Weiss VM32 Mill Conversion

[picclock]

Hi routercnc
I visited the Warco factory on their open day, primarily to check out the WM18. The main difference is that it is gear driven and has a less powerful brushed motor. I am sure the ones I looked at had a different base casting to the one shown on their site. Having a belt drive is important to me. Its quieter and when it fails you just replace the belt. I also have a smaller WM14 mill from Warco, which has had broken gear issues. It is far noisier than the 1.5Kw belt driven VM32B despite being only 600W. I did snag a fairly decent vfd lathe while I at the show, but for me the WM18 did not meet my requirements.

Once I had tested the mill to make sure it functioned correctly I proceeded to disassembly. This started with removal of the table end cheeks and threaded x screw. The tapered gib was removed and the table removed. The original X Axis bearings were kept as I plan to reuse them. Removing the table was easy enough but it is heavy. This I slid onto a workmate. The saddle was similarly removed. Both castings were of excellent quality.

I am fitting ballscrews to all three axis, but I think I made my first mistake here. I assumed that because the leadscrew diameter is 20mm that replacing it with a similar diameter ballscrew would be the easy option. The snag is the size of the ballscrew nut which is far larger that the original - oops!.

I have read about machining ballscrew nuts with carbide mills. My advice is don't bother. I took a brand new 12mm carbide mill and attempted to make a small test cut 0.25 mm deep and 0.25 mm wide. The end result after a very short distance was a very blunt endmill . So I attacked it with an angle grinder with very good results. It is very important not to let the nut get hot or it will lose its temper, so cooling with wd40 or similar is a must, with long pauses between grinds.
To work on a ballscrew itself you need to have a lathe with a spindle hole large enough for the ballscrew, a 4 jaw chuck, a custom collet, headstock centering bush, and a nut bearing keeper. The ballscrews for my machine are all 20mm x 5mm pitch. They were bought with the ends machined (BK/BF15), so if you do this you must remember the length quoted by the sellers includes this. The threaded end is 60mm and the other is 13mm, so ballscrew thread is length quoted - 73mm for 20mm ballscrews.
As a guide the lengths needed for this machine are X=840 table length+end cheeks@30mm each (depending on what you do with the free end). Y= 450mm (inc ends), using a 10mm plate for the thrust bearings. Z = 650mm inc ends.

The collet is simply a length of aluminium tube to fit the ballscrew. This is cut to the length of the jaws in the chuck plus a 2 cm. Four slots are cut in one end to just over the length of the jaws. When fitted over the screw it can be clamped without marking the thread.
The centering bush is just a piece of aluminium bored to the screw OD and made with a shoulder. Approx 2 cms long should do. This is placed on the end of the screw protruding from the spindle and can be fixed in place with the ballscrew nut or a suitably sized oring. It jut stops the end of the screw whipping about.
The keeper is an aluminium cylinder, drilled through 6mm clear. The ends are drilled 10mm to a depth of 15mm or so. The outside diameter is 18mm.

The conventional method is to use deep groove ball races for the axial thrust at the ballscrew ends. This BK/BF system uses two races in which the outer part of the race is spaced apart and the inners compressed until they rotate with little axial movement. balls used in this way run on the edge of the grooves in the casing, which makes me unsettled. Using a thrust race for axial loads the ball is fully supported by the machined recess in the support washer, so a much larger area of contact is used.

In order to use the thrust races the diameter of the ballscrew end needs to be reduced to 12mm for the 51201 thrust races. These have a load rating of 13.3kN, which should just about do The end will be tapped M12x1mm for the clamping nut which will preload the bearings. The nuts are simply made out of 1" bar stock, drilled and tapped in the lathe, with the corners rounded to an OD of 28mm. They are 8mm thick and after parting off the corners are drilled and tapped 3mm for a locking grub screw.

Thats all for now

Best Regards

picclock.