Continuing Articles

[David Clark 13Participant @davidclark13]

Hi John

I use TurboCAD to open drawings.

It has opened all the DXF I have tried it on except

Draftchoice for Windows.

The Illustrator uses Adobe Illustrator which will also open DXF files.

regards David

 

[Anonymous]

Hi Nick,

 

Not the item I was thinking of, but nevertheless an excellent example of an item drawing that cannot be made. I suppose from a philosophical point of view, in going from 3D to 2D some information must be lost, and that lost information leads to ambiguity in the 2D representation.

 

I am familiar with the work of Escher. I bought a book of his drawings many years ago, when I was doing my Ph.D. During that period I tried to read and/or buy at least one non-technical book every fortnight or so. Escher’s drawings exhibit superb draughtsmanship. As Jim has mentioned some illustrate impossible scenes, but he also played with repeating 3D structures and the morphing of a shape from one form to another across the drawing.

 

Regards,

 

Andrew

[Axel BentellParticipant @axelbentell]

Theres been some photography artist making illusions like that above, you can find coffe table books in most book stores on this.

[Anonymous]

Sadly these days, even in Cambridge, you will find the coffee shop, and hence the coffee table, in a bookshop, but sweet FA in terms of books. At least technical books anyway.

 

Regards,

 

Andrew

[Sub MandrelParticipant @submandrel]

If you have the time and the spare grey matter, a book that is still worth the effort some 30 years on is “Godel, Escher, Bach – an Eternal Golden Braid”* by Douglas Hoffstader.

 

Neil

 

*NO the GEB-EGB is NOT a co-incidence.

[Anonymous]

Wow, all the old favourites being mentioned! I’d completely forgotten I had a copy of GEB; must get it out later and have a look.

 

Regards,

 

Andrew

[John OlsenParticipant @johnolsen79199]

Actually you can make something that will look like the above diagram, from one viewpoint only. It will not however, conform to what you assume from lookg at the diagram, eg the angles that we assume are right angles will not actually be square, or else the parts that we assume meet will not actually meet.

I have seen photographs of a wooden model of effectively one side of the above. When set up in the exact position you would think it was all joined up, when seen from any other it is not.

 

The “trick” these diagrams rely on is that we are really only looking at one part of the diagram at one time…each part looks OK in itself, but of course they are inconsistent with each other.

 

regards

John

[Gone AwayParticipant @goneaway]

 

Posted by John Olsen on 18/10/2010 22:07:28:

Actually you can make something that will look like the above diagram, from one viewpoint only. It will not however, conform to what you assume from lookg at the diagram, eg the angles that we assume are right angles will not actually be square, or else the parts that we assume meet will not actually meet.

 

From the CAD point of view, this is another way of saying that while you could draw that figure in 2D CAD, you couldn’t possibly model it in 3D CAD.

 

In fact, this highlights one of the benefits of 3D over 2D CAD: quite apart from deliberate examples such as Nicholas’s figure, it is quite possible to fool yourself in 2D, as it is/was in manual drafting, and come up with something that looks right but isn’t. This is much harder to do in 3D.

 

I know some people feel that 2D is sufficient for them and won’t even consider 3D but I don’t know of anyone who has become proficient using a 3D package that would ever revert to 2D, even for the simplest task. I’m admittedly spoilt have used a high-end 3D package professionally for years before I retired (and still have access to it) but I believe there are affordable 3D packages around for home use.

 

 (I’m talking here of “real” CAD applications that, amongst other things allow you automatically produce a 2D working drawing from the model. Not the kind of thing that featured recently in the magazine which seemed to bear about the same relation to true 3D CAD as, say, CorelDraw or Adobe Illustrator bears to Autocad).

 

[John OlsenParticipant @johnolsen79199]

Hi Sid,

 

I have been using the  Personal Edition of Alibre design, which is of course the poor mans version of Albre, which I gather is itself not as fancy as say Solidworks. It does have its limitations, but I find it very good for general model engineering stuff…for the first time in my life I can produce decent drawings of things much faster than I can make them. I have been using an older version of Turbocad for years (Version 6) and found that very handy, but the 3D modelling takes me into a whole new way of operating, and is very much faster generally. There is the odd time when I find that I don’t quite know how to model something to best advantage, but then I am still very much a learner with the package. I have generally only tried the 2D sketch and extrude approach, there is also a 3D sketch facilty that seems to have some possibilities. One thing that seems to be true is that things that are hard to model are going to be hard to machine too. I haven’t figured out yet how I can get nice tapered elliptical section spokes for flywheels for instance.

 

A really nice feature is the automatic production of the drawings from the model. You really only need to choose the views and then move the dimensions to somewhere sensible, deleting any duplicates and adding the odd one where needed.  The best thing is that it also does nice isometric views, which are really good to give the general look of the proposed component. Then when you change something later, it is not too hard to go back and change the model and produce an updated drawing. Plus by making an assembly of all the parts, you can see how it is all going together before commit yourself too far.

 

This package only cost NZD172, which must be about the equivalent of about GBP50 or so, depending how the economy is going. Not too bad for what it does.

 

regards

John

[Gone AwayParticipant @goneaway]

Posted by John Olsen on 19/10/2010 00:48:05:

 

One thing that seems to be true is that things that are hard to model are going to be hard to machine too. I haven’t figured out yet how I can get nice tapered elliptical section spokes for flywheels for instance.

That’s one of the nice things about 3D. It’ll often tell what’s going to be difficult to make. If it’s something you’re designing, it’ll often lead you to make simplifying changes.

 

While it’s generally true that, if it’s difficult to model, it will be difficult to make, the reverse isn’t necessarily true though.

 

Another thing that I like to use it for is to get my head around a complex assembly from a magazine or book. By modeling the parts and  assembling them you really get to learn what’s happening and how it goes together. (To say nothing of finding the odd dimensional error in the published design).

 

Not only that, it’s fun!

Edited By Sid Herbage on 19/10/2010 01:49:21

[Anonymous]

In the distant past I used EasyCAD; excellent 2D program and very fast. Even better, it pissed off the ‘draughtsman’ at the company were I was working at the time, as I was faster and more accurate than him. Several computers later I switched to QuickCAD. This was an ok package, but essentially just an electronic drawing board.

 

I now use the ‘Expert’ version of Alibre; not cheap, but a lot cheaper than Solidworks. Alibre has its limitations, but it has allowed me to design parts and assemblies that would not have been possible in the timescales otherwise. Rather oddly I got on with the 3D capabilities of Alibre with no problems at all. It’s odd because professionally I have used ProEngineer and hated it.

 

Elliptical spokes should be fairly simple. Draw a plane at the bottom of the spoke and another plane at the top. On each plane draw an ellipse of the appropriate size. Do a loft between the two sketches and hey presto an elliptical spoke.

 

Regards,

 

Andrew

[John OlsenParticipant @johnolsen79199]

Well, I just found another feature that also accomplishes it. Draw an ellipse the size of the insiide end, tell it to extrude a boss of the right length, and tell it to put a draft angle of about -1.5 degrees (according to taste and length of spoke) and Bob is your parental sibling. Of course I couldn’t machine a spoke that shape to save myself, although I have machined a nice curved spoke flywheel from solid. Actually with some low cunning I could do the individual spoke shape, but not while it was part of the rest of the wheel. I would do it between centres on the shaper, with one centre raised to give the taper.

 

Hey, I only figured out about putting planes where I wanted them yesterday…

 

There are some funny quirks, like you go to set some dmensions for a rectangle, the first is OK, then the second it wants to pick the same two sides again. No, you stupid machine, I wanted the other two.  But in a couple of days I have knocked up an almost complete set of models for the Rina engine, apart from the carburettor, which I guess I will have added within a day or two.

 

regards

 

John

[ChrisHParticipant @chrish]

All of the above is very interesting, but what does it have to do with, and in, the thread of “Continuing Articles” in ME (and MEW for that matter)?  The thread has been hijacked!

[John OlsenParticipant @johnolsen79199]

Well, it is a convoluted connection, but the reason some series take a while and have gaps is that it takes time to get the drawings knocked into shape…so anything that can either speed up the process for the illustrator at the magazine, or better yet enable contributors to send in drawings that don’t need too much cleaning up, is relevant to the original topic.

 

regards

John

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