Our original milling machine has undergone several modifications since we purchased it.
First was a new longer bed and then a diy CNC conversion with the control box being relocated, and in December 2009 we purchased a belt drive kit to make it quieter to use. This was followed by a RPM monitor a couple of weeks ago using a optical sensor.
The one thing the mill has always needed was to be made more precise for CNC milling and the original lead screws had too much play in the threads to enable us to work with small tolerances. We had planned to get ballscrews and ballnuts when we first did the CNC upgrade but they are very expensive to buy and had difficultly obtaining the sizes we needed.
Last week a pair of 8mm shaft ballscrews with 18mm ballnuts appeared on ebay for under £100 so we purchased them to upgrade the X and Y axis on the mill. We will upgrade the Z axis later but we are looking into alternatives for the current cast iron column and runners.
Under the X and Y axis beds is approximately 20mm of clearance to fit the new ballnuts so there was little room for error.
We replaced the old steel nut mounts with new aluminium versions which where tapped to 15mm for the ball nuts.
The ends of the ballscrews was turned down to 6mm diameter using the lathe to fit new brass ends to fit against the thrust bearings.
Top ballnut mount fitted to the bed on the Y axis.
Testing the ballnut to ensure it was aligned correctly along the axis.
Once the ballnut mounting block was checked, we partially assembled the X axis to measure the length of the ballscrew with the thrust bearings and motor coupling fitted. The other end of the ballscrew was then trimmed and turned to 4mm diameter and a small bearing fitted to steady the shaft at the far end.
Underside view of the X axis showing the ball nut and stepper motor mountings.
New thrust bearings ordered from Simply Bearings in the UK, a pair of 19mm outer, 6mm inner thrust bearings and a single 8x3x4 bearing for support. The brass part in the lower left of the photo fits on the turned end of the ballscrew to fit better against the thrust bearing face. The motor coupling fits against the other side of the thrust bearings inside the motor mount to stop and left-right movement.
Thrust bearings and motor couplings.
Brass end to motor mount block under X axis.
The lower ballnut mount was made to replace the older steel nut and this was approx 4mm higher than the top mount.
Y axis ballnut and screw fitted to the mill base.
The reassembled mill.
Initial tests show that he mill can now move at over 3 times the previous speed using normal lead screws when cutting.
The mill was purchased from Axmister tools and new materials from ebay and other metal suppliers.