With the 'clapper' engine on hold awaiting a valve and elbow engine awaiting materials I decided to build the opposed single acting twin version of 'Scotty' - 'Double Scotch' - so called because it has two scotch yokes, one controlling the valve and one the movement of the pistons.
IMHO the original design needs some changes in the steam passage/valve area - producing a 90mm x 5mm diameter bore good enough for the valve pistons is difficult; long series drills are needed but it's then too long for reaming and too narrow for boring!
The designer suggests drilling carefully from each end then reaming all the way through. Pah!
I had the remake the part twice as the this hole in the first one came out a little skewed - it would probably have worked but would have irritated me no end - no fun when you have no mill and need to make the frame from 10mm thick steel (I don't have aluminium as called for in the plans).
In the end I decided to make a bigger hole and fit a piece of commercial brass tube (6mmx0.5 wall) as a sleeve. Crank, main bearing and piston rod are made, next the cylinders.
Made the 'spool valve' assembly from 1/8" silver steel turned down to 3mm at each end to take the 5mm dia valves, loctited in place - the plans call for soft soldering discs to brazing rod - doesn't sound that accurate to me
Still a bit too firm a fit at present but at least my modification seems like it'll work.
circlip: let's go 'round his house and relieve him of that obscene amount of freebies posted elsewhere. At least he spared us the pictures this time ;D ;D
Al: Seriously, thanks for the offer but I'm ok for the time being - I bought a 4.5mm and a pack of 10 (!!) 6mm long series bits. If anyone wants a postage-cost-only 6mm just holler, I can't see me using them all in a lifetime!
Made the flywheel/eccentric. First time I'd tried turning anything off-centre in the 4-jaw and it didn't com e out too badly - the diameter and thickness of the eccentric part is spot on but it's a tad further off-centre than the plans call for I don't think it'll make much difference though as I can take up the difference in the manufacture of the valve yoke thingy.
This engine is getting pretty heavy for it's size; I put it on the scales and it came in at just under 2lb! Maybe I should have used that aluminium girly metal after all LOL.
Would that be a British Standard tad or one of these new-fangled mertic ones?
It was supposed to be 2.5mm off centre but it's nearer to 3 so it's a decimo-imperial fractionlet. I couldn't think of a way to measure the offset other than make a fine punch mark in the right place then use a centre in the tailstock to get it aligned, then run a tool across it to make sure the whole face was true. It took ages and I almost changed plan - a separate eccentric disc fastened on to the flywheel would have been easier to make but then I'd have the same problem positioning it before fixing. Hmmmm.....
Al, the flywheel is a bit rough and ready to be honest; I got chatter aplenty while trying to scoop out some of the material and the mass-reducing holes are a very strand shape when viewed closely, not 'triangular' and certainly not round, but they'll do
I had to loctite the air/steam inlet on top as there isn't enough 'meat' to put a thread in above the valve tube and I couldn't get the thing hot enough to silver solder, so I wouldn't be surprised if I manage to break it off at some point!
Nice one Paul. Should you ever have to do something similar, you might think of putting a 2.5mm packing piece under one jaw. That should work with a 4 jaw. And there are probably lots of other ways of doing it too. The job's looking good though. John