Old School Drawing Exercises and 2D CAD

Another recent thread asked which free 2D CAD package could draw a spoked wheel. The question led to a number of recommendations, ranging from do it manually to buy a special Gear drawing package that outputs G-Code so the part can be made by a CNC machine.

Actually, almost any CAD tool could do the job. As can traditional drawing board methods. After all CAD is just old drawing methods on steroids, power assisted, turbo charged, semi-automatic, and all modern conveniences. First problem is CAD software has to be learned, second is they have to be applied to create results. Applying tools once they been understood is hard too! Even simple tools like files are difficult to use properly, and imagining a shape buried inside a lump of metal - design - is another skill again!

Draughtsmen were trained thoroughly and much of it remains relevant. Their 19th century exercises make one think!

In that spirit, I offer a couple of examples. In the CAD package of your choice, or paper and pencil, how easy or difficult is it to replicate them?

First, two lines at an angle (any angle), connected by a pair of reverse curves. This is a common problem laying out railway lines, where the line is run around an obstacle rather than build expensive tunnels or bridges. In this example the tight curve is radius = 1 and the broad curve is 1.75

Second example is decorative:

Not difficult except the swirls are 36° off horizontal.

Bonus question, are the five curvy segments all the same area or not? Can your CAD tool measure areas?

Dave

Edited By SillyOldDuffer on 01/07/2020 11:03:21

Always think that if you know the old school ways it makes CAD easier, my swirls at 45 deg as I could not be bothered to hunt down the adjustable set square

Alibre has answered the bonus question, I'll post the answer later after a few other have posted.

Good grief, SoD must be really bored!

Andrew

If Dave is bored this afternoon then maybe he could have a go at machining his spiral pattern on the end of some bar and see if he can get it to look like this

Sorry you both fail on the railway lines one - no transition curves

Draughtsmen (and most were men, though my Mum was a tracer in a drawing-office), were not only taught drawing thoroughly, but also how their creations would be made. They usually started as apprentices, and ones who back far enough were expected even to do things like making accurate cubes by chisel and file - despite the firm paying good money for the shaper and mill.

I have seen CAD examples that must have been real so-and-sos in the workshop, CNC- or conventional, thanks not to using CAD but to the draughting tool taking precedence over the engineering tool. And we all know just how difficult some cars can be to service, thanks to poor or no consideration of accessibility to what should be accessible, such as lamps.

Where does the fault lie? Poor training? Or good training but skewed too much to theory and design-tools? An assumption of the "Computer says X so X must be right" ?

Some of the old-style, professional, orthogonal GA drawings we see now via our hobby scale-replicating early machinery, or preserving the full-size, were fantastically detailed, needing considerable skill and experience over many days of patient work. These can still be produced in CAD - most of such a drawing is only standard lines, simple shapes and hatching, just lots and lots of them - and much more quickly.

The experience CAD user is no less skilled at draughting than his or her manual predecessor for the same level of drawing complexity - possibly more so. CAD can do awkward things like merging arcs and plotting pitch-circles and arrays, but it is not intuitive. Nor very efficient if it demands creating an isometric model then taking the orthogonal views from that; but even before the forbidding complexity of 3D, CAD adds a deep layer of skill of its own even just to stich a few basic shapes together.

(Some CAD makes, including TurboCAD, offer a straight orthogonal/isometric choice, even if the publishers appear not to know the word for a 2D drawing. Others, including Fusion 360 and Alibre, seem to insist 3D-first. Yet the workshop needs orthogonal elevations, not pretty pictures - though the fitter, operator and repairer need the isometric assembly-drawings.)

Yes, the skill at the keyboard, parallel-motion board or T-square on elm board is important, and may be very high-level, but is the intermediary.

What really count are the skills of designing, and the skills of making.

Both F360 and Alibre start any item with a two dimensional "sketch" eg you draw one elevation, then the extrusion is really nothing more than adding another elevation to give the part depth just as a drawing side elevation would give depth to the front view.

Dave's part I showed above was just a series of three arcs on a two dimensional plane for each area which I chose to give different heights to.

Another one for you to have a go at machining Dave

Areas are all the same.

Pretty easy with Fusion, and the areas are equal.

Pendant alert! Although I know what he means I'm not with Nigel's use of the word 'orthogonal'. Might explain why publishers don't use it much. It doesn't appear in my various Technical Drawing books either (All at least 40 years old, so maybe I'm out of date!).

Pretty sure Nigel means orthographic drawings, named from the technique of orthographic projection, which is the best way of technically representing a three dimensional object on a two dimensional medium like paper. It typically consists of three views of the same object, from the top, side and front. The builder performs the mental gymnastics needed to create the real thing in the workshop. Isometric, Perspective and other projections can be produced from orthographic drawings to create an impression of the 3D reality. Any 2D drawing package can do projections because the user can create them himself. They're all 2D fibs because paper is flat and there is a better way - 3D.

3D modelling is very different. Objects are modelled not drawn. They aren't isometric drawings or orthographic projections. Instead models exist as virtual solids with creators working in height, width and depth simultaneously and from any angle. Models can be rotated or sliced at will from any direction.

Models aren't limited to dimensions either - materials can be specified, allowing weight, strength, heat-flow, electrical resistance and many other real world characteristics to become intrinsic. Plus many models can be assembled together in cyber-space, thus proving the real world objects will too. I'd argue design errors can be spotted faster in 3D than in 2D - features ambiguous in 2D are obviously wrong when seen in 3D.

Modern manufacturing may not require 2D drawings at all; chaps working professionally from 2D drawings are becoming rarer, because - if CNC can do the job, CNC is cheaper. But when 2D drawings are needed for production or prototypes they're easily generated from 3D models, it's not a problem.

Learning to use 3D is hard work at the best of times. Expecting 3D modelling to be a form 2D drawing is a recipe for disappointment. 3D needs another mindset. Once mastered 3D is truly wonderful for designing complex objects. Less useful for simple jobs, much of what I do is scribbled on the back of an envelope.

Dave

.

Dangling from a thread ?

Incidentally ... Orthogonal simply means ‘at right angles’ or ‘mutually perpendicular’

MichaelG.

Not to us geeks : In computer programming, orthogonality means that operations change just one thing without affecting others.

What I dislike about computer-speak is the constant disappointments. Orthogonality is conceptually related to 'Loose Coupling', which has nothing to do with wife swapping. No joy with 'Back-end Penetration Testing' either, and that one is dangerous to Google!

Dave

Another Geometrical Drawing. What's the radius of the largest circle that can be drawn inside this isosceles triangle? Score double for solving it with only a compass.

Dave

.

If I try really hard ... I can just about believe that there may be some underlying logic in that abuse of the English language : If a thing is considered ‘uni-axial‘ then changing it has no effect upon things that are orthogonal to it.

[or something like that]

MichaelG.

No wonder the softies miserably failed to create a working track 'n' trace application. It used to be said that the ideal was an English-like programming language; to which the riposte was that all it would prove was that softies couldn't write English.

Andrew

If you can solve it using a magnetic thing, surely that deserves considerably more than double.

If you can solve it using a pair of compasses, that also deserves more than double as drawing the straight lines that form the angle bisectors and whose intersection defines the inscribed circle centre is quite challenging using an instrument for drawing circles.

Referring back to the original challenge, I am dubious that any railway would be constructed from simple circular arcs tangent to straight lines. AIUI, transition curves are always necessary (and drawing those, whether manually or on a computer, is somewhat tricky).

I plead not guilty to both charges, M'Lud.

A compass is a drawing instrument, also known as a Pair of Compasses, but 'Compass' in this context is correct.

When there's plenty of room to lay out a railway line there's no need for transition curves and few early railways had them. My example didn't give a scale, but if the radius of the tight curve is a mile, or even a kilometre, I suggest there's no need for transition curves.

But the point is a good one. Laying out a practical railway the design of curves is complicated - it's necessary to take trains safely round tight curves as fast as possible.

Question for transition curve experts: what form should my reverse curve example take on a standard gauge line if the small radius is 100 metres? And what speed limit should be applied if the line is laid flat? Even more difficult, what improvement can be obtained by applying super-elevation, and what's the optimum tilt assuming the train normally averages 70mph?

Dave

Edited By SillyOldDuffer on 01/07/2020 22:02:10

70mph round a 100 metre curve? I'd guess you're tilting it 90 degrees and fitting ejector seats...

pgk