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In other way we can say calculation of Flexible Feed Gear ratio (FFG) for different ball screw for linear Axis.

for this we need to collect following data.

• Machine Detection Unit
• Number of position feedback pulses produced per revolution of servo motor.
• Pitch of the ball screw.

Machine Detection Unit: Minimum unit for machine position detection.(Normally it is given 1 Micrometer for linear axis)

Reference counter (Parameter 1821)=Grid interval/Detection unit
Grid interval=Amount of travel per rotation of the pulse coder or Ball Screw Pitch

Reference Counter=Ball Screw Pitch/Detection Unit

Number of position feedback pulses produced per revolution of the servo motor: It is assumed to be 1 million (1,000,000).

Pitch of ball screw : It is provided by Machine Tool Builder . You can find it in MTB’s maintenance Manuals.

Here we are taking example of a turning machine whose x axis ball screw pitch is 10 mm and Z axis ball screw pitch is 12 mm and there is no gear box in axis.

For X axis : Ball screw pitch is 10 mm & detection unit is 1 Micrometer.

Reference Counter(Parameter 1821)=Ball Screw Pitch/Detection Unit

Reference Counter=10 mm/1 micrometer=10×1000/1=10000

1mm=1000 micrometer

Value of parameter 1821(Reference Counter) =10000

Calculation for Flexible Feed Gear Ratio(Parameter 2084/Parameter 2085)

Flexible Feed Gear Numerator/Flexible Feed Gear Denominator=Reference Counter/1000000(Pulses/rev)

Flexible Feed Gear Numerator/Flexible Feed Gear Denominator=10000/1000000

Flexible Feed Gear Numerator(Parameter 2084)/Flexible Feed Gear Denominator(Parameter 2085)=1/100

Parameter 2084/Parameter 2085=1/100

Value of parameter 2084=1

Value of parameter 2085=100

For Z axis : Ball screw pitch is 12 mm & detection unit is 1 Micrometer.

Reference Counter=Ball Screw Pitch/Detection Unit

Reference Counter=12 mm/1 micrometer=12×1000/1=12000

Value of parameter 1821=12000

Calculation for Flexible Feed Gear Ratio

Flexible Feed Gear Numerator/Flexible Feed Gear Denominator=Reference Counter/1000000(Pulses/rev)

Flexible Feed Gear Numerator/Flexible Feed Gear Denominator=12000/1000000

Flexible Feed Gear Numerator(Parameter 2084)/Flexible Feed Gear Denominator(Parameter 2085)=12/1000=3/250( We can’t take this value in decimal so we put only 3/250)

Parameter 2084/Parameter 2085=3/250

Value of parameter 2084=3

Value of parameter 2085=250

With the help of above example we can also replace 10 mm pitch ball screw with the 8 mm ball screw in case of emergency or running machine on a temporary basis.

We did not recommend to change ball screw pitch because there are lot of factors associated with motor’s rating but in case of emergency you can use this calculation.

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