Murray's Hypocycloidal Engine - Antony Mount

The way I do these is to use a 1/16" drill to first drill out each end and then get as many holes in between as I can. I Then change to a 1/16" milling cutter and make a series of plunge cuts working along at about 0.010" for each plunge, I think Anthony suggested this method in more than one of his engine builds and it seems to work better than milling along the slot. If the slot is deep you may need to mill from both sides.

This will leave you with a round ended slot and to make this square I have a warding (flat pointed) needle file that I have ground the teeth off of in the flat faces so I just have the edge teeth which means you don't make the slot wider while filing the ends.

J

Thank you for that, Jason.

Looks like I'll have to splash out on that 1/16 milling cutter (presumably 2-lip slotdrill?), then.

I wasn't sure it was worth squaring off the ends - it might be easier to file radii on the key & wedge, but I'll jump off that bridge when I come to it...

I had been thinking about modding a flat Swiss file to make the rectangular punch affair I was proposing, and taking off the face teeth to stop it opening the slot is a good one - ta.

2 or 3 flute will be OK

OK, so I bought a 3-flute 1/16 cutter - and, as I suspected I might, broke it on the second pass. Probably should've wound the speed up higher than the 1000-odd I was running at.

So I went back to plan A*, drilled 3 'oles and milled between them with the pilot of a BS2 centre drill. Broke the pilot on one end too, but carried on with t'other. Then poked out the tiny bit of remaining metal with a Swiss file and cleaned up.

The other mistake I made was misread the drawing of the crankpin, so the capped journal for the split-bushed bearing was too short. I could've, maybe should've, remade it, but I cheated and tapped it 5BA and turned the fixed cap down to journal size (3/16) and interposed a stepped washer so's it wouldn't bind.

Here's the resulting subassembly:-

As a macro shot it looks rather agricultural, but it's nice enough to normal vision.

Lotsa work to make 1/4 teaspoon of swarf...

NIce one

Ah, thanks, and I meant to ask about gaskets.

My previous engines from Stuart's and PM Research had gaskets in the kits.

I used to make gaskets for my old Beeza A10 (back when the world was young and men were strong) from cornflakes packets (and red Hermetite where the faces had been bruised and butchered, mostly but not invariably by previous owners...).

But they're way too thick for model engines. I bought some 10 thou gasket paper but found it dead fiddly.

I was wondering whether the use of plastic gasket from a tube is regarded as excommunicable heresy, or whether the stuff is actually more trouble than it's worth from the future maintenance viewpoint?

Edited By Mick B1 on 19/05/2018 19:32:05

I use the liquid gasket on all my engines

OK, thanks - I'll suck it an' see.

Mick,

This stuff is great for gaskets, ebay item number 173028673495.

0.1mm PTFE sheet, high temperature resistant, 300 x 1000 mm.

Very easy to cut and shape with a small sharp craft knife and "cheap as chips" to buy

I used it for gaskets on my small jepson engine, gave perfect seals all round.

Comes from China but worth the wait.

Geoff

I use liquid gasket, I used to use oiled brown paper from paper bags, looks like the paper bags are going to make a come back.

Ian S C

Beat me to it

Right. Still haven't got to gasket-making stage yet.

Yesterday I was making the reinforce arcs for the laser-cut ring gear supplied wi' kit. I've drawn in the arc where they meet the inner toothed ring when machined to the drawing, and shaded in the bits I gotta make.

Oh, momma, I really hadda do some baad things here...

In order to get the arc radius right(-ish) at the base of the reinforce, the only easy way I could see to do it was to use a long 3/16" toolbit in my boring bar/flycutter, set out to the 1.708" radius of arc, then set a turned blank in the vice 1.5" above centre height - pretty close to the top of my vertical slide travel.

Shape of the toolbit reflected my earlier punt at doing a sort of interrupted trepanning cut through the face to save chopping away all the metal on the underside. I didn't think I'd get away with that, and when it became clear I wasn't going to I abandoned it before I broke the tool or wrecked the blank.

So then I let down the slide 20 thou at a time and bored out the arc on the slowest feed longitudinally. Took Gawd knows how many cuts.

But eventually it worked, as you can see in the first pic. I was offering up the turned ring gear to see if the workpiece would fit between the inner and outer diameters over the last 10 thou or so, and it came right within about 2 thou of the expected vertical slide setting.

Then I could carefully part off the 2 reinforces on a slow-speed interrupted cut.

Now I gotta soft-solder the things in place prior to a final face skim to clean up.

'Spect that might be another story...

Edited By Mick B1 on 16/06/2018 10:32:09

Edited By Mick B1 on 16/06/2018 10:32:55

I couldn't get the flux-cored soft solder I use on electrics to take properly, so I tried silver solder.

That worked, but I've got a couple of stray blobs oozed out. They won't (AFAIK) interfere with owt, but they look untidy.

Think I'm gonna hafta Dremel out as much as I can with those little rotary diamond file affairs, then finish off with the point of a Swiss file ground like a watchmaker's turning tool.

Unless anybody would please tell me a better way?

Well, the Dremel (actually Rotacraft) didn't really work - there was too much risk of carving into the steel underneath the silver solder - but I found a titchy little parting tool I'd ground for titanium earings I was turning on the little Sieg C0.

So I mounted that in the Warco pointing at the chuck, gripped the inner gear ring on the outside jaws, set the tool to kiss the recessed face and rotated the chuck by hand until the reinforce hit the tool. It peeled off a strip of silver solder. I had to back off a bit on the thickest bit of the blob and do it a few thou at a time, and of course it left a tiny triangle untouched where the flat-faced tool couldn't reach, but maybe 20 minutes' work cleaned off almost all the blobs. Then I blacked the part with gun-blue creme, washed it and emeried the o/d and front face.

Maybe my standards are low, but I think that's now OK. I wish I could get rid of the joint line on the lower reinforce, but I'm not yet sure I care enough to take the trouble - I thought about filling it with soft solder but there's probably some solder-hostile crud in there by now so I don't know how easy - or risk-prone - that might be.

Maybe I'm making a meal of stuff some of you probably think is nursery-slope work...

Looks good to me, you can always leave the crank arm over that joint when the engine is on display. Even if it had soldered up a bit tighter the silver solder can tarnish over time and leave a black tell tale.

Thanks, but sadly I can't - the crank arm's behind the hypocycloidal gear set, and the piston rod bigend only goes straight up and down between the inner radii of the reinforces. Anything I did to widen it would probably just look as if it was there to hide summat...

Just had another look at Anthony's design and yes it can't be hidden. Short of cutting your own from a slice of CI bar there is not that much that can be done, the internal gears are not that hard to cut.

Think I'm just about finished for piece parts now, except for the soddin' frame feet, which I think I'm going to bolt on rather than silver solder, after the last nail-biting experience.

I simplified the main bearing housing to a square u-shape and again bolted that on, there being no discernable reason not to.

I saved myself a 48 mile roundtrip or two-day delivery wait by drilling the tapping holes for the 8BA main bearing retainer bolts 1/16" instead of the proper 1,8mm, and it's cost me not 1, but 2 broken taps. Stupid of me to try it, really - I get away with it once, and then think it's an acceptable technique. Blind holes - not a hope in Hades of gettin' 'em out. And I couldn't be ar$ed to remake the bearing housing - not at this stage - but I did find I could resharpen what was left of the broken plug tap. And if I drilled 5/64" or 2,00mm for tapping, the thread cut much easier and was still strong enough (which I hadn't at first expected).

So one of the bearing retainer bolts is in a bit of a 'not to drawing' position. Whether or not I ever fix it probably depends on how persistently it embarrasses me to see it later...

I've done the feet, bolted them to the A-frames and to the baseplate, also done a casual assembly with 4 of the longitudinal tiebars.

There's 2 thou clearance between the crankshaft diameter and the holes in the bearings, which are 4 inches apart, and it just slipped through and turned freely, just like that.

Now, it's true that the Polly Models kit has some of the fastener holes and the bearing box cutout already in place, but there are still a lot more than enough places you can introduce variations to build up some problematic cumulative errors.

So, at the moment that's looking good. Still plenty of opportunities to screw up yet, though...

Right. 'Tis done - more or less. I might make oil cups for the main bearings and eccentric strap, and I'll probably paint the frame, cylinder and the flywheel except its rim, but it runs from a big bicycle pump - though my 12V tyre inflator doesn't seem to shift enough air. Unsurprising as the cylinder swept volume's much bigger than my Sturat Beam, which it can just operate.

Took me several tries to get it running.

First I tried a Nylon 66 piston ring. Maybe that would've worked if I'd worked up a mirror-smooth bore, but as it was I couldn't get an air seal without a tight frictiony fit. So I took some cotton kitchen string and smeared it with LM3, and that finally gave a smooth-fitting seal.

There was a few thou shimmy on the eccentric because I'd started it from a bit of CI that was pretty much exact length, and that was just fouling the protruding tip of one of the bearing support bolts once per rev, till I found a way to chop the tip off without dismantling the whole thing.

The Loctite 271'd interference fit of the mainshaft to the crank throw wasn't up to the torque, and shifted - so I drilled 3/32 through both and pinned it with silver steel. Meant I had to fiddle the valve chest cover with its 11 x 8BA studs and nut off and on again so as to reset the lost valve timing. Growled away half a morning on that.

But the stupidest issue was my inlet assembly. I had a hollow hex tube feeding the valve chest, and a plain inlet pipe screwed in at a right angle to that. Screwed it in so far that it completely occluded the 'ole into the hex tube, didn't I? So the compressed air wasn't even reaching the valve, never mind the cylinder. I really don't wanna say how long it took me to find that. Duh.

There are two weird-shaped tie-rods from cylinder to stator ring. Only the Michigan original has these, and I can't really see the point of them in the model as the four 5BA bolts holding cylinder to frame should easily be adquate. The drawing's not completely dimensioned and there's no obvious easy way to calculate the compound angle, so I fabricated them from steel components screwed together, which took quite a while as suck-it-and-see seemed the only method.

So. I've got a nice new model now.

Is it because of the laser pre-cut frames and gears that I now feel a bit of a cheat? I strongly doubt that I'd ever've worked up the commitment to build such a thing without - dunno how I'd've done the gears at all with the kit I have.

Thanks to all who've answered questions and provided advice, even if I haven't always followed it.