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Hello to all.

I am very bad at math and a novice at construction but determined to find the answer to my question. I posted on a mathematics site and the answer I got was very technical and unclear. I thought I would post to a woodworking forum and approach it from another angle. [sorry] :thumbdown:

I want to build a wood mold for a planter based on a design I have drawn in a 3D program however I dont know how to determine the angles of the wood cuts. [figure below]

I think it would be a trapezoid shape. The info I have is

width 18" top
width 10" bottom
height 14"
I believe the top angle is 74º and bottom angle is 106º

Here is a pic of what I want to make. The pic shows a solid object but it isnt. I am designing an empty box/mold for an outdoor container to hold plants/small trees. I will probably make the sides out of 3/4" plywood.



The link to a Cinema4D file is
http://users.adelphia.net/~logiclab/Images/trapezoidPlanter_01.c4d

I need to fine out the cutting angles where the pieces meet. After I do that, I am going to
make a second smaller version that fits inside so I can pour concrete inside the two molds. I need 1.5 inches of space between the two molds which would be the thickness of the sides and bottom.

I hope I have explained this correctly and thanks very much for your help. [I wish I could figure this out myself]

Regards,

spindoctor
 

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If i was using solid wood and it was a final product I'd miter joint the corners, but since it is a mold, I'd butt joint the pieces. No special angles to cut there, cut the pieces the size you designed and screw the parts together. 2 sides as per drawings, 2 sides same size minus plywood thickness on each side.
 

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Discussion Starter #4

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Cabinetman

You mean all the angles are the same? I figured most of them were the same. I was unsure just what that number was.

According to an answer I received on a math forum, that number is
47 1/6º [bottom of first link]

http://www.mathlinks.ro/Forum/privmsg.php?folder=inbox&mode=read&p=338932

http://www.mathlinks.ro/Forum/privmsg.php?folder=inbox&mode=read&p=339093

logiclab

I didn't do any math, other than figure the inner piece. Looking at the outer one it appears the angles would be the same for the inner ones yielding an even space for the void. I assumed that since you drew it out and figured out the angles for the elevation you posted, you were correct. If a solution came out to 47 1/6 deg, that's an odd number to set up and cut. I would draw out the elevation to scale, like 1:1 and then use a protractor to see what the angles are. For critical and repetitious pieces (even 4 can be repetitious) make slight adjustments to the size to achieve even degrees. Even half degrees can be a problem at times.






 

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Hi MrRodeoCC

I think your idea is a good solution. This is just a mold and not a finished piece of livingroom furniture.

Forgive my complete ignorance. Is this a butt joint?



and




Thanks MrRodeoCC.

spindoctor
 

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Pianoman
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Half of 106 is 53 divided by 2 is the miter for the base and bottom edge of the sides. The virtical is a compound 45 degree cut. Should be done on the table saw with the miter slide. I`m old school...so I would use a T-bevel to keep the angle... cut a shorter peice on 45 like it was a piece of crown... then set the miter slide to match that angle...that would tell me what the angle is...Rick
 

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Discussion Starter #8
So the vertical cut is a 45º cut on both sides. I think what threw me off was first you have to make the bottom cut which gives it the slant.

thanks pianoman

spindoctor
 

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Pianoman
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Spindoctor, 106 degrees is 16 degrees past 90. So...16 less than 45 is 29 degrees...which is your miter cut for the base and bottom edge of sides. sorry about that! Rick
 

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Here is a full Schematic for what you are wanting to do. I'd build two identicle boxes then cut a 2-3 inch slot both ways through the second box to make the filler piece. the schematic shows a 2 inch cut out.

With a 2" cutout it makes a side wall for the cement of about 3/4" so adjust the width of the cutout to produce the wall thickness you are looking for.
Best of luck,
Randy.
 

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May be a novel idea, but I would do a simple template of the pieces, and use a protractor to determine angles as I progressed. This can be figured mathematically, very easily, but IMO a quick cardboard template and a few quick jigs might produce great results without a headache.
 

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Thanks, I appreciate the compliments.

If ya'll are interested, that was produced on Turbo CAD. It's an awesome program for this kind of work and it's only about $100.00. The best part is it does all the math for you.
 

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Hi MrRodeoCC

I think your idea is a good solution. This is just a mold and not a finished piece of livingroom furniture.

Forgive my complete ignorance. Is this a butt joint?



and




Thanks MrRodeoCC.

spindoctor
Yes those are butt joints, what Graphiti shows are mitered cuts, which would be perfect if it weren't a mold, the butt joint is more suited for molds due to durability, unless you used a jig to pin the mitered corner, but thats too much work for a mold. Don't forget to build in a vacuum release so the molds will separate easier, drill a hole in the bottoms of both parts thread a bolt in and cut off the excess, then once the concrete is set, remove the bolts and tap the molds and remove. If you get a vacuum trapped on your molds and cant get them apart, with butt joints you can at least unscrew the sides and get it apart that way.
 

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Actually, I sugested the way I did because it's 4 identicle pieces and a bottom angled to match. With this method you can make the pieces quickly and accurately enough for consistant results from the concrete. With the butt joint you have to cut the edges a few degrees from square to get them to mate well (about 4 deg.)and the outer surface of the inner mold piece would require alot of hand work to ensure easy mold sepperation and smooth inner contours...

The main idea behind my design is to build disposable molds, quick and easy to make and no big deal if you have to destroy it to de-mold the planter.
Either way, good luck.
Randy.
 

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way to complicated for me...
1. start with the small box lets call it. layout 4 pieces aprox. 15"wide at the top and about 12.5 high, find the center line on eace piece, measure 7.5" on each side of the center line ( top) now from the center line on the botom measure 3.5" on either side of center, connect the marks. and cut all 4 pieces. assemble and fill in the narrow end it should be about 7" square. now turn it up side down and put it away. ( you have to subtract the thickness of the material you are using to get exact dimensions)
2. the out side box do it the same way except you won't need to fill in the bottom by using the 18"wide on the top 14"high on the sides and 10" on the bottom.
3. attach the small box to say a 3/4" piece of plywood. ( turned up side down so the 7" bottom is now the top), place the big box over the top and position it properly, now attach a picture frame around to keep it in the same place.
4. fill with concrete, let cure over night, and remove the big box, remove the casting, and do it again.

5. to release the mould you should spray with a release agent, ie. diesel, applied to all surface that will come in contact with the concrete.
 

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notes to my last post:
You should have all the interior corners filled with either cove moulding or caulking, and inside edges rounded over, as concrete will chip on sharp corners, the inside botom edge of the outside box should allso have cove moulding on it.
Water will not work as a release agent.
The picture frame it to hold the outer box in place, only to keep it centered over the inside box and just so the outside box can be taken off at will.
 
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