A Jig for Marking Clock Wheels for Cutting

The jig that I am about to describe assumes that you have already turned a disk (wheel blank) to the desired diameter and have drilled the hole for the arbor (axle). The measurements assume the wheel to be of wood, to be 5/16 inch thick and to have an arbor hole of 1/2 inch (although the design may also be used for an arbor hole of 3/8 inch, if you use a second jig arbor). The jig may be adapted for use on metal wheels. I will assume the use of modern tools, throughout. Especially necessary will be a drill press.

One of the problems that this jig is meant to address is the phenomenon of accumulated error. If you were to take a clock wheel blank of 3 inch diameter and desire to mark it for 56 teeth, those teeth would have a particular spacing (in this instance, on the order of 0.1683 inches). If you were to simply take a ruler or compass and try to mark such a distance evenly around the circumference of the wheel, each marking would be in error by some miniscule amount, for such is the inevitable consequence of hand work. For the sake of argument, let us postulate that each marking would be out by about 1/64 of an inch, which is about the width of a pencil mark on wood. Over the course of the entire circumference, this could add up to the space of five (5) teeth! If this error were to be always in the same direction, it would mean that your wheel would have 51 teeth, or (if the error were in the other direction) 61 teeth. In reality, the errors tend to "wander around"... and that would result in a wheel with somewhere between 51 and 61 teeth. Hardly the sort of accuracy needed for marking a clock wheel. This is the full form, substance and curse of accumulated error!

Since the physical error (the 1/64 inch used in the previous example) can only be reduced to a particular lower limit, what you want is to find a way to reduce the effect of this physical error on the finished product to an acceptable level.

On a wheel with a 3 inch diameter, the 56/64 inches of possible error amounts to nearly a staggering 10% of the total circumference! If the wheel were 20" across, though, that same 56/64 inches would represent only about 1% of the total circumference (i.e. about half a tooth width). This jig takes advantage of this physical property to reduce accumulated error.

Start with a round disk of 5/8 inch thickness and about 20" diameter (the exact diameter is not critical, but the larger it is, the less error will show up in your finished wheels). I like to use dense particle board for this, but 5/8 inch plywood could be used. I prefer the particle board because I know that there will not be any voids inside it that could deflect a pin set into any hole drilled into it.

Locate the approximate center and lightly tap in a small nail. Using this nail as a center point, draw several concentric circles on the disk, starting at the outside and working inward in 1/2 inch intervals. You will need as many circles as you have need for wheels with different numbers of teeth. (To draw the circles, I use a piece of wood with a nail hole in one end and several pencil sized holes drilled along it, but a piece of stout string would do, instead.)

The wee hole left by the small nail will be the guide by which you later locate center, so make sure that you only make one (1) hole and that it doesn't get confused with any other holes or marks on the disk.

Take a straight edge and line it up on the center hole, then draw a line running out from the center to the edge. The starting points will be where this line crosses each of the rings that you have just drawn (see figure 1).

Figure 1

Ensure that the table on your drill press is set to be exactly perpendicular to the drill. This is necessary!

What you want next is a 1/2 inch hole that is exactly perpendicular to the plane of the large disk. Since most drill presses are not large enough to do this to a disk that's 20 inches in diameter, we've got to "cheat" a little.

Cut a disk of some soft wood (such as pine) to 3 inches diameter and drill as follows:

- in the center of the disk, drill a 1/2 inch axle hole of such depth that just the tip of the drill barely goes through to the other side. If you use a forsterner bit, this means that you will have a hole that is 1/2 inch in diameter almost all the way through, with just a pinhole in the middle of the bottom; and

- Drill holes for three screws all the way through, but no closer than 3/4 inch from the edge of the hole in the middle. Drill the holes to be larger than the threads of the screws, so that when the screws bite into the material below the small disk, tightening the screws will pull the small disk to the large one. Drill countersink holes for the screw heads.

Prepare three finishing nails by filing or grinding their points to be very sharp. Pre-drill three holes just a bit smaller than the diameter of the nails and, from the back side, drive in the prepared finishing nails until their points protrude from the other side by about 1/16 inch. Carefully cut off the heads of the nails using a nipper, end cutter or hack saw. (If all you have is a side cutter, drive the nails so that the points are just barely visible on the other side, use the side cutter, and push the nails in until nothing protrudes from what had been the head end. That should leave about 1/16 inch of very sharp point sticking out of the face of the small disk.)

Take the same nail that you used as a center for drawing the circles on the large disk and push it through the tiny hole at the center of the small disk. By putting the nail into its hole in the large disk, you have now centered the small disk on the large disk so that the axle hole (the 1/2 inch hole) in the small disk is at the exact center of the circles you have drawn and exactly perpendicular to the large disk.

Secure the small disk to the large one with screws (using the screw holes that you already drilled). Make sure that the heads of the screws are not sticking up above the level of the surface of the small disk.

This small disk now provides you with the 1/2 inch hole that is exactly perpendicular to the plane of the large disk, and exactly centered on the circles that you have already drawn. It also provides a means by which the wheel blank can be held from turning during the marking process (i.e. the nail points). See figure 2 for a side view of this disk.

Figure 2

You will be cutting wheels of various tooth counts on this jig, such as 56 teeth, 50 teeth, 48 teeth, etc. Your best advantage is to use the outermost rings for the higher tooth counts and the inner ones for the lower tooth counts. I'll walk you through the process for the most common sized wheel: the 48 tooth wheel.

Please excuse me while I shift gears from inches to millimeters (mm). Let's assume that the ring that you'll be using for your 48 tooth wheel is located 230mm from the center. The circumference would be 2 X radius X pi, which would be 460 X 3.1416 = 1445.136mm. Dividing that number by 48 teeth gives a pitch of 30.107mm per tooth. Take a compass and set it as closely to that measurement as you can. Make sure that the pencil on the compass is reasonably sharp, since width of line errors will quickly add up.

Label the ring at 230mm as being the 48 tooth ring. Start at the "starting point" and measure off segments on the line. When you get back to the starting point, you will likely be off by a bit. Count the markings (to make sure that you got 48) and adjust the compass accordingly. Repeat the process until you get 48 evenly spaces marks around the 48 tooth ring.

Take the disk to the drill press and drill a small hole at each of these marks. You will later be dropping a pin into this hole to act as a guide, so select what you'll be using as a pin (usually a small, headless nail) and select a drill size that's large enough that the pin may be put in and taken out easily, but not so large that the pin will "rattle around" in the hole. Do not drill all the way through the disk; leave about 1/8 inch at the bottom undrilled. That way, the pin will not fall through the disk.

If you are going to use only 1/2 inch arbors in your clock, this next step is easy: just get a 1 1/4 inch length of 1/2 inch dowel to use as the jig axle. Put the axle in the hole in the middle of the disk and push your wheel blank down upon it until the nail points push into the backside of the blank. Set your marking bridge (with a 1/2 inch hole in it) onto the axle and proceed to mark the wheel. (I'll get to how to make the marking bridge in a moment.)

If you intend to use various sized arbors, you will need to make a jig axle for each size arbor you will be using. These will need to be turned on a lathe to exact measurements. The bottom end of each axle will be 1/2 inch, the middle of each axle will be the particular arbor size and the top end of each axle will be the size of the smallest arbor that you intend to use. The marking bridges will all be made with a hole that is the size of the smallest arbor.

The marking bridge is the key to this whole jig. It consists of a horizontal bar of wood, drilled at one end to fit over the jig axle, and with a short pedestal at the other end, the bottom of which sits upon the large disk and is high enough to allow the bridge to remain horizontal when there is a wheel blank on the jig. Affixed to the bottom of the bridge is the template, a thin piece of metal (I use copper) with a notch cut out of it that represents the space between the teeth of the wheel (see figure 3).

Figure 3

The template is secured in such a way as it does not move, once installed.

Remember, for each number of teeth, you can have wheels of differing diameters. Each bridge is good for one diameter/number of teeth combination and should be clearly labelled.

To use the jig, put your turned wheel blank on the axle and press down to seat it on the nail points. Place the bridge on the axle at one end and on the disk at the other. Place a pin in the starting hole for the ring that represents the number of teeth that you wish on the wheel. Hold the bridge up against the pin and mark out the template. Move the pin to the next hole and repeat the process. Do this for each hole. When you are finished, you will have a disk with all the teeth clearly marked out and ready for cutting.

Photo of jig