Stuart Twin Victoria (Princess Royal) Mill Engine

Got the main flat surfaces completed on both straps, and machined the holes. Everything lines up fine, and all but one of the holes look to be good quality (at least when assembled using a twist drill as a spigot). The remaining hole has a slight bulge, but it’s cylindrical over about 270 degrees so it should be ok at least in function. I don’t think the holes are worth reaming after all:







I did notice a flaw in one of the halves, which I hadn’t picked up on before:



I don’t think it will machine out at the diameter specified. Is there any way of filling it before machining that wouldn’t be too obvious when finished (I won’t be painting the straps?

Anyway, next job is to make the four bolts from mild steel.

Thanks.

If you have any offcuts of the same material you could plunge mill a hole and turn up a small plug to Loctite into place

I don’t have any off-cuts, but I wonder if it’s the same material as the main bearing pedestals that I no longer need?

The other thing is I’m not that keen on the flat bar eccentric rods. I’ve seen some with parallel round bar screwed into a flat plate with a boss for the rod, the plate is in turn bolted to the eccentric. If I got some of the right diameter rod I might consider doing that, then I’d have the rectangular tail off-cuts spare.

I might see how much material I’d have for a pad on the eccentrics and take it from there.

Would it be possible to solder the plug in place rather than Loctite? I guess I’ll be soldering the joint faces anyway, so I could do the plug at the same time before final machining.

You could do, just be careful that any hot flux in the bottom of the hole does not pop the plug out.

I thought you were going to just bolt it together for boring?

Thanks Jason, I’m making some ‘fitted’ bolts at the moment, so depending on how they end up I might just bolt it.

I think the bearing pedestals are the same material as the straps, so I guess I could bore right through and plug to avoid any push-back, then machine?

Yes likely to be the same so would not affect the cut

Re: your 'plug' to cover strap-blemish:

How about drilling all the way through a small size (say 1mm), then drilling the 'plug' size part way through.

This would allow flux to escape down the 'pilot vent' and save re-machining inside the straps? The pilot hole would be invisible once assembled. Additionally I note that your flaw appears to be very close to the strap-edge = watch for drill break-through! Maybe worth using a suitable sized end-mill as a drill?

Regards,

Phil

Thanks Phil, yes the idea is to use the end mill again - I realise it's close to the edge, but at least the flaw tapers, so the very edge of it might get machined away anyway.

I'll see how it goes with the milling - I might just mill right through it it's possible. As you say whatever the inner end looks like, won't be visibe anyway.

I got the bolts made today:

I used the end stop and gauges to get everything consistent:



Made a small fixture for getting the heads the same thickness, and the chamfers identical:



And a split fixture for the mill, to get the flat offsets identical:



All seemed to work OK:







After all that, they do assembly very snugly, so I don't thing soldering will be needed for the subsequent machining:



I might add some small 45 degree chamfers to the ends next to the bolts - I think they look a bit clunky as they are now. I was going to make them a semi-cylindrical radius, but the bolts aren't centred on the pads, so that might look a bit odd.

Unfortunately the I don't think there's enough room to make a flanged boss for the eccentric rod, so I'll stick with the rectangular ones as per the article.

Still struggling a bit with the straps - a bit of fettling needed tonight, but I think I’m getting there.

The casting flaw I didn’t pay much attention to: it turns out only about 0.3mm needs machining from the o/d, even roughly measuring a Blue-Tack positive impression showed it would definitely still leave a hole:



As suggested, I milled a 3mm hole, but went straight through the flaw and into the I/d. Then turned some similar material (from the spare bearing block) to be a tight fit, then tinned with solder, tapped into position and sweated in place. I misjudged the proximity of the hole to the edge (it now has a very thin wall), but in the end it held:



After filing back, I think it will be fine once finally machined:

I guess the reason for the separate central ring is to make machining to a good fit easier, since the eccentric boss can be turned in one pass rather from each side?

Presumably though because of the fitted bolts and lock-nuts, a tight fit can to some extent be compensated for?

At the expense of contact area

I’ll hopefully get to making the eccentrics this weekend. I’ll give the parting tool method a go, although judging the right diameter I think I’ll find tricky.

I’m thinking I might turn the outer side of the ridge first, checking fit with half the strap overhanging, then when it’s spot-on, measure and machine the chuck side to the same measurement.

I’ll hopefully get to making the eccentrics this weekend. I’ll give the parting tool method a go, although judging the right diameter I think I’ll find tricky.

I’m thinking I might turn the outer side of the ridge first, checking fit with half the strap overhanging, then when it’s spot-on, measure and machine the chuck side to the same measurement.

Continued with the straps this morning, marking out for setting up in the 4-jaw chuck:









Zeroed in the chuck:



Bored:



Internal groove turned:



Now for the external milling:



Too cold for comfort in the workshop today, so I’ll start again tomorrow.

Made the fixture for milling the outside profile:





The central hole is for a 10mm end mill so I can centre it inthe R/T a bit more easily.

Hopefully the clamp load will be enough to hold it together without bolts.

Set the fixture in the R/T:



And milled the outer profiles:



I used my EBay slip gauges, hot-glued together, to support the tail:



They seem ok - one is a bit off around the oiler pad for some reason. I think the pads are too small for an oil pot, so I guess I will need to spot-face them anyway:



I’ll also put chamfers along the square bolt boss edges.

These are a real pain. I’ve been playing catch-up and chasing my tail with various dimensions ever since the first hole went wrong.

It’s finished!





Well, this 3D printed version is - it's approximately OO scale:



Jason B kindly gave me the CAD model he's done for his single-cylinder version, and I mirrored it and moved the crankshaft and linkages slightly to form a twin. Then printed a base and wrapped some brick paper around it for a bit of scale.

It's now in place inside a mill building on a work colleague's OO railway (with the flywheel powered by a geared motor and rubber belt).

Edited By Dr_GMJN on 18/04/2023 22:17:19

That looks good Doc, what made you change to the smooth faced flywheel?