# 40deg miter saw setting will cut an outside 80 and 100 deg miter ?



## Nickp (Dec 4, 2012)

What do you guys think...?






According to "See Jane Drill", setting the miter saw to 40 deg will cut an outside molding corner for both 80 and 100 deg imperfect corners.

She says for angles less than 90 deg, just divide by two and set your saw...80/2=40...

For angles greater than 90, let's say 100, take the angle, divide by two, subtract from 90 = the miter setting on the saw...100/2=50...90-50=40, set your saw to 40.

So...setting your saw at 40 will magically cut both an 80 deg corner and a 100 deg corner...

Did I miss something...?


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## Stick486 (Jan 4, 2013)

but will the profiles meld???...


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## TenGees (Sep 12, 2012)

In her examples, one is 45 plus 5 (50) and one is 45 minus 5 (40). Unfortunately my table saw won't go to 50, so I'd have to cut at 40 with the material vertical.


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## Stick486 (Jan 4, 2013)

he plan doesn't work...
the toe to heel measure is different at different angles...


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## Cherryville Chuck (Sep 28, 2010)

Exactly. The length across the diagonals (the miter face) are different. That's why you split the angle. On a 45* angle the length of the miter face is 1.414 times the thickness of the board. The steeper the angle, the longer that face gets. You can make the corners meet but one board will have a gap behind it.


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## Flipsaw (Mar 11, 2016)

Aren't you worried more about the faces matching then the gap behind when the angle is not the "perfect" 90?


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## Nickp (Dec 4, 2012)

My point was more that given two different angles, 80 and 100, her formula(e) comes up with the same answer, a setting of 40. Just can't be.

Anytime you cut the pieces with a setting on the miter saw less than 45 deg, when you put the two board faces together it will create an angle greater than 90...whether they are vertical or horizontal...try it.

So if you have an acute angle, let's say 88, you can't set your saw to 44 to bisect the angle...when you put the two board faces together it will make the angle greater than 90...in this case, 92 deg...for 88 deg, you need to set your saw to 46 deg...

In order to bisect acute angles and have the two pieces fit exactly, face to face, your saw needs to cut angles greater than 45...

I was just pointing out that her formula, and the general thinking of just "bisecting the angle", doesn't work for all angles (angles quite a bit less than 90)...unless the saw can be set to quite a bit greater than 45. This is usually not a problem for interior walls, because the errors are generally small, but would be for decks and other construction.


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## Nickp (Dec 4, 2012)

Flipsaw said:


> Aren't you worried more about the faces matching then the gap behind when the angle is not the "perfect" 90?



True...but much harder to deal with when the profiles are deep...


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## TenGees (Sep 12, 2012)

I agree that the formula is wrong. There they are maybe cutting the material different ways for over or under 90 degrees???

Find a different instructable. 

Here's what I was talking about and in those examples it happens to be 40 degrees.


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## Herb Stoops (Aug 28, 2012)

I don't understand any of this. I have done it for 40 years and bisecting the angle always worked for me. on inside or outside corners.
we never tried to figure out the degrees, just had an angle bisecting guage and found the total angle and it gave us the center point which we layed out and cut the angle, But a power mitersaw might be different.
It has to work, if you lay a flat board on top of another flat board and mark where the point and heel are and cut them you have the same angle on each board.
I am not saying the if you bisect a 90°angle, and a 100° you get the same bisected angle, you get half the respective angle. 


Herb


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## Nickp (Dec 4, 2012)

Herb Stoops said:


> It has to work, if you lay a flat board on top of another flat board and mark where the point and heel are and cut them you have the same angle on each board.
> Herb


You're absolutely right, Herb...if you use the two board method you described, you will never be wrong...


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## Nickp (Dec 4, 2012)

TenGees said:


> I agree that the formula is wrong. There they are maybe cutting the material different ways for over or under 90 degrees???
> 
> Find a different instructable.



The video came up as I was looking at where to install Boost and EGT gauges on my Cummins...


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## Stick486 (Jan 4, 2013)

Nickp said:


> The video came up as I was looking at where to install Boost and EGT gauges on my Cummins...


gotta love (or hate) google...


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## Herb Stoops (Aug 28, 2012)

Nickp said:


> You're absolutely right, Herb...if you use the two board method you described, you will never be wrong...


I have reread this thread a couple of times now and I see what you guys are saying and I agree the formula is wrong,and you are right,on a deck or laying flooring you would be in big trouble.
Herb


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## Taffygeek (7 mo ago)

Nickp said:


> What do you guys think...?
> 
> 
> 
> ...


Yes, mitre saws at 0 deg cut a 90 degree angle to the fence. If you cut a 40 degree angle you will end up with a 40 & 50 degree cut equalling 90 degree. It's the 50 degree one you need for the 100 degree angle and 40 for the 80 degree angle. If you mitre saw goes up to 50 degrees keep the orientation as you would normally. If doesn't turn work around cut 40 use the 50.


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## jw2170 (Jan 24, 2008)

G'day @Taffygeek , welcome to the forum.

Great to see new members sharing their knowledge...


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## old55 (Aug 11, 2013)

Welcome to the forum @Taffygeek


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