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Building an engine bigger than designed

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115/02/2013 15:55:31
65 forum posts
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For my next project I am considering Mogens Kilde's Double Diagonal engine as serialised in ME No 4383 - 4394 starting in Aug 2010. However it is a little on the small side for my taste so I would like to make it a bit bigger, say 1.5 times drawing. I know little of actual design or drawing and since Mogens has drawn it in metric units it would seem to be a straightforward case of increasing all measurements by 50%.

Does the team think my simplistic approach will work or are there complications that I will only find when I get in to making swarf or even worse when I try to run it?

Jim

JasonB15/02/2013 18:13:11
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Generally it works I've done it several times

Things to watch out for are fixings. Eg a M2.5 will not easily increase by 50% to M3.75 so decide what works better M4 or M3.5

Same with nominal diameters a 3mm x 100 rod is off the shelf but its not much fun turning 150mm of rod down to 4.5mm

Stub Mandrel15/02/2013 21:23:17
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I'm sure it will work fine, but you may find things start to look a little 'chunky' when blown up, just as models to scale can be abit too delicate. Just use your judgement and if it looks good it probably is.

Neil

Clive Foster15/02/2013 21:25:33
3171 forum posts
113 photos

Usually not a functional issue when scaling up but its wise to keep an eye on the strength / stiffness aspects of components. At smaller sizes components often have to be thicker or of larger than "scale" size to ensure that they are not too floppy to function well. The fixings and screw sizes used are often relatively oversize too so that ordinary range threads etc. can be used rather than the super weeny watchmakers versions.

By reason of familiarity such relative over-sizes can be seen as perfectly acceptable on a very small model but if carried over onto something rather larger ( i.e small model) the effect can be jarringly wrong. I've no idea where the line between very small and small falls but I'm sure we have all seen examples of nice work being wasted because such things ensure that it will never look right.

Clive

 

PS Neil types faster!

Edited By Clive Foster on 15/02/2013 21:26:26

Stub Mandrel16/02/2013 11:11:15
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> PS Neil types faster!

PWND!

Neil

 

Edited By Stub Mandrel on 16/02/2013 11:12:06

116/02/2013 16:40:10
65 forum posts
1 photos

Thank you all for your input. Looks like I have my next project lined up.

Jim

Takeaway17/02/2013 17:18:47
108 forum posts

If that engine is a wobbler/oscillator should be no probs. Only time I ever went wrong when upping the size was on the crankshaft of a now finished engine. It did not occur to me at the time but the counter weights or webs (hope I have the terminology right) on my crankshaft should NOT have been doubled in size - instead they should have been doubled in WEIGHT!

The finished crankshaft was hopelessly out of balance and so was I after I'd bashed my head against a brick wall.

Gordon W18/02/2013 09:50:21
2011 forum posts

Also, don't forget the area of, say, a piston is 4 times when the bore is doubled. This affects the required strength of fastenings etc. but maybe not in model sizes.

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