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Chapter 4. Maple Mike: > The Clubholder Assembly - Pg. 119

MAKE: PROJECTS Eccentric Cubicle sanding block will leave you with perfect circles and a dense layer of sawdust covering a 6' radius. I cut the spacer disk with a 2 1 / 4 " holesaw, then stacked and glued the whole she- bang using 1 / 4 -20 threaded rod and nuts and some extra 2 1 / 4 " disks to really clamp the pieces together tightly. A word of warning: once this lot is glued up, your chance of applying a proper finish inside the gap is next to nil. You can prefinish the inner surfaces of the 7" disks now if it's an important factor; I missed this wee inconvenience until well into the process, and by then it was too late. So sue me. Let's take a moment to consider cams. Consider them irregularly shaped wheels. In their simplest forms, they're a circle rotating around not its centerpoint. The technical term for this motion is eccentric, which may in no small part explain my affection for the damned things. Taken to ambi- tious extremes, they can be shaped to provide startlingly complicated periodic changes in a mechanism. That's just one particular application. An equally strong suit for cams is providing mechanical advantage. In a par- ticularly overt segue, I'd like to draw your attention to the short discourse on "Bicycle Hub Cam Lock Hack" (located at the end of this chapter), which is a useful primer on the usage of cammed levers in clamping mechanisms. It's not really essential reading for this build, as both the club- holder drive pulley and the trigger mechanism that we'll be building in about five pages harness the sublime charms of the cam in other ways, but it is another example of the versatility of the mechanism. Camming the pulley that takes the bungee tension onto the clubholder lets you play with the power curve like crazy. When combined with variable positioning of the club around the radius of the swing arc and multiple latch points for the trigger, you get an extremely flexible "test bench" mechanism for elastic-based power systems. Plus, you can really torque the bugger up for maximum destructive power . . . not that that's important or anything . . . nope . . . not at all. The 3 / 4 " eccentric offset of the pulley component stretches the bungee considerably during the first 90 degrees of rota- tion from a fully slack state, then rather less so during the final 90­100 degrees. The nature of the elastic is such that the tension increase per inch of stretch rises almost expo- nentially; the end result when camming is factored in is a significantly more evenly distributed and efficient delivery of power into the system. We need to fabricate the aforementioned cammed pul- ley component. The cutting diagram details the two 3 1 / 2 " discs that you'll be laminating together to form the blank for the cam; you'll only need to drill out the center hole for the Figure 04.06: The rough-cut club holder disks moment. Rough out the disks, glue 'em up, and leave 'em in the clamps overnight. [Figure 04.07] A lathe would be helpful in rounding out your cam blank and forming the groove, but it's not really necessary. If you have a drill press, now might be an appropriate time to consider fabricating the lathe attachment detailed at the end of Chapter 7. Failing that, run threaded rod through the center hole, add lock washers and a few nuts, and chuck it into your drill/drill press: a steady hand and 40-grit sand- 4 paper wrapped around a piece of 1 / " wood stock will shape the requisite 1 / " deep groove in short order. Be patient, and 4 refresh your sanding surface frequently until you're at the right depth, then smooth out the surface down to 150 grit. Remove it from the chuck and drill a 1 / 4 " hole 3 / 4 " off center. This is the new and eccentric rotation point of the pulley. Drill and tap the hole for the bungee tether point into the rim of the disk as shown. It's threaded #8-32, and yes, you can use machine taps to cut threads into hardwood. [Figure 04.08] Double-check your component orientation and alignment before attaching the eccentric pulley to the back face of the clubholder permanently. The original center point of the pul- ley is a now a dowel point to strengthen the entire assembly. Use it as a target and drill through the assembly with a 1 / 4 " bit. 108