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Mark J

Drawing a twisted block

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I am working in Sketchup and trying without success to draw a twisted block.  What I am talking about is a 6 x 6 x 4 block where the top 6 x 6 face is rotated 45 degrees with respect to the bottom face and each top corner is still connected by a line to the appropriate bottom corner.   @Ronn W I am trying for a shorter version of what you drew for your Twisted  Leg Table project.  Was this in Sketchup? 

I can twist the top relative to the bottom face, but no matter what I've tried I can't create a curving side that appears smooth.  I am trying to depict a turning blank that I will use for a project and I want to try out different proportions and different degrees of rotation.  I could try another design program, but I am reluctant to have to put things on hold while I climb another learning curve.  

 

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Are the legs / corners actually curved, or simply the illusion of curved because of the twist?

I would tackle it by creating two faces the required distance apart, rotate as desired, then connect the apprepriate corners with a straight line. I think sketchup will create a face when all the bounding edges are connected. 

The line could be a splined curve, as well. But I find those difficult to control.

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31 minutes ago, wtnhighlander said:

Are the legs / corners actually curved, or simply the illusion of curved because of the twist?

I am actually confused on this point, which is one of the reasons I want to do a 3D drawing.  I believe the answer is the lines are straight.  

In any case I have tried the method you described, but no joy.  I also tried making a cube and rotating the top face, but no happiness there, either.

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3 hours ago, Mark J said:

I can twist the top relative to the bottom face, but no matter what I've tried I can't create a curving side that appears smooth.  I am trying to depict a turning blank that I will use for a project and I want to try out different proportions and different degrees of rotation.  I could try another design program, but I am reluctant to have to put things on hold while I climb another learning curve

This post is going to make people's heads hurt.

Drawing the leg in sketch up was a b****.   I am not sure that I can explain it with words only.   First the shape is not a real twist, it Is an illusion.  Be aware that the if the base is 6" square the other end of the piece, if you do a 45 degree "twist", will only be 4 1/4"  square (not a 6" squre) since that top square is formed by connecting the midpoints of each of the 6" sides (like a diamond shape inside a square.) You are not really "twisting" the piece.  So you have a large 6" square that is the bottom and a 4 1/2" square that is the top.  To see what the squares look like as you transition from the bottom to the top, place a dot on the edge of the 6" sqaure  1" from one of the corners (clockwise).  Do that at all 4 corners.  Connect those dots to form a square.  That is the size and shape of the leg at 1/3 of the height .  Now draw another set of dots that are 2" from the corners.  Connect those dots.  Now do it again. This set of dots will be at the center of the sides of the 6" square.  These are at the top of the piece and describe the 4 1/2" square.  if you stack these squares one above the other in sketchup and draw lines from the corner of each square to the corner of the square above it you will create traingles that sketch up will fill in as a surface.  I hope.  It's almost easier to make the leg that to draw it in sketchup.

The legs that I made cannot be turned on a lathe.  Look again at my post pics and the jig that I used.  This was chisel and spokeshave work.

Hint.  The 4 guide lines that you will have to draw on the face of your piece from a bottom corner to the midpint of the top edge are sacred.  They are not cut way. They remain straight lines in the finished piece. I think you can sse them in my pics. My legs had a full 90 degree "twist".  My 4 guide lines went from bottom corner to top corner.  The result is  a leg that gets thinner from the bottom to midheight and returns to full size from the midheight to the top.

It's almost easier to make the leg that to draw it in sketchup.

I hopr this helps.  good luck.

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Thanks, I'm gonna have to read that a couple of six times while I have Sketchup open.  Might have a question for you later.  

And no, I am not going to turn a piece of wood into this twisted form, I'm going to make the twisted blank and then turn that into a twisted bowl.    

Maybe.  

If the wood spirits permit.  

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It took me a few tries to understand what you were getting at, but I think I got it.  I was able to keep the squares the same size by copying the original square, moving it in 1 inch increments and then rotating each copy by 15 degrees (45 divided by 3).   

1012601692_twist1.thumb.jpg.3e101faf19d8d1c08b1d093304451a4e.jpg

20792309_twist2.thumb.jpg.aa4e96ebcdd3ac1a7745a031e4598e8f.jpg

But OMG what a crap load of work!  It's a good thing this is a hobby otherwise I could never make this much effort make any sense.  

Thanks @Ronn W I can only imagine how much time you had to spend hacking at this program to get it to do this.  

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Yeah, it's a lot of work.  What size are your top a bottom?  It's very hard to tell from the drawing.  It looks like you have them the same size.  They should not be.  Also, If you are using my method, then that "vertical" sides should be a straight line from top to bottom.  How do I say this?......the right hand most corner of the top in your sketch should be directly above the center dot in the edge of the base square below it.  I am not say that you can't build it with the top and bottom the same size, but rotated, but it's not the way I did it.

Oops, the grandkids just arrrived, bye.

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Top and bottom are both 6 x 6 and the height is 4.  

Your observation is correct.  I took your idea and figured out how to modify it to keep the top and bottom squares (and all the squares in between) the same size.  So it doesn't taper to the middle the way your table legs do.  i.e. it's more like a cylinder than an hour glass.

So here is one consequence.  If I were going to make this shape out of a real piece of wood with spokeshaves the way you did, I would have to start out with a cylindrical piece of wood.  

 

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In that case, your drawing looks about right - nice extrapolation.  Cylinder - yeah I agree.  Your edge lines will be spirals - better you than me.  I had enought trouble with straight guidlines.  I'm sure it's obvious to you but your piece has no flat surfaces or straight line.  Every surface is curving in 2 directions.  The one saving grace with mine is that at any level I could lay a straight edge across the leg so a spokeshave could be used with short strokes.   I'll be interested to see how you finally do it.

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4 hours ago, Ronn W said:

I'm sure it's obvious to you but your piece has no flat surfaces or straight line. 

Well, actually, not until I got this drawn.  I'm pretty good at thinking three dimensionally, but I couldn't get my head around this shape.

 

4 hours ago, Ronn W said:

I'll be interested to see how you finally do it.

Gee, me too!  Sitting here... no idea, none. 

I'll sleep on it, maybe give it a couple of days.  Something might come to me.  

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Good morning Mark J.  In cad this would be a simple loft function.  There is a plugin I guess is free called curviloft.  Do a search in youtube for sketchup loft and you will find it.  Basically there are two sketch planes, one for the bottom of the leg, one for the top.  Draw your profiles on these planes and the loft command does the rest.  The lines connecting the corners is a helix.  same thing as a screw thread.

It's actually pretty easy if you need help I can draw something up in Solidworks and translate it out to something sketchup can open.  Making the twist is a whole other matter.  Can't help lol.

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On 11/11/2018 at 6:57 PM, Ronn W said:

I'll be interested to see how you finally do it.

OK, here's an idea:  a 4 axis CNC machine.  Or a really complicated rotisserie jig spinning a bit big enough to cut the entire cove.  :o

I'm asking about 4 axis machines in the CNC section.

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I found something on curviloft for Sketchup.  It looks pretty slick.  I have to check into it further, but I think the learning curve might be steep.  Here's a ten minute video worth a look if someone else is interested. 

https://www.youtube.com/watch?v=MqOzK23XUVE

So I'm still thinking about making this shape and I've come up with this.  One can approximate the shape by stacking up and gluing very thin (think veneer) squares of wood with each rotated say 1 degree from the square below.  

368348019_TwistedBlocks20.thumb.jpg.63da69439cf45930b1858b6fc279bc5d.jpg

If each square plate has the grain running the same way, then as the piece rises the grain direction slowly turns until the top and bottom plates are perpendicular.  But my segmented woodturning friends tell me this will be OK as the stresses will be distributed across many layers (and a whole lot of glue).  

I suppose if you were to try to make a solid piece in this shape you could use the same idea with a 4 axis CNC machine and a large cylindrical blank.  Then probably some hand sanding for the final surfacing.  

Incidentally, drawing this was a lot easier.  Make the first plate, then copy rotate and stack.  Now copy these two together, copy, rotate and stack.  Now copy those four together, then eight the 16, etc.  So I was able to build up the model pretty quickly.  

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