case mod 101: cutting a freestyle, low-profile window

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There are dozens of different bits for a rotary tool, but the fiberglass-reinforced cutoff wheel remains a consistent favorite for cutting steel cases. Regular cutoff wheels work as well, but they shatter more easily. Cutoff wheels make it easier to cut a straight line freehand than other bits. Having a flexible shaft is a nice feature, but by no means is it necessary. Many times a shaft attachment comes in a kit with the tool. If your tool didn’t, don’t worry. It will cut just as well without it.

flex shaft and reinforced cutoff wheel
the Dremel multipro with Flex shaft and fiberglass reinforced cutoff wheel
flex shaft grip
the thinner grip of the flex shaft is easier to handle, but the tool has to be supended overhead

The real secret to making clean cuts is to let the speed of the tool do the work for you. A cutoff wheel is grinding away the metal along its edge. The tool motor spins that bit at up to 35,000 rpm, but as soon as you touch it to the work piece, friction slows it down. Using a light touch, draw the edge of the wheel along the line you marked, against the direction of rotation. You will see a very shallow groove form in the material. Sparks may fly in the direction you are moving the bit (probably not if you’re cutting aluminium, but almost certainly for steel). Even, steady hand motion is the key to keeping the bit traveling in a straight line. Here’s what the groove looks like after a single pass.

sparks fly
oh yeah! sparks are flying!

Once you get a groove started, the bit will naturally want to follow it. Try to keep the depth of your groove fairly even. Do not force the bit through the workpiece. Draw it across the workpiece with a light touch, and back off the pressure if you hear the motor slowing down. If you use the flex shaft, you won’t want to use top speed, but with the bit mounted directly to the tool, you can. You may find that backing the speed down a bit from the highest setting makes the tool easier to control. Practice on some scrap until you have a good feel for the tool.

light pressure
light pressure on the bit, and let the rotation do the work
cut the groove first
the groove is not cut through the workpiece

After a number of passes, you’ll feel the tool break through the panel. Right at this moment is when the bit has a tendency to jump out of the groove. The lighter your touch, the less of a problem this will be. Resist the urge to put the bit through the opening; it will jump around if it catches an edge.

cut through the workpiece
this cut has just broken through the back

Cutting a rounded corner is trickier than a straight line. It’s harder to advance the tool in a steady motion, but if you cut from inside the curve, you can place your hand and pivot the tool, adjusting your grip to get the radius you want. It’s natural for the width of your groove to increase a bit on a curve since the bit is straight. Repeated passes over the same path will get the job done. It’s better to leave a bit of material behind, especially on curves. It’s easy enough to sand or file the edge down once the cutting is finished.

corners are tricky
corner work is tricky, but you can handle smooth corners fine with a rotary tool


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