Dug’s Automata Tips, Techniques and Tricks No.4
June 15, 2011 by Dug North
Attaching Cams, Pulleys, Cranks, Gears, and Handles to Wooden Shafts
To get things moving in an automaton, some parts must be attached firmly to round wooden shafts. But how? And which way is best for your project? Read on! The techniques discussed below apply to cams, pulleys, eccentrics, cranks, gears, handles, and other parts. I’ve used the term “cam” throughout to simplify the descriptions.
Glue the Cam to the Shaft
If you don’t plan to remove the cam from the shaft, glue is certainly an option. However, if you need to fix or change the mechanism, it’s nice to be able to take it apart easily! That said, glue has its place — especially for simple automata with few parts.
Tip 1: Try using a small amount of cyanoacrylate glue (e.g. Super Glue or Krazy Glue). These glues have a low shear strength. Should you need to remove the cam, a sharp blow parallel to the shaft can break the glue’s bond, hopefully without damage. Apply a bit of acetone or nail polish remover first to soften the glue.
Tip 2: If you don’t want wooden parts to come apart, use polyvinyl acetate (PVA) wood glue, commonly known as “yellow glue” or “carpenter’s wood glue”.
Drive a Fastener Through the Cam
Drill a pilot hole into the edge of the cam and into the shaft, then insert a pin or faster. Generally, this method is easiest to use on thicker cams that are not too large. You have several fastener choices:
Small Nails (also known as brads, panel pins, and gimp pins)
A small-headed round-wire nail can be driven though the cam and into the shaft. Try to get the nail all the way through the shaft and into the opposite side of the cam. If the nail has a raised head, drill a slightly larger opening to recess the head. This is a simple, tidy solution that can be used on smaller diameter shafts, although the nail can be difficult to remove without damaging the wood.
Small Wood Screw
A long, narrow wood screw can also be used. This approach requires a thick cam. It also helps if the cam has a spot on its circumference that is not too far from the shaft. You’ll want to countersink the screw head so that it’s below the surface of the wood. This method is reversible, strong, and particularly useful for attaching hand-cranks to the ends of shafts.
A short length of brass rod can be inserted into a hole drilled all the way through both cam and shaft. The hole in the shaft should be no larger than 1/3 the diameter of the shaft. Try to drill the hole through the exact center of the shaft. Because the rod can be pushed through and out, this arrangement is easy to take apart.
Wooden pegs can be used in a similar manner to the brass rods mentioned above. Pegs made from toothpicks can be used on somewhat smaller shafts and are nearly invisible in cams made of Baltic birch plywood. Hardwood pegs of a larger diameter are easier to remove, however.
Tip 1: Use a V-block to help you drill straight into the edge of the cam. This helps to ensure that the pin enters perpendicular to the shaft and is parallel to the cam’s sides.
Tip 2: Toothpicks are between .075 to .086 inches in diameter. Use a drill bit between #49 and #44, working your way from the smaller to larger size to get a perfect fit.
Use a Screw Eye and Screw
For this method, a screw eye is turned into a pilot hole in the shaft. Slide the cam onto the shaft, against the screw eye. Finally, drive a small wood screw through the eye and into the cam. A well-chosen flat head wood screw will seat neatly inside of the screw eye. This method is easy to disassemble, provided that other parts of the mechanism are not in the way. This method may be used for somewhat thinner cams.
Tip 1: Over-tightening the screw can cause the cam to tilt on the shaft. If this happens, use a thicker cam and ensure it fits tightly on the shaft. You can also bend the screw eye slightly to offset a tilt.
Tip 2: Seat the screw eye into the shaft such that its neck is buried completely into the shaft. This allows the lower portion of the screw eye to bear some of the rotational force.
Collar Glued on to the Cam
Glue a small block of wood, round or square, onto the cam to serve as a collar. Then, attach the collar to shaft with a fastener. The collar should be securely glued to the cam. For the strongest glue joint, orient cam and collar such that the long grain of the wood is used as the gluing surface for both pieces. The hole running through both cam and collar should be carefully aligned. This technique is the best choice for thin cams, and is the easiest to disassemble.
Tip 1: If aligning the holes in the two pieces proves difficult, use the base end of a drill bit the same size as the shaft to align the parts while gluing them. Rub a bit of candle wax on the drill bit to prevent the wood glue from sticking to it.
Tip 2: A tight fitting wood screw with its tip ground flat can act a set screw that presses against the shaft to hold the cam in place. This arrangement will allow you to reposition the cam on the axle. Be careful though: don’t apply too much force to the screw or expect it to hold under a lot of pressure.
There are many other ways to attach parts to wooden shafts, but these methods should serve you well for most situations. And as for attaching things to metal shafts: we shall have to leave that topic for another time!
A quarterly column by automata-maker and enthusiast Dug North
Copyright 2011 Dug North
Warning: The topics covered in this column include the use of tools and materials that have the potential to cause damage to property and/or bodily injury. Your safety is important and it is your sole responsibility. Always read and follow the safety instructions that come with tools and materials you use. Wear safety glasses, use guards and other forms of safety equipment, follow safety precautions, and use good judgment. Seek the guidance of experienced outside sources whenever required.