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PostPosted: Tue Jan 30, 2018 11:18 pm 
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Apologies for my maths or lack of it, but this is really bugging me.

I have 3 peices of timber 5ft in length (1524mm) 280mm wide and 55mm thick.

One is going to be horizontal 840mm off the floor, the other two will meet it at either end and angle down until they touch the floor. I want it to be as neat a join as possible where they meet the horizontal peice, and also where they meet the floor i wanted to angle them flat parallel to the middle one so where it meets the floor it is flat not on an edge although thats not as important as where they meet.

I have even gone to try drawing it to scale in a program to work it out but struggling to get my head round it if anyone could help. How its supported isn't an issue ive got that bit sortd its getting it to meet nice and snug im struggling with.

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PostPosted: Tue Jan 30, 2018 11:27 pm 
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Ive worked it out to 26.5 degrees if anyone would be kind enough to confirm ive worked that out properly?


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PostPosted: Wed Jan 31, 2018 12:54 am 
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i make it 25
as in 140-90=50 divided by 2=25
and the other end i am having a brain fart but but as its 90 and 50 must either be 40 or l40 lol :scratch:
that assumes i am right on the first end :lol:

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PostPosted: Wed Jan 31, 2018 10:18 am 
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If you've already drawn it to scale (often the easiest way) just transfer the angle to a sliding bevel and set the saw up to that. Make a couple of practice cuts with some spare timber, just a bit of batten will do, doesn't have to be the full width.

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PostPosted: Wed Jan 31, 2018 11:31 am 
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Using this site here
http://www.cleavebooks.co.uk/scol/calrtri.htm
The right angle triangle solves as:

So I make the top angle 56.6 + 90 = 146.6 deg.
That assumes 840mm to the top of the hz. beam, and the full 1524 used on the angle.
Do check my figures though!

However I agree with ayjay. Take the angle off a scale drawing with a slide bevel, and bisect the top angle to obtain the mitre.
http://www.proudlybuilt.com/uncategoriz ... er-angles/


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PostPosted: Wed Jan 31, 2018 12:01 pm 
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I agree with Dave.

Maybe you can remember your school maths.
Sin (Angle) = Opposite / hypotenuse = 840 / 1524 = 0.551
or Sin (Angle) = 0.551, thus Angle = ARCSIN(0.551) = 33.4°

You can use the google scientific calculatorto calculate the ARC SIN (Press INV first)


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PostPosted: Wed Jan 31, 2018 12:10 pm 
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Does the maths work there Dave?. Using Pythagoras the sum of the squares of the two sides is equal to the square of the hypotenuse. So (1270x1270|) + (840x840) = 2,318,500. And the hypotenuse squared is 2,322,576. The sides length will affect the angles. Not criticising I just could not make it add up.


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PostPosted: Wed Jan 31, 2018 12:23 pm 
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i think the confusion may be "0" on a mitre saw is actually 90% :dunno:

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PostPosted: Wed Jan 31, 2018 12:38 pm 
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dewaltdisney wrote:
Does the maths work there Dave?. Using Pythagoras the sum of the squares of the two sides is equal to the square of the hypotenuse. So (1270x1270|) + (840x840) = 2,318,500. And the hypotenuse squared is 2,322,576. The sides length will affect the angles. Not criticising I just could not make it add up.


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I just took the figures off the site "as is"
Looks like there's a slight rounding in the calcs there.
if you use this site
http://www.calculator.net/triangle-calc ... &x=80&y=21
You get 1271.604 for the "adjacent"
So 1271.604 squared = 1616976.733
plus 840 squared =705600
Which added together = 2322576.733

(Thank whatsit for calculators!)

The angles are near enough for the real world I reckon.

I didn't bother to work any of it out myself to start with, but I can remember the trig solution as

Sin = Opp / hyp
Cos = Adj / hyp
Tan = Opp / Adj

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PostPosted: Wed Jan 31, 2018 12:52 pm 
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dewaltdisney wrote:
Does the maths work there Dave?. Using Pythagoras the sum of the squares of the two sides is equal to the square of the hypotenuse. So (1270x1270|) + (840x840) = 2,318,500. And the hypotenuse squared is 2,322,576. The sides length will affect the angles. Not criticising I just could not make it add up.


DWD



Think it's a rounding error. I see it as 1271.6 which corresponds with cos(33.448) = 0.8344 which also equals 1271.6 / 1524 (adjacent / Hypotenuse)


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PostPosted: Wed Jan 31, 2018 12:57 pm 
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I see, a slight variance plays quite a bit with the calcs. We must have confused the OP :lol:

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PostPosted: Wed Jan 31, 2018 1:04 pm 
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well he is setting the bevel on his miter saw. I doubt he can get it that accurate past 33.4 degs My miter saw is lucky to be nearest to 5deg :lol:

Best advice i think is as you guys say, practice cuts, or ensure one slide flush and then alter the other angle to suit.

when I do cornices, plinths from oak I ensure that I have extra few mm for shaving down. What goes on paper never seems to translate in practice.


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PostPosted: Wed Jan 31, 2018 1:11 pm 
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Taking the angle off with a slide bevel, then bisecting the angle and using that to set up the cut is the most accurate way I have found in the real world. I've tried various angle gauges and calculations at different times, they don't really work in the real world. Angle scales marked on machines are notoriously inaccurate. Apart from anything with most of them you have parallax error, and error from the thickness of the marks.


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PostPosted: Wed Jan 31, 2018 1:27 pm 
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It put me in mind of:

A mathematician named Hall
Had a hexahedronical ball,
And the cube of its weight
Times his pecker, plus eight
Was four fifths of five eighths of f*ck all

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PostPosted: Wed Jan 31, 2018 7:08 pm 
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ayjay wrote:
If you've already drawn it to scale (often the easiest way) just transfer the angle to a sliding bevel and set the saw up to that. Make a couple of practice cuts with some spare timber, just a bit of batten will do, doesn't have to be the full width.


Totally agreed sliding bevel is the way to go. I've cut probably tens of thousand of angles in my time and there's barely a handful where knowing the actual angle has ever been needed. It's far to easy to overthink this whole thing.


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