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TB036 Rev4 Measuring Machine Axis Overall Turns Ratio Axis Calibration
“Overall Turns Ratio” is the Kinematic relationship between the Axis Motor and the Machine mechanical system
The Overall Turns Ratio is an absolute value; there is only one right answer for Overall Turns Ratio for each axis of every
machine tool
The Overall Turns Ratio is a physical value determined by the ballscrew pitch and how the axis motor is belted/geared to
that screw
Overall Turns Ratio is used to calibrate commanded distance with actual machine axis distance moved
Units for the Overall Turns Ratio for an axis in inches is the number of turns of the Axis Motor to produce 1" of machine
travel Turns per Inch
Units for the Overall Turns Ratio for an axis in mm is the mm amount the machine moves for one revolution of the Axis
Motor Mm per Revolution
Overall Turns Ratio for Rotary Axes number of Degrees that the Rotary axis moves per one revolution of the axis
motor Degrees per Revolution
For example a 5 turns per inch ball screw with an axis motor belted at 2 1 yields an Overall Turns Ratio of 10
Said another way it would take 10 revolutions of the axis motor to make the axis move exactly 1" of travel
Not all screws or racks are perfect 5 turns per inch so a more accurate adjustment can be made to fine tune the
commanded movement to the actual movement The 'fine' Overall Turns Ratio is determined by measurement
The process to determine the perfect machine axis Overall Turns Ratio is typically done in two stages
Stage 1 a “Rough” Overall Turns Ratio value determination is done with simple math
Stage 2 a “Fine” Overall Turns Ratio value determination performed by comparing the commanded movement to an
accurate measurement of the actual movement of that axis
BEFORE PROCEEDING it is imperative that the Encoder counts per Revolution and for Acorn/AcornSix users the “Steps
per Revolution” is configured and set properly DO NOT try to adjust of the Overall Turns Ratio without first verifying the
Encoder counts per Revolution and for Acorn/AcornSix users the Steps per Revolution is configured and set properly
This is a simple test to do
1 Verify counts/steps per revolution is set properly for each axis motor by setting the Overall Turns Ratio = 1 and then
commanding movement of the motor 1” The motor should rotate exactly one revolution Use MDI to command the axis
to move 1”
G91 X1 F20 ;move X axis 1” at 20 inches per minute The motor should rotate exactly one revolution
For metric users set the overall turns ration motor mm/rev = 25
G91 X25 F1000 ;move X axis 25mm at 1000 mm per minute The motor should rotate exactly one revolution
If this test fails Review the Encoder counts/Motor Steps Per Revolution settings in the axis motor drive and set CNC12 to
the same value and repeat the test to verify
Note Once the Encoder counts/Steps per rev has been set and verified DO NOT adjust it to change the distance moved
by the machine Steps/Encoder counts per rev is NOT used to Calibrate Axis movement
If you are coming from Mach or other DIY cnc controllers read this post
“ Steps per Revolution and Overall Turns Ratio don't do it like Mach did it! ”
https //centroidcncforum com/viewtopic php?f=61&t=5823
\tech\Tech bulletins\TB036\TB036 Measuring_Axis_Overall_Turns_Ratio_rev4 odt 2 13 24
2 Set the “Rough” Overall turns ratio for each axis
Using math determine the final mechanical gear ratio between the axis motor and the machine
Inch Examples
5 turn per inch pitch ballscrew with direct drive axis motor Set Overall Turns Ratio = 5 revs/in
5 turn per inch pitch ballscrew with motor pulley to ballscrew pulley 2 1 reduction Set Overall Turns ratio 10 revs/in
5 mm pitch ballscrew with direct drive axis motor Set Overall Turns Ratio = 5 08 revs/in
10 mm pitch ballscrew with direct drive connection to axis motor Set Overall Turns Ratio = 2 54 rev/in
10 mm pitch ballscrew with motor pulley to ballscrew pulley 2 1 reduction Set Overall Turns ratio = 1 27 rev/in
Rack and Pinion Pinion has 18 teeth Rack has 5 teeth per inch Pinion is belted down 3 1 Set Overall Turns Ratio =
1 2 rev/in 18/5=3 6 3 6/3 = 1 2
MM Examples
5 turn per inch pitch ballscrew with direct drive axis motor Set Overall Turns Ratio = 5 08 mm per rev
5 turn per inch pitch ballscrew with motor pulley to ballscrew pulley 2 1 reduction Set Overall Turns ratio 2 54 mm/rev
5 mm pitch ballscrew with direct drive axis motor Set Overall Turns Ratio = 5 mm/rev
10 mm pitch ballscrew with direct drive connection to axis motor Set Overall Turns Ratio = 10 mm/rev
10 mm pitch ballscrew with motor pulley to ballscrew pulley 2 1 reduction Set Overall Turns ratio 5 mm/rev
Rack and Pinion Pinion has 18 teeth Rack has 25 teeth per 100 mm Pinion is belted down 3 1 Set Overall Turns
Ratio = 24 mm per rev 18/25= 72 revs/100 mm 72 revs * 100mm = 72 mm 72 mm / 3 = 24 mm per rev
Video showing a ‘trick’ with CNC12 to have CNC12 do the mm to inch conversion for you
https //youtu be/zNySv5b6CJ8?si=Qw4uNvxCyaQikc6Y
Once the Rough Overall Turn Ratio has been set verify that the commanded axis motion matches the actual machine
movement
a Power down the CNC control power back up and re home the machine
b Set the WCS to zero position 0 0000
c Command an axis to move a large easy to measure amount such as
G1 X10 F20 ;move X axis to the 10” position in the positive direction at 20 inches per minute
d Now the measured actual movement of the axis should be VERY close to the commanded value
If not STOP double check the Overall Turns Ratio values as there is a mistake Reset and always power down
and restart the cnc controller and re home the machine before running another test after every change to the Overall
Turns Ratio
If good then you may be done!
Many CNC Routers and Plasma Machines and some Mills do not need any more adjustment of the Overall Turns Ratio at
this point
Precision Lathes and Milling machine owners will often want to spend the time fine tuning the overall turns ratio to
squeeze out the last few tenths 0001” of accuracy
If you do not have the screw pitch and/or the pulley or rack information to calculate and set the Rough Overall Turns Ratio
number as described above then another method can be used You guess at it and make a some measurements and do
some basic math You have a couple of choices on how to do this
A Use the Centroid Provided Axis_Calibration CNC macro Located here \cncm\system\Axis_Calibration cnc
which will guide you through the process and automatically do the calculations for you
B Perform the same steps manually
1 Set the Overall Turns Ratio to 5
2 Send machine Home Set WCS zero position G92 x0y0z0
3 Line up a machinist rule or tape measure in a place that will be easy to measure the axis actual movement amount
4 In MDI issue a command of G1 X10 F20 if metric use X250 F500 press cycle start
5 Measure the distance the axis actually moved and set your motor revs/inch or motor mm's/rev if in metric as follows
Press F1 Setup → F3 Config password = 137 Press enter → F2 Mach → F2 Motor
Imperial – How to compute Overall Turns Ratio motor revs/inch
a Commanded distance DRO / Actual Distance moved measured = Multiplier
b Multiplier * Current revs/inch = Corrected Overall Turns Ratio revs/inch
Example Commanded Distance 10 0000” / Actual distance moved 5 = 2
multiplier 2 * current revs/inch 5 0000 = 10 ← New motor revs/inch
Repeat the test Steps 2 5 and verify the Commanded movement matches the actual movement
Metric – How to compute Overall Turns Ratio motor mm's/rev
Distance moved measured / Commanded Distance DRO = Multiplier
Multiplier * Current mm's/rev = corrected mm's/revs
Example Distance moved 125mm / Commanded Distance 250 mm = 0 5 “Multiplier”
Multiplier 0 5 * current mm's/rev 5 0000 = 2 5 ← New motor mm's/rev
NOTE After changing mm's/rev power cycle and re home the machine and reset any WCS locations
Repeat the test Steps 2 5 and verify the Commanded movement matches the actual movement
C Use a Spread Sheet Calculator to do the Calculations for you Free download
https //centroidcncforum com/download/file php?id=27052
Fine Overall Turns Ratio Adjustment
Not all screws or racks are perfect pitch so a more accurate “fine” adjustment can be made to fine tune the commanded
movement to match the actual measured movement
The 'fine' Overall Turns Ratio is determined by repeated position commands and VERY accurate axis movement
measurement
Making sure they are parallel/perpendicular w ith the table put a gauge block or anything else that can be measured
accurately up against a 123 block – a 6” 150mm block works nicely as shown below
1 Jog in slowly to gauge block in one direction only to take up the lash
2 Zero indicator and axis part zero
3 MDI – command Z move up to clear top of the gauge block
4 MDI command a slow movement along the axis the known length of the gauge block
5 Read the actual position on the indicator
6 Calculate and enter the new revs/inch OR mm/rev value
Imperial – How to fine tune calculate motor revs/inch
Commanded distance / Distance moved = Multiplier
Multiplier * Current revs/inch = corrected revs/inch
Ex Commanded Distance = 6 0000” = ~ 999417 multiplier * current revs/inch
Distance moved = 6 0035”
multiplier 999417 * current revs/inch 5 0000 = 4 997085 ← New motor revs/inch
NOTE After changing revs/in power cycle and re home the machine and reset and WCS locations
Metric How to calculate motor mm's/rev
Distance moved measured / Commanded distance DRO = Multiplier
Multiplier * Current mm's/revs = corrected mm's/rev
Ex Distance moved meas = 150 021mm = 1 00014 multiplier * current mm's/rev
Commanded Distance DRO = 150mm
multiplier 1 00014 * current mm's/rev 5 0000 = 5 0007 ← New motor mm's/rev
Perform this procedure 1 or 2 more times being sure to reset your part zero after any change in the ratios You should be
able to accurately position to within a 0001” 00254mm
NOTE After changing mm's/rev power cycle and re home the machine and reset any WCS locations
Related Subjects and Documentation
Discussion and videos of Machine Calibration for Acorn and AcornSix users that most of it applies to other platforms as
well https //centroidcncforum com/viewtopic php?f=61&t=5823
Ballscrew Compensation
https //centroidcncforum com/viewtopic php?f=61&t=5461&p=80095&hilit=ballscrew#p80095
BackLash Compensation
https //centroidcncforum com/viewtopic php?f=63&t=5404&p=46199&hilit=backlash#p46199
https //www centroidcnc com/dealersupport/tech_bulletins/uploads/37 pdf
Volumetric Compensation
https //centroidcncforum com/viewtopic php?p=80013#p80013