Westbury Whippet Project:
We are still progressing toward making the Whippet crankshaft. We have the shaft bushings, turned to fit the Nose Piece, that will be used to finish the main journal to the correct running fit. Now we need the conrod so the crankpin can be finished to a close running fit in the big end. The kit came with a cast bronze rod that is embossed with a "K". As this rod has bosses on the sides for big end cap screws and is almost twice as long as it should be, I conclude it must be for the ETW Kiwi. Good. The casting is a bit crude anyway. The Whippet notes say the rod can also be made from "light alloy" and I like that idea much better.
As noted in the Corrections Section of the Introduction page, the published ME plans gave a between centers distance that was 0.250" too long. I only spotted this by the change in lettering style on the Hemingway/Woking plan reprint and confirmed it by calculation; the correct distance should be 1.625". As this is a large rod, it is easier to make it by profile milling on a rotary table, so the first step is to make a quick CAD sketch and use that to determine the "tool path" for a 3/16" milling cutter.
A couple of blanks are prepared from 3/8" thick 2024-T3 plate, placed on parallels in the mill vice, and center drilled using DRO coordinates to get the center to center distance precise. The table X axis dial could also have been used. I'll drill and ream these on a lathe faceplate as the geometry of my joke of a mill/drill would make this operation impractical.
It's a very quick operation to hold the blank in place with a tailstock deadcenter, clamp-up lightly, and tap into precise position with wobbler and DTI. The holes are then drilled 1/64" under size and reamered. This is the same reamer that made such a mess in the nose piece. In 2024, the finish is perfect, pointing to the cast alloy being the problem child.
While on the faceplate, witness circles for the ends were turned 10 thou oversize. These are being used here to mill away the rod faces to the longitudinal profile required. This step could just as easily have been done later, but it will provide a reference for one of the finish depths when doing the circular work (think about it).
Poor man's CAD/CAM in action, aided by a GHT Rotary Table. This little shop-made table has major divisions centered on the T slots, so using bushes and clamps, a blank is aligned on a slot, the table stops set to the arcs shown on the CAD sketch, and the profile around the bosses milled. The big end is done in two steps, flipping the blank for the other side. The "pip" at the bottom of the rod is an "oil flinger" that will dip into the wet sump at BDC and stir it all up into a mist that hopefully gets everywhere required.
The little end profile can be completed in one sweep. Each swing of the cutter is accompanied by about 40 thou of down feed. A business card under the blank acts as a warning and allows us to stop before milling into the table. After the full depth is reached, the table is moved 0.005" for a single final, "normal direction" finishing cut. Naturally, the mill Y axis had been aligned with the rotary table center before the profiling operations began.
The last operation is to connect up the dots. To align for this, two 3/16" rods inserted through the big and little end holes, and rested on the vice jaw tops. After clamping, they were removed. As the big and little end bores are 3/8" and 1/4" respectively, the conrod axis takes up the required 2° slope with respect to the jaw tops. After milling, the intersections are blended with a needle file and the rod polished all over. I didn't bother with the "H" section profile called out for the bronze version of the rod as the alloy version is already much lighter.
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