6. Cutting and Drilling Pencils and Brass Rods

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Pencil wood is relatively soft. During sumo combat, would the forces split the ends of the pencils that connect to the rod? If so, it would be time consuming to replace the pencil. Either the robot would be disqualified or it would need to compete with a missing arm.

To prevent breakage at the most stressed point, I created a sort of tubular washer. In many applications, a washer spreads the load over a larger area. By applying metal tubing to the ends of the pencils, both rotational wear and combat stress could be absorbed by tougher material.

No industry-standard tube seems to fit firmly on a pencil. Instead, I turned to a metal tubing assortment from MicroMark (part #83260). There, I found an irregular tube (either it has thicker walls or it is metric) that has an inner diameter of approximately 0.28 inches.

Marking and cutting off brass tubing with a miniature circular cutoff saw.

Marking and cutting off brass tubing with a miniature cutoff saw.

I marked off 1/2-inch lengths on the hollow brass tube. The actual finished desired length is 3/8-inch, but the saw is going to cut away some material and the ends will be shortened when the burrs and rough spots are milled or sanded down.

I used a circular cutoff/chop saw from MicroMark. It holds round tubing easily and uses thin blades that reduce the amount of wasted material.

A hacksaw can be used, but you might need to insert a wood rod in the middle to keep the tubing from being crushed as it is cut. A standard tube cutter should be avoided, because it rounds and curls the ends of the tubing somewhat. Although this is a nice feature for some purposes, the rounded ends will prevent the tube from fitting onto the pencil.

Pencils cut to length with a brass tube fit onto the cut end.

Pencils cut to length with a brass tube fit onto the cut end.

Four pairs of pencils are cut to arbitrary lengths using a hacksaw with a fine-toothed blade. If you want matching pairs, you should tape a pair of pencils together and cut them at the same time.

Although not strictly necessary, I machined the cut ends smooth on a milling machine. The center of the pencil is graphite, not lead, so there isn’t a health hazard. Graphite can lead to aluminum corrosion (it prevents the oxide from forming a protective surface), so don’t machine graphite around aluminum parts.

Drilling a pencil with a brass tube cap in a milling machine vise.

Drilling a pencil with a brass tube cap in a milling machine vise.

The hexagonal (six-sided) shape of a pencil makes it easy to hold in a vise. A flat piece of scrap material can be placed underneath to make sure the pencil is lying flat. Make sure the brass end is not resting on the scrap piece, because it is slightly thicker than the rest of the pencil which means the pencil won’t be flat.

Since this isn’t a precision operation, you can line up the drill visually. Use a drill size equal to the outer diameter of the brass tube that will be inserted into the hole.

The pencils are almost finished...