A custom woodworking machine uses Ethernet-compatible Baldor Electric linear and rotary servomotors to control dual sanding belts and a swinging work holder that precisely finish fret boards for acoustic guitar manufacturer, Martin Guitar.

In guitar manufacture, fret board machining is a critical operation because it affects two factors important to an instrument‚’s quality and playability: the board‚’s straightness and accuracy of the radius. Skilled technicians produced the required finish with a special swinging jig that held the fret board in position for the motorized sanding belts. However, it was easy for particles to clog the belts and cause vibration, forcing additional processing to achieve the required finish.

When Martin Guitar planned to upgrade this process, Engineering Project Manager Fred Walters asked a local machine builder and CNC consultant, Brian Rasley of Rasley Enterprises Inc. to investigate and suggest improvements, including the addition of another sanding belt. Rasley determined that a more controlled sanding process that removed material in small increments by means of rough and smooth sanding belts could enhance the finish quality as well as the likelihood of a right-first-time result.

Rasley‚’s proposal required more flexible and powerful sanding belts that he would create. He discussed this aspect with Baldor, a major supplier of electric motors to Martin Guitar. Baldor‚’s application engineer advised the use of vector motors to drive the belts, and discussed how the other required movements could be implemented through servomotors integrated into a real-time system using an Ethernet-compatible network called Powerlink.

Together, the engineers conceived a motion control architecture that uses six axes of motion. Two vector motors power the rough and smooth sanding belts. Two servo motor axes with ball screw jacks raise the belts after the work piece has moved across the sanding faces. Finally, the swing-arm jig that holds the fret board is powered by two axes of motion: a servo motor to swing the fret board in an arc, plus a linear motion axis that moves the swinging arm across the sanding belts.

The linear motion is the most critical of the axes. The selected motor offers cog free action. The layout of its magnetic elements eliminates the tiny ‚‘ripples‚’ of movement that can take place as the motor shifts between magnetic poles, providing a very smooth action. This smoothness is also aided by a stiff aluminum mounting frame designed by Rasley Enterprises.

The Powerlink Ethernet compatible network communicates with MicroFlex e100 motor drives, H2 ac drives and vector motors, BSM servomotors and LMCF motor controls from Baldor. 

Rasley also developed an algorithm to control the sanding process to remove material in small increments, in both rough and smooth steps, while exploiting the width of the sanding belts to aid
uniformity and reduce the clogging effects of sawdust particles. Different woods also benefit from variations in the sanding process, and Rasley developed a touch-screen human-machine interface that allows operators to load the appropriate sanding routine.

In use for around six months, the machine boosted the basic speed of sanding by 60%, as well as virtually eliminated rework. Previously, 40% of fret boards needed additional processing to meet the finish standards. Additional processing is now running at around 3%, and Martin Guitar is investigating modifications to the upstream wood milling process that might lower this figure further.

The electrical architecture of the machine is based on a Powerlink-compatible controller called NextMove e100. It controls all six axes of movement: four through Powerlink connections to MicroFlex e100 motor drives, as well as stop/start and speed control of the two sanding belt axes (controlled by H2 ac drives and vector motors) by means of digital I/O signals. BSM servomotors drive the rotary axes. The LMCF motor controls the linear motor axis.

Powerlink simplified the electrical design. One network cable links the four precision motion axes. The other axes connect directly to the controller. The Mint programming language includes high level English-like commands for common motion tasks, providing ready to use software routines for much of the motion. Two simple lines of code were all that were required to interpolate the two rotary and linear axes that control the swing arm jig.

Baldor Electric Co.
www.baldor.com