Reducing drawings

[Anthony Thomas 2Participant @anthonythomas2]

Hi all, newbie question here. I like the Julius de Waal drawings, which are all over the internet, but they are usually way too big for me. I shudder to think how big a boiler would be required to run a twin at, say 50mm bore by 80mm stroke. My question is this. Can I simply divide all the given dimensions by, say, 50%, (or 75% or whatever) and still end up with a working model?

All advice gratefully received! Cheers, Tony

[Anthony Thomas 2Participant @anthonythomas2]

[JasonBModerator @jasonb]

Funny enough Julius tends to do just the opposite as most of his drawings are enlarged versions of an original design. If you look along the bottom of most drawings you will see the source and can look that up to see what the original size was.

Within reason most parts will be OK just being halved etc though you will have to watch that some parts don't get too flimsy or make the call up/down to suit nominal sizes,

Also be aware that he has not built any of these engines so there are the odd errors, fits that are questionable or parts that just can't be assembled from time to time

 

Edited By JasonB on 18/08/2021 13:34:24

[Ramon WilsonParticipant @ramonwilson3]

Just bear in mind also Tony, that if you reduce dimensions by say 50% you quarter the volume of the part.

I have recently done this to another design by scaling to 5/8 of original size. All dimensions simply multiplied by .625.

Doing that will not change the design providing you don't begin to round up or down the dimensions to any great extent as doing so will soon have an accumulative effect.

It does mean having to work to some unusual dimensions at times.

Good Luck – Tug

[Anonymous]

Posted by Ramon Wilson on 18/08/2021 13:55:55:

….reduce dimensions by say 50% you quarter the volume of the part….

Not so, if linear dimensions are halved then areas are reduced to a quarter, and volumes are reduced to an eighth, of the original. Basically an inverse square law for areas and an inverse cube law for volumes.

Andrew

[Ramon WilsonParticipant @ramonwilson3]

Correction accepted Andrew – Perhaps I should have said it's a quarter of its physical size 

 

Edited By Ramon Wilson on 18/08/2021 15:50:47

 

Yes I know it's still an eigth 

Edited By Ramon Wilson on 18/08/2021 15:52:38

[Mick B1Participant @mickb1]

Plus if you " …simply divide all the given dimensions by, say, 50%…" you'll actually multiply them by 2!

But if you didn't already twig that, you'd surely find out on the way…

In one DO I worked in several decades back, they had an enormous G-clamp, allegedly made by an apprentice scaling from a 10:1 drawing. I still don't know whether I really believe that.

[Ramon WilsonParticipant @ramonwilson3]

Posted by Mick B1 on 18/08/2021 15:52:36:

Plus if you " …simply divide all the given dimensions by, say, 50%…" you'll actually multiply them by 2!

But if you didn't already twig that, you'd surely find out on the way…

In one DO I worked in several decades back, they had an enormous G-clamp, allegedly made by an apprentice scaling from a 10:1 drawing. I still don't know whether I really believe that.

Er Mick, I did say multiply by the desired percent reduction (or indeed an increas in scale) eg 50% but as you say you will soon find out

Tug

[brian jones 11Participant @brianjones11]

I would call these 3d sketches, artists illustrations, nowhere near regular engineering drawings for manufacture. Decide on some basic dimensions say bore and stroke and work outwards from there. After all you will have to interface with real world available components, nuts bolts, tubes, material thicknesses etc

Also work out how big your flywheel needs to be as scaling down half say wont work for example

But then I never went for model engineering and seeing old men sat on 5" gauge steam locos running round a back garden seemed a peculiar taste to me

[Ramon WilsonParticipant @ramonwilson3]

There's nothing 'wrong' with Julius's drawings as far as a model project is concerned. As Jason says they are unproved but the information is there to enable a build. Mistakes happen on all drawings to the most part certainly a rare occurance not to have any. For my own part I do not round up or down save the third digit and never rely on stock size material to fit – everything done to dimension. That's a choice we all have.

One of the issues facing the scaled down model is fastenings but that's not insurmountable – my good friend Chris Collison has scaled down to half size many of the Anthony Mount designs and others – here is the half size Mc'Onie running in front of the original on my workbench. The other is a halfscale Galloways Non Dead Centre engine from the same source. He has recently finished a Stuart Twin Victoria to bring to well over twenty his 'halfers'

I believe he told me his smallest fastening is 12 BA and he works the other fastenings in relationship.

As far as I am aware they are direct halfscale dimensions with no compromises save air/steam passages which are enlarged if the part allows for better air flow – though they run faster than their original none have suffered any flywheel issues

Proof indeed that this is a viable concept Tony so don't be put off your aim – you will have a lot of calculations to do (see another thread ) but your thoughts and intention are definitely feasible

As before Good Luck

Regards – Tug

Edited By Ramon Wilson on 18/08/2021 18:31:28

[JasonBModerator @jasonb]

Posted by brian jones 11 on 18/08/2021 16:26:19:

I would call these 3d sketches, artists illustrations, nowhere near regular engineering drawings for manufacture…………..

Well plenty of models have been built from his drawings and although a little crowded for my taste the isometric views and assembly drawings are a big step up from the usual model engineering drawings. Particularly for the beginner who may have difficulty visualising a part from traditional plans & elevations and/or not have much idea of how all the parts fit together.

I have scaled down and up quite a few engines both steam and IC and never had a problem with the flywheels having to be scaled differently, what is the reason that it won't work?

Another batch of scaled down engines by the late Tony Boxall, mostly Anthony mount designs again every part scale by the same amount

 

Edited By JasonB on 18/08/2021 18:44:49

[Anonymous]

Posted by JasonB on 18/08/2021 18:39:44:

…..never had a problem with the flywheels having to be scaled differently, what is the reason that it won't work?

Presumably because the volume of the rim, and hence the mass, has gone down by a cube law. However, for a model, probably running off load, I doubt it will be significant.

Andrew

[Anonymous]

Posted by Ramon Wilson on 18/08/2021 18:22:48:

………with no compromises save air/steam passages which are enlarged if the part allows for better air flow……..

That's interesting. I'm in the process of re-designing the steam ports , slide valves and ultimately the valve gear for my one third scale traction engines. The port sizes are mostly copied from full size, scaled as per the model engine. I didn't think the ports needed enlarging. My reasoning went along the lines of the port areas have gone down by a factor of nine but the volumes are down by a factor of twenty seven. So the flows are reduced well below the reduction in area. Calculations for the model engines seem to indicate flow rates and Reynolds numbers well below what I would expect for a full size engine.

Of course I could have got my reasoning wrong – what does the team think?

Andrew

[JasonBModerator @jasonb]

Posted by Andrew Johnston on 18/08/2021 19:15:01:

Posted by JasonB on 18/08/2021 18:39:44:

 

…..never had a problem with the flywheels having to be scaled differently, what is the reason that it won't work?

Presumably because the volume of the rim, and hence the mass, has gone down by a cube law. However, for a model, probably running off load, I doubt it will be significant.

Andrew

But so would the capacity of the engine have gone down by the cube law. Or is the momentum to the 4th power?

I expect your traction engine flywheel is quiet close to 1/4 the diameter of the full size just as my 1/6th scale one is and has not needed to be made a larger diameter or have an excessively thick rim.

And lets face it most "scale" models all have scale flywheels be that 1/2, 1/12th or even smaller than the full size, seldom would they have a larger out of proportion flywheel but interested to know Brian's reasoning.

Edited By JasonB on 18/08/2021 19:58:48

[Ramon WilsonParticipant @ramonwilson3]

Andrew – I can't comment on the technical apsect from a personal view as I have never had need to do similar.

However I believe the reason is a result of your comment previously, that scaling by half reduces volume by one eigth. If the passages are small in the first instance of the model scaled then they become somewhat inefficient at a half reduction. The one thing that can't be scaled of course is the medium passing through them.

In the frequent discussions I have with Chris he has referred to doing this – not on all engines he has scaled – but on quite a few to get a reasonable running engine without recourse to high inlet pressures

Flow rates and Reynolds numbers wil be relevant on all but Chris is much like myself – he wants to make an engine that just works reasonably well – the enjoyment being in the making far more than the performance derived.

Tug

[Anonymous]

Posted by JasonB on 18/08/2021 19:42:16:

But so would the capacity of the engine have gone down by the cube law. Or is the momentum to the 4th power?

I expect your traction engine flywheel is quiet close to 1/4 the diameter of the full size………….

I think momentum goes down by the fourth power. Consider angular momentum which is:

radius x mass x angular velocity

If we assume a scaled flywheel, and the same rpm, the mass goes down as a cube law and the angular velocity (radians per second) stays the same. But the radius goes down linearly. So overall the angular momentum scales roughly as the fourth power.

I'm pretty sure my flywheels are close to 1/3 scale, the rim looks pretty thin although they seem to be roughly scale compared to pictures of the full size engines.

Ramon: I agree that the medium behaviour doesn't scale. That could explain why an engine that is significantly smaller than full size, say a tenth or less, might have a problem with scale ports/pipes.

Andrew

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