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Scaling drawings

Scaling Doris

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Mike Wainwright17/02/2017 17:59:41
149 forum posts
8 photos

I recently found the write up for Doris on the Internet and thought I would change the scaling to 5" scale instead of the 3 1/2 scale it, was drawn in. Is there a formula for scaling plans. Do I just enlarge all important dimensions by 1.429 and then use the nearest equivalent material. I am drawing it in solidworks as further practice in using the program.

Does anyone have the missing issues of the build notes as the file is missing the last 6 episodes


BDH17/02/2017 19:04:43
732 forum posts
66 photos

I'm no expert on this but I would suggest that you are correct in your reasoning .On my CAD program (DesignCAD) there is the facility to draw in the chosen scale, but you then need to check available material sizes.

Rik Shaw17/02/2017 19:10:14
1115 forum posts
310 photos

Hello Mike – Just thought you might like to know. Years ago I built a single cylinder stationary steam engine from plans supplied by ME. I decided to double the size and just doubled the dimensions.

All was OK apart from the crankshaft and the counterweights (counterweights - is that the correct term?) . Doubling the dimensions made the volume of the counterweights much more than double so that when I completed the crankshaft I found it to be hopelessly out of balance and had to remove a lot of the mass of the counterweights to bring the crankshaft back into balance.

Many will say “oh that’s obvious” but it didn’t occur to me at the time.


IanT17/02/2017 19:35:38
1105 forum posts
107 photos

Sounds about right Mike - in essence, change the scale and then go right through the resultant new drawing and make sensible adjustments, particularly to all those holes/shafts/clearances that require it. It sounds simple enough but actually it will involve a lot of work, although if you are 're-drawing' in 3D CAD, then you are already committed to a fair amount of the effort required anyway.

I've never 'up-scaled' any drawings but have tried 'down-scaling' them in the past, as well as 'improving' other designs that (eventually) I found less than satisfactory for my own use. These days (as a rule) I prefer to use any existing drawings of a 'target' engine as a general guide (to give me a few 'hints' as to how best to approach the work - a starter for 10 if you like) but when possible, just work straight from the original GA and scale things exactly (easy to do with a CAD system). Otherwise you can end up 're-scaling' other peoples design 'features' (which also includes their mistakes & compromises) which might not be required in your version and which also might not be possible to build/make functional either. This may be more pertinent when 'down-sizing' (rather than the other way around) of course. My efforts have generally involved 'halving' 5" designs for G3 use.

So it's certainly possible and made much easier with access to a modern CAD system but it's still considerable work - and non-trivial. For most people, my advice would be to simply find an existing design (in your chosen scale/gauge) and build that one instead.



David Reeves 117/02/2017 20:43:32
8 forum posts

LBSC's Doris is based on an LMS 'Black Five'. Martin Evans has published a design for a 5 inch gauge Black Five which you might wish to consider instead - if you want a super detailed, accurate scale model.
Or you could adapt your upscaled Doris using Martin Evans's design as a good guide but be prepared to spend a lot of time on CAD work rather than cutting metal!

The "Words and Music" for Doris appear to be downloadable for free from John Tomlinson's website at

Dave Reeves

Mike Wainwright17/02/2017 20:47:47
149 forum posts
8 photos


Thank for the link but that is where I got the notes but that download is missing the last 6 episodes


Nigel Bennett17/02/2017 20:59:18
245 forum posts
6 photos

Don Young did a black 5 in 5"G... published in his LLAS magazine. Probably a lot easier to use existing 5"G drawings at 1:1. LBSC's boiler would also require a good coat of looking at if scaled up.

julian atkins17/02/2017 22:54:19
1186 forum posts
352 photos

Hi Mike,

Reeves do the Don Young 5"g LMS Black 5 design. It is pointless attempting to scale up an old out of date 3.5"g design to 5"g, and you would be far better off with Don Young's design.

The tapered barrel and belpaire firebox isnt easy neither are the piston valve cylinders.



David Reeves 118/02/2017 16:41:48
8 forum posts

My apologies. The 5"g Black Five design I was thinking of was by Don Young not Martin Evans.

Tim Stevens18/02/2017 16:59:18
826 forum posts

Yes, Rik Shaw, the term 'counterweight' is quite correct.

But I cannot really understand the problem you found with scaled up counterweights causing a mis-balance. If all the parts which are to balance each other are made of the same stuff and truly to scaled-up dimensions, you should be balancing extra mass in the piston & conrod,assembly with extra mass in the counterweights. All these masses will be affected in the same way by scaling up (or down). [Bear in mind, though, that perfect balance is never possible without lots of extra complication, so perhaps what was just about OK when small, became too obviously out when enlarged?]

But I'm sure someone can put me right, if I have missed something.

Cheers, Tim

MalcB18/02/2017 17:11:55
245 forum posts
28 photos

I dont think it will balance Tim.

If you scaled an alloy piston say 100% it will not have the same weight as a steel block scaled 100% dimensionally because of their varing specific gravities. Mass will scale but the weight wont I dont think, maybe I am missing something though.



Edited By MalcB on 18/02/2017 17:13:27

Neil Wyatt18/02/2017 17:58:34
13394 forum posts
576 photos
68 articles

I think Tim's right. Doubling linear dimensions will make all masses increase 8 times, whatever the material. I also think he's right about a small design being OK but the out of balance being much greater when scaled up.

The big problem is scaling down is when making something smaller makes all its components more flexible.


Martin Connelly18/02/2017 19:33:11
620 forum posts
67 photos

MalcB, weight is mass times gravity so if the mass is scaling up by a factor of eight when linear scale is doubled then weight will also go up by a factor of eight unless gravity changes.


MalcB18/02/2017 23:56:51
245 forum posts
28 photos
Posted by Martin Connelly on 18/02/2017 19:33:11:

MalcB, weight is mass times gravity so if the mass is scaling up by a factor of eight when linear scale is doubled then weight will also go up by a factor of eight unless gravity changes.


Hi Martin,

Agree mass x specific gravity = weight.

So lets do an exagerated example of two materials I can remember the specific gravities of:

2 cu inches of steel ( 0.27lbs/cu in ) and 2 cu inches of lead ( 0.36 lbs/cu in ).

This equals 0.54lbs and 0.72 lbs = 0.18lbs difference

Scaled by factor of 8 = 4.32lbs steel and 5.76lbs lead = 1.44lbs difference.

Therefore the difference does scale up by eight.

So unless i have missed something, yes i think i agree with whats being said.

Martin Connelly19/02/2017 08:22:01
620 forum posts
67 photos

If you have balanced weights then scaling them up will result in balanced weights. 1kg of steel balanced by 1kg of lead will become 8kg of steel balanced by 8kg of lead. If you have an imbalance to start with then the imbalance will change with scaling.

Problems occur because volume increases by the cube of the scale but area increases by the square of the scale. If you double the scale the displaced volume of a cylinder increases by a factor of 8 but the inlet orifice only goes up by a factor of four. This can result in reduced output where increased output was expected.


Ian S C19/02/2017 08:42:47
6777 forum posts
224 photos

In practice a scaled up piston is relatively lighter than the small original one.

Ian S C

Mike Wainwright19/02/2017 10:39:03
149 forum posts
8 photos

Thanks for everyone's help on this. I am going to try to model the running gear and see how it looks.

I will use standard materials and hole sizes. I have got a guide on some of the dimensions from the Reeves website so I have a starting point for the wheels and axle boxes.

This is purely a CAD exercise to see how it all works out.. I will post a picture of this when I have made some progress


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