Feeney Construction Log Page 3:
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Crankshafts for single cylinder engines fall into two categories generally called "full-throw", or "overhung". The overhung variety is the sort we are accustomed to seeing in model engines. The other type, as used in the Feeney, is what we'd expect to see in our lawn-mower (or motor-cycle, or maybe our weed-eater, or giant R/C accident waiting to happen). The purpose of a motor is to produce power, and the enemy of power is friction. We take great care to machine the crank-pin of an overhung crank to be axially in line with the crankshaft. But when running, it is seldom in that position as pressure will be bending the crank web one way, then the other. This constant loss of axial alignment between the big and little ends of the conrod increases the friction and wear, especially on the the big end of the connecting rod (which is generally a softer material than the crank pin). As a full-throw crank is supported at both ends, the pin cannot deflect, but now we have more bearing area to worry about (and extra complexity in the crankcase manufacture).
The shaft will have to be machined first so we can jig off it for the throw. The best and easiest way to do this is between centers. First, centers are marked on each end of the casting using a surface plate and marking block. Each end is then center drilled after making the marked ends run true in the 4 jaw chuck. There is another way using a vertical slide in the lathe, but it's more effort than using the 4-jaw.
Now, finally, the tip: we have to prevent the centers from springing the casting together in the gap. This would result in a shaft that is not straight when the pressure is removed. To prevent this, we fashion a short screw-jack from a piece of 8-32 bolt cut just shorter than the gap. Nuts are screwed onto each end so that they can be unscrewed to make the thing a firm fit in the gap (the inner faces having been dressed flat with a file first). Obviously we must not over-tighten the nuts or we'll be causing the same problem we are trying to prevent! The gizmo is jiggled into somewhere close to the phantom axis of the shaft in the gap and brought to finger tight. A light tweek of the spanner makes it firm without turning it into a jack. The lathe centers can now be brought to bear and heavy cuts applied with impunity.
Turning the crankpin requires that the shaft be mounted with its axis perfectly in line with the lathe axis, but offset by half the throw. There are many ways to do this, but I decided to use a special jig. Digging through the junk box, I located a piece of 1" square steel with some holes in it that would serve. The first step was to mill one side flat, leaving a lip that will be used for alignment. Two holes are drilled for bolts that will attach the jig to my angle plate with the lip hanging over the front edge.
The jig block is now center drilled in the mill vise using the mill table to position the two centers precisely one half of the crank throw apart. One will be bored and reamed a close fit for the crankshaft. The other is a centering reference for that will be used to position the jig when turning the throw. After center drilling, a 1/8" drill is used to carry the center pilot all the way through the block.
The jig is now bolted to the angle plate on a lathe face plate and the journal center accurately centered using a "wobbler" and DTI. The jig is now drilled and bored for a very close sliding fit of the crankshaft. The old hole to the right serves no real purpose, but does no harm either. After boring is complete, the jig is removed and slit on the band saw from one edge, through the old hole, and into the bored hole. This allows the jig to clamp the shaft journal. Note how the lip on the jig will allow us to replace it on the angle plate with the bored hole perfectly in line with the lathe axis.
The shaft is inserted and the pin centered under the 1/8" hole. After clamping up, it goes under the mill and the 1/8" hole is used as a guide to drill through the shaft on the axis of what will become the crankpin. Why? Well, because the plan shows a 1/8" hole here for some unknown purpose,but more importantly, we can insert a 1/8" drill rod pin that will absolutely prevent any tendenct of the shaft to accidentally rotate in the vicious, brutal steps that are about to come.
The jig goes back on the plate, sans the shaft, and the "wobbler" and DTI are used again, this time to set the second center drilled hole to run true. This is a one axis adjustment because of the way the centers were drilled under the mill. I try to get less than 0.001" of wobble indicated on all these operations. It's not hard—rotate the faceplate by hand to the maximum DTI reading, tap across by half the error, and repeat until satisfied.
Now we insert the shaft with a drill rod pin inserted in jig and the hole we drilled in the crankpin earlier. The shaft is pushed right up against the jig and the bolt that passes through the sawed slit in the jig tightened up. Everything is now ready to start forming the finished crank pin and inside faces of the crank webs. The pin as cast is massively oversize and I found taking it down a few thou at a time painful in the extreme. So...
Faceplate and all goes over to the rotary table under the mill where a slot drill can be used to munch away most of the excess. This was done in a series of cuts, indexing the shaft 45 degrees between each one, aiming to leave the pin about 0.020" oversize (across the flats). Note how I've had to slide the shaft out on the drill rod pin to get the cutter holder and quill to clear the angle plate. That pin is proving to be uncommonly useful!
Finally, back to the lathe to finish up the pin using a tight-left finishing tool fround from 3/8" square HSS. The tip of this tool is first aligned with the lathe axis using the rear portion of the crankshaft. In use, it needs an appreciable overhang, so very light cuts are the order of the day. Ten points go to whoever spotted that this shot was taken while I was determining that roughing down the as-cast pin using this tool was not a practical plan. However, finishing the pin with it after the bulk had meen milled away worked just fine.
The prop driver is keyed to the crankshaft by a short 1/8" diameter pin inserted in the threaded portion. The driver—which has no knurling according to the drawings—was faced and bored in Yet Another Pot Chuck. Following this, my handy-dandy Hemingway key-way shaper replaced the cross slide to cut a perfect keyway. Photo 11 above shows the gadget in place, and Photo 12 shoes how a piece of 1/8" diameter HSS has been ground back to the diameter to make a simple, relieved keyway broach.
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