Hi Tin,
Thanks for your reply, I eventually got the courage to take off the bearing blocks of the X and Y axis (cast iron I think) and bored them on my Taig lathe to accept two sets of thrust bearings from either end, these are some 16mm OD, 8mm id and 5mm thick which are loose, the ball race is sandwiched between two steel washers with locating grooves they are supposed to be for RC helicopters. I can not detect any significant improvements in smoothness but the back lash has dramatically improved for both axis, about 0.05mm or 2 thou. I have decided to use direct motor coupling hence the use of two sets of thrust bearings rather than the thrust radial bearings which I think would have been required had I chosen to use a toothed pulley and belt arrangement these bearings are not so easy to source here in the UK or at least I have not been able to find them. I hope I have not miscalculated.
For the Z axis I had to take the lead screw off and machine off approximately 10 mm off the threads on my Sieg C1 ( at last something usefull ) to make room for the two sets of the bearings as the Sieg's Z axis "bearing block " does not have much meat on it to bore. The bearings are sitting on the top and bottom of this block. This has had the most dramatic effect on the smoothness and reduction of the back lash on the Z axis. I have not however, measured the back lash yet and I really wonder why Sieg does not fit the bearings as standard as they do make a lot of difference.
Now I have to start thinking about machining the motor mounting blocks for all 3 axis, the X and Y are somewhat straight forward but again the Z axis is different.
Will post once I have got everything sorted out . Hope it all works.
Thanks for the reply again.