Mini-challenge

jeff hammond

SU does not manage angles under 0.001° in rotation, another frustrating inaccuracy.

right.. it will, however, rotate to smaller angles than .001 if there's a point to snap to..

As you said before, Your "true tangent intersection" and also (I'll take his word for it) Jeff's DC are the solutions to go by.

well, the dc as used in this case is just being used as a calculator that remembers the formulas for me so i don't have to re-figure it out every time… i wouldn't say it's a dc in the traditional sense..

but as TIG mentions, there are still some inaccuracies in there.. mainly because the DC still only uses six decimals for it's results.. and at some steps, i need to use those results further down the line in the calculations so eventually, some combo of length/width/height in my dc will probably result in a rounding error.. so i concur, the DC i use isn't absolutely perfect either.. there is room for error in there.. but so far, in the situations i've needed it for, it's given me good results every time.

@wo3dan said:

SU-team (now that you're not caged by Google anymore) please, it's high time for a construction circle tool to solve these issues.

AGREE !!

@tig said:

This started as an exercise in drawing something using only SUp's native tools that is actually impossible to do to 100% accuracy; however, it can be done manually to a reasonable accuracy.

fwiw, the reason i keep stressing SU native tools is because i'd like to encourage people to find a geometrical relationship in this situation.. something like -- divide the height into thirds, divide the width in half, take a segment from each then add the board width and viola.. this is the length of the crossmember .. something similar that would be repeatable every time regardless of dimensions.. i'm beginning to think this type of relationship may not occur in this situation.. (but i'm hard headed.. i guess i would need proof that it is impossible to find such a geometrical relationship )

if the challenge goes on long enough and enough people come to realize that you just can't use the rotate tool for this, maybe some other avenues will be explored (but i've explored so many avenues over the years on this.. many of which are being posted in this thread.. and nothing has proven fruitful as of yet )

@tig said:

If you made the same 300mm rail using True Intersection it would measure in the SKP as 300mm exactly - although in fact it's maybe ±0.0000001mm off too - but Ruby and SUp always rounds answers to suit themselves - just as SUp will do if it ever gets built-in tools to do this.

right, there are still inaccuracies in the math methods.. however, you'll generally get errors that are smaller than .000000 and sketchup will report them as perfect.. the thing with very,very close in sketchup is you may get invalid geometry when things are super close.. as in, if i drew a rectangle and raised one corner by a super small amount, it will still function as a rectangle (i can delete an edge then redraw it while locked to an axis and the face will re-form etc..).. but then, i may try intersecting something with that plane and i'll get ghost edges or hidden lines that shouldn't be there but they are.. stuff like this:
http://forums.sketchucation.com/viewtopic.php?f=11&t=43950&hilit=hidden
i'm not sure where exactly the breaking point is in sketchup as far as invalid geometry goes but when i see things like .0001mm, i get a little scared

andybot

@unknownuser said:

fwiw, the reason i keep stressing SU native tools is because i'd like to encourage people to find a geometrical relationship in this situation.. something like -- divide the height into thirds, divide the width in half, take a segment from each then add the board width and viola.. this is the length of the crossmember .. something similar that would be repeatable every time regardless of dimensions.. i'm beginning to think this type of relationship may not occur in this situation.. (but i'm hard headed.. i guess i would need proof that it is impossible to find such a geometrical relationship )

I think in this case, we're trying to hit a tangent, and thus with the segmented circles in SU, we will never get there with the native tools as they are now.

mac1

@jason_maranto said:

OK, I broke down and looked at the math on this -- it seems dirt simple to do so I think this is the solution (based on the math).

[attachment=0:n6vj9qbd]challenge solved.skp[/attachment:n6vj9qbd]

In this instance the desired width is 2 inches.

Best,
Jason.

You have a closed form solution to this?
Hcos(theta)-d(1+cos(2theta)) /2-Ssin(theta)=0
Where H post height; d dimensional lumber width; s post spacing; theta rotation angle at post base.
If you have a closed form solution should not the measured width be exactly 2.00000??
IMHO all focus of the accuracy is questionable. Jeff is not building a watch. One will have to deal with dimensional lumber tolerance, crowning the lumber and maybe using a side winder to cut angles not to mention the angle layout. Maybe a good chop saw is at hand.???

jeff hammond

mac1
I was hoping it's already been established in the thread that the challenge isnt about accuracy from a real world construction standpoint. I mean, depending on the time of day (temperature) and humidity, a board will expand/contract far more than the results being given in the thread.

moujiik

Hi

May be like this?

Moujiik

Chalenge.skp

jeff hammond

Jason. the problem with that is you're going 90° from the diagonal. which isn't the same thing as going 90° from the edge of the board.

jason_maranto

All that mathematical notation is greek to me, but here's what we know:

we know the diagonal length and position (call it a hypotenuse if you want but it's really a diagonal to me).
we know the desired width of the beam.
we know that the angle should be 90 degrees on the opposite corners from the diagonal.

As far as I can see with that information we should have no trouble drawing the thing directly using nothing but the line tool and a few guides... because the actual numbers (aside from what we already know) should completely irrelevant.

Here's a video -- it works, and consistently (not to mention dirt simple)... it's just that SketchUp seems to have lower than perfect tolerances for the 90 degree guides.

[flash=960,720:5o5ld983]http://www.youtube.com/v/OfHaC40zvxM?rel=0&hd=1[/flash:5o5ld983]

http://youtu.be/OfHaC40zvxM

Best,
Jason.

jeff hammond

@moujiik said:

Hi

May be like this?

Moujiik

im curious to see what you've come up with. I'll be able to download your file in an hour or so.

jason_maranto

I'm not sure why you think so -- all 4 corners are 90 degrees, and since we know where 2 of the corner are why not use them...

It works fine and you get about as accurate results as any other non-plugin solution put out here.

Best,
Jason.

jeff hammond

@jason_maranto said:

I'm not sure why you think so -- all 4 corners are 90 degrees, and since we know where 2 of the corner are why not use them...

It works fine and you get about as accurate results as any other non-plugin solution put out here.

Best,
Jason.

nah.. the corners won't end up square when doing that

jason_maranto

OK... did you watch the video? Those results are as accurate as any other non-plugin result I've seen in this thread -- it works, you can say it shouldn't if you want to but it does.

I guess the thing here is I don't think square corners matter anyway, since they will be trimmed off -- what matters is the width... I mean after all this is a cross brace.

Best,
Jason.

moujiik

you're right.
And it's the same solution as Tig's.
I miss my real wood planks. It is so easy to do that for real!

moujiik

jeff hammond

yep. watched it.

the corners need to be perfectly square if thats where you're drawing a line to determine the boards width.

upon finishing your method, delete the diagonal (as to avoid confusion) then measure the angles of the board's corners. they won't be 90°

jeff hammond

@moujiik said:

you're right.
And it's the same solution as Tig's.
I miss my real wood planks. It is so easy to do that for real!

moujiik

sometimes.. I'll upload a picture a little later which shows a situation where you can't just put a long board in there and trace/cut it.

another reason for having a precut board is that you can use it to plumb a wall with. (instead of needing two people do it -- one to use the level and one to trace the board)

jason_maranto

Yeah, I already said that several posts back -- but it is consistently the same regardless of the distance between the beams, which is why I said I think there may be an accuracy issue with the engine somewhere.

If the method was faulty there would random results each time... right?

Best,
Jason.

jeff hammond

@jason_maranto said:

Yeah, I already said that several posts back -- but it is consistently the same regardless of the distance between the beams, which is why I said I think there may be an accuracy issue with the engine somewhere.

If the method was faulty there would random results each time... right?

Best,
Jason.

in this particular case, the fault isn't sketchups. the approach is geometrically flawed.

edit-- for instance, doing what you're doing.. the wider you decide to make your board, the further away from 90° you'll get in the corners.

jason_maranto

Gotcha, yeah, I see that now -- so it was just an illusion created by the long rectangles/aspect ratio of the beams here.

I guess nothing is as simple as it seems with math (which is why I avoid it as much as possible).

I'm ready to just use Fredos tool and call it a day

Best,
Jason.

jeff hammond

@jason_maranto said:

Gotcha, yeah, I see that now -- so it was just an illusion created by the long rectangles/aspect ratio of the beams here.

I guess nothing is as simple as it seems with math (which is why I avoid it as much as possible).

I'm ready to just use Fredos tool and call it a day

Best,
Jason.

me too. fredo solved it..
I'm still curious to see an alternative solution but as far as actually drawing the thing, the fastest method will win and I think it's going to be fredoScale's latest addition.

fredo6

Jason,

You're right.
The native Rotate tool seems to find inference in alignment of edges with others. You have to play around with it, but it seems to find it in the end.

Jeff Challenge3.gif

Fredo

jason_maranto

Yeah I was trying for stupid simplicity, and just ended up with stupid

BTW I'm playing with the protractor a bit more and it seems to be snapping to something, I can't tell what -- and it's not giving me any feedback, but the result is the missing correct 90 degree reference... maybe they (SketchUp Devs) already solved this problem and we just never found it.

It's strange though because you have to kind-of slide the protractor along a guideline until it just decides to snap for some reason.

Best,
Jason.