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CNC Retrofit of Churchill Lathes


  • To replace ageing GE controls on 4x Churchill lathes.
  • To replace Contraves spindle drives, but retain existing motors.
  • To replace position feedback encoders with new units.
  • To replace unreliable turret resolvers with new units.
  • To re-create the main part-program used in the current control system.

The machines previously allowed the input of component weight from a seperate weighing machine at a specific point of the part-program. This data was then multiplied by a constant to give a Z axis datum shift offset to the facing-off tool used in the program.

It was imperative that this facility was retained with the new CNC system.


The old GE units were becoming increasingly unreliable and costly to obtain spare parts for.

It was apparent that replacing the control system with a modern counterpart would reduce downtime and ultimately save money.

Various new controls would have been suitable for this application, including Fanuc 0i, Mitsubishi M64 and Siemens 802D. Robtec prepared quotes for the installation of all these controls, and the customer decided that the Siemens 802D represented the best value for money.

A Control Techniques Mentor M75R drive was selected to replace the pre-existing Contraves spindle drive.

A panel was designed and constructed by Robtec to allow the new Siemens control to be mounted on the same backplate as the old GE control.

Copies of the original Churchill Electrical drawings were used to allow the new system to be integrated with the original machine I/O and axis drives.

Full PLC design and new electrical drawings were completed before installation work began, thus reducing the machine downtime to a minimum.

Full de-commissioning of the old unit, and installation and commissioning of the new control took approximately 2 weeks.

Heidenhain encoders replaced the pre-existing position measuring devices. A 360 count gray-coded encoder was selected to replace the turret resolver.

The data from the separate component weighing machine was taken as a binary input into the control. PLC code was written to decode this data, convert it to floating-point and allow it to be accessed during the part-program cycle.

A part programme was written by Robtec which allowed the component weight data to be seen as a message on the screen and verified by the operator before the program was allowed to continue.

Upon completion of each machine, full training on the new control system was provided for the operators and process engineers.