A novel motion-control scheme on a custom woodworking sander has automated the precision finishing of fret boards for highquality guitars.
Dual sanding belts and a swinging workholder on the machine are controlled by a combination of linear and rotary servomotor axes. Formally, fret boards at guitar maker CF Martin & Co. were made by hand, relying on the know-how of highly skilled technicians. They operated a special swinging jig that held the fret board and presented it to a motorized sanding belt. But particles tended to clog the belts and cause vibration, often forcing fret boards to undergo a second sanding for the right finish.
A better method came from machine-builder Rasley Enterprises Inc., Bangor, Pa., which helped devise a more-controlled sanding process which removed material in small increments by means of rough and smooth sanding belts. The company worked with Baldor Electric Co., Fort Smith, Ark., on a motion-control architecture that employs six axes of motion to automate the sanding process.
Two Baldor vector motors power the rough and smooth sanding belts. Two servomotor axes with ball-screw jacks raise the belts once the workpiece has moved across the sanding faces. The swing-arm jig that holds the fret board is powered by two axes of motion: a servomotor to swing the fret board in an arc, plus a linear-motion axis that moves the swinging arm across the sanding belts.
A linear motor with cog-free action handles the linear motion, the most critical of the axes. The motor sits in a stiff aluminum mounting frame and employs a special layout of the magnetic elements that eliminates any ripples of movement that can otherwise arise as the motor transitions between magnetic poles.
The sanding scheme exploits the width of the sanding belts to aid uniformity and minimize the clogging effects of sawdust particles. Different woods also benefit from variations in the sanding process. A touchscreen human-machine interface lets operators load the optimum sanding routine.
The architecture of the machine is based on a Powerlinkcompatible machine controller called NextMove e100. It controls all six axes of movement. Four are via Powerlink connections to Baldor MicroFlex e100 motor drives. Stop/start and speed of the two sanding belt axes (controlled by Baldor H2 ac drives and vector motors) are via digital I/O signals. Rotary axes are driven by Baldor BSM servomotors. And the linear motor axis is implemented by a Baldor LMCF motor.
Using Powerlink simplifies the electrical design of the machine in that just one network cable links four motion axes. The other axes connect directly to the controller. Programming is through Englishlike high-level commands on a Baldor Mint development environment for Powerlink. In particular, two simple lines of code interpolate the two rotary and linear axes controlling the swing-arm jig.