Challenge: Create an Impossible to Make Object in CAD

[SillyOldDufferModerator @sillyoldduffer]

In one of Nigel Graham’s Steam Tractor threads, he mentions the possibility of a designer drawing something impossible to make in the workshop with CAD and Martin gives some useful tips on how to avoid such situations, especially the need to provide maintenance and assembly access.

But, given today’s range of manufacturing methods, I was hard put to come up with an impossible to make object myself.   This is the best I could do at short notice.   It’s a cube with an internal rectangular blind U-bend, shown transparent, to be made of mild-steel.

 

No particular tolerances.

Now although this presents some difficulties, I don’t believe it’s impossible to make, so I’ve failed my own challenge.   And, so far, none of the real parts I’ve thoughtlessly designed in CAD, have ever turned out to need a major rethink.   I believe it’s because the process of modelling in 3D-CAD is similar to what I do in my workshop – a simple mix of additive and subtractive processes.  Thus 3DCAD modelling tends to highlight stuff that will be awkward to make in the real world.   In theory CAD allows unfettered madness, in practice it tends to suggest sensible approaches.

Can anyone come up with a CAD object that really is impossible to make, as opposed to being merely unnecessarily complicated or overly expensive to manufacture?

Dave

[duncan webster 1Participant @duncanwebster1]

Hollow sphere with a ground internal surface.

[JasonBModerator @jasonb]

If we are allowed cast steel then your object would be quite simple to cast with a core.

 

[Nigel Graham 2Participant @nigelgraham2]

It’s not how the drawing is made that is the root of the problem: you can make the same mistake with manual drawing or even just the proverbial vape-packet sketch.

Indeed, one of my old engineering text-books does touch on this aspect, including among things ensuring that the appropriate spanner can fit a nut in a recess or tight corner, and rotated sufficiently.

I have seen professional detail-drawings of parts that might not be impossible but would certainly be very hard to make, such as calling for an M3 thread 3mm deep full, to be tapped in blind holes in material only 4mm thick. Another was a component with a deep, stepped hollow as inside a cone-pulley, with a rectangular recess to be machined in one of the steps (probably for a plate-key) specified as having 1mm radii internal corners.

Those were CAD-drawn, but I have seen other, manually-drawn items not especially hard to make but absurdly over-complicated for their task, vastly increasing the production cost.

So, yes CAD can make it easy to design difficulty but that’s not at all axiomatic nor even a CAD weakness particularly; and I agree that the fact of digitally adding or cutting material ought to AID the designer specify something physically feasible.

One could even imagine for example, temporarily transplanting a spanner to that nut, on screen, to make sure the physical thing can be bolted together.

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Which leads to your other point: the parts should be not only feasible to make, but also feasible to assemble / disassemble.

[FulmenParticipant @fulmen]

That’s noe a CAD problem, those are just as easy to produce on paper. It’s what happens when the designer doesn’t understand the manufacturing process. I used to do the same, had a machinist buddy that I always pestered with work. And every time it was the same story. “Sure, you can draw that. But how am I supposed to make it?”

Then he got a home shop. And the answer turned to “Fine, the lathe is over there. You try to make that.”

 

These days you can 3D print almost any shape, but it still has huge limitations on cost, materials, precision and surface finish.

[duncan webster 1Participant @duncanwebster1]

All draughtsmen/design engineers should spend some time in a machine shop. Not to become expert machinist, but to become familiar with what machines can do. The modern practice of having drawings done by one company to be made by another also takes the foreman out of the loop. The first DO I worked in had a doorway into the machine shop. When a fierce looking guy in a white coat appeared with a drawing in his hand, a hush would descend, wondering who was going to get blasted with ‘how the **** am I supposed to make that’.

One of the worst examples I’ve seen was from a friend of mine who did sub contract machining. A plate with holes all round the outside, 9/16 diameter, clearance on 1/2 bolts. The hole diameters were tied down to +0, +0.002 on diameter and to be within 0.001 of TP. Clearly the draughtsman didn’t have his thinking head on, but they wouldn’t listen when told how much extra it would all cost.

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[jimmy bParticipant @jimmyb]

Metal 3D printing has made a lot of “impossible” designs possible.

 

I had some photos of incredibly complex parts from an exhibition, but as always, can’t find them!!

 

The best item i have seen is honeycomb core for inlet/exhaust valves.

 

I’ll carry on looking for the photos, as they really are rather good!

 

Jim

[Gary WoodingParticipant @garywooding25363]

You could make this with a 3D printer, but certainly not with anything else. Its chevron planetary gears that actually work, but cannot be dismantled. If made as separate parts they cannot be assembled.

[duncan webster 1]

On duncan webster 1 Said:

All draughtsmen/design engineers should spend some time in a machine shop. Not to become expert machinist, but to become familiar with what machines can do. The modern practice of having drawings done by one company to be made by another also takes the foreman out of the loop. The first DO I worked in had a doorway into the machine shop. When a fierce looking guy in a white coat appeared with a drawing in his hand, a hush would descend, wondering who was going to get blasted with ‘how the **** am I supposed to make that’.

…

There’s always the possibility that the foreman is the problem!  Quite often a senior man, experienced, but out-of-date.   This is the guy who can’t cope with metric and sees no value in computers or anything else requiring an old-dog to learn new tricks.   Whereas the Drawing Office blights the company with poor designs, a bad foreman brings his employer down by insisting everything be made using only the methods he understands.   May not entirely be his fault because the dreaded Account-Ant may have spent his entire career discouraging investment in new technology because it costs money!  And even if the accountant was agreeable, absentee owners often blew the profits on Horse Racing, the Stock Exchange, and their playboy lifestyles!

Watched a Youtube video a while ago.  It was about a Canadian factory making shells for WW1 circa 1917.   I was struck by the almost total absence of general-purpose lathes and milling machines.   Mills and lathes were present in considerable numbers, but they had all been modified to perform one particular specialised function.  The workers were mostly semi-skilled, or unskilled.    For example, a lathe with a hydraulic chuck had a powered cross-slide purpose made to cut a zig-zag groove around the shell to stop the copper driving band slipping when forced into the gun’s rifling during firing.  Presumably the zig-zag cut was driven by cams under the cover.   The driving band, apparently impossible to fit, was squeezed into place with a large hydraulic-press, details not shown, possibly a trade-secret.

Another striking machine was a multi-head drill purpose made to drill 5 accurately placed holes simultaneously.   This machine would be utterly useless without the necessary jig, itself held in a designer clamp, and only then if a new user needed exactly the same  pattern of 5 holes.   I guess a Production Engineer had this drill designed to his specification by the Drawing Office, and then the foreman was told how to manage it on the shop-floor.   A small operation might combine the Foreman and Production Engineer roles, but this is quite dangerous because the jobs require different skills, and both are time-consuming.

By the by,  I don’t know how to model a zig-zag slot running around a cylinder in 3DCAD!  Sketching the zig-zag is easy but I don’t think FreeCAD will let me wrap a plane sketch around a curved face.   I think SolidEdge can do it, not tried yet.

Here’s the shell, the zig-zag slot is cut around the inside of the groove highlighted in green:

Nothing difficult about making the shell body on my manual lathe apart from the zig-zag, but I think a CNC machine would cut one without much bother if only I could draw it!

Dave

 

 

 

 

[JasonBModerator @jasonb]

Now you have got yourself into impossible to draw parts, easy with the right software when you know how to use it😊

[RobinParticipant @robin]

How about a Dyson sphere? 😀

[JasonB]

On JasonB Said:

Now you have got yourself into impossible to draw parts, easy with the right software when you know how to use it😊

Exactly what’s needed, presumably Alibre?

Sad story of my life: not knowing what the right tool is, followed immediately by not knowing how to use it properly!    Started with straight slotted screws, where it took me years to realise the importance of matching the screw-driver blade to the slot, and that the d*mn*d things were all different.  And then finding that there were two incompatible cross-head types that gouged each other!

And of course the zig-zag object is neither impossible to make or impossible to draw.

Dave

[Robin]

On Robin Said:

How about a Dyson sphere? 😀

Looked it up, and although a megastructure big enough to enclose a star would be extremely difficult to make and build,  it’s not inconceivable that an advanced civilisation might be able to knock one up!

Certainly well beyond human technology for the foreseeable future.   Not difficult to make a model though, and it would work if one could also make a star small enough to fit inside it.  Now there’s a project…

Dave

 

[Peter Cook 6Participant @petercook6]

Coincidentally there is a paper on the June edition of the Monthly Notices of the Royal Astronomical Society which presents anomalous data from an infrared survey of five million stars that suggests partial dyson spheres as possible candidates to explain the findings.

[JasonBModerator @jasonb]

Yes Alibre and rendered with trial keyshot.

See if you have something called “wrap” that allows you to wrap a sketch around the surface then extrude or cut it.

[SillyOldDufferModerator @sillyoldduffer]

…<br class=”bsp-quote-title” />See if you have something called “wrap” that allows you to wrap a sketch around the surface then extrude or cut it.

Yes, but it seems the Workbenches I’m used to don’t support that.   Can be done in FreeCAD by downloading the Curves Workbench and the Lattice Workbench, and combining the results with boolean operations.   The workflow seems quite convoluted to me! Described in this youtube video.  There may be an easier way

The example screen-shot from the video is of a tyre tread, itself created by tracing a bitmap image of a real tyre:

Creating tyres, or tyre moulds, is a Model Engineering requirement as is the other application mentioned in the tutorial.   That’s making rollers for embossing patterns into modelling clay,  jolly useful if a model railway layout needed lots of brickwork, floorboards or paving slabs.

Dave

 

 

 

 

[Nigel Graham 2Participant @nigelgraham2]

That’s a neat application, Dave, making pattern rollers!

The cylinder with the zig-zag groove reminds me of something on this Forum a while ago, a gearbox being made I think for a miniature car, in which the selectors included channel-type cams with elaborate helical-arc grooves. I don’t know which CAD package the contributor used to create his impressive, translucent assembly-model.

…..

Years ago I worked for a small electronics company that gained a lot of its work from various State organisations. One research laboratory would send us “drawings” of the metalwork for experimental electronics enclosures, not prepared in the drawing-office but by the scientists themselves.

They would draw the things in pencil on graph-paper then send photocopies for us to interpret – and what was indeed meant was hardly good engineering. Inspired (?) by an American children’s TV show popular at the time, we nicknamed the customer, “The Muppet Labs.”

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One day I commented to one of the Inspectors on the apparent over-engineering of some fairly small items we serviced for the Royal Navy. “Ah!” he replied, “It’s not just to withstand the ship ploughing through storms or being hit by shells. It has to withstand Jolly Jack Tar too!”

[Paul LousickParticipant @paullousick59116]

How about this.

It was drawn in CAD but only in 2D (the challenge did not specify what type of CAD. also could be done freehand)

[Michael GilliganParticipant @michaelgilligan61133]

Following Paul’s theme …

My Dad ran the workshop at a northern university, and one day some clever chap produced a sketch of this:

https://upload.wikimedia.org/wikipedia/commons/thumb/8/84/Poiuyt.svg/601px-Poiuyt.svg.png

and asked if Dad could make one for him …

 

Of course, he accepted the challenge, and of course he succeeded … by soldering it up from wire, exactly as per the sketch.

 

The silly prankster had provided no additional specification … and a lesson was learned.

MichaelG.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I drew this in paint on the first desktop computer that I had, it isn’t my creation as I copied it in from a pencil drawing, but I think it would be impossible to make a 3D one by any means.

Regards Nick.

[Nigel Graham 2Participant @nigelgraham2]

That arrangement may be impossible to model in 3D CAD, and certainly would be in real wood, but would Paint and similar work in a very different way because they process only 2D images?

Anyone ever played with Pov-Ray? In computing terms it is an artistic version of CAD by making 3D-impression images, but is purely for pretty pictures not serious design tasks. That could well give you images impossible, or at least very hard, to model physically. I don’t know its current form but the version I have is entirely command-line driven, though surprisingly not ever so difficult to use, at least at a basic level.

‘

Wasn’t the triangle, and perhaps the fork, devised by M. C. Esher?

Artists have tried to make physical models of some of Escher’s architectural optical-illusions, but probably not convincingly. It necessitates distorting parts of the model, defeating the object.

 

A portico on the front of a shop in Dorchester can give a very peculiar, Escher-esque effect when seen in certain lighting conditions from a window-seat in a cafe diagonally opposite and slightly higher in elevation.

 

 

[Michael GilliganParticipant @michaelgilligan61133]

First description of the ‘fork’ is attributed to Schuster

In 1964, D.H. Schuster …

My Dad’s challenger made the request in 1966

MichaelG.

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Ref. https://en.wikipedia.org/wiki/Impossible_trident

 

[Nicholas Farr]

On Nicholas Farr Said:

Hi, I drew this in paint on the first desktop computer that I had, it isn’t my creation as I copied it in from a pencil drawing, but I think it would be impossible to make a 3D one by any means.

Regards Nick.

I like it up to a point, but it’s only the colour scheme and lines that fool the eye into thinking it’s a 3D object.  If they are ignored the X,Y coordinates are valid and so are the Z-coordinates provided they are all the same.  In other words, the object is flat, and I just made one by printing the shape on a sheet of paper and cutting it out:

Dave

 

[duncan webster 1]

On duncan webster 1 Said:

All draughtsmen/design engineers should spend some time in a machine shop. Not to become expert machinist, but to become familiar with what machines can do. The modern practice of having drawings done by one company to be made by another also takes the foreman out of the loop. The first DO I worked in had a doorway into the machine shop. When a fierce looking guy in a white coat appeared with a drawing in his hand, a hush would descend, wondering who was going to get blasted with ‘how the **** am I supposed to make that’.

I did something like that in about 1980, when I was a Tool Designer for a firm that made teleprinters. I was already an experienced machinist by then. I don’t remember the detail but it involved a specialised jaw for a milling fixture, and it wasn’t the component manufacture that was impossible but the assembly.

Fortunately, unlike some of the other guys in the design office, I had a good relationship with the toolmakers and the bloke in the white code already had a suggested solution when he came through the door.

I lived. 🙂

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dave, I don’t agree that the colour scheme (which I added) fools the eye. The hand drawn pencil drawing was just lines without any shading on a plain sheet of white paper, and represented four straight planks of wood, and had the same affect as my coloured example, so to convince me, you will have to make one out of four pieces of wood, or metal if you like, which are joined together as shown.

Regards Nick.

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