What Did You Do Today 2025

[JasonBModerator @jasonb]

It looks like you got the stud hole that you used at the junction of the two in the wrong place. looking at the photo if the holes are 1/4″ then it is about 3/16″ closer to the central recess than all the other holes. If you had picked any of the others when you machined the flat you could have got away with it.

 

[Nigel Graham 2Participant @nigelgraham2]

Not quite Jason. That’s where I had to chop both but especially the larger cover about a bit to make them both fit. The holes all lie on two pitch-circles that are tangents at the common one. I drilled them before forming the flats, but I think what threw it was mis-reading my own drawing for the various diameters.

Mulling it over I realised this construction can never give the effect I wanted, but can be done by making a single plate shaped like a figure ‘8’.

 

Studying the one photograph I have of the original engine, it’s clear that the covers were not circular, but more ‘D’-shaped, the bolt-hole pattern reflects that, and they were quite deeply dished, but the image is at too low an angle and too far away to show the true form.

So I have just spent something like 3 hours re-drawing them to something like the photo:

The LP cover studs all lie on a circle except for the central one – displaced slightly to centre between the cylinders.

The HP studs are all at 60º angles but have two displaced outwards so the outer ones lie parallel with the straight edge. The central hole is on the pitch-circle and was the leading one for generating the rest.

There is no real reason why the arc in the LP cover recess cannot continue round to meet the straight wall, with a fillet formed by the cutter.

I’ve also realised the underside can stay flat. These covers, unlike the crank-end ones, do not need closely register with the cylinder bores.  It could be a single plate but I thought that would be harder to make flat all over.

Overall length just over 5 inches. Height 3/8″ , recesses 3/16″ deep. LP cover 3.25″ dia.

[Nigel Graham 2Participant @nigelgraham2]

Post-breakfast look at my drawing… the one a few posts back up this thread.

Of course – that’s where I went wrong!

The dotted inner circles represent the registers for compound cylinders. Enlarging the HP bore and reducing the LP to make two identical cylinders, moved their common centre so trying to use non-identical covers in the same way cannot work. That was what I had failed to realise, despite the dimensions!

I created the 3D model above in TurboCAD, though not quite as CAD is intended to be used. The supplied “Student 2D” template makes all that fancy geometry fairly easy, and I saved that as an orthogonal drawing. Then copied-&-pasted it to a new drawing from the “Normal 3D” template, for the extruded image saved as a separate file. (I do not know how to derive the elevations from the model, in TurboCAD, nor how to copy drawing elements properly from one drawing to another.)

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