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>> Presumably CAD also allows you to do things like "find a point 3/5
>> along the length of this edge".
>
> AFAIK there is not usually any automatic way to do that, you'd just need
> to add a formula to one of the point dimensions that says something like
> "Edge1.Length * 3/5". That way, whenever the length of that edge
> changes, the point is always fixed 3/5 of the way along.
Right.
> You could then
> draw some geometry referenced to that point, then of course that would
> all move with the point too. It gets a bit like POV, where you can just
> define a variable for something like "screw position", and then drive
> lots of other features like holes and cut-outs in lots of parts from
> that one variable. The benefit of this is when your customer says "ok
> we need that screw moved 1.2mm" it only takes you a few seconds to
> update all your parts rather than an hour or two.
SketchUp allows you to draw precise sizes easily, and it allows you to
line things up easily. What it does not do is allow you to "link" things
so that when something changes, other things change to match. You'd have
to propogate any necessarily changes manually yourself. SketchUp makes
it quite easy to adjust everything by exactly the right amount, but you
still have to know what to adjust and how to adjust it.
>> the best feature of all is the way surfaces "stick" to each other as
>> soon as they touch...
> ...
>> Also, you can't delete any item smaller than a certain size on screen.
>> You have to zoom in first. And the minimum size is significantly
>> larger than you'd expect.
>
> I would expect there would be settings for these sorts of things?
The delete size, maybe. (I haven't found the setting anywhere though.
There are surprisingly few settings you can adjust.) The sticking
appears to be a design feature - annoying as it is. (You can prevent it
by using layers or grouping surfaces into seperate entities.)
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>> Now, sure, if your window is rectangular, the push/pull tool makes it
>> easy enough to continue. But what if you made a *circular* window? Or
>> some even more complex shape?...
>
> ...you need to get a proper 3D CAD program? :-)
On the other hand, weren't you the guy complaining that "propper 3D CAD
programs" are too crash-happy? At least SketchUp is reliable. ;-)
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> On the other hand, weren't you the guy complaining that "propper 3D CAD
> programs" are too crash-happy?
Not me, I can't remember the last time Catia crashed on me. Don't ask me
about the thermal simulation software I use though...
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Invisible wrote:
> Yeah. How SketchUp does intersection is that you line up the two
> objects, press "intersect", and then delete the geometry you don't want.
Blender too. I usually move the original forms to another hidden layer.
> Manually. Which, for curved surfaces, takes a really long time.
> (Because, naturally, those surfaces aren't really curved. Stupid
> idea...)
What makes you think really curved surfaces would be faster?
BTW, if you want curved surfaces, subdivision surfaces is the way to go
(no idea if Sketchup handles them). That way, you may keep and handle
operations on very simple "cage" mesh while having a very "curved" final
surface... I'm guessing intersecting between 2 "low-poly" cage meshes is
way faster than between 2 high-poly ones. You intersect 2 low-poly
cages and give the resulting mesh a subdivision "modifier"...
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>> Yeah. How SketchUp does intersection is that you line up the two
>> objects, press "intersect", and then delete the geometry you don't want.
>
> Blender too. I usually move the original forms to another hidden layer.
Well, it does mean that you have one "intersect" operator rather than a
seperate union, intersection and difference. But for complicated meshes,
it takes forever...
>> Manually. Which, for curved surfaces, takes a really long time.
>> (Because, naturally, those surfaces aren't really curved. Stupid idea...)
>
> What makes you think really curved surfaces would be faster?
Treating a curve as one edge, rather than 25,000 edges, makes it far
faster for a human to work on. (I make no claims about how easy or hard
it is for the computer to work with.)
> BTW, if you want curved surfaces, subdivision surfaces is the way to go
> (no idea if Sketchup handles them).
Nope. SketchUp handles only straight lines. The whole program is
optimised for working with geometry with lots of lines in it. If you
want fluid, organic shapes, SketchUp is a very bad choice.
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>> On the other hand, weren't you the guy complaining that "propper 3D
>> CAD programs" are too crash-happy?
>
> Not me, I can't remember the last time Catia crashed on me. Don't ask
> me about the thermal simulation software I use though...
The only CAD program I've ever heard of is AutoCAD. Last I checked, it's
slightly expensive. ;-) Then again, given that it's complex specialist
software with a limited market demand...
I doubt I'll ever need to simulate thermals. Mechanical stress, on the
other hand...
(Do you have any idea what the "bending moment of reinforced concrete"
is? I haven't.)
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> Last I checked, it's slightly expensive. ;-)
I have no idea about AutoCAD, but I think a base license of Catia is about
5K. For personal use, ermm, no not really :-)
> I doubt I'll ever need to simulate thermals.
Me too :-) But in my job I do. A few weeks ago we had to choose between two
microprocessors, the standard 85 degree C max version, or the special 125
degree C max version. Obviously there's a price difference, and our product
must work up to 75 degrees ambient. Do we order the 85 degree version or
the 125 degree version?
> (Do you have any idea what the "bending moment of reinforced concrete" is?
> I haven't.)
The material doesn't matter, bending moment is just a description of the
internal torques of a bent object. If you cut a bent object (eg a bent
ruler) obviously the two halves would spring back to their normal flat
shapes, the bending moment is the torque you would need to apply to each
side of the cut to keep the original bent shape.
If you're mathematically minded, you can calculate lots of interesting
things by repeatedly integrating any applied forces to the object with
respect to distance. The first integration gives you the shear force in the
object, the second gives you the bending moment, the third gives the
curvature (multiplied by some constant) and the fourth gives you the
deflection. The constant you need depends on the cross-sectional shape and
the material properties, but it's relatively straightforward to look up
and/or calculate.
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>> Last I checked, it's slightly expensive. ;-)
>
> I have no idea about AutoCAD, but I think a base license of Catia is
> about 5K. For personal use, ermm, no not really :-)
LOL! OK, where's my latest Insight spam... OK, Autodesk AutoCAD LT 2010,
I'm *so* not trying that. :-P
>> I doubt I'll ever need to simulate thermals.
>
> Me too :-) But in my job I do.
Yeah, well, you make small hot things, right? ;-)
>> (Do you have any idea what the "bending moment of reinforced concrete"
>> is? I haven't.)
>
> The material doesn't matter, bending moment is just a description of the
> internal torques of a bent object. If you cut a bent object (eg a bent
> ruler) obviously the two halves would spring back to their normal flat
> shapes, the bending moment is the torque you would need to apply to each
> side of the cut to keep the original bent shape.
I was chatting to some guy who apparently designs bridges and so forth.
Maybe not "bridges" as such, but things like motorway flyovers, ramps,
etc. Apparently it's kind of important to know about such things...
Personally, I know very little about this kind of thing. Although I did
read somewhere that at high speeds, materials behave as if they are
harder and more brittle than they are. Which would explain why jumping
belly-first into a pool of soft, liquid water is so God-damned painful. o_O
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> LOL! OK, where's my latest Insight spam... OK, Autodesk AutoCAD LT 2010,
>
> I'm *so* not trying that. :-P
There used to be a free version of ProEngineer called ProDesktop which was
really good. Unfortunately they stopped that line for some reason, and now
there seems to be nothing available for home users in terms of quality 3D
CAD.
> I was chatting to some guy who apparently designs bridges and so forth.
> Maybe not "bridges" as such, but things like motorway flyovers, ramps,
> etc. Apparently it's kind of important to know about such things...
I would imagine so, you don't want a motorway flyover collapsing because a
queue of heavy trucks stops on it. Also important to know how much your
bridge is going to expand/contract based on temperature and design for that,
otherwise you'll end up with a buckled road...
> Personally, I know very little about this kind of thing. Although I did
> read somewhere that at high speeds, materials behave as if they are harder
> and more brittle than they are. Which would explain why jumping
> belly-first into a pool of soft, liquid water is so God-damned painful.
> o_O
When you test the hardness of materials you generally apply the force quite
slowly, so can I well imagine that applying forces at a similar speed to
that which the material can deform will result in a seemingly much harder
material. I suspect for things like road surfaces they generally test with
real life stuff and don't just go on the suppliers hardness rating.
Although you never know, when they built the Newbury bypass they had to do
it again a year later because the trucks had ripped up the surface.
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>> LOL! OK, where's my latest Insight spam... OK, Autodesk AutoCAD LT
>>
>> I'm *so* not trying that. :-P
>
> There used to be a free version of ProEngineer called ProDesktop which
> was really good. Unfortunately they stopped that line for some reason,
> and now there seems to be nothing available for home users in terms of
> quality 3D CAD.
Well, I mean seriously, it's not like I'm DESIGNING AND BUILDING A PIPE
ORGAN FOR MY HOUSE. [I'd just like to emphasize again that SOMEBODY
ACTUALLY DID THIS.] I just like building stuff for fun. ;-)
>> I was chatting to some guy who apparently designs bridges and so
>> forth. Maybe not "bridges" as such, but things like motorway flyovers,
>> ramps, etc. Apparently it's kind of important to know about such
>> things...
>
> I would imagine so, you don't want a motorway flyover collapsing because
> a queue of heavy trucks stops on it. Also important to know how much
> your bridge is going to expand/contract based on temperature and design
> for that, otherwise you'll end up with a buckled road...
Well, when you see a huge bridge over the river Severn or something,
it's obvious that some serious engineering went into it. But a small
ramp up a hill? How hard could that be? Quite hard, apparently...
It still blows my mind that in some parts of MK, you drive down a road
and there's a road crossing 30 feet below you, and a few yards later
another road crossing 30 feet above you. And all of these roads are
roughly level along their entire length, and level with the surrounding
ground plane. That must have taken some *serious* hard-core earthworking
to do in the first place...
>> Personally, I know very little about this kind of thing. Although I
>> did read somewhere that at high speeds, materials behave as if they
>> are harder and more brittle than they are. Which would explain why
>> jumping belly-first into a pool of soft, liquid water is so God-damned
>> painful. o_O
>
> When you test the hardness of materials you generally apply the force
> quite slowly, so can I well imagine that applying forces at a similar
> speed to that which the material can deform will result in a seemingly
> much harder material. I suspect for things like road surfaces they
> generally test with real life stuff and don't just go on the suppliers
> hardness rating. Although you never know, when they built the Newbury
> bypass they had to do it again a year later because the trucks had
> ripped up the surface.
They were saying that if, say, a bullet hits things, they tend to
shatter rather than deform. Apparently due to the velocity of the
bullet, the materious "doesn't have time" to deform, so it shatters
instead. Sounds pretty weird to me, but that's Wikipedia for you...
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