Backlash Compensation

Backlash Compensation

Each of the mill's three axes is moved linearly by a rotating lead screw. The lead screws are driven by stepper motors to which they are connected by toothed belts and pulleys.

Whenever one of the three lead screws changes its direction of rotation, the slideway it drives should begin to move immediately. This is the ideal situation. In reality this does not happen. There is a small amount of slack in the bearings, the lead screw and other components which has to be taken up first. The amount of slack should be very small, just a few 1/72 of a millimetre, the equivalent of a few steps of rotation of a stepper motor.

Because these steps have been made, the computer running WELmill will record a mill slideway as having moved when, in fact it has not. The slack has just been taken up. The result will be that the machined component will be slightly dimensionally inaccurate: see the link to "Accuracy of Machining" below.

WELmill is able to allow for backlash so that components are dimensionally accurate. In Options - Settings - Machining there is a dialogue in which X, Y and Z backlash compensation figures can be entered. The figures are the number of stepper motor steps required to take up the backlash. Two methods of determining these figures are described below.

But…………….

Before taking measurements and making any changes to the three default zero values, do the backlash tests described in Accuracy of Machining to get a clear idea of how the machine actually performs.

After entering backlash compensation values, run the tests again to check the performance of the mill. If the test piece is oversize, too much compensation has been applied.

Dial Test Indicator Method
1) Go into "Use the Keyboard to Move the Tool" mode.
2) Mount a metric dial test indicator such that it can measure movement in the X direction (left - right movement of the mill table).
3) Move the mill table so that the dial test indicator plunger is depressed. Remember the direction of movement.
4) Zero the dial test indicator.
5) Zero the WELmill measurement display on the PC screen.
6) Move the mill table in the same direction as in 3 above until the WELmill measurement display reads 1mm. Take great care not to overshoot 1mm. If you do, you must begin again. You should find that the dial test indicator needle has registered 1mm of movement.
7) Now move the mill table in the opposite direction until the WELmill measurement display reads zero. Take great care not to overshoot 1mm. If you do, you must begin again.
8) You will find that the dial test indicator has not quite returned to zero. This indicates the backlash distance.
9) While counting the steps, single step the mill table in the same direction as in 7 above until the dial test indicator does read zero. It will probably be only one or two steps. Make a note of the number of steps.
10) The number you noted is the number you enter in the X field of the backlash compensation dialogue.

Repeat this process for the other two directions of movement.

Opti-Set Method
As an alternative to a dial test indicator, the Opti-Set device, suggested for measuring tool offsets (see the link below to Measuring Offsets), can be use to measure backlash. This is the method:

Z Axis Backlash
1) Place the Opti-Set on top of the slightly opened vice jaws. With no tool in the milling head, bring the cleaned rim of the spindle down on to the Opti-Set plunger, a step at a time, until the green LED goes out and the red LED turns on;
2) While counting the number of steps, raise the milling head, a step at a time, until the red LED goes out and the green LED turns on again;
3) Counting the steps again, lower the milling head again until the red LED turns on again. This should require the same number of steps as in 2);
4) Enter the number of steps in the Z field of the backlash compensation dialogue.

Y Axis Backlash
1) Hold the Opti-Set against the cleaned back of the machine vice or mill table. Align the Opti-Set plunger with the Z slideway. Use Y axis movement to bring the plunger just into contact with the Z slideway;
2) Continue the Y axis movement, a step at a time, until the green LED goes out and the red LED turns on;
3) While counting the steps, reverse the Y axis movement until the green LED turns on again;
4) Counting the steps again, reverse the Y axis movement until the red LED turns on again. The count should be the same as in 3);
4) Enter the number of steps in the Y field of the backlash compensation dialogue.

X Axis Backlash
1) Hold the Opti-Set against the cleaned cover of the X axis stepper motor (RH end of the mill table). Use X axis movement to bring the plunger just into contact with the mill end casing;
2) Continue the X axis movement, a step at a time, until the green LED goes out and the red LED turns on;
3) While counting the steps, reverse the X axis movement until the green LED turns on again;
4) Counting the steps again, reverse the X axis movement until the red LED turns on again. The count should be the same as in 3);
4) Enter the number of steps in the X field of the backlash compensation dialogue.

Accuracy
You are recommended to look at the Accuracy of Machining entry in this Help file: see below.

Accuracy of Machining
Measuring Offsets
What moves - the milling head or the table?