- During turning the bed is protected to avoid chips coming between the bed and the carriage
- On the small lathe the carriage is pulled down on the bed (no play) by 2 preloaded ball bearings mounted on a 3 mm steel bar that functions as a spring. On the large lathe, the weight of the carriage itself is large enough to keep the carriage on the bed (no play) during turning.
- During CNC turning the top slide gib is tightened to avoid play.
- The final pass is always 0.02 mm when using insert tools or 0.01 mm when using HSS tools.
I do CNC turning the same way I do conventional turning. That means backlash is eliminated during the turning pass by always moving to tool towards the workpiece. That way the tool determines the cutting depth, not the workpiece.
For example turning a outside taper, the thicker part is at the chuck side (Z-).
Wrong procedure:
Start at the thinner side (end of the bar) and turn in the direction of the spindle (Z-). To position the tool (cutting depth) you move in the direction of X-. During turning de tool is gradually moved in the direction of X+. The change in direction causes backlash. The workpiece has to push the tool in position. The results are depending on the backlash, the drag of the cross slide and the material you are turning etc.
Right procedure:
Start at the thicker side (spindle) and turn in the direction of the end of the bar (Z+). To position the tool (cutting depth) you move in the direction of X-. During turning de tool is gradually moved in the direction of X-. There is no change in direction so there is no backlash. The tool determines the cutting depth.
If the lathe was equipped with backlash free ball crews and driven directly by the stepper, then there would be "no" backlash and it wouldn't matter how you turned the taper. However, if you have trapezoid spindles and geared steppers like me, you have to CNC turn the "conventional way" to get good results.