next I machined the ballscrew and made a quick tube to fit over the end and transferred it into the ballnut. This enables fitting the ballnut without all the balls dropping out...



Belville washer pressure piece and one screw machined to closely fit the ballnut on the cylindrical portion. This constrains the ballnut rotationally.



Sliding the ballnut on:



Assembled and preloaded:



Assembled with Z-motor mount, not shown:



The 6.5kW (allegedly) brushless motor I had was the same diameter of the somewhat limited 150W DC motor this lathe came with, so it bolted straight in with the mount it came with. I just had to make a shorter shaft (so the original covers fit) and machine the pulleys. Although this motor initially seems excessive, it's arguably well matched since it has plenty of torque over the full speed range, I probably wont need to use the 1:3 ratio.



I've not had chance to do anything with the lathe for a couple of weeks, so I've left it like this:



I will soon add plates to the Z-motor mount to seal the belt, similar to Z.

Now I can cut hemispheres!


Enough for now... more to follow shortly.

*Well near enough zero backlash..