Drawing 622’s Stephenson gear

I just finished drawing up the valve gear for 622 in OnShape.  This is certainly the most complex part I’ve drawn yet in the tool, and the idea is to have it 3D printed as a single piece.  Sadly that means it won’t work, but in the interest of getting the locomotive running, I’m going to leave it in mid-gear, and pretend the wobbly bits needn’t wobble.

One interesting outcome of drawing this in OnShape was that I could use a copy of the right side sketch for the left side.  This is something you couldn’t do with SketchUp as accurately.  To make the left side, I took the right side sketch, and simply moved the ends of the eccentrics to rock the Stephenson link.  Okay, it was over-constrained to begin with, and I had to figure that out, but once I had, I could choose the angle of the left link.

Indeed, I can still go and change the angle, and the whole drawing will recalculate to accommodate it.  This sort of revisionism is what gives me confidence to go ahead and draw these parts before finalizing the distance between the frames.  It’s a very powerful drawing program!


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