As it turned out there was an issue with my ‘final‘ casting which meant another go at it. The tenth try.
The bore for the crank shaft is 0.625” and is counter bored to 0.875” at both ends. At the front bearing the counter bore extends 0.375” into the casting. I had not allowed enough depth in the casting here, it’s hollow above the main bearing for the oil breather or filler, and the counter bore broke through into the cavity.
But number ten is progressing.
I started by machining the lower surface with a fly cutter. The goal is to get the height of the casting to 2”, so once I have the lower surface machined flat I flip the casting over and machine the top where the cylinders mount. Lots of checking on the surface table that the top and bottom surfaces are parallel. Then keep removing metal off the underside until the height dimension is achieved.
My next operation was the bores for the cylinders. The casting pattern was made longer to accommodate shrinkage and its close but actually a little too long. It’s barely noticeable but to keep it that way I bored the number 2 cylinder first and then used that as the datum for the other three, they being 1.6” apart.
So then the the front of the casting needs to be 1“ forward of the rim of the front cylinder bore. Front of casting milled down accordingly.
This front surface and it’s relationship to the cylinders is very important because it’s the datum used for the position of all the crank bearing supports.
The crank bearing mounts were milled to required widths. The rear of the casting was also machined down to the required overall length at this point (which is what the oil pump assembly attaches to).
At this point I made up the crank bearing caps from a previously shown casting and drilled them for the bolts.
The drawings spec the bolts as 4BA which I translated to 6-40 UNF And made a bunch of them from SS.
Drilled and tapped the casting for the bearing caps…. and disaster!. I was sooo careful. Had bought new taps etc. but the tap wrench I used managed to jam in the vertical axis and I stripped the thread on one hole.
I thought about just re threading a size up but in the end opted to try one of these low temp aluminum brazing rods. Fill and re drill.
The trick to success with using this stuff on castings is to get the casting hot enough overall so that you can melt the rod by playing a flame from a gas torch over the area. These rods melt at about 650F so well below the melting point of the casting but the casting will wick away heat very effectively. In fact the instructions that came with rods warn that an oxy acetylene torch is required on castings. Instead I heated the casting to 500F and then used a propane torch to get the localized area up to the melting point of the rod. I had drilled out the original threaded hole to something larger. You need to be sure the rod is melting into the bottom of the hole properly.
The operation went well and I re drilled and tapped the hole. Back in business! The caps were bolted down with 0.02“ shims in place.
At this point I was ready to drill for the crankshaft. On the mill I bored the front and back of the casting 0.875” dia to required depths. This is where things went wrong on the last casting.
I considered drilling the crank bore on the mill but in trying to set it up it just felt too awkward so I opted for the lathe.
I made up a bracket to support the casting on the saddle out of 3/16 angle iron. The holes were all slotted so it could be adjusted for height etc. The casting is secured to the bracket using three
M6 bolts through the cylinder apertures. There are packing pieces made up to fit in the casting for the bolts to screw down on.
I made up some 0.875” dia blanks to fit the end counter bores that were held in the chuck and tailstock to assist with alignment. Also a dial indicator to ensure the casting was horizontal.
I had made a 1/2 “ on center boring bar intending to get to final dia with it, but chickened out and went with a reamer. The bar needed to be at least 15 inches long and it just felt too flexible to get reliable diameters with it. I might practice using it on something less critical at some point. Reamers at this size are $$$.
The hole for the cam shaft was done in a similar manner. I made some slotted extension plates to raise the casting up. In some ways this is a trickier operation than the crank because the drill goes quite deep into the side of the casting so it is always wanting to veer off with the asymmetric loading.
The internal side walls of this casting need an extensive amount of machining out to allow for the swing of the con rods around the crank and in retrospect it would have been better to do this first as it would have eased the pressure on the drill. Oh well.
I should mention that the drilled holes were started with a center drill at each web, then drilled out and the next web center drilled progressively. Only the last drill and reamer really went all the way through non stop.
The results seem OK, that is the hole for the crank came out where it was supposed to!
The bronze bushings were turned and a trial fitting of the crank shaft made. The shaft sits in the bearings and can be rotated freely without that telltale tight, loose, tight , loose feel of misalignments. I need to make some new shims for the bearing caps, l lost the originals, their probably in the shop vac with 10 thousand other shim looking bits of metal.
The first couple of attempts at making the bronze bushes were not successful. I did the usual slitting of a bronze bar then silver soldered them together. Drilled and reamed but the solder joint broke both times.
I had better luck with two flat bars of bronze half the dia thickness, held together in the four jaw.
Whilst drilling and reaming I also held the stick outs together with a C clamp (at slow rpm). So.. good news… no slit saw, heating the bronze or messing about with solder.
But… the technique is more expensive, not twice but getting there.
RS.
