Motion Scenarios: Grinding and polishing

Consider a toothed bandsaw for cutting lumber. Automating its occasional regrinding (for resharpening) certainly reduces cost, but requires synchronization.

One motor turns the grindwheel at a controlled speed, and another pulls the bandsaw to that tool; another motor lowers the grindwheel into the blade, while still another angles the wheel to shape individual teeth.

For these axes, the newest designs take axis synchronization a step further — to linear interpolate movements. In this technique, the beginning and end of each axes' moves are concurrently timed so that their speeds and accelerations work in harmony. (Read the September 2010 Motion System Design article on contouring for more information about the mathematics of interpolation.)

In fact, evolved synchronization is the hallmark of cutting-edge grinders.

Interpolation-capable controllers can finish even complex parts such as cams to a mirror finish. One such controller from Aerotech Inc., Pittsburgh, called the A3200, can store multiple cam profiles and define the motion required to machine them in CAM tables for fast changeover time. Preprogrammed individual lobe profiles can be ground to exact tolerances in one operation.

Software support

Collisions are another issue with grinders, one that's increasingly addressed by software. With some such tools, users create machine models that account for add-on and moveable parts of grinding and polishing machines — including supports, tailstocks, steady-rests, and special chucks.

Tool-grinding programming software NUMROTO 3.5.1 from NUM Corp., Naperville, Ill., is one package used on machines that produce and resharpen tools. Its collision detection function allows exhaustive simulations of new grinding programs; it also imports compensation profiles generated by external measuring machines, to help grinder manufacturers engineer more compact machines.

Continuous removal-rate monitoring alerts operators to grindwheel overload. Here, the software automatically flags even brief overloads, to optimize feed rates and cycle times.

Mechanical considerations

Coordination of axes requires that the physical parts of a machine remain aligned. To this end, Nicolás Correa VERSA milling machines now include TwinDrive rack-and-pinion drives from REDEX ANDANTEX for their X and Y axes. (ANDANTEX USA Inc. is in Wanamassa, N.J.) The new TwinDRIVE consists of two planetary reducers that ride on one rack; they're mounted on opposite sides of a standard machine frame with oversized screws to eliminate the looseness associated with intermediate assembly elements. The two motors are setup with a master-slave electronic configuration (also called electronic preload) to eliminate backlash. A spheroidal-graphite cast-iron housing extends off the main machine frame and is optimized with FEM to prevent deflection during acceleration.

The miller's interchangeable heads include a mechanical one with a resolution of 0.02° over the table's 14,500 × 5,000 × 1,500-mm work area. On the X axis, these drives move up to 50 tons at 1.8 m/sec², or up to 70 tons in custom designs. Speeds in X and Y axes are 30 m/min.

High dynamics boost positioning precision: The rigidity of the reducers together with the integrated pinion provides X-axis K_V = 1 with 50,000 kg of moving mass, and Y-axis K_V = 2 with 9,200 kg of moving mass. Swiss company Güdel manufactures the racks; paired with the servo reducers, they reduce pitch error. In fact, under closed-loop control with a Heidenhain linear encoder, the setup offers X-axis repeatability of 5 µm and precision of 7 µm, and Y-axis repeatability of 4 µm and precision of 6 µm.

“This range of milling machines incorporates rack-and-pinion drives in the Y axis for the first time; interpolation with the X axis is better than … with a ballscrew,” explains Javier Hernando of Nicolás Correa.

Rack-and-pinion drives are also integrated into the company's floor-type moving-column millers that machine medium and large parts.