Which CAD program have you been using for your design work?

Do any of them have the ability to change the rest of the design if you change the size or location of a part?

I don't think a book would help you. Being a designer all my working life in a few different fields you start with what you know, ie your power unit as you seem to know what you want to use, and work out from there. Use photographs to give you the exterior outline ideas then work out what goes in between. Design is all about working things out to give you an end result, the mathematics of scale is all part of it. You start with a sketch or series of sketches outlining what you want and work in from there. The cad system you are using is just a tool to create working drawings so the system you use doesn't matter so long as you are confident in it. The drawing time can be a long process, especially if you are only working on it a couple of hours here and there.

Not really nuts I've seen quite a few, the Germans tend to make Lanz ones, the Italians small tracked Fiats and the popular one in the US is the Holt 75, though not a dozer it's still tracked. though these tend to be a bit smaller at 2-3ft long.

Nice Fiat 55L build photos on this site and the finished model

I suppose a tracked Bobcat would be a good donor machine and make it outline or stand off scale rather than a full replica. Watch where you can use all steel tracks, may beed rubber inserts at the least but making your own is quite common at least in teh smaller sizes.

Edited By JasonB on 06/03/2023 11:21:06

Edited By JasonB on 06/03/2023 11:22:48

Design is the most difficult part of engineering because all options are open. Materials, technologies, shape, size, ergonomics, appearance, cost, strength, performance, and a host of other decisions like is it going to be bolted, soldered, brazed, welded, glued. riveted or clipped together!

I see the design process as eliminating options to get to the goal. The starting point is defining the purpose, because that eliminates many alternatives. If the purpose is to hammer in nails, don't make the striker out of cheesium, true from toffee hammers to nail-guns.

The purpose is usually a long list of Requirements, divided into mandatory and desirables. Important to get the requirement correct at the ouset because the cost of fixing mistakes multiplies by 10 at each stage.

In theory design should be "top down" rather than "bottom up". The idea is to get the big picture correct before tackling details and not letting trivia steer the design. Top down favours team work because parts of the design can be farmed out. Most modern design is multidisciplinary teamwork. Barnes Wallis, b1887, d1979, said he felt lucky to have started at a time when it was still possible for an individual to design an entire aircraft. Not how the Wellington was designed: in 1932 Barnes Wallis led a large team who implemented his 'big picture' vision, and he didn't have much input into complicated sub-systems like the radio installation, or what Rolls-Royce did with the engines.

In practice, a singleton or small team, can benefit from a "Middle out" approach. Details may have to be solved first, and perhaps the requirement has expensive low-value features, or the design process revealed a too-good-to-miss opportunity. The trick is maintaining control: all too easy for projects to drift off course, or lose touch reality. Top down is best practice, but it pays not to be too purist!

Design being hard work liable to go wrong, designers should reuse standard components and designs as much as possible. Modellers are in the happy position of being guided by the prototype. LBSC didn'tt have to derive the shape of his locos, or tackle much from first principles. He didn't have to invent the multi-tube boiler, injector, regulator, pressure gauge or valve-gear from scratch. "All" he had to do was scale full-size designs down in a way allowing them to be assembled a small workshop, and then add the "words and music" enabling an amateur to build one. Not trivial and LBSC designs include many modifications to get reasonable performance out of his engines, including a giant perched on the tender to reach the controls!

As full-size and model design principles are so similar, there are plenty of books. I favour those written before about 1960 because they tend to be less academic, but many have been updated, helpful for things like FEA. Shigley's Mechanical Engineering, 8th Edition (2006) .

I don't know of any books on model design. I have one with a promising title, but it's actually about models for testing full size designs. Interesting, but not model engineering as we know it.

In Jelly's case, I suppose the first requirement to nail down is whether or not he wants to ride on the model.

• If riding-on the design problem is carrying the driver's weight, with a certain degree of comfort, so he can see where he's going, and positioning the controls so they can be operated properly. Is the driver a slim, agile, 12 year old, or a creaky left-handed 6' 5" pensioner who has eaten too much cake? The controls can probably copy the original layout, tweaked and extended to improve access.
• If remote controlled, all the driver on-board problems disappear, to be replaced by other issues. Radio Control is a possibility, but servos have to arranged to drive the model's controls safely. Working controls whilst walking alongside with a stick is a simpler form of remote control, but introduces other issues, like a model that moves faster than the driver!
• Computer control is a possibility. The model runs to a program, but senses obstacles and stops if necessary. Could be combined with a remote "dead-man's hand" where the operator can stop and start the machine manually, and the machine stops if the signal is lost.

Considering options, the designer has to watch practicalities. A mechanical engineer able to build a ride-on model, might discount computer control because he doesn't have the necessary skills. The contents of my junk-box often constrain what I do!

CAD is a powerful design tool, not magic, more a "power assist". The designer still has to know what he's doing. CAD build the design as a 'virtual' object, existing realistically inside a computer, by understanding most of the rules that apply to the real object. Saves lots of time by highlighting interferences, impossible configurations, weaknesses, assembly problems, part mismatches, over-weight etc, that are very difficult to see on paper. Sadly CAD is skilled work in it's own right, taking time and effort to learn.

Dave

The world of engineering is littered with the beautiful corpses of unfinished projects

I think most points have been covered but the one I have learnt by bitter experience that means my model steam-wagon is over a decade behind any sensible "schedule" , is that you do need think a long way ahead, of many possible consequences.

Not my strong point! Hence my often re-making or at least altering parts made ten or even twenty years ago, because I could not foresee what would happen if I make this bit that shape rather than the other.

One trap I fell into many times was over complicating parts, leading to messy, un-prototypical things that just wasted steel, electricity and hours of work.

Another was putting assemblies in locations that years later, make it hard to locate others. For example I am trying to design its two-speed transmission cramped by the driving-chain and the engine-case, and driven by a crankshaft I have unwittingly designed and made from solid, a good couple of inches too short..

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This is where design come is, and I mean design not just drawing.

It does not matter if you use pencil and paper or CAD. Each, the alternative to the other, is valid in its own ways, pros and cons; but still a means to an end, not the end itself. My prototype's 1908 design was drawn using loose tee-squares on elm boards, and I now use both paper sketches and orthographic drawings (but not 3D ones) in TurboCAD.

If you are already skilled in either you have a head start but you still need understand your objective and source information. If like me you have no more than a few old photos of the exterior you have a lot of designing to do. CAD does score there even in 2D, if only by speeding up the drawings and making them easy to modify. It still does not design the thing for you!

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I have seen no texts on designing miniatures. Lots on the metalworking skills, yes; a few books about materials, gears and the like; little or none on how to design entire machines replicating ones built many years ago.

So, in the absence of original works drawings or at least full service-manuals for the prototype, what you can very usefully do is seek "full-size" general engineering text-books of the prototype's era.

In my case, my oldest book is of 1911 vintage, so pretty well contemporary to my project; but most are of 1930s vintage. Their value is of showing how things were proportioned and joined, bearings were made, parts secured to shafts, etc; in the era of interest.

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You do as others say need consider how you will drive it, and if you need ride on it, you risk having to compromise its static show appearance, to accommodate you for operating. A tractor or bulldozer, like a traction-engine, has its controls at the back so that you might be able to drive it from a trailer. That video of the Hornsby engine shows a full-size driver on a cantilevered seat is not a very stable combination. A model steam-wagon, or any sort of lorry, has to be large enough for the driver to sit on it; and my thought is to use its fully-planked platform for static display but a special, pierced, one for driving from.

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Incidentally I don't know who owns it now (the one named at the end of the video?) but that Hornsby tracked steam-tractor and its wagons, in the video shown by Jason, were built by Steve Baldock, of the Model Steam Road Vehicle Society. He registered it for road running, but as it is tracked he had to take an extension driving-test. He reported that the examiner was so impressed and intrigued by this vehicle – unique in both prototype and model and rare for having wheel-operated steering – that he had a go at driving it himself! A peculiarity not obvious in the video, or has been ironed out by track adjustment, was that it put only the middle four or five treads flat down at once, giving it a strange pitching tendency in motion. The original was built for hauling goods between the harbour and gold-mines inland, in the Yukon; and sprouted a large, enclosed cab for the weather conditions there.

I faced a similar problem at the start of my steam lorry project. Design in the main building blocks as far as you can on paper. Then make a full size cab area mock up in wood and junk. That allows you to sort out seating and get a stab at ergonomics. One observation is that controls like levers and pedals take up more space than you think thay will.

Rest of the design was done by overlaying the design on Fowler origial drawings, scanned to jpeg and scaled to model size.

Hope that helps,

Martin

Jelly, these just popped up on Youtube and I thought of you