Aluminum Angle Stock Gearmotor Mounting Bracket

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Aluminum 90-degree angle stock is a popular material for making motor mounting brackets. Aluminum is lightweight, corrosion resistant, inexpensive, and readily available.

Because angle stock is extruded in an 'L' shape, it can be thinner and lighter than flat thick pieces attached at a right angle by screws. Angle stock is available from McMaster-Carr, MSC Direct, and local hardware stores.

A chop saw with a metal cutting disc cuts through aluminum angle stock.

A chop saw with a metal cutting disc cuts through aluminum angle stock.

Angle stock can be cut using a hacksaw. But, that method is slow and can result in a bent angle and meandering edge. A table saw can be used, although it can be somewhat awkward to push long stock over a short table.

My preferred method is to use a chop saw with a metal-cutting disc or metal-cutting blade.

The freshly cut edge of angle stock may be rough. Mill and grind (or sand) to make a nice clean edge.

The freshly cut edge of angle stock may be rough. Mill and grind (or sand) to make a nice clean edge.

Depending on the blade and machine tool, the edge of the angle stock may be rough and crooked. If the edge is crooked, it can be squared-up on a milling machine. Or, the edge can be left as-is since the angle of the edge of the bracket is often not critical.

But, rough edges should be ground on a grinding machine (or Dremel) and sanded smooth to avoid metal burrs from getting into your robot or cutting your hands.

Angle stock mounted in a milling vise with support from wood stock. Left: Drilling holes (good). Right: Milling slots (bad).

Angle stock mounted in a milling vise with support from wood stock. Left: Drilling holes (good). Right: Milling slots (bad).

Because of the size and shape of angle stock, it can be awkward to drill holes in it on a drill press. The setup pictured above is okay for drilling, but lousy for milling.

One side of the angle stock is held firmly in the vise with the face side raised up for drilling. If the face of the angle stock were to rest directly on the top of the vise, the drill would mar the vise as it penetrated the angle stock. So, some flat wood stock is placed between the top of the vise and the face of the angle stock. Not only do the wood blocks protect the vise, but the wood blocks also prevent the angle stock from bending as it is drilled.

However, when I tried to mill out some slots, the thin aluminum angle stock twisted away slightly, resulting in jagged slanted slots. I was lucky that I didn’t break an end mill. If the workpiece moves during machining, it isn’t secure. And the golden rule in machining is: always, always secure the workpiece.

A better approach is to first drill some mounting holes. Then, create a plate or fixture that can be held on the milling table or vise, and screw the face of the angle stock to the fixture. Don’t rely on holding the angle stock from the other side of the angle. The material is simply too thin and too far away from the machining point to be rigid.

Left: Wood blocks hold up the face of angle stock during machining. Right: Nasty burrs in architectural-alloy aluminum.

Left: Wood blocks hold up the face of angle stock during machining. Right: Nasty burrs in architectural-alloy aluminum.

As you can see, wood blocks work well to hold up the thin aluminum angle stock against downward forces. But, the blocks don’t provide any support against lifting or side-to-side forces.

One of the common issues with machining aluminum is that it is a bit gummy. That is, it doesn’t cut cleanly. In particular, extruded aluminum angle stock is primarily used in architectural applications where corrosion resistance is valued over machinability. Therefore, expect to do some extra grinding and sanding to get all of those hole edges clean.

An unusually complex setup to hold two pieces of angle stock while machining a large hole using a rotary table. A large hole for a motor in a finished angle bracket.

Left: An unusually complex setup to hold two pieces of angle stock while machining a large hole using a rotary table. Right: A large hole for a motor in a finished angle bracket.

Usually, only the motor shaft is going to stick through the bracket, thus only requiring a simple drilling operation. However, for some reason that currently escapes me, I decided to have the entire motor pass through the face of the bracket. That meant that a relatively large hole (38 mm diameter) needed to be machined in the angle stock.

To save time (and to further complicate matters), I decided to cut both motor brackets simultaneously. A 1/2-inch diameter pilot hole was already drilled when the angle stock was in the milling vise. That way, both brackets could be centered in the same imaginary circle by inserting the shank of the drill during this setup. After tightening the bolts, the drill is removed.

The large hole is made by cutting out the circumference of the circle drawn on a paper template taped to the workpiece. Insert a narrow diameter end mill and line it up on any point of the paper circle outline. With the end mill spinning, gently lower it down to begin cutting a short depth of metal. Rotate the dial on the rotary table so that the entire circle passes under the mill. Lower the mill further and repeat the rotary table revolutions until the face is entirely cut through.

Because this setup relied on holding the face of the aluminum angle stock rather than the other side, the final hole is clean and circular (refer to the picture above -- on the right side). Compare the clean large hole with the slightly jagged and slightly angled slots. A well-secured workpiece makes all the difference.

Reinforcing a Motor Bracket

Aluminum angle stock is available in a variety of thicknesses. But, with increasing thickness comes increasing weight and cost. For larger motors, sometimes it is more effective to brace the sides of the bracket than to increase the overall thickness.

Scrap aluminum braces are machined square with a rounded corner to fit into an angle bracket. Right: Notice the bulge in the bend?

Scrap aluminum braces are machined square with a rounded corner to fit into an angle bracket. Right: Notice the bulge in the bend?

Braces can be made from scrap pieces lying around the work shop. The scrap just needs to be thick enough (around 1/4-inch) and solid enough to accept screws.

Machine two edges square. That is, make two edges straight and at a 90 degree angle with respect to each other. Then, grind or sand away the corner of the brace to mate with the angle stock. (You don’t need to remove as much material as I did.) The angle stock is purposefully engineered to be thicker and rounded in the corner to avoid stress fracturing.

Aluminum is nortoriously difficult to weld because it oxidizes so quickly. Most people do not possess the proper equipment or training to weld aluminum. Therefore, the aluminum brace should be attached to the bracket with adhesive or screws.

A brace is both epoxied and screwed onto an angle stock bracket.

A brace is both epoxied and screwed onto an angle stock bracket.

Flat head screws are used on the bottom of the motor bracket so that it will sit flush. Standard pan head or other style screw heads can be used on the bracket face.

It is more difficult to drill holes for flathead screws, especially in thin material like angle stock. So, I experimented with attaching braces with epoxy rather than screws.

If the metal is carefully cleaned and a strong (not 5-minute) epoxy is applied, I believe that epoxy might be acceptable for light-weight robots. But, because epoxy is vulnerable to being peeled, and this is a place where a peeling force occurs, screws are more reliable.

Of course, you can simply epoxy the brace in place to make it easier to machine the screw holes. After that, who cares if the epoxy flakes apart?

For the final portion of this article, we'll see how to attach a pulley to a wheel for a belt-driven motor drivetrain.