Pantograph shape cutting machines, first patented in 1917, used oxygen-fuel cutting torches for such work. These little workhorses are passe' since "electric eye," and later CNC, technology came about. However they still might have a use for those lacking the space or the need for a more sophisticated system.
Pantograph machines can cut simple shapes just as smooth as a CNC machine. They use a knurled, magnetic rotor traveling around the perimeter of a 1/8" or thicker steel template. The torch tip, directly below, duplicates the shape in steel. A slight size adjustment in the template must be made to offset the diameter of the tracing rotor. The formula for an external shape is to make the template smaller by 1/2 the diameter of the rotor minus 1/2 of the kerf, or gap created by the torch. The opposite is true for internal cuts. They can cut straight lines using a straight edge for a template.
Since the steel template is just as difficult to make as the part being duplicated, the process is most useful for reproducing multiple copies of the same shape. One-offs can be cut by using a wooden template, and holding the knurled rotor (a rotary burr) against the edge of the template, changing the direction of manual pressure as it goes around corners. Internal shapes can be cut by using a template with the desired internal cutout. The motor shaft is moved to the inside of the template, and the magnetic rotor then placed back on the motor shaft. Starting at the edge of a drilled hole simplifies the operation.
There are a number of "tricks of the trade" in pantograph work, just as there are in CNC production. For example, if a steel template has a sharp internal right angle, the magnetic tracer will stick to both edges and stop moving. Solution - undercut the leading edge of the corner so it has less mass than the other edge, and therefore less magnetic attraction.
Earlier pantograph machines required rather complex tracing heads that provided the magnetic pull for the rotor. Today, with the advent of rare earth magnets such as iron boron, inexpensive magnetic tracers can be made by simply stacking a couple onto a rotary burr. A 3/4" x 2" length of drill rod on top of the magnets acts as a reflector plate, and adds about 40% to the magnetic pull.
Inexpensive gear motors, combined with a cheap model railroad transformer can supply adequate variable speed, reversing power.
Long before getting into CNC plasma cutting, I built a number of pantograph cutters. Below are some photos starting with the first one I made (which I still have) up to the last one, which used 8020 aluminum extrusions in its construction.
- 1979 pantograph sm.jpg (955.75 KiB) Viewed 18069 times
- with manual oxy fuel torch.jpg (701.07 KiB) Viewed 18069 times
- manual acetylene torch.jpg (1.86 MiB) Viewed 18069 times
- manual plasma torch.jpg (1.81 MiB) Viewed 18069 times
- Pantograph plasma.jpg (714.68 KiB) Viewed 18069 times
- machine torch.jpg (1.94 MiB) Viewed 18069 times
- street rod parts.jpg (1.39 MiB) Viewed 18069 times