A Tandem Compound Mill Engine

[deltatango]

Crikey just looked at the date on the last post, nearly a year since I last reported any progress! Hope some of you are still watching.

When I got the engine assembled with the slide I found a very disturbing wobble that pulled the crankshaft from side to side. This was "one of those times" in a project when I must have needed a break and I simply set everything aside and moved on to other things; there are plenty of those around here.

When I motivated myself to go back and diagnose the problem it didn't take very long. I first suspected that the con-rod bearings were out of line with each other but work with a surface plate and V-blocks proved that one wrong, alignment in both planes was very good (and very pleasing!). Spinning the crankshaft between centres quickly showed the crank was wobbling, how in Hell's name did I manage to drill and ream the hole for the crankshaft so far out of square? Anyway, re-drilling the hole and making a concentric steel bush fixed the problem - and removed any excuse for not making progress.

With the con-rod sliding, and the crankshaft turning, nice and smoothly the eccentrics needed the rods and brackets to extend their reach towards the still imaginary valves. This lot needs to end up looking like:



The eccentric strap palms were carved from the solid on the end of a piece of bar:



parted off and cleaned up to thickness:



with the sides squared off and the stud holes drilled these improved the look of the eccentric straps:



I'd bought in an offset tailstock centre a while ago with the tapered eccentric rods in mind and the rods came out of FC stainless rod with shallow cuts - mercifully it really was free cutting and there wasn't any work hardening:



This all went together to look like:



David

[crueby]

Great to see you back at this engine!

[Kim]

Nice work on the eccentric rods, they look great!  They certainly are long and spindly - tapering those must have been a worrisome operation!

Kim

[deltatango]

Chris and Kim,
Good to have you back!

My first thought when I looked at the drawings of the rods was that it would be difficult bordering on impossible to avoid the tapers going fish-bellied. I was also doubtful about the "free cutting" claim on the SS I'd bought. Turned out I was wrong on both. The tool was a very sharp Crobalt bit in a Diamond toolholder and I had the RPM high with the self-act driving and the rods turned out well.

David

[Jo]

Crikey just looked at the date on the last post, nearly a year since I last reported any progress! Hope some of you are still watching.

Nice to see some more progress on this David, I won't admit to how many years it has been since I worked on my one   and I still have not worked out what to do with such a large model if I ever finished it   

Jo

[deltatango]

Jo, When I first looked at the drawings I circled the cross-compound version as my choice. If Bob P had been willing to ship the castings I could well have gone with that one but Bob (and later on airline weight limits) saved me from myself there. Even the tandem compound is covering 3/4 of my fitting bench AND it is getting heavy, really heavy. If I'd started the big one I think I'd be changing my mind about now and making the two sides as individual engines. For some reason right-hand engines seem to be more popular (could be wrong on that, just an impression I've come to) so a left-hand might make a change. Just a thought...

The drive to the valves is split into two with a set of rockers mounted on two "fulcrum brackets" to use the term from the drawings. These came as a pair out of one block of steel:



holes drilled in two planes:



sides milled to the correct angle:



which gets the parts to here:



which when cut in two and filed with the help of buttons look like:



The rocker pivots and shafts were simple turning jobs and then the fit to the base was tried:



Only small parts but very satisfying to make.

David

[derekwarner]

David......an image from a few days back has some intrigued

May I ask, what are the hexagon double-ended inserts in the edit bigends? eccentrics?.....too simple for a screw-in grease lubrication?

Derek

[deltatango]

Derek,
They are meant to be dummy oil pots. The original drawings show a 3/16"x40 ME tapped hole with an oil passage through to the eccentric but I can't see anything drawn that would keep the dirt out. I'm pretty sure the originals would have been be lubricated with oil rather than grease. I guess they'd look more convincing with a knurled top rather than the hex but I don't have a microscopic knurling tool. I'll think about some of these little details a bit later on. I'll also have another look at what Tug did with his.

David

[steam guy willy]

Hi David , When I need to reproduce small knurlings I push a small /medium or large crosscut file against the round part and push down really hard with the headstock free to turn. The file needs to be quite sharp though >>>>

Willy

[Jasonb]

The wheel that usually runs against the flint on a lighter makes a reasonable fine straight knurl on softer metals.

[deltatango]

Willy and Jason,
Thanks for the tips re very fine knurling. I haven't owned a cigarette lighter for a very long time, I'll buy one when I get out again and have a look.

It's also a while since I had any really sharp files either and they aren't easy to find around here these days, I'll have to look on line.

David

[deltatango]

The various valve and eccentric rods are supported part way along by rocker arms that are pivoted to the brackets described in the last session. The rocker for the slide valve is a single piece, for the HP cylinder the steam and eccentric valve drives each have their own rocker. The original drawings don't have any suggestion as to how these are to be made, probably they were intended to be silver soldered steel fabrications. It was seeing Tug Wilson's model that started me off on this one and I seem to have picked up his liking for JB Weld as well, so the rockers started off as rod and flat strip:



and these were prepared by drilling and milling:





then the arms and pivots drilled for pins:



I carefully avoided handling the camera when JBW was around so the last picture is of the finished slide valve rocker:



Most of the fillets of JBW turned out well, one or two will need a little bit of cleaning up later. The pivot pin is shown because I made those first to help with lining it all up before the epoxy set and that worked out well.

Next it's back to machining the last five iron castings for the HP cylinder - more on that later.

David

[propforward]

Excellent work here! Great progress.

[deltatango]

Thank you Stuart - all and any encouragement helps!

[Admiral_dk]

Great looking progress David 

What's next ?

Per