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How do I calculate compound mitre angles?

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Rob Fritz

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Jul 5, 1999, 3:00:00 AM7/5/99
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Hi Everyone,

I'm attempting to build a mantle shelf for my fireplace. Looking down on
it, the shape is supposed to be:

|_________|

It seems fairly simple -- three pieces and the open side attaches to the
wall like a regular shelf. A simple 45 degree mitre makes the corners
square.

But I wanted to slant all three pieces in at 45 degrees as well. Looking
from the front,
the shape is:

__________
\_________/

and from the side:
_
\_

But when I used my table saw and cut all the angles at 45 degrees, looking
down, the resulting pieces fit together in a shape like this:

/______\

It's obvious to me that the inward 45 degree slant I'm trying to achieve
somehow changes the normal 45 degree mitre angle. And I'm sure one of you
experts knows the formula for what I understand is called a compound mitre.
Any info appreciated.

Thanks.

Rob
rfr...@geologics.com

Kevin Goom

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Jul 5, 1999, 3:00:00 AM7/5/99
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By my calculation you need to set your miter gauge at 35.26 degrees from its
normal perpendicular angle, and tilt the blade 30 degrees. I suggest you try
this on some scrap first (I'm not that confident).

I have just written a small program in Excel to calculate the geometry and
tablesaw setup for any pyramid (or truncated pyramid) of any number of
sides. If there are any math wizzes out there, I would love to send you a
copy to check the veracity of my program. If all checks out OK, I would be
pleased to share it with anyone who is as frustrated as me when trying to
figure out compound angle cuts. I also intend to use it for calculations for
stained glass lamps.

Kevin

Ronald Murray

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Jul 5, 1999, 3:00:00 AM7/5/99
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Hello Rob

Try
www.augusthome.com/etipindx.htm

I believe there is one there for cutting miters. There are lots of other
tips as well. Good luck.
ron

C.E. "Chuck" Ring

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Jul 5, 1999, 3:00:00 AM7/5/99
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Rob,

Go here:

http://209.122.88.33/index.htm and use compound as your keyword. You should get
at least three articles/charts which will help out.

Chuck

--

C.E."Chuck" Ring
MailTo:cr...@concentric.net
Edgewood, New Mexico USA
http://www.woodworking.org
http://209.122.88.33/index.htm

Jgtimp

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Jul 6, 1999, 3:00:00 AM7/6/99
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It's easier to make these cuts if you have a miter saw ("chop saw") or a radial
arm. If you have one of these tools, place your moulding upside down, with the
top (ceiling side) against the table and the bottom,(wall side) against the
fence. Then, just cut your 45 degree miters as normal. What you're basically
doing is cutting the pieces as they'll be installed, only upside down. If you
want, you can make a wedge to fit in the 45 degree void behind the moulding to
steady it and keep the angle constant with all your pieces, or clamp a stop
block to the table to butt the top up to. It helps to mark with a pencil the
general angle you're going to cut, since it's easy to lose your orientation
when cutting crown mouldings. I advise doing some test cuts in scrap first
until you're comfortable with the procedure.

By the way, not all mouldings come with a 45 degree angle on the back side.
Some angles vary. If you're going to use the trigonometry tables to do it on
the tablesaw, you have to first know what angle the back of the moulding is cut
at.

-JIm Gott

cut_...@my-deja.com

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Jul 6, 1999, 3:00:00 AM7/6/99
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Go to this site for a formula;
<http://www.wwforum.com/faqs_articles/miter_formula.html>

Dale Williams


Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.

Rob Fritz

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Jul 6, 1999, 3:00:00 AM7/6/99
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Thanks to everyone who replied!

The funny part is that I tried some basic web searches on wood working and
didn't find anything useful. These are some great sites.

Thanks!

Rob

John McGaw

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Jul 6, 1999, 3:00:00 AM7/6/99
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You can go to www.wwforum.com and then into the "articles" section to
download a free Windows program I wrote that will perform such calculations
for anything from 3 to 16 sides and various side slope angles. In your case
you would tell it that you are working with 4 sides.
--
John McGaw
Knoxville, Tennessee, USA
-----
! Remove "no-spam-" from address to reply by e-mail !
-----
This is not your average boring web site!
This is MY average boring web site: http://www.public.usit.net/jmcgaw

Rob Fritz <rfr...@geologics.com> wrote in article
<93120030...@news.remarQ.com>...

HineJS

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Jul 11, 1999, 3:00:00 AM7/11/99
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Kevin:
I'd be most interested in your excel program. Could you please send me a copy?

-Jeremy

KenJr

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Jul 11, 1999, 3:00:00 AM7/11/99
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On 11 Jul 1999 21:23:06 GMT, HineJS says...
I wouldn't mind a copy myself. Post it here if you can so we can all
enjoy it.

If you should email it to me you will need to remove the "_remove_" from
the address line.

Jack Lemley

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Jul 11, 1999, 3:00:00 AM7/11/99
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I'd like a copy of the Excel program you've written.....

Thanks
Jack
jle...@usa.net

Ted Rummel

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Jul 13, 1999, 3:00:00 AM7/13/99
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On 05 Jul 1999 19:56:20 PDT, "C.E. \"Chuck\" Ring"
<cr...@concentric.net> wrote:

>Rob,
>
>Go here:
>
>http://209.122.88.33/index.htm and use compound as your keyword. You should get
>at least three articles/charts which will help out.
>
>Chuck
>
>Rob Fritz wrote:
>>

Come on boy's!!
Surely there is a carpenter out there who can explain how to read a
framing square. This makes all the miter cut angles used in rafters or
anything else without having to know triginometry (which I don't).I'm
not a carpenter either so I can't help but I've watched it done.

Ted

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