3. Motor-Gearbox To Shear-Gearbox Coupler

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This project automates PCB cutting by connecting an electrical motor to a manual shear. The motor shaft attaches to the shear’s pivot (where the lever arm was) using a brass coupler.

A brass coupler with set screws that hold a round motor shaft on one end and a square pivot shaft on the other end.

A brass coupler with set screws that hold a round motor shaft on one end and a square pivot shaft on the other end.

The coupler has a round hole on one end for the gearmotor shaft. The round shaft is held in place by a pair of set screws.

On the other end, the coupler has square hole that matches the pivot shaft on the shear where the manual lever is normally attached. A pair of setscrews keeps this end attached, although technically the coupler would operate without these screws. Basically, it would work similarly to a square socket wrench.

Simple motor couplers with round shaft holes are easy to make on a lathe. However, this motor coupler needs a square hole, so a milling machine is used instead.

Because this coupler won’t be spinning rapidly, it doesn’t matter if the holes are drilled in the exact center of the rod for balance, nor does it matter if the coupler shape is round (a square or irregular shape would actually be fine). But, regardless of the outer shape, the round shaft hole and the square shaft hole must be centered relative to each other.

Finding the center of a solid brass rod using an indicator and a digital read out.

Finding the center of a solid brass rod using an indicator and a digital read out. (Note: This is a side view where #1 is the rear of the machine)

There are lots of techniques for finding the center of a circle, disc, or rod. Here is a good technique if you have a mill/drill with a digital read out (DRO) and a lighted indicator (such as the Borite Machinist Mate electronic edge finder -- McMaster-Carr #2039A12, $26.73).

  1. Place the visual indicator tool into the chuck or collet.
  2. Place the rod into the v-groove of a machining vise.
  3. Position the tip of the indicator until it touches the back side ➀ of the rod at whatever appears to be roughly center.
  4. Zero out the y-axis of the digital readout.
  5. Without moving the x-axis, lift and position the indicator until it touches the front side ➁ of the rod. For my 1-inch diameter rod, the digital readout now reads 0.95. It isn’t exactly an inch because I wasn’t exactly centered. No big deal.
  6. Mentally divide the digital readout value in half (0.95 ÷ 2 = 0.475) and move the y-axis until the display reaches that value (0.475). It is now centered vertically.
  7. Reset the y-axis to zero.
  8. Repeat these steps for the x-axis by touching the left side ➂, zeroing the x-axis, touching the right side ➃, dividing the read out by two, moving to that position, and zeroing the x-axis.

At this point, the indicator is in the center of the rod and the digital readout reads 0,0. This technique works equally well for finding the center of rectangular, hexagonal, and other axis-symmetrical workpieces.

Drilling the hole for the motor shaft.

Drilling the hole for the motor shaft.

In this case, the motor shaft hole diameter (8 mm) is narrower than the square hole (12 mm) for the shear pivot. Therefore, the motor shaft hole is drilled first, all the way through the coupler. Not only does this remove material to make it easier to drill the larger square hole, but it acts as a pilot hole to ensure the shafts are centered.

Don’t remove the coupler from the vise to flip it over. There is no need to do so. The hole on the bottom is the same as the hole on the top. Removing the coupler at this point would only increase the chances of misaligning the next hole to be drilled.

Not only are short (stub) length drill stiffer, but they can fit into smaller drilling machines without having to move the table or workpiece.

Not only are short (stub) length drill stiffer, but they can fit into smaller drilling machines without having to move the table or workpiece.

The square hole is fairly large and fairly deep. It is best to remove as much extraneous material as possible with a drill, as that is more efficient.

The standard length 12 mm diameter drill is too long to fit into the drill chuck without moving the table or removing the coupler. But, that would defeat the whole purpose of getting everything nice and centered. Fortunately, I routinely use short-length drills as they are stiffer and they also have the advantage of fitting better in small milling/drilling machines.


The larger drill hole centered nicely over the motor shaft hole.

As you can see, since nothing has been moved, the larger drill hole is nicely centered over the previously drilled motor shaft hole. A lot of material has been quickly removed by the drill that otherwise would have slowed down the square milling operation.

Machining a Square Hole

Believe it or not, there are special drills that can actually create a square hole. These are called Harry Watt drills or rotary broaches (available from Slater Tools). They’re based on the Reuleaux Triangle.

Unfortunately, I don’t have such fancy drills.

Milling an internal square hole with a clockwise motion of a 1/8 inch end mill.

Milling an internal square hole with a clockwise motion of a 1/8 inch end mill.

My technique simply uses a small diameter end mill (1/8 inch) to cut out the corners. Each pass mills away a tiny amount of material until the desired depth is reached.

Making progress on milling a deep square hole by gradually cutting away the corners of a round hole.

Making progress on milling a deep square hole by gradually cutting away the corners of a round hole.

I used the digital readout to determine when each corner was reached. To avoid errors due to rolling past the desired points, I actually milled a square that was slightly smaller than the final desired dimensions and then skimmed off the final amount in one last pass. The whole thing took a couple of hours.

If you don’t have a digital readout, you can add adjustable physical stops or LED switches to the x and y axis. For initial positioning, simply press the tip of the mill against the far edges of the larger round hole that you drilled and set the stops.

Another possible approach would be to drill out the corners with a narrow drill and then use a file to square them up. Or, for this application, a large end mill could cut the coupler from the side (like the letter 'C' with a square inside) and a flat plate could be screwed on to make the fourth side.

Drilling set screw holes with an edge of the square hole parallel to the bottom of the vise.

Drilling set screw holes with an edge of the square hole parallel to the bottom of the vise.

Finally, place the coupler lengthwise in the vise, with a v-groove holding it firmly in place. For maximum structural strength, be sure to drill perpendicular to a flat side of the square hole where the material is the thickest.

In contrast, the corners receive the most stress and are also the thinnest points. Holes and set screws in the corners would weaken the coupler and wouldn’t make good contact with a flat side of the shear’s pivot.

Of course, none of this really matters if the apparatus fails miserably under load...