Motion control system upgrades guitar production

Novel Ethernet-based motion control architecture employs six axes of motion to automate the sanding of fret boards for Martin Guitar.

A custom woodworking machine built using Ethernet-compatible motion control components from Baldor Electric has automated the precision finishing of fret boards for the renowned acoustic guitar manufacturer, Martin Guitar (CF Martin and Co).

By using dual sanding belts and a swinging work holder controlled by a combination of linear and rotary servomotor axes, this machine now replaces a previous manually-actuated sanding process.

Four axes of motion provide ultra-precise and smooth machining that enhances both the speed and quality of fret board production.

Machining the fret board is a critical operation in guitar manufacturing.

The board's straightness, and accuracy of the radius, makes a major contribution to the instrument's quality and playability.

Like many of the production processes at Martin Guitar, fret board production relied on the knowhow of highly skilled technicians.

They produced the required finish by means of a special swinging jig that held the fret board, and presented it to a motorised sanding belt.

It's easy for particles to clog sanding belts, and cause vibration, and fret boards often needed to be put through this process twice before the required quality of finish was achieved.

Martin Guitar planned to upgrade this process and began researching improvement ideas along with determining key requirements that a new machine needed to achieve.

Martin's Engineering Project Manager Fred Walters asked a local machine builder and CNC consultant, Brian Rasley of Rasley Enterprises, to investigate the process and see if it could be improved - especially by means of adding another sanding belt.

Rasley spent time with the project and production managers along with the operators as well and all agreed that a more controlled sanding process, which removed material in small increments and by means of a combination of rough and smooth sanding belts could enhance the quality of finishing as well as the likelihood of a right-first-time result.

More flexible and powerful sanding belts would be a key component of the new solution and Rasley decided that he would need to create them, and discussed this aspect of the application with Baldor - a major supplier of electric motors to Martin Guitar.

Baldor's applications engineer advised Rasley on the use of vector motors to drive the belts, but also discussed how the other movements required for the process could be implemented by servomotors, and integrated into a real-time system using a new Ethernet-compatible network called Powerlink.

Together, the two companies conceived a motion control architecture that employs six axes of motion to automate the sanding process.

Two vector motors power the rough and smooth sanding belts.

Two servomotor axes with ball screw jacks are used to 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 servomotor 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, and for this task, Baldor suggested using a linear motor with a "cog free" action.

In this motor, a special layout of the magnetic elements eliminates the tiny "ripples" of movement that can take place as the motor transitions between magnetic poles - providing an ultra smooth action.

This smoothness is also aided by a stiff aluminium mounting frame designed by Rasley Enterprises.

Using this basic automation architecture, Rasley Enterprises developed an "algorithm" for controlling the sanding process that removes the material in small increments, in both rough and smooth steps, while at the same time exploiting the width of the sanding belts to aid uniformity and minimise the clogging effects of sawdust particles.

Different woods also benefit from variations in the sanding process, and Rasley provided operators with a touch-screen human-machine interface that allows them to load the optimum sanding routine.

Now in use for around six months, the machine has boosted the basic speed of sanding by around 60%, as well as virtually eliminating rework.

Previously, around 40% of fret boards would need to go through the sanding process a second time to make sure they met the quality of finish standards demanded by Martin Guitar.

This is now running at around 3%, and Martin Guitar is in the process of investigating how modifications to the upstream wood milling process might lower this figure even further.

The electrical architecture of the machine is based on a Powerlink-compatible machine controller called NextMove e100.

This controls all six axes of movement: four via Powerlink connections to Baldor MicroFlex e100 motor drives, as well as stop/start and speed control of the two sanding belt axes (controlled by Baldor H2 AC drives and vector motors) by means of digital I/O signals.

The rotary axes are driven by Baldor BSM servomotors.

The linear motor axis is implemented by a Baldor LMCF motor.

A Baldor touch screen human-machine interface (HMI) is also connected to the machine controller.

Using Powerlink simplified the electrical design of the machine.

One network cable is all that's required to link the four precision motion axes, reducing the sise of the cabinet and the wiring task.

The other axes are connected directly to the controller.

Programming the system was also simplified by using Baldor's Mint development environment for Powerlink.

This machine control language provides high level English-like high level commands for common motion tasks, effectively providing ready to use software routines for much of the motion required.

In particular, two simple lines of code are all that is required to interpolate the two rotary and linear axes that control the swing arm jig.

"Using the Powerlink architecture gave us a powerful and robust control platform on which to build this machine", says Brian Rasley of Rasley Enterprises.

"Combined with the capabilities of the Mint programming language and technical support from Baldor, we were able to focus efforts on optimising the sanding process, to deliver the best-possible quality of finish for the fret boards".

"Powerlink reduces the electrical footprint of this machine probably by a third", adds Baldor's David Lunn, the applications specialist who helped Rasley Enterprises staff to engineer the motion control aspects the machine.

"A useful by-product of using this architecture is that it becomes very easy to expand the system at later stages, to add further automated functions such as inspection, or to integrate the machine into a factory network".

"All this can be achieved easily, largely in software, and without adding an extra controller".

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