scaling

Hi All

Just signed up for Model engineer and am a relative beginner to model making.

I have finished the Stuart 10V and the Stuart Beam although not under steam yet.

I was wondering is it possible to double the size of a model just by doubling the dimensions on existing drawings.

Castings.i realise,might be a problem but just thinking of a next project so to speak.

What a mine of info this forum is.Keep it going guys.

Yes it's quite possible to enlarge or reduce a design I've done it numerous times and the castings can either be fabrications held together with weld, silver solder, JB Weld etc or a combination of those. You can also carve the part from a solid block or bar of metal too or make your own patterns for castings just depends on the actual part and what equipment you have available.

What engine do you have in mind?

Hi Jason

Wow that was quick.Thanks for the reply.

As i have the 2 Stuart models and drawings it was either of these.I did think that some casting might be a problem

but glad to know it can be done.Is it a case of simply doubling dimensions of a small model rather than getting into scaling.

I have a Warco 290V lathe and an Axminster Mill so maybe a tad limited as to what i can achieve but very pleased with the results so far.I have experience of machining from when i was a capstan and centre lathe operator back in the day.(too far back)

I would be cautious ! If you simply want the model to go round OK but if you want it to do work then some calculations may need to be done. Cylinder bore, port ways, steam feed pipes bore Etc. Good luck. Noel.

I would have thought that flywheels would be a problem with linear scaling.

Depends on how efficient the result needs to be, and whether the materials retain the necessary strength to weight ratio. At model sizes, I'd guess 4:1 scaling is unlikely to cause much trouble because builders tend not to scale everything, our work has to be kept practical. For example a correctly scaled full size steam pipe should be much thinner walled on a model. However, modellers use pipe of about the right scale outside, but small bore inside. There's no need fret about wire-drawing and thermal losses.

Usually easier to scale up than down, but there are plenty of exceptions. A 100x model bee couldn't fly.

Dave

A lot depends on what you start with to scale up, Engines like the 10V, Beam and a Minnie Traction engien may be a little chunky if just doubled as they are not "scale models" but their flywheels will certainly have plenty of mass at the right radius to allow for slow display running as well as being put to work. Looking at drawings for the 10V and a No 4 which is double the bore and stroke of the 10V and there is not a lot of difference between the No4 dimensions and what the doubled up 10V total and the No4 is as at home ticking over as a display model or doing actual work.

Other subjects that have been scaled down from full size will tend to keep their correct proportions and provided not too much artistic licence was used by the original designer there should be little issue going part way buck up in scale.

One other option you could consider is just buying a few castings from the likes of Stuarts. As mentioned above the No4 is approx twice the size of the 10V so you could get say just the A frame, cylinder and flywheel castings and them make the rest from scratch. This would be a good way of easing yourself into the realms of completely making from scratch and is something I have done a number of times

Your equipment should be well upto the job, I have a 280 lathe and X3 Sieg mill, If you look at this album of mine about 75% of the engines there are scratch built with a good proportion of those being up or down scaled from the original design,

Peter, you can scale any model to your desired size but as others have said various factors need to be taken into account beforehand.

As Jason I have scaled several model projects - commercial 2.5cc model aircraft engines to a constant 5cc for example and an Anthony Mount Waller Engine to 1.333. Currently I am working the other way and building a marine compound engine to 5/8ths of the original drawing dimensions.

Something to take into account when going up is mass - a straight forward 'twice the linear dimension' will give eight times the volume for instance - quite an increase in material costs for a start. Heat absorption or retention is another if an IC engine or an engine running on steam.

Construction techniques is another - something adequate for the original size being overkill or inadequate for the scaled item, up or down.

Personally I would think of scaling (in your instance) as a percentage of the original and convert each dimension directly - and work to it - no rounding up or down on anything. Doing the latter quickly leads to having to change things to suit further on and usually, if not always, brings further problems down the line.

Castings are not always an essential - it's amazing what can be produced from stock material and JB Weld for instance. Plus the benefit of firm datums, soft homogenous cast iron if bought as such and no casting alignment problems either.

On that note "The world's your oyster" just pick your subject bearing the above in mind and go for it, there are some great designs out there that just beg for being made a bit larger or indeed a tad smaller.

Best - Tug

Edited By Ramon Wilson on 15/11/2022 16:56:47

"Usually easier to scale up than down, but there are plenty of exceptions. A 100x model bee couldn't fly. "

The size of insects is restricted by their respiratory system and the laws of physics.

Often the details in a model that has been scale down is a simplified version of the original and and material sizes are slightly larger than calculated to suit available plate and bar sizes. (smaller standard sizes may be too weak).

If a drawing is scaled up from a small model and a slightly larger, standard size plate, etc is used than that which was calculated, you may end up with a chunky model that does not have all of the detail of the original design. Always best to work from full size if available.

Posted by Paul Lousick on 16/11/2022 10:53:42:

Often the details in a model that has been scale down is a simplified version of the original and and material sizes are slightly larger than calculated to suit available plate and bar sizes. (smaller standard sizes may be too weak).

If a drawing is scaled up from a small model and a slightly larger, standard size plate, etc is used than that which was calculated, you may end up with a chunky model that does not have all of the detail of the original design. Always best to work from full size if available.

I'm afraid I can't agree with that surmising Paul, my good friend Chris Collinson has scaled down a considerable number of steam engine designs to half size all of which work very well if a little fast. As said above though, like myself he works to the dimension and does not change things to suit stock material unless there is no option - that is where matters begin to go wrong as one change leads to another.

Here's a pic of his two M'c Onie Oscillators one standard size the other half size

and another of his Galloway

Scaling does bring issues depending on the amount scaled but it's a well worth objective and does give a personal touch to a project.

Tug

Edited By Ramon Wilson on 16/11/2022 11:12:50

For someone doing it the first time there is a chance that they may end up with something a bit chunky or just as much something a bit skinny depending on what the original model designer may have done and what rounding up or down they choose to use but with the more you do and with reference to originals where possible you can end up with a better model than the original one.

Peter if you have a spare moment or two you may find reading through a couple of threads of engines where I doubled them up, The first was Anthony Mount's Benson engine which being an early one of mine was almost all just multiplied by 2 and it runs well. The second again one of Anthony's designs for the Easton & Anderson grasshoper was after I had a few under my belt and I was able to find some more detailed photos of the original that Anthony did not have so ended up with a more correctly proportioned and detailed engine than his design which is the opposite to what Paul suggests.

I'm sure Ramon can find a link to his Waller engine which has some good descriptons of how to replicate castings and is another good read for those that don't just want to stick with what's available commercially.

There is also the problem of using another persons drawing that may have been interpreted differently from the original set of drawings, Paul's entablature engine would be a good example where parts that were square on the original are round on the redrawn (but freely available on the net) one and the error continues onto the CAD model.

The next progression once you have found out that you don't need castings to make a model engine will be to model something that has not been modelled before. You can go by an old photo, etching or engraving and design from that rather than using existing plans. I quite enjoy doing this from small almost "toy" engines that can be made 1:1 replicas through to larger prototypes at whatever scale you wish which is often related to the size of parts you can machine such as the largest flywheel you can turn and proportion everything back from that.

Edited By JasonB on 16/11/2022 13:31:08

Just be aware when scaling, things can get very big very quickly. A 1” x 1” x 1” cube is one cubic inch, a 2” x 2” x 2” cube is eight cubic inches, ie eight times the volume/ mass ! If you want a 3d object twice the size each dimension should be multiplied by 1.26.

Peter - Further to Jason's post some further thoughts etc.

'Scaling' in itself is easy enough but as Jason says the original design plays a big part too so it is something to keep in mind.

As I already mentioned the direct doubling up leads to an eight fold increase in mass of all components so the final result can be way over what would be expected if designing a similar sized engine from scratch and conversely the opposite if scaling down, some components can then be too weak for their designed use.

Some of the IC engines I have made, all scaled to give a constant 5cc capacity, have suffered from the increase in mass situation. This Super Tigre 1cc was notable for this - the outcome though a direct copy (within the scope of machining from stock) was considerably heavier than you would expect for a commercial 5cc engine. It made a great project though.

The Waller engine that Jason refers too was also described on Home Model Engine Machinist (HMEM) some time back. This will take you there

The current thread I have in progress on here with the Marine Condensing engine will also show that direct scaling (down in this instance) works ok but I stress again that all dimensions are scaled by .625 and the result worked to, however odd they might be to the original 'stock drawing size'.

Obviously the quality of the original design plays a big part but if it's a proven design with the bugs ironed out then most will allow an increase or decrease to suit ones personal choice.

Enjoy yourself - Tug

Edited By Ramon Wilson on 16/11/2022 17:55:54

The real problem with scaling (in my view) is that when you double the lengths, the areas become 4 times, and the volumes (and the weights) eight times. This sometimes works in your favour (for example a half-size model plane will be easier to fly, as its weight goes down more than its lift as you get smaller. But with steam engines it tends to work the other way. Big engines tend to be more efficient (etc) than small ones.

In detail, there can be other problems - with standard components in particular. Finding a bearing which is exactly twice as big, or half the size, of your drawing is sometimes very easy, and sometimes impossible. Another example is sparking plugs.

The dimensions on a drawing can be scaled, but not the physical characteristics of the materials. No-one is able to make half size electricity, or resistance, or double density, or fingers small enough or muscles strong enough to manage with scaling at the extreme.

It can be done, though, as long as you are prepared to use your common sense, and your ingenuity, and curiosity. That's what you joined for, isn't it?

Regards, Tim