Tuesday, October 20, 2009

CNC Machines: Performing Alignments and Volumetric Compensation

CNC (Computer Numerical Control) machines have successfully transformed the manufacturing industry from using hard-wired machines, whose operating parameters could not be changed, to machines that can cut curves as easily as straight lines and can produce complex 3D structures with ease.

Thermwood Corporation is currently the oldest CNC router company and continues to offer products for the woodworking, plastics, and aerospace industries.

An important process for their CNC machines is axis alignment verification. Their tools, however, were very time-consuming and just weren’t providing the desired solution. They also wanted a solution to performing volumetric axis compensation.

FARO Laser Tracker became that solution for Thermwood. The amount of time required for machine alignments has been cut to less than half. What once took up to eight hours to complete can now be accomplished in approximately three. In addition, they can now perform volumetric compensation, which would not have been possible without the Laser Tracker.

Axis Alignment Verification

How it’s done:
• Initial Laser Tracker measurement setup is performed in the early stages of machine assembly
• SMR (spherically mounted retroreflector) is fitted onto a special fixture, which allows it to be attached to each rail and moved manually along them
• Fixture is then fitted with precision rail bearings, which allows it to track the rail very precisely
• Machine assembly is completed
• Servo drive system is fitted
• Computer control is installed
• Machine is again checked using the Laser Tracker
• Additional minor calibration adjustments are performed if necessary

Volumetric Compensation

How it’s done:
• Positioning data is acquired in a matrix pattern on multiple planes
• The entire work envelope of the machine is mapped
• A correction table is developed by
-Triangulating between the various points in volumetric model space
-Measuring the error for each position
-Formulating a table of correction data to be loaded into the machine control and accessed during program execution
• Method of compensating for error during the program cycle is essentially the same as with linear compensation, except during program execution the control is required to
-Simultaneously track and compare axis movement on three distinct orthogonal paths
-Affect positional corrections correspondingly on all three axes, in phase with program execution

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