I've used a sliding T-bevel at times. But it does not tell you what the actual angle is.
As for calculating angles, look up sine, cosine, and tangent in Wikipedia (or ask a resident 9th grader in Trigonometry:thumbsup to find the equations for calculating angles and lengths of right triangles (adjacent, opposite and hypotenuse). Also use the Pythagorean theorem. Most calculator will calculate sine/cosine and tangent, but you will need to first convert degrees into radians.
But I suspect there are some basic angles used in woodworking and that the more experienced woodworkers in here do not do much angle calculating. They probably just divide a 45-degree or 90-degree by however many parts they need. Or they measure with a T-bevel and cut without worrying about what the exact angle is.
If I have to find an angle, sometimes I'll draw up something real simple in CAD. I sometimes use the Bosch Miter Master if I'm replicating an angle and just got the Wixey digital angle tool for saw setup.
I use this tool http://www.mitergage.com/ to measure the angles I need to cut after I modified it.
I added a new indicator mark 180 Deg. out on the inside scale this then gives me a number I can take to my chop saw
You know, this is why alot of your old timers started coping inside corners. There is never or hardley evergoing to be a perfect corner!! I use a sliding T-bevel to start, and then hone in on it.You know... in a corner the t-bevel looks like 90 so miter cut is 45.There are standards everyone tries to stick with... that is, in building. 90 45 22.5 11.25 or 15 30 60 120 and so on. inside miters for an octagon are 22.5, a dodecagon has 12 sides. I believe the miter cut is 15. this might be old school... b ut it never slowed me down. Rick
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