Currently drilling and tapping the joiner holes for the rod bosses, I need to get a M6 plug tap to finish this off. Also gradually cutting the centre sections for the rods, this is 4 x 9" cuts in 6mm plate, so is taking a while. Just doing a bit each day so as to not wear my arm out or get bored and rush it. Also contemplating building a 16mm scale loco as a side project, this would be built to sell to help fund a bigger lathe. The little Taig isn't big enough to machine the blowfly cylinders, and the crowdfunding campaign I have going is not going very far.
Yey, 2 hours work and I have a hole. Just bored out the crankpin hole in one of the front coupling rod bushes, this includes a 2mm deep 20mm counterbore for the retaining collar to fit in.
That was quicker than I expected; all the boses are now machined to size, so I can get on with boring the crankpin and joiner holes next. Will have to sharpen up a boring tool first.
That's one boss machined to size, and now all nice and square and shiney. Five more to go. Hey look! I cleaned the lathe benche (didn't realise how messy it was till I say that last photo). Sitting on the bed is the finished-to-size boss, with another mounted in the 4-jaw chuck ready to be machined to width. Lathe belt is off, as I took the pic' just as I was done for the night.
Just to show I am actually doing stuff, here's the coupling/connecting rod bosses (as per earlier drawing) getting machined to size in the lathe; to give some idea of scale, that chuck is 4" diameter. The other 5 bosses are sitting on the cross slide for the photo. All have been machined to length, currently machining to height, next machine to width. Then I'll bore the holes for the crankpins and bushes, and then drill the holes for connecting them together into a finished rod. This all looks messier in close up.
Well that's one dead (or at least blunt) hacksaw blade, now waiting for some new ones to arrive, which should be a few days. I could get a pack of cheap blades froma local store, but those cheap carbon steel things are utterly useless; akin to cutting a steak with a stick of butter. So I've ordered some good high speed steel blades. But as I've got a couple of the coupling rod bosses cut to (rough) length, I've started machining those to square them up; the combination of rough drawn black steel and my battling on with a blunt hacksaw blade on the last one means they're far from square now. As I was setting the job up I gave the 4 jaw chuck a good clean and some fresh oil; apparently went overboard on the oil as I got a face full of it when I first started the lathe, oops. But none-the-less the little Taig lathe is good for this kind of small job, and even with mild steel it cuts without fuss. Still took an hour to face the ends of one boss down to size, 6 to do total (4 for the coupling rods, plus 2 for the connecting rod), so longer task than one might think for something so simple. Plus that's only 2 faces done, 4 more faces per boss to do, then bore out the pin holes, and make the bushes. Hopefully I can find plenty of time to work on it though.
My arm is threatening to go on strike, still cutting metal. Still at least when I've got it all cut the rest of the work on these rods will just be machining; which I can pretty much sit down and do. On the plus side it's still less than half the cutting work needed for the original design, so I shouldn't complain too much.
Giving the coupling rods some thought, I'm considering deviating from the plans; partially so they're easier to make, but also to give them a bit more shape, and centre the bearing surfaces on the rods' forces. The standard Blowfly design calls for 25x7mm steel, shaped into a dogbone, but I'm thinking of using 2 pieces of 25x10mm for the rod ends, joined by a length of 12x6mm which would be threaded and silver soldered into the ends. This would give what to me is a more pleasing shape, while resulting in considerably less hacksaw work. Also, unlike the original design which has the bushes overhanging the rods by several millimetres on one side, this would centre the forces of the rods on the bearing surfaces, which could reduce wear (would also centre the oil hole in the bush, thus better lubrication). Here's a quick workshop drawing I made tonight of the intended coupling rod design.