The sides of the block were machined next. The features on them are very similar to those on the full-size block, and the realism they add make this engine much more than just another runner. George's attention to detail is especially remarkable since he planned to manually machine the entire engine.
I tweaked the design and placement of some of the features in George's original design in order to limit the number and sizes of the end mills needed to machine it. The radii of the fillets were limited to a minimum of 3/32" to avoid the difficulties and surface finish issues sometimes encountered with tiny long stick-out cutters running on a 5k rpm mill.
Waterline roughing operations with .040" vertical steps were used on both sides of the block. These operations were compiled for 1/2" and 1/4" diameter end mills to leave .007" radial and axial excess stocks for finishing. The final finishing operations were also waterline, but rather than fixed steps they were compiled for .0003" maximum scallop heights using 1/4" and 3/16" ball cutters. The sides' few truly flat surfaces were finally finished using a 3/16" end mill.
Even with my tweaking, simulation results showed spaces between several port-side filleted features that would have required a long reach 1/16" end mill to entirely machine finish. These were manually filed down later using a tiny spherical carbide burr in a hand-held pin vise. The block's total six-sided machining time on my Tormach came out to some 27 hours making it the most complex part I've machined to date.
A fixture plate drilled with the block's bottom hole pattern was also machined. This plate will eventually be used to hold the workpiece for the oil pan while it's being machined. It's also needed to support the block while drilling the angled holes for the dipstick and distributor. The bore for the distributor is especially critical since it will establish the mesh for a custom machined 72 pitch/14 tooth 45 degree helical gear set used to drive the distributor from the camshaft. The distributor bore won't be done until these gears have been machined and are in hand so their center-to-center distance can be verified. The less critical dipstick hole, however, was drilled as a warm-up exercise for the distributor bore. Both holes are to be drilled at 60.43 degree angles, and each is referenced to the camshaft bore.
The angle plate setup used to drill the 3/32" dipstick hole is shown in one of the photos. This same setup will be used later for the distributor bore. After carefully establishing the exact drilling angle, measurements on the exit point of the resulting hole showed the actual angle had ended up 0.4 degrees too shallow. If this had been the distributor bore, the resulting .010" mesh error would have been enough to bind the gears, and the block would have become scrap.
After several hours of detective work, I concluded that regardless of the beefy appearance of my ten pound angle plate, it lacked the rigidity needed to hold the setup angle during the drilling of the tiny dipstick hole. A shimmed support block under the front edge of the movable plate should hopefully eliminate the problem for the distributor bore.
Bead blasting using generic glass media from Harbor Freight hid the machining marks and allowed me to see the places that needed manual touch-up. It also gave the block's exterior a nice cast metal looking finish. Only two operations remain. In addition to the scary distributor bore, the top surface will be faced to remove its excess stock once the liners are installed. - Terry
