2. Machining a Gearbox Cover and Matching Adapter Plate for a PCB Shear

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I considered a number of different methods of attaching a motor to the Micro-Mark shear. Upon closer examination, I was pleasantly surprised to discover that the shear’s side cover detaches from the main body. This makes it really convenient to drill bolt holes in the cover without metal shavings getting into the mechanisms.

Side cover removed from the Micro-Mark shear reveals the integral gearbox with an advertised 3:1 ratio.

Side cover removed from the Micro-Mark shear reveals the integral gearbox with an advertised 3:1 ratio.

Looking at the shear’s gearbox, we can see that it:

The gearbox cover raised up in a vise for drilling.

The gearbox cover raised up in a vise for drilling. (See lubricant note in text below.)

The shear’s gearbox cover is placed in a vise on a drill press or a mill/drill machine. Flat pieces of metal (called parallels) are located underneath the cover so that the drill can penetrate the cover without drilling into the vise itself.

The gearbox cover is likely made of cast iron. Therefore, relatively slow machining is required to prevent the drill from dulling prematurely.

The end of this article will be an example of “how NOT to” rather than “how to”. To begin with, cast iron should be machined dry to prevent tool dulling -- not machined with lubricant as portrayed in the above photograph. (Thanks again, Kirk.) I used Tap Magic, which is acceptable.

Starting a large hole in cast iron with a countersink.

Starting a large hole in cast iron with a countersink.

To avoid interfering with the gearbox mechanism, I selected flathead screws that are countersunk into the cover. (It turns out, if the hole locations are chosen carefully that there is probably enough room for standard screw heads.)

I began drilling with an Ultra Tool TiCN-coated solid carbide countersink, assuming it was the sharpest and sturdiest way to start a hole in a ferrous metal. However, the hole size is sufficiently large that my relatively small milling machine shook too much.

Drilling holes before countersinking is easier on light machinery.

Drilling holes before countersinking is easier on light machinery.

A gentler approach is to remove material with a smaller drill before countersinking. There is simply less material for each tool to remove.

I couldn’t decide as to whether the holes should be threaded to hold the bolts in place, or whether to use a clearance hole. My concerns with threaded holes are that the bolts might become angled or that they wouldn’t sit flush since the bolt threads don’t seem to extend completely up to the flathead. I decided to go with a clearance hole and to rely on nuts to clamp the whole thing together. That worked fine, but I wonder what would have happened if I had tried threads.

Left: Flathead screws inserted into the cover before placing the cover back onto the shear. Right: The cover installed on the shear shows possibly enough space for hex-head bolts.

Left: Flathead screws inserted into the cover before placing the cover back onto the shear. Right: The cover installed on the shear shows possibly enough space for hex-head bolts.

The finished holes are sprayed with a rust preventative (Boeshield). Then, flat-head bolts are installed with nuts tightened to hold them in place.

Finally, the gearbox cover is reinstalled onto the shear.

Motor Adapter Plate

The bolt hole pattern is based on the open locations available on the shear’s gearbox cover. The bolt diameter (1/4 inch) and number of bolts (four) is based on the need for strength. None of this takes into account the pattern, size, or quantity of holes on the prospective gearmotor. Therefore, an adapter plate needs to be made to attach to both the shear’s bolts and the motor’s bolts.

A paper template designed on a computer can be used to locate holes.

A paper template designed on a computer can be used to locate holes.

The shear’s bolt pattern is entered into a computer drawing and then combined with the Escap motor’s bolt pattern. The paper template is printed out and placed onto a 3/16-inch thick piece of brass. The template does not need to be exactly centered onto the brass, since the edges don’t matter in this design.

All that really matters is the spacing of the holes with respect to each other. Any misalignment between the sets of holes would cause a misalignment of the shafts. Given the large forces for this project, alignment is critical.

To avoid alignment issues, the holes in the adapter plate are all clearance holes. That is, all of the bolts pass through with some room to spare. By doing so, the motor shaft and shear pivot can first be perfectly aligned with the coupler, and then everything tightened up on the adapter plate.

Besides the bolt holes, a 30 mm diameter hole (1 3/16 inch) must be machined in the adapter plate to accommodate the front of the motor gearbox.

Machining a very large hole using a metal hole saw.

Machining a very large hole using a metal hole saw.

It is possible to buy reduced-shank drills in ever increasing diameters to gradually drill out a large hole. Or, the hole can be bored out using an adjustable boring head.

But, a faster method for cutting large diameter holes is to use a bi-metal hole saw. It is similar to a hack saw, but in tubular form.

I selected a Morse 1-3/8″ hole saw (McMaster-Carr #4008A251, $8.22). The diameter is slightly larger than desired, but I already had it on hand.

As with large diameter drills, it is vital to slow down the drill speed with hole saws. Morse kindly includes a speed chart on the hole saw package, suggesting no more than 330 RPM for brass at this diameter. (My Digital RPM display came in handy.)

An advantage to using a hole saw is that a round slug of material is leftover from the process, rather than just drill shavings. So, I have a small brass disc for a future project.

The adapter plate screws onto #10-32 tapped holes in the motor gearhead.

The adapter plate screws on via #10-32 tapped holes in the motor gearhead.

The motor gearhead mount already includes four holes. I tapped them for #10-32 threads. The finished brass adapter plate is then screwed onto the gearhead.

Let’s see how to make the coupler to bring this all together...