Weekly Update on the 1 by 1 - Part 23

I really shouldn't call this a Weekly update because my last update was far from a week ago.  Part 22 was January 30th, so over two months ago!  At the time, I was talking about preparing my piston's connecting rod to be cut out via CNC.  Since then, lots of complicating factors got in the way; big and small interruptions to life, ranging from one of our senior cats having a bad annual physical leading to several tests, lots of things to do and angst to go with it, to a one week house repair project resisting completion for a month, to my not being happy with either my CAD or CAM of the design and then finding an interesting new way to do it.  I do have a bunch of other hobbies, too, after all.

I've been back to it, but haven't started machining the part yet.  Let me pull up the drawing as it's provided with the plans, and shown here in January.  

One of the complications with this part is that I can't just import this into my CAD program, create a solid model of the finished part and then make it.  That big hole on the right end of the crankshaft is 3/8" and needs to be 0.375" exactly.  That means a 3/8 drill bit isn't accurate enough, and what you might think to do is to drill it undersized and then ream to final size.  The problem is that if you then cut the right end off it, you now have something that's neither circular or 0.375" diameter.  It's missing the width of the cutter.  

If you read the that big (four line) note at the bottom in that figure, you'll see the designer says to drill and tap the two holes for #5-40 socket head cap screws that you can see in outline above and below the big hole, then cut off the bottom cap.  Once it's cut off, attach it with the screws you just tapped for, and then drill and ream the big hole.  

That changes everything.  A slight complication is that's assuming the saw cut is .030" wide.  My saw cuts .047 wide.  All this led to re-drawing the part to revise the solid model to make it look like what I need to machine, moving that cap on the right enough for my saw blade.  It's tough to see but that raised area on the right isn't circular.  In CAD, once the two pieces were separated by the width of the saw kerf, I filled in the gaps with solid material, so the .047 segment is straight in those areas.

This is a two sided part, completely symmetrical top to bottom (in and out of the screen in this view).  A brilliant modeler on a forum I read regularly came up with an approach I'd never heard of for machining two sided parts like this.  He machines the part halfway in thickness, fills the space around the part with epoxy, turns the blank hunk of metal over and repeats the machining on the second side (after verifying the machine is still zeroed properly). This operation ends with the parts fully machined and suspended in the epoxy.  A short stay in a toaster oven causes the epoxy to break down and the part can be popped out.  There's fewer steps here than a lot of machining operations.  

I've lifted three pictures of his machining four connecting rods at once for a model of a Ford 300 six cylinder engine. 

Once the top side is machined, the missing aluminum is replaced by Devcon epoxy. 

Then the work is flipped over and re-fixtured on the mill to machine off the aluminum on the back of the connecting rods. Once that's done the parts are ready for the heat treatment to break down the epoxy.

He still has some machining steps to do before they're usable, but easy stuff for him.  

My approach to this part is to learn as much as I can and try some techniques I haven't done before.  For one thing, I'm going to do a roughing pass and a finish pass.  Both will use 1/4" diameter cutters and I'll use finer steps between passes on the finish pass.  My mill's spindle (driving the cutter) is slow for most work in aluminum and especially for a 1/4" diameter cutter.