Stuart S50

[Allen Smithee]

Last week I picked up a job lot of stuff from a friend who is clearing out a lot of model engineering and aeromodelling stuff because he has realised that the number of years he has remaining is almost certainly less than the man-hours of projects he has been hoarding. Much of this I will be looking to pass on, but that's another thread.

Amongst this hoard was this box containing a Stuart S50 kit:



I thought it was incomplete, but I've checked it off against the BoM and it's all there (screws and small metal parts are in those two film tubs). As you can see it has been started, but I'd estimate it's about 20% finished. I'm really just an aeromodeller who dabbles in the dark arts, but for some reason this one has caught my imagination and I think I may actually finish it off. So if people can bear yet another S50 thread I will document my amateur attempts for all to laugh at.

Doing some reading around I note that these were very prone to chilled-casting syndrome, especially in the steam chest and the steam-chest cover. If you look at the photo you may be able to see that this steam chest casting is actually bronze rather than iron, so perhaps this was a response. The steam chest cover is still iron, but if that turns out to be chilled it's a shape I can always mill from a bit of mild steel if I need to.

I regard the silly practice of quoting all sizes in fractional inches as a deliberate challenge; proper metric dims would be ideal, but if you're wedded to inferial dims what's wrong with thou FFS!! At this moment I'm wondering whether it will be simpler to do a quick redesign to allow me to use my existing metric taps/dies/drills/reamers or just to buy in a few inferial tools and hide them when they're not needed...

Actually I do have a box of assorted BA taps an dies that I was given, so the fetish with 5BA and 7BA may not be a problem. But I've never owned a 3/32 reamer and I don't intended to start now!

Looking closely at it I think there are a few things I'll change. The connecting rod and even the crankshaft run steel-in-cast-iron, and I think I want to bush these with brass or bronze on principle. There's no oil hole in the big end, which seems sub-optimal but easily fixed. And the niggardly piece of 1/4" x 3/8" MS bar supplied for the connecting rod has almost no machining allowances, so I'll probably make it from 1/2" round bar stock instead. I might also replace the cast-in dummy nuts with real ones to look nicer (if I can be bothered). They supplied a big chunk of mild steel bar to make the crank web from, but it seems over 80% of it would end up as swarf and as it's essentially just a shape with two tapped holes in it I'm thinking of cutting it from 1/8" plate instead, and making a flanged bush which I'd silver solder into the middle to provide the support for the shaft. Or something.

I've never done a steam engine before so may I as a typical clueless newbe question? I'm looking at the steam valve which is supplied as a tiny bronze casting. I can see how it is actuated by the pushrod from the eccentric, with the end of the pushrod screwed into a bit of brass that sits snuggly between the pillars on the back of the valve. Am I correct in thinking that the valve block itself just floats, being pushed onto the face on the side of the cylinder by the incoming air/steam pressure? If so I presume I just need to get the seating face flat on some wet&dry and then possibly lapped onto a piece of glass (or something) mor is there a precision fitting aspect that I'm missing?

TIA,

AS

[Jo]

Hi Pete,

Stuart models normally provide good quality Cast Iron and you rarely find hard spots (they had a bad period but seem to have improved since then  ). Having seen this kit I think you have a good set 

Cast Iron is actually a good bearing surface against steel as long as it is well lubricated.

Yes the valve block floats and the air/steam pressure will hold it against the valve face. You will get both surfaces smooth enough rubbing them on wet and dry on a flat plate.

May I suggest you try making the bits to drawing before you try modifying them or making improved ones. 

Jo

[Jasonb]

yes the valve needs to be able to lift off the port face slightly to allow any trapped condensate to escape and avoid a hydraulic lock. Just some fine wet and dry on a flat surface to rub the casting on should do, i don't always bother with that but will depend on the finish on the casting if it has already been machined.

M3 and M2.5 would be OK preferably with smaller than ISO hex sizes and you could get away with drilling the valve rod hole 2.4mm and cut a M2.5 thread onto the supplied 3/32" material.

When the drawings were first bone most would have been working with a steel rule and firm leg callipers so fractions suited those methods.

[MJM460]

Hi Allen,

Looks like a very nice acquisition indeed.  I don’t think anyone ever complains about another steam engine build on this forum.  I for one am looking forward to it.

I also try and stick to metric tooling and dimensions, but have found that occasionally, the gaps between rational metric dimensions are just too big and something old fashioned just seems to fit better. But many times, nearest metric sizes work fine.  Just need to check clearances, and importantly the gaps between features to ensure that the small differences in dimensions do not accumulate in an unfortunate way. So I have purchased a set of ME taps and dies.  I also occasionally resort to a drill size from a set I purchased many years ago. 

As Jo suggests, I have just used wet and dry agains a sheet of glass for the valve face and my engines run fine, though I have to admit that I am not sure how I could determine if it was passing too much unless it was so excessive that the engine would not run.  Allowing the valve to lift means that the cylinder can discharge condensate instead of locking up on the condensate that accumulates when warming up from cold.

Getting beaten to post faster than I can type, so better post anyway.

It won’t fly but I am sure you will enjoy it anyway.

MJM460

[Allen Smithee]

May I suggest you try making the bits to drawing before you try modifying them or making improved ones. 

Invoking MIL-TFD-41?

Where's the fun in that??? 

AS

[propforward]

That's a very nice acquisition - looking forward to seeing your build.

[Roger B]

No more Mil specs please that time in my life is passed   Make It Like The F**kin’ Print for once

[Allen Smithee]

For the uninitiated - we had a shop floor senior manager who didn't like to see any concessions rejected by Engineering because it implied his lads had tried to ship stuff that was unacceptable. So if a concession/permit request wasn't approved the engineer woukld write "Rework in accordance with Mil-TFD-41".

Mil-TFD-41 = "Make It Like The F'ing Drawing Four Once"

AS

[Jasonb]

There was quite a detailed build article in Model Engineer some years back by "Tubal Cain" which is still available on the ME website and may be helpful for your build. Just change the PDF number at the end, there are about 10 parts.

https://www.model-engineer.co.uk/sites/7/documents/sally-1.pdf

This photo of the bed casting being marked out gives a good idea why fractions were used as you can't find many steel rules marked in 1/1000ths

[Allen Smithee]

I was waiting for paint to dry on my current project, and it was too windy to go flying, so I thought I'd make a start on this (but this will be a sporadic "when I'm not doing other stuff" build so don't hold your breath). I've never built a steam engine, never build from castings and my metalworking skills are (by the standards of this community) rudimentary at best. But the best learning is by doing, so here goes...

I looked at the drawings and the con-rod took my fancy. As I said previously, the kit includes a miserly piece of gash 1/4"x3/8" bar for this and I had already decided that was too much like hard work, so I found a piece of 1/2" EN1a round in the stock box and used that instead.



It's a simple-enough part with some taper-turning, but it's infuriatingly dimensioned - measurements from random features rather than picking a datum and referencing everything to that. But why do things properly when you can waste everyone's time doing it strangely...



The books always talk about setting-over the tailstock for mild tapers, but I hate doing that so I put my better boring head in the tailstock instead. I'd forgotten that I'd made a dead-centre for it, but it was there in the box waiting for me:



The setup is a bit strange because my QCTP gives me limited access near the tailstock, so initially used the whole 12" piece of bar and started taking the round part of the rod down to 7mm dia before doing the final taper turn. Despite the unsupported length it cut nicely even without a steady:



Then the feck-up fairy dropped in for coffee. The centre in the boring head suddenly started to smoke, and then burned out completely. That's when I remembered that I had machined the centre, but hadn't hardened it...

Looking around the workshop my eye fell on a small live centre in my Unimat 3 box. It wouldn't fit into the nice Jones & Shipman boring head, but my other boring head took larger cutters so I quickly turned up an adapter sleeve and I was back in business. I also played around with the position of the compound slide and toolholder to allow me to taper turn on a shorter length:



Then it was just a matter of moving to the mill to face off all sides to dimensions, pilot-drill little and big end bores and trim off the surplus length I'd left on the bar so I could hold it in the chuck. This is still around 0.1 oversize on thicknesses to allow for cleaning up after I drill and bush the bores. The phone's flash exaggerates the machining marks (that's my story and I'm sticking to it):









So here's the roughed-out part - I've left the ends square while I decide how to round them. I think once the big-end is bushed I'll make a little jig with a pin that I can stick in the milling vice and round it against an end-mill. But the little end will probably have to be hand-filed because the bore isn't concentric with the radius for some obscure reason.



Incidentally - this is my part sitting next to the piece of gash material that Stuart suggests you could make it from. No doubt an experienced machinist could do it, but why? Is an extra 1/2" of material at each end to give you something to hold it by really too much to ask...



To be continued...

AS

[Admiral_dk]

Then the feck-up fairy dropped in for coffee.

Yea - don't you just hate some uninvited guests     

Nice part so far 

Per

[propforward]

Looking good!

I can't help thinking that the kit suppliers have a right old laugh when they think about what bar they supply for parts. Seems like more often than not the kit stuff goes in my offcuts box while I find something more suitable.

[Allen Smithee]

Then the feck-up fairy dropped in for coffee.

Yea - don't you just hate some uninvited guests     

Sadly she's a regular visitor...

AS

[propforward]

I’m pretty sure that little beach lives in my shed somewhere.

[Baltic]

Hi Allen,
I feel your pain, I am currently building the Hemingway knurling tool (nearly finished),

Some the the tooling called up is
BA taps, Metric, and Imperial??
I will be onboard following S50 your Build,

Gary