Fadal control procedure

How to Enter Backlash on a Fadal CNC Machine

This guide covers fadal cnc machine backlash from measurement to verification: where the compensation values live, how to measure real lost motion with an indicator, and how to prove the fix with a test cut. It is written for maintenance techs and machinists who need to hold ±0.005 mm on older iron. Read it before you touch the tables.

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X, Y, Z axesDT and CNC 88 controlsTest-cut verification

fadal cnc machine backlash entry on a vertical machining center

On this page

1
Key takeaways
2
What fadal cnc machine backlash actually is
3
Symptoms that tell you to check the backlash value
4
Prepare the machine and the measuring setup
5
Mistakes that make the number worse
6
Step by step: how to enter backlash on a Fadal CNC machine
7
What the measured value tells you

Quick answer

Key takeaways

Backlash is stored as compensation, not a repairThe control adds the value to the commanded move. It hides lost motion; it does not remove wear.

Enter whole counts, not millimetersAt 0.0001 in per count, 0.0008 in equals 8 counts. Round to the nearest whole count.

Measure warm and measure twiceA cold machine reads differently. Warm the ways for 15 to 20 minutes, then take two runs.

Compensation direction mattersSign errors double the error instead of removing it. Confirm with an indicator before cutting.

Big values mean mechanical troubleIf one axis needs more than about 20 counts, inspect thrust bearings and couplings first.

Basics

What fadal cnc machine backlash actually is

Backlash is the lost motion between the ballscrew and the nut, plus any play in the thrust bearings and the motor coupling. When the control reverses direction, the screw turns but the table does not move for a short distance. On a Fadal, the control compensates for this by adding a fixed number of counts to the first move after a reversal. That number is what you enter in the backlash tables.

Fadal controls, including the CNC 88 and the later DT series, keep a separate value for each axis. Some machines also carry a second value for the opposite direction. The value is stored in counts, not in inches or millimeters. One count equals 0.0001 in on a typical inch machine, so a value of 12 means the control offsets 0.0012 in of lost motion.

Two things get confused in shops. Backlash compensation is a software offset. Mechanical backlash is wear. If the screw, the nut, or the angular contact bearings are loose, the compensation number grows over time and the machine still cuts poorly in contouring work because the offset only applies on direction changes, not inside a continuous arc.

1
Applies on reversal onlyThe offset is injected when the axis changes direction, not during a steady cut.
2
Direction-specificX plus and X minus can carry different values on a worn machine.
3
Stored per axisX, Y, Z, and any rotary axis hold their own numbers.

When to adjust

Symptoms that tell you to check the backlash value

The clearest sign is a step or witness mark on a part where the cutter changed direction. On a pocket wall, you see a line at the quadrant points. On a bored hole, the diameter reads different at 0° and at 180°. If the step appears only on one axis, start there.

A second sign is a finish that changes with feed direction. Climb milling one way cuts clean, and the return pass rubs. That usually means the tool is dragging until the lost motion is taken up. Measure before you blame the tool or the fixture.

A third sign is positional drift in a test program. Command a move of 100.000 mm out and back to zero. If the indicator reads 0.010 mm or more off after the return, the axis is carrying slack. Repeat the test three times. A consistent offset points to backlash. A random offset points to something else, often a loose encoder coupling or a sticking way.

1
Witness marks at quadrantsA visible line where the axis reversed mid-contour.
2
Out-of-round bores
3
Repeatable return errorThe axis comes back to a different number every time by the same amount.

Before you type

Prepare the machine and the measuring setup

Clean the way covers and wipe the table and spindle nose. A chip under the indicator stand will show up as backlash and send you chasing a number that is not real. Warm the machine with a 15 to 20 minute spindle and axis warm-up cycle. Thermal growth changes the reading, especially on the Z axis.

Mount a dial test indicator with 0.001 mm or 0.0001 in resolution on the table, and touch off on a solid reference such as the spindle nose or a gauge block held in the spindle taper. Do not indicate against a vise jaw or a fixture plate that can flex. The stand must be rigid; a magnetic base on a painted surface will move.

Write down the current values for every axis before you change anything. Photograph the screen. If the new number makes the part worse, you need the old values back in seconds. On most Fadal controls you reach the backlash tables through the service or parameter pages, and some entries sit behind a service access code. Ask the integrator or the machine dealer if you do not have it.

1
Warm-up first15 to 20 minutes of spindle and axis motion before any reading.
2
Rigid indicator mount0.001 mm resolution, mounted on the table, not on a flexing fixture.
3
Record old valuesPhotograph every axis before editing.

Common mistakes

Mistakes that make the number worse

The most common error is typing the measured value straight into the table without converting to counts. A machine set for 0.0001 in counts will read 0.0006 in as 6, not 600. If you enter 600, the control offsets 0.0600 in on every reversal, and the first cut will scrap the part.

The second mistake is guessing the sign. If the value goes into the wrong direction column, the control adds the error instead of removing it. The step on the part gets twice as big. Always re-measure after the write, and if the error doubles, reverse the sign immediately.

The third mistake is compensating for a mechanical fault. A Fadal that suddenly needs 30 counts on one axis has a problem in the thrust bearings or the coupling. Compensation will make the test indicator look right while the machine still chatters and loses position under load. Fix the hardware, then set the number.

The fourth mistake is measuring against a flexible reference. If the indicator base is on a vise or a thin plate, you are measuring the fixture, not the axis. Move the base to the table or the spindle nose and run the test again.

1
Counts versus inchesConvert before you type. 0.0006 in is 6 counts, not 600.
2
Sign and directionIf the error doubles after the write, the value is in the wrong column.
3
Do not hide a faultA sudden large value is a mechanical warning, not a tuning task.

Procedure

Step by step: how to enter backlash on a Fadal CNC machine

Do one axis at a time. Finish and verify X before you start Y.

1
Zero the indicator on the axisJog the axis in one direction to take up slack, stop, and set the indicator to zero. Use a slow jog, not rapid, so the servo settles.
2
Move away and come backCommand a move of 0.500 in (12.7 mm) in the same direction, then command a return to the start point. Read the indicator without touching the machine.
3
Repeat three times and averageDo the out-and-back cycle three times. If the readings are 0.0006, 0.0007, and 0.0006 in, use 0.0006 in as the working value. Throw out any reading that jumps by more than 0.0002 in and find out why.
4
Convert to countsDivide the measured lost motion by the count resolution. At 0.0001 in per count, 0.0006 in equals 6 counts. Round to the nearest whole count. A metric machine uses a different resolution; check the control manual before converting.
5
Open the backlash tableGo to the service or parameter page that holds the backlash values. Note the axis and the direction column. Enter the count in the field for the direction you measured.
6
Enter the value and store itType the number, press enter, and store or save the parameter. Do not power down until the control confirms the write. Some Fadal software requires a separate save step.
7
Re-measure the same axisRun the out-and-back test again. The indicator should read within 0.0001 to 0.0002 in of zero. If it reads worse than before, the sign is wrong. Reverse the sign or move the value to the opposite direction column.
8
Cut a test part and check sizeMachine a 50 mm square pocket or a bore and measure at four points. Repeat for Y and Z. Only then move to the next axis.

Judgment

What the measured value tells you

Use this table after you have three consistent indicator readings.

Measured lost motion	Likely cause	What to do
0.0001–0.0005 in	Normal screw and nut wear	Enter the counts and re-verify
0.0006–0.0020 in	Worn thrust bearings or loose locknut	Enter compensation, then inspect the bearing pack
Above 0.0020 in	Loose coupling or failing ballscrew	Stop cutting; repair before compensating
Readings vary by more than 0.0002 in	Loose encoder, sticking way, or dirty scale	Clean and inspect; do not enter a value yet
Error only in one direction	One-side wear or a bad direction column	Confirm the direction field before typing
Error grows through the day	Thermal growth, not backlash	Warm up longer and re-measure cold and hot

Compensate, then verify on the part

Enter the counts, store the value, and cut a test part before you trust the number. If one axis needs a large value, repair the hardware instead of hiding it.

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FAQs

Frequently asked questions

How often should I check fadal cnc machine backlash?

Check it whenever a part shows a witness mark at a direction change, after any crash, and after replacing a ballscrew, thrust bearing, or coupling.

For a machine running production, a quarterly out-and-back indicator test takes about 20 minutes per axis and catches drift before it reaches the part.

Can I enter backlash while the machine is running a program?

No. Stop the program, bring the axes to a safe position, and enter the value with the machine idle. Writing parameters during motion can produce an uncommanded offset on the next reversal.

Store the value, then run the test cycle again before you load a real part.

Why does my Fadal need a different value for X plus and X minus?

The two directions load the thrust bearing faces differently. On a worn machine, one face carries more clearance than the other, so the lost motion differs.

Enter each direction from its own measurement. Do not copy one value into both columns.

What if the value is correct but the part is still out of tolerance?

Backlash compensation only acts on direction changes. If the error appears inside a continuous cut, look at tool deflection, thermal growth, or a loose workholding setup.

Also check the machine level and the way lubrication. A starved way can cause stick-slip that mimics backlash but changes with feed rate.

Does backlash compensation affect surface finish?

It can improve a finish by removing the step at a reversal point. It also adds a small jump at each direction change, which shows up on a mirror finish as a faint line.

For finishing passes, keep the compensation value accurate and use a constant direction of cut where the geometry allows.