While I'm not done with the conn rod, I completed my first experimental piece - which my system turned into scrap very early in the process - but while it isn't possible to complete this, it actually came out fairly well.
As I mentioned in the previous post,
several of us have had a lot of discussion of the issues I had with my CAM program. A lot of the story - as it happened - is there.
So let me bring this thread up to date.
What I had started out wanting to do was to duplicate the approach I saw
@Mayhugh1 use to make his tiny, two-sided parts in his Ford 300 build, which is to machine one side of the part about half the thickness off the blank, fill the area just machined away with epoxy, flip the part (while maintaining the machine's zero) and then machine away the rest of the excess material on the second side. My approach was to do a rough and a fine pass - so two passes on each side. The rough pass was going to leave margin around the final geometry, using a technique called waterline cutting which is largely cutting away the excess material around the part. I elected to do a fine pass in a completely wrong approach, and it led to a butt-ugly result (which is a comparison that's offensive to butts everywhere). I fixed it by doing a different type of machining strategy. Details over on the CAM discussion.
Once the second side was finished, I baked it in a toaster oven at 250 for two hours. It fell apart perfectly.
Those yellow chunks in the front left are the epoxy, and the leftover, machined metal is in the back. The first side machined is facing you.
As I say, the dimensions I've checked are all within a reasonable tolerance of the intended size (.005" or less), and while a long way from done, if not for the machine screw-up on the first pass, it would be usable. The machine screw up is visible here.
The machine screw up happened in a way I've never had my system screw up before, but in the bottom of the square area on the right, you can see that it's pretty short on the front (first pass side).
From the second side, it looks like this:
It's about .075 extra cut away along that one edge.
I've done one (and only one) conn rod, before and that was for my Webster. For fun, here's the two rods alongside each other. Webster in white.
I did the Webster's in a completely different way, using a larger end mill and cutting the contour of the part and then relieving around the circles in G-code I wrote manually. I put the cutter near the left circular feature so that the end mill was leaving the final shape, and then cut a circle around the center of that feature. Then I moved the cutter to the larger feature on the right and repeated the approach. As a bonus, it cut the area between them to the right depth. The tool paths looked like this
The trick with this rod is that the raised area that looks circular on the big end isn't circular. It's two pieces separated by the thickness of my slitting saw (.047") and then filled in solid. It's still possible to do this sort of tool path with that distortion, I just need to cut two semicircles separated by that distance. I'd post a picture like this but on this scale, with each of the smaller squares 1/4", the .047" straight line segment isn't visible.
So that's where I am right now. If I sound a bit undecided on how to approach making the real part, that's because I am. I'm not sure if I'm going to do the epoxy trick again, or screw the conn rod blank onto a fixture before I attack it.
