I’ve been away a while playing trains but have now made a start on the Nemet Bobcat 15cc 2 cylinder 4 stroke I/C engine serialized in ME from August 2012. I’ve never made an I/C engine before but this is suitable for beginners so I thought I’d give it a go. The construction articles are pretty well written and work through the fabrication of the parts with words and pictures. Just a case of following along really.
Cut to the chase and I got stuck into the cylinders, liners, crankcase, and nose, timing gears cover and in about 6 weeks got to here.
At this point the next thing was the crankshaft. I started on the crankshaft last weekend and wish I hadn't bothered. I'd purchased an expensive piece of EN8 'key steel' that was 15mm x 30mm and set about fly cutting it down to 12mm. I've now got an expensive EN8 banana that's less than 12mm in the middle. I now realise that I maybe should have skimmed it on both sides or even heated it to red heat and let it cool slowly before I started. I could go and buy another piece but I'd be worried that in the machining it would bend even more.
So I thought of fabricating the crankshaft with 12mm and 10mm silver steel and silver soldering it together. I realised that the throw is only 10mm and but by putting a 10mm shoulder on the bearing ends and the big end journals are 8mm that's 1mm to play with. If I made some sort of jig for the soldering I should be able to keep it all in alignment. At least better than the piece of EN8 I purchased.
So I gave it a go and its come out OK so here’s my words and pictures for anybody wanting to do a fabricated crankshaft.
I used some of the off cut of the EN8 bar I’d purchased to make 3 crank webs, 12mm silver steel for the main shaft, 10mm silver steel for the crank journals (ignoring the 10.5mm larger dimension on the drawings) and another piece of 10mm silver steel for the rear bearing journals.
In all a simple mill, drill job for the three webs and turning job for the three shafts.
The webs are both drilled/reamed 10mm for the main shaft and rear bearing. For the crank journals the two outer webs drilled/reamed 8mm and the inner 10mm.
Loosely fitted together.
The next question was shall I silver solder and risk distortion with the heat, and how would I jig the assembly, or shall I Loctite and pin. I decided on the later.
To be able to hold the job and add the curved top edge to the webs I drilled an extra 3mm hole in the middle web. To enable a through shaft to be used in the lathe
The lower bar is a temporary alignment bar to ensure the outer webs are properly aligned.
With the crank journal and temporary main shaft Loctite the two outer webs with the inner web flipped out of the way and clamp up to set.
Flip down the middle web Loctite and position and clamp to set.
When set drill and pin the three crankshaft webs with 2mm pins
The pins will be tidied up later
Modify the temporary main-shaft with a 3mm section in the middle
Reassemble
Drill and bin the temporary shaft to the main-shaft web 1.6mm. This will be removed later.
Turn the radius on the top of the webs
Remove the pin and temporary main-shaft ready to install the main-shaft and rear bearing journal. At this point I had been wondering what I’d use as a jig to align the main-shaft and rear journal while the Loctite set. Then I realized I already had a jig.
Just don’t get Loctite everywhere.
Now a case of pining the main-shaft and rear bearing journal with 2mm pins, trimming and filling the pins flush and drilling and tapping for the counter balance weights.
Hopefully that will be useful to somebody, I'm far happier with this version than I think I'd have been with the solid version. It took me the weekend to make, I feel I'd still be out there if I was doing the one piece.
Pete
