Calculate an angle

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  #1  
Old 11-19-18, 11:25 AM
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Calculate an angle

I want to add trim in a room that has non-standard angles along the walls. I know there is an electronic instrument to read the angle but this will be a one time project and I would prefer to determine the angle without this expense. My geometry lessons are 60 years in the past. My only known option at this point is trial and error and that eats up a lot of time and trim.
Can anyone tell me how to calculate the angle?
 
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Old 11-19-18, 11:38 AM
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Buy a sliding t bevel, match it to the angle on the wall, then bisect that angle. (Half of it is your miter.)

A stupid proof way for a newbie might be to place 2 index cards on each side of the corner so that they overlap perfectly... point to point. Tape them together with scotch tape. Then draw a line from the long point to the short point. (Where they intersect) If you cut on the resulting line they should be mirror images of each other. Miters need to be the same. For example, 22.5 and 22.5. Not 22 and 23.
 
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Old 11-19-18, 02:54 PM
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Another method I've used a few times, almost identical to Sleeper's but with a slight deviation, is to use a knife to trim a single piece of paper to fit the existing angle, then fold it in half and you have your miter angles. Point of either one though is that a precise measurement is only going to be as precise as the graduations on your saw, so what you really want is a gauge to set our saw, and either of these methods will give you that. Otherwise, if you're determined to do it with a tool, a sliding T as Sleeper mentioned will do it, or what I like is my Starret 505P-7, which are relatively inexpensive.
 
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Old 11-19-18, 05:32 PM
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You need two dimensions (adjacent side and opposite side) to find the angle. First divide opposite dimension by the adjacent dimension. Take the answer and look in a table of tangents to find the angle in degrees. A calculator with trig functions will do it for you. Take inverse tangent (opposite/adjacent).
 
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Old 11-19-18, 05:38 PM
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Old 11-19-18, 05:42 PM
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I don't think I've ever seen a YouTube video with such an overwhelming amount of "thumbs down". LOL!


Good tool, bad video?
 
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Old 11-20-18, 04:27 AM
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Update
Opposite and adjacent lines must intersect at 90 degrees for the tangent function to yield correct angle.
 
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Old 11-20-18, 05:02 AM
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All of the above, and use scrap wood to make test pieces first!
 
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Old 11-20-18, 09:36 AM
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beelzebob:
You need two dimensions (adjacent side and opposite side) to find the angle....
You need at least three dimensions to prove any triangle. In certain instances of right triangles you can get by with only two but only because one angle in a right triangle is always a given.
 
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Old 11-20-18, 03:40 PM
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Opposite and adjacent lines must intersect at 90 degrees for the tangent function to yield correct angle.
This method uses trigonometric tables as stated in post number 4.
 
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Old 11-20-18, 05:26 PM
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For petes sake, spend $10 for a carpenters protractor and do the job right, you'll have the tool for the rest of your life!


https://www.doityourself.com/forum/a...1&d=1542759986
 
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Old 11-20-18, 05:31 PM
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That was in post #5 marq.
 
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Old 11-21-18, 05:37 AM
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While all responses are helpful, OP requested help to calculate, not measure, the angle.
 
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Old 11-21-18, 05:45 AM
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B-bob: I think the answers that have been given will help me do the job. Thanks to all who have responded. Some of the videos on line have been very instructive.
 
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Old 11-21-18, 05:51 AM
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That was in post #5 marq.
Sorry, post #6 dissuaded me to venture to that video!

OP requested help to calculate, not measure
Actually if you read the pose he inquires:

I know there is an electronic instrument to read the angle
So there for, to calculate what the angle cuts would be you first have to measure.

Dont need anything fancy or electronic, basically a simple protractor!

This is where DIY get the bad rap, not using the money saved to buy the tools needed so the job doesn't look like it was a DIY project.

Lets guide and teach how to do it right!
 
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