120 cc 7 cylinder radial

[Pat1311]

Because I didn't have any C45 in the right diameter to give the cam disk another try I made a start on the cylinders.

The start of making 7 or even 14 of the same parts.

I also made a fixture to mill out the conrods,took me 2 hours but it will shorten the time being spend on the 6 conrods by a lot more.

I made the cylinders out of 65 round 2007 aluminium,the plans say to make it out of square stock because of the square flange on the bottom but I find it easier to start with round stock,there's a bit more wasted but I don't like the interrupted cut involved turning square stock.

 

[Pat1311]

And a quick mock up and test fit just to see how it looks.

Next will be the conrods and then another try on the camdisk and the rest of the gears.
I decided to make the gears myself,I ordered a set of module 1 gearcutters,see how that goes.

 

[Pat1311]

Next parts to make are the con rods.

Not very complicated but the holes for the big and small ends have to at the right distance and the bores have to be exactly 7 mm in my case.

I ordered some flat stock 7075 alu,this was a bit hard to find so I had to order it in Germany,a bit expensive but it saves a lot time not having it to make out of round stock.

 

[Pat1311]

Next were the big end pins and 2 cover plates that secure the pins.
I also had to make 3 bronze washers.

 

[Pat1311]

The pins fit very nicely,the rods can rotate freely and there is no play at all.

Next to make were the gears and a bush where the gear shaft runs in ,but first I had to make an arbor for my new bought module 1 gear cutters.

I needed 1 10 tooth gear on a shaft and 2 25 tooth gears.

The 10 tooth was easy on the rotary table,just 9 complete rotations per tooth.
The 25 gear was 3 rotations and 12 holes skipping on the 20 hole dividing circle.


Everything fits very nicely.

next to do is the second attempt on the cam disk

 

[Cogsy]

Great work so far. Radials just look so 'pretty' inside. I'm almost tempted to seek out a set of plans but I think I've got enough on my list for now.

 

[petertha]

Coming along nicely!

So looks like neither big or little ends of the connecting rods have bronze bushings in them, right? Just the aluminum itself? That's the same I see on comparable designs, some have, some don't. Some guys put them in on their own even if plans don't call for. But I think the amount of remaining stock material is already pretty skinny.

Getting back to your crankshaft, maybe I missed but are you cross-pinning the counterweight/crankpin assembly to the shaft itself? I'm making my 5-cyl crank now, also out of solid. Not as large as yours but still lots of material to remove. Was thinking out loud about an assembled crankshaft vs. solid. I might even make one for the heck of it so was wondering.

 

[Pat1311]

No,there's no pin in there.

I thought about pinning the shaft but was advised not to because the shaft is a shrink fit in the counterweight and it would only weaken things.

And there's allready not much "flesh" around the shaft.

I'm by no means sure if it is going to be strong enough but the alternative was making it out a lump of 90 mm round C45 turning it excentric with an interrupted cut and I don't think my little lathe is up to that.

My thought was just to try first the easy way and if it fails I can allways go and try the hard way.

 

[petertha]

Food for thought. A keyway method & cross pin method.

http://modelicengine.la.coocan.jp/k070601.htm

http://modelicengine.la.coocan.jp/h9090903.htm

I know, I've been thinking about same issue. A shrunk joint or glue (locktite) joint hopefully would 'fail' with crankping/counterweight spinning around shaft & (hopefully) minimal internal damage to engine. Pins coming loose would be more dramatic. OTOH, maybe designed not to fail?

The way I look at it is the crank pin itself is a pretty hefty diameter, supposedly sized not to shear under cylinder firing. Maybe its over designed, who knows. Anyway, the crankshaft 'joint' we are talking about should be somewhere around that same strength?

Maybe you could make a simple test piece using same size, material, interference method & apply torque with torque wrench. If value meets expectation of engine rating, will give you much more confidence. If it breaks, saves you an ugly problem down the road?