Although I can etch circuit boards at home, I usually order commercially manufactured PCBs to save time. One company’s prototyping service offers an inexpensive fixed price for three 3.5″ x 2.8″ printed circuit boards. Since many of my circuits are smaller than that, I place multiple designs on a single board. This is called panelizing.
Upon receiving the finished boards, they need to be cut into the smaller circuits. A table saw or a Dremel rotary tool with a cut-off disc works well, but produces a lot of dust. A hacksaw also works, but leaves the edges ragged.
The recommended approach for cleanly trimming boards or separating panels is to use a shear. A shear consists of two heavy-duty sharp blades bolted to a large base. The sharp blades slice through the board starting at one end, just like a pair of scissors cutting through a piece of paper. The shear produces straight edges with no dust.
For Christmas, my friends gave me a combination miniature shear/brake from Micro-Mark (#83213, $200). It is rated for cutting circuit boards up to 1/16-inch thick. But, it takes a lot of effort to do so. I’m a big man with plenty of shop muscle, yet I need to lean down on the handle to force the blade through the board.
Having to strain means I don’t get to concentrate on holding the board in place. This tends to result in an angled or imprecisely aligned cut. Also, I am reluctant to use the shear for larger batches of boards, because it is exhausting work.
Larger industrial shears use hydraulic power controlled by a foot pedal so that your hands are free to position the workpiece. I thought, “Why not simply add a gearmotor and foot switch to the Micro-Mark shear?”
A PCB placed between two shiny blades of a small shear, with a gearmotor replacing the manual hand lever.
The goal of this project is to replace the hand lever with a reasonably powerful gear motor that I can operate with a foot switch. Because the shear was a gift, I self-imposed the requirement that the modifications be downgradable to the original configuration. That is, none of the alterations should be permanent.
It turns out that was a really good requirement. This story has a sad ending.
As you read through these pages, try to guess which parts break.
As stated earlier, the shear needs a lot of torque to cut through printed circuit boards. So, I knew the gearmotor needed to be powerful. In this case, I selected a 24V DC Escap motor (#34L 11 219E 5) with 2178:1 ratio gearmotor (P42 14 0 2178). The output shaft rotates at approximately 4 RPM @ 24V. The motor was purchased on eBay for $50 including shipping.
A brass coupler connects the motor to the shear. Bolts and a plate hold it in place.
Screws attach the gearhead to a 3/16-inch thick custom adapter plate. Four 1/4-inch bolts connect the adapter plate to the shear’s cover.
A square hole in a motor coupler mates with a square-shaped handle pivot on the shear.
The most complicated part is the custom-made coupler that connects the motor shaft to the square handle pivot -- where the manual lever normally attaches.
The pivot has a square shape. Due to the torque required, it seems best to include a matching square hole in the coupler, rather than a round hole with a wimpy setscrew.
All of the fasteners (bolts, nuts, and screws) are stainless steel for strength and rust resistance. The custom coupler and adapter are brass for rust resistance, reasonable strength, and ease of machining.
On the next page you'll see how to machine the gearbox cover and adapter plate. After that, the machining of the coupler with a square hole is described. And, finally, the damaged parts are examined.
