Wednesday, October 6, 2010

Working Large Marquetry Pictures

One of the comments I received on a previous post on this blog is how to handle large designs. Keep in mind that workers in America, England and European countries outside Paris usually don't use the chevalet de marqueterie as a cutting tool.

The use of overhead saws is one way to solve the problem. I have in the school an overhead saw which I constructed of welded frames, wood tables and an enormous old Sears Craftsman motor. The need for a larger cutting saw became obvious early in my marquetry career. My first large project was in the 1970's when I received an order for a table top that was 4' x 15' in size. Even though the design of the top allowed it to be cut into smaller sections, the largest section had a 33" length, larger than the 28" throat depth of my chevalet.

I struggled through with that project using only the chevalet. That meant that when I was cutting the 12 different packets, each 33" in length I would hit the saw frame or the depth stop on the chevalet on a regular basis. I would sit and cut the design until the packet would hit the tool, then stop. By careful manipulation of the packet, I would then rotate the packet 359 degrees around to continue the cut a few millimeters, stop and then rotate back to continue the cut. It was difficult to say the least.

Years later, after I had been accepted to study at ecole Boulle, I discovered that the school had several 19th century iron jig saws, including an overhead saw, which was suspended from the ceiling. This saw had a base, with a speed control operated by the foot, and a separate frame which hung from the ceiling and contained a wood bow for returning the action. The only connection between the top and base was the saw blade.

As soon as I returned to work, I designed and constructed a similar saw. At that time, the ceiling height of the shop was 8 feet, and I engineered the upper structure to fit. The top frame was welded steel and suspended from a "T" shaped plate. By using a tripod attachment, it was fairly easy to adjust the alignment. The square steel tube was filled with loose lead shot, which I figured would neutralize the vibration. The base was also welded to conform to my design, and included a foot lever that was connected to the leather belt drive.

I determined what diameter wheel would reduce the drive speed of the motor to 180 rpm, and made two wheels. The drive wheel was connected to the drive shaft, and the second wheel was loose on the shaft. Thus, by pushing on the pedal, the drive belt would slide over from the fixed wheel to the loose wheel, causing the speed to drop gradually. By carefully pushing on the pedal, I could adjust the speed from 180 rpm to a complete stop.

The top structure was designed to be adjustable so that the blade alignment was exactly vertical in both directions. The ash bow was made to return the blade and lubricated with some grease. The bow was connected to the upper saw clamp by leather belts purchased at the sewing machine store. (Yes, you can still purchase leather belts at some stores,)

When I built the second floor on the shop for the school, I made the ceiling height 10 feet. That required me to order another welded steel frame to make up the difference in height. It took several men to move the saw. I attached the "T" frame to the ceiling rafters so that it is bolted in 4 places along the top of the "T", with each bolt connected to a ceiling joist at the same distance from the wall. The longer arm of the "T" is only connected to a single joist at one bolt position, several feet away from the wall. The reason is that, during the day, as the roof moves heats up and cools down, the joist moves slightly. I can easily correct the blade alignment by turning the single mounting bolt a half turn one way or another.

The saw also has a table which lifts up for larger projects. The distance to the wall is 54" which is the same dimension for the inside width of my manual veneer press. I could have made the distance larger, but I decided to make it the same as the press.

I also am posting a picture of the chevalet with the longer saw frame. This is one of the normal features of the chevalet. You can replace the saw frames easily to adapt to the size of the project. I always choose the smallest frame which will complete the project.

I do not want to go back to those days when I had to cut 33" packets in a 28" saw.

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