Motor Mount Block:
  1. Screw Holes
  2. Drilling
  3. Clamping

2. Drilling Holes for a DC Motor Mounting Block

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To make the motor mount, begin with a chunk of non-brittle material. Most plastics and metals will do.

A metal block (especially aluminum) may allow the motor to run cooler -- acting like a large heat sink. Steel has the added advantage of providing a return path for the magnetic field, which may theoretically improve motor efficiency. But, steel rusts.

Plastics are generally easier to machine and usually lighter. Don’t choose a brittle plastic like acrylic, because it will crack. Instead, choose Delrin/acetal, polycarbonate, nylon, polyethylene, or polypropylene.

You can square up a piece of scrap block on a milling machine. Or, simply purchase an almost-ready rectangular bar stock piece from a material supply shop like McMaster-Carr or MSC Direct. Most bar stock comes in lengths by the foot or half-foot. Just cut it down to the length of the motor using a hacksaw. The rough end can be milled clean or left as is.

Measure the diameter of your motor with calipers or look it up on the datasheet. Note that most miniature electric motors have metric diameters.

Drilling a large hole in the center of a plastic block to insert a motor.

Drilling a large hole in the center of a plastic block to insert a motor.

In the block, drill out a hole that matches the diameter of the motor. Because this type of mounting block uses clamping action, you can get away with drilling a hole that is slightly larger than the motor diameter (if you don’t have the right drill size).

If the motor diameter is larger than the maximum diameter of your drill chuck (usually 1/2-inch), you can purchase a reduced shank drill (also known as Silver & Deming) that has a larger cutting end than the holding end. Of course, if you have a boring bar accessory or a lathe with a four jaw chuck and boring bar, then you can bore out a hole of almost any size.

Drilling two holes in the side of the motor mount.

Drilling two holes in the side of the motor mount.

On the side of the block, you'll be drilling two holes for screws. Screws will eventually tighten the motor hole around the motor. (You might want to skip ahead a page to see a picture of the mount in action, and then come back.)

Half of each screw hole needs to be clear (loose / no screw threads) and half of each screw hole needs to be threaded. To accomplish this, first drill the hole with a tap size (smaller) drill and then, without moving the x-y table, swap in a clearance size (slightly larger) drill and plunge it only halfway.

For example, if the desired screws are #4-40, then the first hole should be drilled all the way through with a #43 drill. (That’s the correct size hole for adding #4-40 threads.) Then, without moving anything, switch to a 1/8-inch drill (or #30-#32). (That’s the correct size hole for a screw to be inserted or withdrawn without being turned.) Drill the same hole again, but this time only half way. By doing so, the bottom half of the hole can have threads tapped in it and the top half of the hole allows the screw to drop right in.

Once the holes are drilled, insert a tap to create the threads.

If this is too confusing, you can drill the entire hole with a clearance-size drill and use a nut on the other side, instead of threading the block. But, that’s not as elegant as the design shown here.

There’s only one more step...