Source of this article http://www.kentechinc.com (adapted to MAXNC machine).
What is backlash ?
The axis motion that makes up your machine tool is done through the use of ballscrews attached to your machining center's table and spindle housing or your lathes tool turret. The nut for the screw is usually attached to the table or turret and is connected to the ballscrew which is connected to your drive motor. As the motor turns the ballscrew, the nut moves the table or turret and your machine has motion. All ballscrew assemblies have some "slop" or backlash at assembly - the match between the screw and the nut. Basically backlash is the amount of motion the screw has to make when reversing direction before the nut and therefore the table or turret start to move.
How is backlash compensated?
Using the machine tools CNC controller, the builder can tell the controller how much motion is lost when the axis reverses direction due to the backlash. This value is stored in the machines parameters and when the particular axis goes to change direction, it looks in this parameter to know how much motion it needs to have (how many revolutions of the screw it needs to make) before the axis will physically start to move. The value of the parameter is usually in MM, although they may be in INCH settings in some instances.
Why should I care ?
As the machine tool wears or as contaminants get onto the ballscrew and therefore in the nut, the original backlash settings lose their accuracy and therefore effect the accuracy of the machine tool. Positioning problems arise, straightness problems arise, as do a host of other related problems. Basically, the machine does not meet the specs like it did when it was new. As mentioned above, sometimes contaminants can get onto the screw and then get carried into the nut. Although most nuts are protected against chips and debris, poor conditions can sometimes force the debris into the nut causing premature wearing of the screw and a pronounced backlash problem. Those contaminants can range from coolant to cutting chips. That is why it is essential to keep the machine areas clean and free from an excessive amount of chips. If chips are allowed to accumulate, they can become packed and when the machine tool moves, it forces the chips under guards and into areas where they shouldn't be. Eventually they get forced into the screws and nut areas causing un-repairable problems. Ballscrew replacement is not a cheap repair. Keep the expression: "An ounce of prevention is worth a pound or cure" in mind when planning your maintenance efforts.
What can I do about backlash ?
The normal method for adjusting the machine's backlash involves adjusting the backlash parameter values. This can be done by a qualified technician or you can give it a try. Outlined below is a brief but complete explanation of how to check for backlash and how to adjust it.
How often should you check it ?
Recommended time frame would be about every 3-6 months. If you create the following sample programs in your memory and leave them there or upload and download them from a shop floor PC, you shouldn't spend much more than one hour or so keeping your machine accurate and at the same time you'll be checking for any other damaging problems. For example, if you see the backlash changing drastically, you might find a way lube problems or chip build up problem before they cause bigger problems.
How much backlash compensation is acceptable ?
As mentioned above, all machines have some backlash adjustment, even when brand new and at ship time. As the machine wears, that value needs to be increased. Normal wear might have .005" - .010" adjustment in a ballscrew. If the value needs to be more than .010", it might be time to take a deeper look. Also, you need to check the backlash at various areas of the screw as it might be wearing more in one area than another. One example might be on a machining center where the set-up people always mount the vise or fixture in the middle of the table. Looks good but also causes a massive amount of wear in one confined area. the best scenario is to mount the vise or fixture all over the table, changing the location for every job - speading the wear around evenly. The best way to check the backlash is to first clear out the current parameter value in the control. The various parameter numbers for the variety of FANUC controls are listed further down in this page. First, write down the current values, then clear them by setting them to zero. Then make the machine move through the memory mode. We have found discrepancies in the past between the machine's handle or MPG mode and the memory mode, so we recommend you run the machine through MDI or through the machines memory mode. Below are a couple of sample programs that you can use to gather your backlash data (modified for MAXNC). Remember, the backlash is the amount of wasted motion when the particular axis changes direction. If possible, check the backlash at different areas of the screw. On a machining center, mount the block in different areas of the table and check. On a lathe, check the backlash as various distances away from the chuck. If the values are different in the different areas, this could mean that the screw is worn in one place different than others. On a lathe, this tends to happen close to the chuck where the majority of the cutting is performed. You can's do much about to prevent it on a lathe but on a machining center, you can help yourself by mounting the chuck or fixture in various places on the table to allow for even wear. If you find big differences in the backlash in different areas, it may be too late and you may have to replace the screw.
Machining Center Backlash Adjusting Program.
If you have a Vertical or Horizontal machining center, the following program will give you an idea of how to create a program to test the backlash for each axis. The following are sample programs. Start the program with an indicator mounted to the spindle, touching a block mounted on the table, touching the right side of the block.

               (this is for metric (millimeter) setup X-AXIS)   
   G00G91X1.00   
   X-1.00   
   M00 (STOP: make first reading)   
   G00G91Z2.00   
   X-10.00   
   X10.00   
   Z-2.00   
   M00 (STOP: get the backlash)   
   M99 (repeat the cycle: almost 5 value can be read)   
   (this is for metric (millimeter) setup Y-AXIS)   
   G00G91Y1.00   
   Y-1.00   
   M00 (STOP: make first reading)   
   G00G91Z2.00   
   Y-10.00   
   Y10.00   
   Z-2.00   
   M00 (STOP: get the backlash)   
   M99 (repeat the cycle: almost 5 value can be read)   
   (this is for metric (millimeter) setup Z-AXIS)   
   G00G91Z-1.00   
   Z1.00   
   M00 (STOP: make first reading)   
   G00G91Y2.00   
   Z10.00   
   Z-10.00   
   Y-2.00   
   M00 (STOP: get the backlash)   
               

Note that the Z-AXIS programs has movement inverted: this because I read the backlash on the left of the table for X, on the rear of the table for Y, but UNDER the motor-mount for Z.
You can let the program run a couple of times to make certain that you get the same readings at the M00's in the program. The difference between Reading #1 and Reading #2 is the amount of backlash in your X axis. You can use the same style program making changes as required to perform the same function for the other axis as well. Basically, you just want the machine to move one way then back, stop so you can and collect the indicator reading, then move the other way and back and collect that reading.
these are images that show backlash measurements (one image for each axis) using the above programs (X, Y, Z) .
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In my new machine the values found was:

• X_AXIS = 0.05 mm (that means a backlash value of (0.05 / 25.4)*4000 = 8 steps)
• Y_AXIS = 0.02 mm = 3 steps
• Z_AXIS = 0.13 mm = 20 steps

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