Weaver Construction Log Page 1.
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Crankcase and Cylinder rough-out
The crankcase starts off as a chunk of aluminum bar of about 1-1/2" diameter, sawn to length, plus a little bit. The first step is to finish turn the front section with a 1/4" radius where the journal blends into the body. The photo shows the roughing out process. Note that the tool is raked back sharply while "hoggin' great cuts" are made so that if it digs in, the cut will be forced shallower, not deeper as it would be if the tool were set raked forward like a regular knife tool.
Next we need to remove all the excess aluminum that does not look like a crankcase. This can easily be done with a band saw, or less easily done with a hacksaw! In either case, marking out is simplified by preparing a full size profile on paper, centred in a circle the same size as the bar stock. Mine was done from the CAD drawings, but pencil and compass could achieve the same result with about the same effort.
The paper template has a hole cut roughly in the middle to accomodate the journal and the blended radius where it meets the front face. It is attached to the face with a standard glue stick. If you're carefull, this will last long enough to complete the butchery. Saw to within about 1/32" (1mm) of the outline. Take care because heat buildup will melt the glue. The next photo shows the four basic stages in crankcase manufacture:
The half-round underside of the crank case can be shaped surprisingly easily and swiftly by (ab)using your lathe as a hand shaping machine. Grip the crankcase by the journal in the 3-jaw chuck. Mount a knife tool, switch on and locate the exact center of the work by feeding in near the center until the "pip" is just removed. Back off from the cut, absorb the (outwards) backlash and zero the dial. Now use the dial to feed out by the crankcase radius. Advance the tool to make a light cut which will be positioned on the crankcase outer diameter. This will be turned off later when the rear of the crankcase is brought down to exact length.
Mount a sharp, substantial parting off tool in the toolpost, rotated 90 degrees to the left from its normal position and adjusted so the center of the tool tip is on center height. Now crank the sadle back and forth with the hand wheel, rotating the chuck by hand a little each time. Trial and error will soon show you how much cut to put on per pass and how far to rotate. Continue until you reach the diameter indicator mark.
This photo shows the process well along towards finished. Note that I've used a thin shim of aluminum (beer can, actually) to protect the finished journal from the ravishes of the chuck jaws. I've also placed a bar accross the chuck face, between the jaws to act as a backstop for the work as shaping forces tend to move it forward and would damage the finish on the blended radius.
A for real chuck stop would be a much better way of doing this. Must make one, someday.
The outside of the crankcase s completed before the interior because this is a rough process and finishing the interior makes the blank weaker. Before removing the crank case from the lathe, put a scriber in the tool post and set it on center height. Then lightly scribe the line that marks the underside of the engine mounting lugs.
Finishing the crankcase profile can be done with files, but it's much easier if the chuck can be transferred to a rotary table, set vertical under a mill. Adjust until the scribed center line is perfectly horizontal with the rotary table at zero, then mill the undersides of the lugs flat to this line. Rotate through 180 degrees and complete the top of the lugs, leaving them 1/8" thick. The cut should just touch the diameter scribed in the backface earlier. While at this setting, you can clean up the top of the case, but bringing it to correct height will be done later.
Now, mill the side flats so the top is the correct width (7/8") and the side blends with the top of the lug. I found the angle by trial and error to be about 8 degrees. You can also drill the mounting holes while this setup is in position.
Now back to the lathe. All the following operations should be made without disturbing the setup to preserve concenticity. Center drill, then progressively drill the crank shaft bore to about 15 thou undersize. Then face the rear of the crankcase until the finished size (5/8") is achieved at the top of the case. The interior can now be bored to correct diameter and depth as measured from the rear face of the crankcase. This is followed by screw cutting the thread for the backplate. Finally, the bore for the crank shaft can be completed with a reamer. The next photo shows the correct way to "float" the reamer into the work with a hard center in the tailstock. The chuck is rotated by hand for this operation and never rotated backwards. Reamers like to cut. That's why we left the hole significantly undersize. I use keroscene as a lubricant for this operation and widthdraw the reamer frequently to clear the chips.
Finish off by cutting a respectable 45 degree countersink where the crank shaft bore enters the crankcase. This is to assure the front thrust face of the crank shaft runs against the face of the crank case, rather than riding on the edge of the hole.