CNC : Eliminating Following Error by Identifying Backlash

CNC Machine Maintenance and the Backlash Phenomen

It doesn’t matter if router, plasma, laser, 3D printer or pick and place, CNC machines have become an indispensable part of any successful manufacturing process. Besides production increase and diversification, CNC machines enable more in-house operations that were previously outsourced. Since they are operated by computers, using actuators, drives and software programs, CNC machine maintenance checks are of critical importance to ensure that the equipment is prepared to run as it should and to eliminate the possibility of errors and breakdowns.

Following error, also called axis-positioning error appears occasionally on CNC (Computerized Numerical Control) software as Num, Ge FanucHeidenhain or others. In the long run machine’s mechanical system get timeworn. The computer doesn’t register the delay of the mechanics and sends information to the drive much faster that the drive can not send it to the axis. The following error is the difference between the commanded position calculated by the numerical control NC and the actual position of the axis sent by the encoder and the incremental measuring.

If the commanded position is in the range of the actual position, the axis may eventually eliminate the error. If the error is out of range and exceeds the maximum, an error message will occur and the servo might fault.

There are two major reasons for the occurrence of a following error in a CNC machine:

  • Difficulty when reading an incremental measurement ruler.
  • The identification of the source of a backlash in the axis drive system, which will be discussed below.

 

The Backlash Phenomenon

Backlash is defined as any loss of motion during axis reversals. For instance, if the X axis is commanded to move in the positive and immediately after that in the negative direction, a certain indicator for existing backlash is the retarded and not exact movement of the X axis.

Backlash is crucial to accuracy and therefore checking it regularly should be compulsory in CNC machine preventive maintenance.

 

How to Identify the Source of the Backlash?

 Does the problem on the axis come from the belt or from somewhere else?

The possible reasons could be V-belt backlash, bearing backlash on the axis motor, nut or ballscrew backlash. It is quite unlikely that the problem comes from the pulleys, because given the width of the belts; it would require a backlash greater than 2 mm. However, to eliminate any doubts, the pulleys can be checked.
 

Priority Checkpoints

  • Is the synchronous belt secured and taut on either side of the two pulleys? First possibility for lost motion is eliminated.
  • Does the axis motor have a backlash?

This requires disconnecting the belt in order to check the motor. This type of motors is a subject to strong radial pressure due to the tension coming from the belt. The radial pressure increases even more during the reversal of direction of rotation, unless the motor pulley is cantilevered. The motor is also subject to axial pressure in case of slight lack of squareness and especially at the moment of inversion in the axial direction.

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  • Controlling the mechanical backlash of the direction of rotation, called "measurement trueness."

To implement this type of simple control with a cost-efficient control device, you will need a comparator or a stylus-based contact instrument.The bias error is determined by the difference between the actual movement of the studied point and the displacement indicated by the CNC machine.

We will control the X-axis related to the spindle.

 

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Position the base of the comparator on the axis to be monitored, in this case the X-axis and place the comparator key on the spindle. Using the electronic handweel, check if everything is well rigid, without any lost of motion.

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Of course, you have already been attentive enough to round the position of the X-axis, as shown on the picture below.

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Manually adjust the dial gauge on Zero Point.

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Using the handwheel, move the axis with 0.1 mm in the + direction and then in the – direction, while watching the comparator, which should display matching to the numerical control NC numbers.

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The X-axis moves with 0.1 mm on the NC, which corresponds to the 0.1 mm movement of the comparator.

In conclusion: There is no axis backlash.

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