This is the current state of my design for the Wallaby:
I need to complete the work on the front of the engine including the Oil Pump housing, ignition triggering system and the distributor. The oil pump housing needs its intake and output on the same side, so I need to work on that. I think the timing sleeve with a magnet for the ignition trigger is straight forward, but I need to design the little housing for the hall effect sensor. The distributor for the two cylinders will be driven from the end of the camshaft.
But I am going to direct my attention to the design and fabrication of the camshaft next. I have deviated from Westbury’s design by using ball bearings at each end of the camshaft. I will be using 1018 cold rolled steel and case harden the cam lobes. This is the current state of the camshaft design:
I know this is not dimensioned using proper GD&T standards, but when I am machining I like LOTS of dimensions.
Below are the lobe designs using numbers gleaned from Westbury’s construction article: Cylinder 2’s lobes are 180 degree mirror images of cylinder 1’s. The nose radii are reference dimensions, to specify them would over constrain the design as all curves are tangent and sufficient to describe the cam profiles.
My machining approach is to use the lathe to turn all features with the exception of the cam lobes themselves. I am going to then use a 4th axis on the mill to machine these. This will be the adventure. I am going to machine some aluminum test articles first.
One thing about model engines (and full size engines as well) I dont’ quite understand is the need to be able to adjust cam timing. The timing of the valves with repect to the crankshaft are rigidly defined by an engine design, so why do I need to adjust the cam position on the cam timing gear? Is it because of variation in manufacture of the timing gear train? Am I just adjusting out slop or inaccuracies in the manufacture of the associated components?
The majority of the last week has been spent working on tool paths for the Camshaft Lobes. This is my first exposure to using a 4th Axis on the CNC Router. Below is my first attempt cutting an exhaust cam lobe on a piece of test aluminum rod. I used a ball end mill for the entire operation.
Operation in process
Operation complete. I was quite pleased with the surface finish and the dimensional accuracy.
I continued to experiment with various tool path options ending up using a 1/4″ flat end mill for the roughing and then a final pass with a 1/4″ ball end mill. I offset the tool .3″ to the side so I used the edge of the cutter, not the center. I worked initially in aluminum, then a final test with steel.
The camshaft print.
First I rough machined the camshaft focusing on the cam lobe blanks. These are 5/8″ in diameter and 5/16″ wide. I went back and forth on how much lathe work I wanted to do before milling the cam lobes. I finally decided to minimize the amount of lathe machining before moving to the CNC router to minimize lost work in case of a failure there.
Instead of creating a tool path for the entire camshaft, I created two tool paths for the intake lobe and two tool paths for the exhaust lobe (roughing and finishing). This required four set up changes as I would re-zero the CNC router and rotate the camshaft to the proper orientation for each cam lobe before starting the machining on each. To insure I didn’t skip a step or mess one up, I created a checklist for the machining of the cam lobes.
Here three of the four cam lobes have been machined from a 1018 cold rolled steel rod on the CNC router.
Back to the lathe to complete the machining. Above I have completed both bearing surfaces, you can see a bearing test fit next to the live center in the tail stock. And started working on the taper that will secure the cam timing gear.
Once I finished the machining and cut off the cam I found that the camshaft was too tight between the bearing holders and the taper need some more material removed to properly mesh the cam timing gear with the idler timing gear. I had to figure out how to chuck up the camshaft again to do this work. Gotta love collets.
Camshaft Installed in the Engine. The outside lobes are exhaust and the two center lobes are intake. You can see the timing of the lobes.
I still have a little bit of clean up on the camshaft, then I need to case harden the cam lobes. I sure hope this does not ruin it.