From prototype to model

[Eric CoxParticipant @ericcox50497]

When you are designing a model based on a full size prototype,how do you arrive at the dimensions that are inaccessible.

For example, on the canal side crane how do you get the overall height or the length of the jib.

[Eric CoxParticipant @ericcox50497]

[Les Jones 1Participant @lesjones1]

Hi Eric,
For the example you give I would suggest the height could be estimated by sighting along a straight edge using some form of angle gauge to measure the angle and measuring the distance you are from the base. Then calculate the height. You may also have to make a correction if the ground was not level. Assuming you cold get directly below the tip of the jib then sight up a vertical spirit level to be sure that you are directly below it. Then again use trigonometry to calculate its length. For more general things then take photographs and then scale from dimensions that you can measure. When taking photographs take care to avoid things that would cause distortion of the image. Try to use a fairly long focal length lens. (Towards telephoto if using a zoom lens.) Try to keep the line from the object to the camera perpendicular to the face of the object you are photographing. If you are using a ruler measuring from the computer screen try to use an LCD monitor rather than a CRT type as these can cause distorsion due to non lineararities of the scanning. There are some graphics programs that can be used for making measurements. If using these distortion on the display would not matter.

Les.

[Ian S CParticipant @iansc]

A bit of geometry would do for the crane, measure out a distance from the crane, then take the angle from that point to the top, then work out the vertical side, you have one side and two angles.

Or (you might have too) do it the way we were taught in Scouts, You pace out 11 units(yards, meters, anything), you get another Scout to hold his staff vertical at this point, step out another unitof measurement, making 12, from there you get your eye to ground level, and sight the top of the crane, you get the person holding the staff to move his hand up or down until it lines up with the top of the crane,. Measure the distance along the staff between the hand and the ground in inches, call the inches feet, and theres the hight.

Another way would be to measure the length of the shadow, and the angle of the sun.  Ian S C

Edited By Ian S C on 25/04/2013 12:52:30

[BazyleParticipant @bazyle]

Surely 90% of this sort of work is done off photos with a few key dimensions having been measured. Nowadays itis even easier with clever programs that can rotate the photo if necessary and insert into a CAD package to do measurements.

[Sub MandrelParticipant @submandrel]

Hello Eric,

I'm not the most accomplished model engineer and I think my models are 'workmanlike' rather than 'works of art', so my hope is that the choice of prototype and some information on how the design evolved is of interest.

As for the crane, Bazyle has it on the nail. I worked almost entirely from photographs and just measured a couple of key dimensions. For the jib, you'll see there are equally spaced cross-members, you onl have to measure one to work out all the dimensions up to the top. For many dimensions, I imported the pictures into Corel draw as bitrmaps and produced vector drawings over the top. I can correct the perspective of photos in photo-paint and draw allows me to scale them by +/- 0.1% so in essence much of it becomes a process of getting a full on view scaled to the right size and 'tracing' the part over the top.

When preparing the drawings I scale the units so all the measurements are in real-world inches. This is a great sense check as when everything is proportioned right, you usally find almost every larger measurement is a round number of inches or feet. Larger dimensions are rarely to fractions or to things like 7" or 11".

There are pictures of the model and the prototype in my albums, make your own judgement about the absolute accuracy. Although i revisited the crane at least twice for extra pictures and the odd measurement my aim was to capture the 'spirit' of it, not micron accuracy!

I used a similar approach for my 'deisel' shunter, hampered by the fact I could not revisit the original, although the web gave me pictures of some similar engines.

The mill engine Nordern was interesting, as there were dimensions and a drawing, but lots of missing information ,and no way (at least I thought no way when I was making it) of getting more data.

The two models I'm working on now, one is of an engine I've actually got full size in my workshop, so that's easy. The other is from an old engraving I scaled up and down (within obvious limits) until nearly all the dimensions came out as neat one-inch multiples.

Final thoughts, if looking at prototypes highlights anything, it's that crankshafts and valve rods may often be over-scale on models. Also, sometiomes the real thing has faults that would be upsetting to a judge if seen on a model!

Neil

[Ian S CParticipant @iansc]

I think there was a bit in ME, back in the 1990s, suggesting that with documentation, faults, repairs, upside down lock nuts, and other oddities can actually gain points if you are copying a particular prototype, but then again you can build your model as the prototype should be. One of the important things in the artical was the presentation of information, about the prototype, and the model, and if the item for display in competion it should be well documented.

Heres a crane at the "Ferrymead Park" on the railway platform.  It's not complete, but still interesting.       Ian S C

Edited By Ian S C on 26/04/2013 12:02:15

[Stewart HartParticipant @stewarthart90345]

I tend to measure a few key features if you can get at it, or gleen information from its description that you can use ie stroke 20" this means the crank pin must be 10" from crank shaft centre etc etc. and use these as reference for scaling off photographs

I try to take photographs square on if you can't do that take a range of pictures from diferent positions then work out and the average value for a feature.

You don't have to scale avery feature once you have a value for the main ones the rest will fall in to get them to fit, also you may have to make comrpmises to take account how you are going to make it.

As a last resort if it looks right it is right.

And don't get too hung up at getting everything spot, at the end of the day its a model just enjoy the process and have fun.

Stew

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