Morning All,
It's been a while , I know, but I have at last found time to make a little progress on this long running build. Along the way, this thread has passed 100,000 views. so thank you all for tuning in so regularly.
I used the time to make a start on the two sets of con-rods. That's a total of 16 con-rods plus a couple more; just in case.
The full size Mercedes Benz engine used a one piece hardened steel crankshaft and two piece hardened steel con-rods running with caged roller bearings. The roller bearing for both the big ends and main bearings run directly against hardened con-rod and crankshaft surfaces.
Here is a photo of a similar con-rod (it's from a full size replicas MB W125 engine). You can see the elaborate split line necessary to achieve prefect alignment of the two halves of the big end bearing. The replica con-rods were machined from solid, nitride hardened and then ground and polished to create the roller bearing surface. The full size crankshaft was also machined from solid, nitride hardened with all the journals ground and polished to size.
I chose to do it differently; I will use one piece con-rods and a built up crankshaft. I do not have the means to harden, nitride and grind the con-rod eyes or the crankshaft bearing journals. Instead I created an alternative design to allow me to use commercially available thin section ball races for the eight big end bearings and also for the five main bearings. With either design there would still a need for high precision. However, my alternative design, should be within the scope of my machinery.
I chose a different poison to Mercedes Benz by following the Auto Union method of construction.
Commercial thin section roller bearings are pressed into the one piece con-rod, they will have dummy big end bolts, just for show. The available thin section roller bearing were a little larger in diameter than I would have liked, which forced the wall thickness of the big end eye to be slightly thinner than the original design. It was for this reason, I felt it necessary to compensate for the reduced thickness by going for the strongest available aluminium alloy, for my one piece con-rods. I went for 2014 (HE15), as being the most affordable high strength aluminium I could find.
I only had a limited amount of the 2014 material to play with. It's much more expensive than 'ordinary' aluminium plate so I had to settle for an off-cut from my supplier. It was necessary to nest the con-rods on the stock to make maximum use of the available material. My table movement is a little under 8" x 4" but I found that it was possible to extract 8 con-rods from one piece of 8 x 5".
Here you can see the first operations. The stock plate was bolted to a jig plate, above a sacrificial stand off, which allows for bolt clearance. The top row of con-rods were machined to the first stage before the plate was rotated through 180* and the machining process was repeated. I used a boring head to bring the con-rods big end eye to the precise diameter, after first roughing out the hole by CNC
Top hat buttons were machined and used to secure and locate the big end while the individual con-rods were profiled and separated from the stock material.
Here you can see the tool path used to machine the outer profile of four con-rods at a time. And below that, the first set of four con-rods released for the stock plate. I used my peristaltic 'no mist' mister to deliver a tiny amount of water soluble coolant and blow the chips away from the action.
After that, what was left of the stock plate was turned though 180*, bolted back on the jig plate to machine the second set of four con-rods. There was very little wasted material left because of the close nesting.
I had set off with the intention of making 16 con-rods plus two spares; just in case. Along the way one rod was lost; so I ended up with only 17. Here they are playing 'ring of roses' and 'piggy in the middle'.
The con-rod in the centre has been fitted with the thin section bearing (SKF 61802). You will notice I have used a sealed 2RS1 version of the bearing. All the big end bearings will eventually be replaced with open SKF 61802 R02 bearings, But there is far too much swarf about to do that at this early stage.
There's more to follow; so stay tuned
Mike