This installment will discuss my process for finishing the outside surfaces of the crankcase and the sump. I have found that it is advantages in my workflow to first create a perfectly square work piece to all of the max outside dimensions. In the past I have left extra material on to be machined off at later stages only to mess up because I misremembered what side I left material on, or what the new datum was supposed to be. In hindsight I would have made an exception with the top of the block and left some extra material so I could fly cut the deck after the cylinder sleeves are installed.
Here is the squared up stock roughed out using a fly cutter on the mill.
I have recently been using a new technique, importing a model of the stock into the CAM program creating the tool paths for machining on the CNC router. The program considers the stock to have been machined by a previous machining operations. I am using the free version of Fusion 360 CAM and am pleased with its capabilities (albeit limited).
I create a simple drawing with the necessary dimensions that I machine to using the DRO on the mill.
Once I rough out most of the material on the mill I clamp it to the CNC router bed and machine. I use a 1/2″ drill bit and a 1/4″ HSS roughing mill at about 1000 RPM on the mill, a 1/4″ carbide end mill running at 8000 RPM on the CNC router, then a 1/4″ carbide ball end mill to finish the final profile.
I spot drill using the CNC router in the same setup as the main bearing surfaces to insure proper placement. I then take the work piece back to the mill to drill the holes as the CNC router has a very limited Z and does not have room for a tool holder, chuck and drill bit.
Here I am roughing out the bottom of the sump on the mill.
When touching off the part, I do not use just the edges, I will touch off both ends of the part, move the axis to the middle of the two touch off points and then set the DRO to the center dimension of the final part. This way if there is any discrepancy between the drawing and the true dimension of the work piece, the difference is split between the ends and is far less noticeable.
The parts just off the CNC machining the outside profiles. Here I use a .010″ step over with the 1/4″ ball end mill. It takes about 45 minutes a side. If I reduce the step over, the machining time increases linearly, and if I increase the step over the machined surface is not as nice. I find that .010″ step over is a nice compromise. We will see how well the sides clean up and look after bead blasting.
On the sump I machine the two sides and the bottom, each in a different setup