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The Modern Luthier
A modern luthier talks about his success with using a
Whatever software is used to create your geometry, it is not important to draw a complete picture of a violin to cut beautiful parts. In my experience, it may just make it harder. It is often easier to create a surface which extends beyond the perimeter of the part, because of what happens to the surface inside the area where you are working. To create an arched surface, the beginning process is quite similar to what you do by hand. You start with the centerline and cross sections, combined with the outline. To do this, you need a CAM program which can do surface milling, pockets, projections, and 3d contours. Multi surface and multi-axis are all beneficial but not absolutely necessary.
(Cello and base plates straight off the CNC Router)
The CNC is actually not difficult to run. It is more a matter of learning what works and what doesn't as far as holding your workpiece stationary, cutting velocities, tool performance, and the like. I have always found valuable vacuum fixtures specific to the particular piece to optimize hold without damaging the part. You have a maximum of 1 atmosphere of pressure (14 psi) to hold a part down, so the best results are achieved with a larger surface area and a vacuum jig that minimizes leakage. Because wood is porous, the pump needs to be able to move a sufficient volume to make up for the leakage through the part. For most of my parts, I use a 1.5 hp rotary vane pump, which is quite adequate for plates violin up to the size of a double bass.
The next issue is tooling: this aspect is key. In my experience, round nose tools as they come from manufacturers are not well suited for use in machining surfaces. I have found, in general, that the tips are ground with insufficient relief angle. As a result, resin builds up behind the cutting edge and burns the surface. The cutting edge will last several times longer if they are reground so the relief angle remains constant normal to the radius of the tip. This also helps to eliminate the burn. Cutting compression occurs when a cutter pares the wood away from the workpiece. The force with which the cutter impacts the wood, the geometry of the cutting edge (including cutting angle, sharpness and relief angle) and the direction of the cut relative to the workpiece, are all key factors in causing cutter compression. How your tool path is chained, the cutter geometry and speed of cut will help to reduce or eliminate this problem.
As a general rule of thumb, if you can reach it from the top you can cut it with a 3-axis CNC. Just like the step from 2.5- to 3-axis tool paths, the jump to 4-, 5- and 6-axis machining is a kind of quantum leap. The cost of software to do true 4- axis tool paths and above can get extreme, not to mention the cost of machines capable of running full 5-axis tool paths. Before I got my 4th axis up and running, I was able to cut necks scrolls on my 3-axis techno machines by developing fixtures which held my parts in each position as if in an indexing between centers. it just makes it necessary to be able to consistently hold the parts in the correct position.The things that I see as most important in a CNC used for instruments are accuracy of cut, consistency of performance, smoothness of motion and strength of the machine. The Techno machines with the current software and controls have very nice jerk-free motion. I do the final surface cut at about 4.5 to 5.5 meters per minute. The control will slow and stop when necessary to keep from hammering the ball screws due to instant reversing of the axes. I would suggest a closed loop system with servo drives. Stepper motors have an annoying tendency to have lost step issues, and without an encoder feedback system, the machine has no way of knowing it is lost. While the Techno machines are not heavy-weight machines, they are definitely capable of producing your parts. The final finish cuts are always the lightest and the object is finish surface, not heavy stock removal. My 19+ year old table top machine is still in use, still producing parts.