Milling the crankshaft channel through the crankcase, fitting the main bearing caps, and line boring the assembly are important foundational operations for this engine. For these operations the crankcase needs to be rigidly supported with its bottom (reference) surface facing up. Although I was able to hold the crankcase in the mill vise earlier for light surfacing of the pan mounting surface, I didn't feel comfortable using the same set-up for these operations. The complex shape of the crankcase makes it difficult to safely support in a vise, and accurately aligning it to all three axes while being supported that way would be very difficult.
I decided to begin using the crankcase motor mounts for work-holding, and so I machined their (top) surfaces parallel to the bottom (reference) surface of the crankcase. Their narrow widths and unequal heights added some complexity, but they gave me full machining access to the bottom of the crankcase.
I've been waiting for an excuse to build a milling table for the cross slide on my Enco lathe, and it looks like the line boring operation that's coming up is a good excuse. I had a half inch ground steel plate in my scrap collection that was just wide enough to hold the Merlin, and so I spent a few days machining it into a universal fixture for my lathe. I attached the crankcase to this plate so it could be mounted on either my lathe or mill table. I set the height of the crankcase so the center of the crank bore was slightly below the center of my lathe's spindle. Later, when it's moved to the lathe, the height will be tweaked with shims.
After another day's effort the crankcase was finally under the mill's spindle and indicated along all three axes. One of Delrin plugs that was used earlier to help position the gear case cover was re-machined to snugly fit the bored opening in the lower gear case where it was used to indicate the crankshaft's centerline.
After milling the channel for the bearing caps, I machined the crankshaft thrust bearing surfaces on either side of the center bearing. Since the Merlin uses an offset prop shaft, the crankshaft doesn't bear a significant thrust component of the prop load. The midpoint of this bearing is the engine's forward/aft zero reference, and so it was used to surface the rear of the crankcase and oil pan to their finished lengths.
In addition to a pair of conventional cap bolts the Merlin also uses a pair of cross-bolts to tie each bearing cap to the sides of the crankcase. These were probably necessary in the 2500 hp version of this engine, but they greatly complicate the machining of the bearing caps in this scale model. The cross-bolts are made of 3-3/4" lengths of .098" diameter drill rod threaded 3-48 on each end. Fourteen .102" holes must be drilled completely through the 3-1/2" wide crankcase where they will actually intersect the cap bolts. The cross-bolt holes can't be moved without offsetting them in their external cast bosses since this would spoil the engine's appearance. The 8-32 cap bolts must be necked down where the cross-bolts would otherwise contact them. The total interference ultimately depends upon how well the trajectory of the cross hole bit is controlled, but it will be a minimum .010" if all goes perfectly.
This interference might be a consequence of the incompatibility of the scaling in this particular area of the engine with the use of standard size model fasteners. I've looked ahead in the drawing package, and I'm afraid this may be a continually repeated story. There's another nasty scale-related issue coming up involving the heads and cylinder liners.
The fixture'd crankcase was reoriented on the mill with one of its sides facing up for the cross-bolt drilling. A Guhring parabolic bit was used to drill the cross-bolt holes without the bearing caps in place. Drilling the holes with the caps in place may seem like a better idea, but I couldn't come up with a way of clamping them securely in place. I didn't want to drill the cross-bolt holes with the caps already bolted in place because even with necked down bolts the drill would have wandered after breaking into the cap bolt holes. Plenty of WD-40 was used as a lubricant since any aluminum stuck to the bit would also cause the bit to wander off course.
The major issue I ran into was indicating the starting positions for the cross-drilled holes. The bosses for the cross-bolts cast into the sides of the crankcase are dimpled to help locate the drill since there are no drawings referencing their positions. I used them for the first set of holes in the rear bearing, but they ended up offset .010" from its center. The starting locations for the rest of the holes were determined by indicating the centers of the cast bearing webs, but these still ended up with errors as large as .005". The errors that accumulate during the drilling of the cross-bolt holes will increase the amount of material that has to be machined from the cap bolts for clearance, and this may weaken them.
The bearing caps, themselves, plus a few spares were CNC machined +.002"/-.000 in 'cookie sheet' fashion. They were engraved with numbers indicating their bearing positions, and they were individually fitted.
With the snugly-fitted caps temporarily in place, the locations of the cross-bolt holes were marked on each side using the Guhring drill held in a pin vise. The caps were then moved to the mill vise where each hole location was indicated using a spindle microscope. The holes were spotted and drilled to half depth from either side which resulted in 12 operations per cap. A .102" reamer was finally run through both holes to smooth any discontinuities at the intersections.
The crankcase was repositioned on the mill with the bottom of the crankcase facing up. Each cap was temporarily held in place with four .100" diameter drill bits inserted in the cross-bolt holes. The cap bolt holes were drilled through to the crankcase using an 8-32 tap drill with the four drill bits being moved as required. After removing the caps the crankcase was threaded for the bolts, and a reamer was run through the cap bolt holes to provide clearances for the bolts.
The diameters of the cap bolts were smoothly necked down to .125" around the point of contact with the cross-bolts. This was sufficient to clear the worst-case cross-bolt, and since it matches the minor diameter of the 8-32 thread there should minimum impact on the bolt's strength. I'm currently using stainless SHCS's for the cap bolts since I had them on hand, but I'll likely replace them with stronger steel versions later. I had only 3-48 lock nuts on hand for the cross-bolts, but since I don't like their looks I plan to replace them with plain hex nuts. The other three holes machined into the tops of the caps will eventually be used to mount injectors that will supply pressurized oil to the bearings.- Terry
