BalancingYet another little task not often looked forward to with any relish is that of cutting the counterbalances into the crankshaft web (con-rod end profiling, 'shaft web cut-outs... is there any task we do look forward to?) I'll offer up my reason for this reluctance. Some parts take a lot of effort to produce, and effort equals time. As the process nears completion, the time investment is large and the opportunity to double the time taken by stuffing up the one in hand reaches a maximum—especially if some final operation requires a less than ideal set-up. Cutting the crank web cut-outs is one of those as it involves an interrupted cut coupled to a problem in workholding.
The purpose of cutting a relief in the crankshaft web is to achieve static balance. It is impossible to achieve perfect dynamic balance on single cylinder engines like ours. The best we can expect is a reasonable compromise by attempting to counter-balance one-half of the reciprocating mass of the engine, namely the piston, wrist-pin, and conrod little-end [LCM]. This is generally achieved by removing metal from the web so that the crankpin will sit horizontal when a weight equal to one-half of the reciprocating mass is acting through the axis of the crankpin. The photo here (from the 5cc Sparey Project shows a shaft set on knife-edges with a donut of the correct weight slipped over the crank-pin. Sadly, even with massive cut-outs, the shaft is not within coo-ee of being balanced. At this point, one can try adding lightness to the piston, but then you run the danger of having an easily distortable piston and subsequent loss of compression. I've had compression punch the crown out of one such light weight piston—a most unnerving occurrence! Obviously, the metal must be removed from the half of the crank web that carries the crank-pin. There are a number of ways of doing this; the picture here shows five of the most common approaches. I'll describe these before looking at workholding. The names used are in no way "official". I've made them up to be descriptive (I hope). Note there are other ways. These are merely the most common.
[A] Crescent Cut-outsA large diameter cutter is used to take a bight out of the web on either side of the crank-pin. The center of the arc is arranged to minimise the amount of metal near the pin, consistent with retaining some strength in the web. The lower extent will be on, or slightly below the diameter of the web. A variant on this scheme is to drill holes in the web either side of the pin. I've even seen old designs that advocate filling these holes with aluminium. The idea being to achieve mass reduction without volume reduction. Another variant is to drill the holes on the opposite side and plug them with lead, tungsten, depleted uranium, or some other easily obtainable heavy metal. [B] Flat Segment Cut-offsThis is similar to [A], but does not require a large diameter cutter and offers some other options for work holding. The key point on this approach, and that of [A], is that the cut-out can extend beyond the diameter line (shown just a trifle exaggerated in the 3D rendering) to improve balance. The reason is that up to a point, the amount of metal being removed by the cut on the "heavy" side is less than the amount being removed on the "light" side, so the ratio heavy to light continues to increase until that critical point. [C] Obtuse Angle Cut-outsThis is a variation on [B], that retains all of the mass on the "heavy" side of the web, so instead of an arc, we have an obtuse angle. The downside is machining is more complex and you need to be careful that the intersection is not so sharp that it induces cracking. The style was used on a lot of old sparkers, and some more recent designs as well. One extreme I saw on an old design reduced the obtuse angle to a right angle! [D] Peripheral Crescent Cut-outsYet another variation on [A], except the cut-out crescents are made in the edges of the web only. A thin ring is then generally shrunk over the web diameter to seal the edge. The objective is the have the effect of a full web on crankcase volume, while achieving some degree of balance. This approach has been used on commercial engines and high performance engines. It does require a thicker web than usual. [E] Web ThinningA common "full-size" practice is to bolt (or otherwise secure) a weight onto the web opposite the crank-pin. Model designers frequently achieve the same effect by machining away the web concentric with the crank-pin. The result is more mass where we need it. Some designs supplement this with [A] or [B] type cut-outs. The down side is complication of the conrod profile to avoid having it clobbered by the counter-weight.
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