Welding Tips and Tricks

These days, most shops with steel shape cutting needs either farm the work out to a laser cutting service, or do it in-house on a CNC plasma table. However, it hasn't always been that way.

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.

I have a dc-2400 victor pantograph i use a fair amount the magnetic tracer is getting weak, could you give me more detail on making a new tracer as you described ?

bw500 wrote: ↑Sun May 28, 2023 1:32 pm I have a dc-2400 victor pantograph i use a fair amount the magnetic tracer is getting weak, could you give me more detail on making a new tracer as you described ?

I think the Victor machines are descendants of the old Heath machines. I had one of those and my magnetic stylus kept coming off the template. I tore down the pattern cutter's magnetic spindle and took out the weak magnet that was in there and replaced it with a stack of neodymium magnets that fit very closely. Here is the disassembled spindle next to the stack of neodymium magnets:



What you see is a brass tube with ID 7/8". There are bearing assemblies that press into both ends. In this shot, the lower bearing assembly is already pressed in. Across the bottom of the picture are shown 4 parts. The first is a bearing. The second is the upper bearing assembly/spindle. The 3rd part is a steel stylus with a crappy knurl job on it. It's just mild steel, shank turned to 5/16", outer end turned to 1/2". With a 1/2" diameter stylus, the torch will cut 1/4" outside the template dimensions (ignoring kerf). The last item is a brass nut that goes over the spindle nose and compresses an O-ring to the shank of the stylus. Upper right is a stack of very powerful neodymium magnets. These are probably 50 times stronger than the factory magnet. Now it requires effort to pull the stylus away from the template.

I would like to mention that not all of these magnetic tracing pattern torches have the weak magnet problem. The machine made by Air Reduction Company (Airco) is called a Camograph and its magnetic spindle is quite powerful. I have owned 3 of these machines and they all have strong magnets. So look for an Airco Camograph. They are very well made and if they were not scrapped they usually still work perfectly. The only exception is that they have double worm gearboxes that need to be opened up and cleaned out and filled with NLGI no. 1 grease. This is considered routine maintenance and it makes these machines purr like a kitten again. Big Camograph fan here.

metalmagpie