Had Another Go

[Nigel Graham 2Participant @nigelgraham2]

My problem with the set exercises was not grasping what was actually happening. As long as what I made on the screen matched what was expected I assumed I was doing it correctly, but I couldn’t really see how or why. I’ve drawn analogies with experiences elsewhere, where I identified basically the same weakness: inability to think in abstract, or to learn without understanding but finding the understanding hard or impossible.

Where I mentioned particularly matrices, it was because they illustrated the problem perfectly. You might learn you times that number by this, then the next “that” and “this”, then…, to match the answer in the back of the book. You have no idea what was “right” about it beyond the Infants’ School level of each multiplication; not what you were really doing, what was really happening, what it all meant. If it was wrong it was only by some silly arithmetical slip or multiplying the right that by the wrong this. Not why it was the wrong this. Similarly with CAD, I could not see the basic principles. So when I hit problems I had no idea what I had actually done wrong where – and it could have been many steps back.

 

No, I’ve not forgotten or ignored Jason’s set of instructions for drawing that frame. I printed them off – then found for some reason a gap in that – but became too demoralised to attempt them. Thar row of cubes was the first CAD model I’d created since trying to model those chassis parts.

 

I know 3D (or come to that, 2D) CAD is not for making scrappy drawings, in that it offers you a way to make very high-quality, very accurate drawings. That comment is about what I’ve managed to produce; which is rough.

That I’ve struggled to learn CAD for so long must show something – no ability to advance beyond a very basic level.

[Nigel Graham 2]

On Nigel Graham 2 Said:

That I’ve struggled to learn CAD for so long must show something – no ability to advance beyond a very basic level.

It’s certainly a good example of why trying to teach yourself something complicated when you don’t know what you don’t know, is unlikely to end well. Those unknown unknowns are a major limitation in any subject, and are why I failed A-level maths – there was no explanation what the various mixes of trig and calculus were actually supposed to do.

[Nigel Graham 2Participant @nigelgraham2]

Well, Jason, I thought I’d better try what you suggested. This is as far as I managed:

The lowest line is where it should be. (I used the dimensions from my sketch of the real thing but that shouldn’t affect it.)

The top one is stuck on that 7.25″ distance. I typed in its correct distance from centre (6″ ) but it wouldn’t move. I deleted it, drew a new line, tried moving it. No. It stays stuck to the rest of the formation.

The image is of that first attempt. I tried again this time deleting the bottom line. That made the two top ones obligingly move to the intended 6″ distance as if that was stored for use.

So tried to add the lowest line and that behaved as above: stayed stuck in line with the others.

Went back to your instructions to see if I’d missed something, but if I have I could not see what.

Tried once again this time deleting the middle, angled line. I could then draw it back in but of course it is slightly longer than it should be because I’d not adjusted the others to the correct overall length. Probably not significant here but it might be on something else.

[JasonBModerator @jasonb]

It looks to be down to how you placed the three lines initially. If you look at the middle and top lines there is a green — constraint next to them which is the co-linear constraint which puts them in line with each other.

If you start by just doing three undimensioned lines starting with the bottom one and letting it snap to vertical, mid one at any angle and then the top one again let it snap to vertical. Then dimension their vertical lengths and then the offset from the mid line you may do better.

Also looks like you did not start the first line the horizontal axis line, though that is not critical

 

[JasonBModerator @jasonb]

Should be like this when you place the three basic lines

[JasonBModerator @jasonb]

This might help

 

[Nigel Graham 2Participant @nigelgraham2]

No – still can’t see what I’m supposed to do where and how.

I managed to create the dog-legged line, then when I tried the next bit saw only a blue box on the grid. What’s that supposed to be?

Developed the C-shape, couldn’t make the Symmetry constraint work – deleted the complete figure and drew it again, now by dimensioning it equally about the centre-line.

Is a closed figure actually that or a set of individual lines? They seem to behave as the latter.

Then couldn’t see what to dimension it from (as with the [600/2] value in your example), and which way. The only visible lines on the screen were the grid. I decided I must have gone wrong by drawing the back of the C on the axis, so copied it sideways from that by the supposedly right distance and deleted the original.

Then what? Where is the guide-line? I kept raising an error called a “Top-Bot-Conflict”, or similar, meaning or explaining nothing to me.  So tweaked the view to see what the heck I’d generated…

I’d generated complete rubbish: totally wrong orientation, totally wrong place, planes all haywire….

 

Hopeless.

If I want to represent angle or channel-section frames in 3D, I’ll have to do so symbolically or partially.

 

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