Cannot cut a proper triangle to save my life

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Forum topic by ColonelTravis posted 08-15-2013 08:13 PM 1917 views 0 times favorited 19 replies Add to Favorites Watch
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1756 posts in 1857 days

08-15-2013 08:13 PM

Topic tags/keywords: triangle plywood

Adding plywood shelves to an odd closet. Shelves look like this (isn’t to scale, just a rough idea):

It took forever to cut this shape correctly, and it came out OK but not exact, but it’s a back room closet, not gonna worry about it this time. But I’m starting out in woodworking and in the future I need to get things exactly right. I’ve got one of those cheap, plastic, yellow angle finders and a cheap, plastic protractor – first and last time I’m using those.

1.) Can someone please suggest proper measuring tools for angles?

2.) How do I mark those angles accurately on a somewhat lengthy piece of wood, so I can keep the cursing and scraps to a minimum?

3.) Using Mr. Pythagoras, I got nothing close to that 58 degrees. That A in geometry 25 freaking years ago has really helped. What am I messing up mathematically trying to come up with that angle?

Very sad and embarrassing. Trying hard to not be an idiot but that is, in itself, difficult.

19 replies so far

View Charles Maxwell's profile

Charles Maxwell

1099 posts in 3770 days

#1 posted 08-15-2013 08:20 PM

If you need angles and dangles go here:

-- Max the "night janitor" at

View PurpLev's profile


8534 posts in 3612 days

#2 posted 08-15-2013 08:22 PM

something with your numbers isn’t right. using t he same 36.5 and 32.5 for the right angle legs I get 48 7/8” for the 3rd leg which you have marked as 37, the 58 degrees angle you have marked I have as 47.1 degrees.

I suggest you rework your numbers with a square reference

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View JustJoe's profile


1554 posts in 2001 days

#3 posted 08-15-2013 08:31 PM

Why not skip the math, grab a couple of large pieces of cardboard or masonite and make a template?

First take a piece of cardboard that fills most of the space, then take another smaller piece and put it on top of the first, move it against the wall so it’s flush and tape it to the first piece. If you’ve got more than one weird wall, then take a 3rd piece of cardboard and move it flush against that wall, tape to the first piece. Then you just carry the cardboard over to the plywood shelf material and trace it.

Or option two for those who are trig challenged but still know how to read a ruler:
Cut a shelf to 32.5×36.5 rectangle.
Measure 16” along the top edge and make a mark.
Draw a line from that mark to the bottom right corner of the plywood.
Cut along the line.
Remove waste
Install shelf
Beam with pride as your wife says “well you actually completed something. Nice. Here’s 30 more projects I want done.”

-- This Ad Space For Sale! Your Ad Here! Reach a targeted audience! Affordable Rates, easy financing! Contact an ad represenative today at JustJoe's Advertising Consortium.

View JoeinGa's profile


7735 posts in 1970 days

#4 posted 08-15-2013 08:33 PM

I just use cardboard and make a pattern. Trial and error till it’s right, and trace it on my wood.

Dang! I shoulda hit “refresh” first. JustJoe said the same thing, just in more detail :=-)

-- Perform A Random Act Of Kindness Today ... Pay It Forward

View junebug's profile


101 posts in 2367 days

#5 posted 08-15-2013 08:41 PM

I agree with PurpLev. Something is wrong with your dimensions

View REO's profile


928 posts in 2037 days

#6 posted 08-15-2013 09:01 PM

Pythagorean theorem has nothing to do with angles. only the relation ship of the lengths of sides in right triangles.

View EEngineer's profile


1098 posts in 3576 days

#7 posted 08-15-2013 09:28 PM

PurpLev is absolutely right! Those numbers ain’t right!

Now, as the owner of an older house (built in 1927), I suspect some of the problem might be in assuming you actually have a right angle. I know damned well that there isn’t a straight wall or right angle anywhere in my house! I think I have measured them all at this point!

-- "Find out what you cannot do and then go do it!"

View ColonelTravis's profile


1756 posts in 1857 days

#8 posted 08-15-2013 10:23 PM

Cardboard template – thank you. Geez.

I took out the shelf and measured it to make sure I put down the right numbers, also rechecked the angle. That photo I posted above is indeed correct. It’s a 40-year-old house, I’m not dealing with square walls and doors, so I have to factor that in but they’re not off by a lot. As for the math, I’ll check out GeoGebra and do some homework.

One more question – is that Incra protractor worth the money?

View 404 - Not Found's profile

404 - Not Found

2544 posts in 2932 days

#9 posted 08-15-2013 10:31 PM

Bit of cardboard, pencil, track saw, you’d have that done in a couple of minutes.

View Dan Lyke's profile

Dan Lyke

1519 posts in 4088 days

#10 posted 08-15-2013 10:41 PM

Which of those are your known measurements? The angle and the opposite sides (32.5)? The angle and the hypotenuse (37)? The two sides?

What you need here is trigonometry, but it’s hard to whittle this down to a tractable example unless we know what your known dimensions are. And I agree with everyone else that these numbers are a little weird: If that bottom dimension (36.5) includes the 16, then the two sides of the triangle are 32.5 and 20.5. Which means either it’s not a right triangle (that base angle isn’t 90°), or that hypotenuse is closer to 38.425 than 37.

-- Dan Lyke, Petaluma California,

View Gary's profile


9323 posts in 3396 days

#11 posted 08-15-2013 11:27 PM

This is just my opinion but, everything Incra does is worth the money. That’s been my experience with everything I’ve bought from them

-- Gary, DeKalb Texas only 4 miles from the mill

View knockknock's profile


445 posts in 2136 days

#12 posted 08-15-2013 11:37 PM

Trigonometry/Geometry: the lengths of the sides of a triangle, define the triangle (2 triangles are congruent if each side of one triangle has the same length as a corresponding side of the other triangle).

View Dallas's profile


3599 posts in 2450 days

#13 posted 08-15-2013 11:46 PM

Cut to the chase…..
Make your angle any way you want to as long as the two sides equal 90°.

Mount the wall piece and hold the angled piece at level.

Scribe a line, cut the line,

mount the angle to wall piece, Wall- Laa your done.

it was more difficult to describe it badly than to actually do it.

You are trying to make too much work out of this.

-- Improvise.... Adapt...... Overcome!

View ColonelTravis's profile


1756 posts in 1857 days

#14 posted 08-16-2013 01:11 AM

Dallas – you are correct. Too much work, good advice.
Really appreciate the input from everyone. Site has been indispensable.

View firefighterontheside's profile


17930 posts in 1820 days

#15 posted 08-16-2013 02:10 AM

I like just Joe’s option 2. You assume the corners in the closet are square. Then you just make measurements and transfer those points to the piece and connect the dots. Something I’ve done was just make an angle copier with 2 pieces of wood connected together tightly. Then you can set in the corner and open it or close it to get the angle. Then put it on your piece and use it to trace the angle. Even though I have used trigonometry in construction especially in the construction of my home, most of the time I do much less math and more scratching my head until I figure out a much less technical solution.

-- Bill M. "People change, walnut doesn't" by Gene.

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