Using Photographs In CAD?

[Nigel Graham 2Participant @nigelgraham2]

Thank you for those.

Yes – I see the point there, about finding the vanishing-points.

I’m impressed by the engine work. I think you do have the advantage there of a much better quality photograph to dimension than mine are.

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Meanwhile I’ve been laboriously drawing a new cylinder-block, on the basis that it would be easier and better in the long run to make it afresh rather than trying to modify the existing one. It took me two or three goes to draw the ports, in the process putting their Sketch and Extrusion numbers out of step with each other, and I had to plot both ends individually.

The bores are now 1.375″, using the existing centre spacing, and I have used the port sizes given in the table on p.54 of Martin Evan’s Model Steam Locomotive Construction. I have not plotted the passages. That’s a bit beyond me, and I may have to do that manually on a print, but I have not yet determined their sizes and dispositions anyway. Since there is a cover stud in the middle it will need two pairs of drilled passages per end.

 

[JasonBModerator @jasonb]

I doubt it will have a massive effect on steam flow if you drilled the passages at right angles then they will be a lot easier to draw (and drill)

Something like this on another engine I made last year. You may need to drill down from the bottom of the ports if your milling cutter can’t reach the bottom.

[Nigel Graham 2Participant @nigelgraham2]

Thankyou Jason.

That is how I made the original block, but without too much thought about further details like cover studs, and how to seal the covers.

I had also cut the recesses as we see at the left-hand end of your example, too far back from the cylinder wall. (“Upwards” on your image.)

Many full-size engines had passages parallel to the cylinder, although made by coring the casting which allowed smooth curves at the ends and entering the bore without emerging through the end surface.

 

[JasonBModerator @jasonb]

Withh your stud in the middle you would be better off to mill two pockets at the ends, one either side of the stud, you can also cut them with a radius that is concentric to the bore so they don’t get too close to the outer edge of the end covers

[Nigel Graham 2Participant @nigelgraham2]

Thankyou Jason –

Yes, a much better layout!

I think that is the first design I have seen using drilled passages arranged radially, rather than in a straight line.

Thinking about how to do that on a twin-cylinder monoblock, I realised I have a plethora of rotary tables and the secret is to make a stub centre to locate the block by the bores, and use parallel passages. (Four RTs, not counting that built into the BCA jig-borer!)

From a machining point of view I wonder if the pockets might be more easily milled in the covers.

I note that although Martin Evans’ book gives a guide to cylinder and valve port dimensions, he doesn’t say a great deal about the sizes of the bit between. Just a sentence advising the passages’ total cross-sectional area “may approach” that of the ports, to avoid throttling the exhaust steam.

I can take my time a bit with designing the engine as there is plenty of other work to do, and indeed I spent some hours today refitting the boiler’s external plumbing as that was becoming a snake’s wedding.

[JasonBModerator @jasonb]

They are only radial at the cylinder end, each is angled slightly differently so they meet th eport in a straight line

You may not need to do that as the ones on the fowler also have to clear the steam jacket around the cylinder nence why they are set back quite a long way from the bore.

[Nigel Graham 2Participant @nigelgraham2]

I’ve started to re-design my cylinders, based on your suggestions – and no, I don’t need avoid a steam-jacket but I do need consider various other aspects.

Trying to squeeze four drilled passages in without risking breaking into a cover stud hole, I realised the two outer ones can overlap the port end walls by half their diameters (drill them before milling the ports so the drill is fully supported). That shuffles everything slightly outwards, giving more metal around the stud. I think I lengthened the port by 1/8″ as well, to help.

I did not touch the thing today, but in a clear-out found a few preliminary drawings that might be usefully copied in Alibre. I’m sure there were others! One which could not be though, is the construction geometry for the steering. That is all built but the drawing has a note about a shallow castor angle – I’d missed that! Easy to rectify, though.

 

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