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Determining the Slope of Crown Moulding
As was mentioned previously, the slope of a crown
moulding is the compliment of its ”Spring Angle” (the
spring angle is measured relative to the wall, and the slope
is measured relative to the ceiling). The two common
spring/slope angles of crown moulding are 38/52 and
45/45. The reason why these angles are represented by
dual numbers is because the actual angle depends on
which direction the moulding is oriented. For example,
turned one way, the slope is 38 degrees, but turned the
other way, the slope is 52 degrees. Note that the sum of
these two angles is always 90 degrees.
For mouldings that are not the standard 38/52 or 45/45,
you will need to measure the moulding to determine its
slope. This is very similar to measuring the Drop and
Projection of a pyramid, but more care needs to be taken
to ensure the measurements are taken from the correct
locations. Important: The slope of a moulding does not
necessarily coincide with the “points” or “Tails” of the
moulding. The slope is measured from the back-side of
the moulding, regardless how long the tails of the
moulding may extend (see image below).
Calculating Compound Angles
There are two separate equations for calculating a
compound angle. One equation is for calculating the miter
setting of the saw, and the other equation is for calculating
the bevel setting of the saw.
()
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
= Slope
eCornerAngl
Cos
2
TanArcTanMiter
()
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
= Slope
eCornerAngl
Sin
2
SinArcSinBevel
If you don’t know the slope angle, but you know the Drop
and Projection, then use the following to calculate the
slope:
⎥
⎦
⎤
⎢
⎣
⎡
=
Projection
Drop
ArcTanSlope
The most common error in calculating compound
angles is using the wrong Corner Angle. As a
double-check to your calculations, for nearly all
cases, the Corner Angle should be less than 100°.
The only time the angle should be significantly
larger than 90° is either for a 3-sided box, or a
wall corner with a sharp edge.
Compound Angle Calculator
A compound angle calculator has been built into this Adobe
®
Acrobat
®
PDF file. If you do not have an original electronic
version of this manual, visit the
http://www.festoolusa.com/ website to download a copy.
Slope Angle Options
Drop
Slope Angle
Projection
Corner Angle Options
Corner Angle
MiterFast
Tool Angle
Results
Miter Setting
Bevel Setting
Note: If you are unsure
about the corner angle
you have entered, check
the calculated “MiterFast
Tool Angle” value. This
number should be no
more than 60 degrees for
sharp corners, but
otherwise, it should
always be less than 45
degrees for most corners.
If it is significantly
greater than 45 degrees,
then you probably have
the wrong “Corner
Angle.”
Supplemental User’s Manual 21
90.0
Calculate
33.9
31.6
52.0
45.0