What is a Compound Miter Cut?

Cutting compound miter cut with miter saw.

As a tyro woodworker, you will come to a point wherein you would need to make a compound miter cut for your projects. If it is your first time making such a cut, you might get confused about how to do it. Thus, it will help to understand what a compound miter cut is so that when your project requires you to make a compound miter cut, you can confidently say that you know what a compound miter cut means.

Different types of cuts on wood.

A compound miter cut consists of cutting two angles: the miter and the bevel angles. The bevel angle refers to the saw blade’s tilt from its vertical position on your table saw. On the other hand, the miter angle refers to the perpendicular cut with a miter of 0-degree. 

When you make a compound miter cut, you make a cut wherein the saw blade is both in its bevel and miter positions. Compound miter cuts come with several applications. But they are commonly used when cutting crown sloped miters and crown molding.

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The Primary Challenge When Making Compound Miter Cuts

When you try to make compound miter cuts, the primary challenge you would encounter is how to set your miter saw correctly to achieve the desired compound miter cuts. You can check out the saw settings on the look-up tables of standard moldings. Yet, if you would create non-standard angles, you need to engage in some mathematical calculations to get the angled cuts right. When calculating compound miter cuts, you need to be mindful of two parameters: the slope and the corner angle. Below is a short description of these two parameters:

Corner Angle

The corner angle refers to the angle between two wood pieces as you view the pieces straight down at the joint. It is easy to figure out. Yet, the determination of this angle is also a source of confusion for many aspiring woodworkers. The reason is that many woodworkers think of angles differently from those of the real mathematicians. 

For example, woodworkers would more often consider the inside and outside corners as having a 90-degree angle, despite the mathematician’s insistence that one corner is 90-degree angled and the other is 270-degree angled. 

So, to avoid confusion, you should measure the corner angle between the two pieces from a straight and perfect line. 

Slope

The slope refers to the angle that every piece produces, relative to the corner’s base, such as in the case of the floor or ceiling. If you are installing a roof-like structure, the slope angle may refer to the roof’s pitch or angle. 

If you are installing a crown molding, the slope will refer to the complementary angle of the crown molding’s spring angle. The molding’s common spring angles include 45/45 and 38/52.

To determine the slope of your crown molding, you need to know the measurement of the spring angle. The pitch complements the spring angle, and the two standard spring/slope angles for crown molding are 45/45 and 38/52. These two angles get represented by two numbers because the actual angle depends on the orientation of the molding. 

For example, if the molding gets turned one way, you’ll get 38-degrees, but turning the other way, you’ll get a slope of 52 degrees. As complementary angles, the sum of these two angles would be a right angle. 

If you are dealing with a non-standard angle, measuring the molding to determine its slope will be best. When calculating this, you need to be very careful and ensure that the measurements are made from the correct positions or locations. 

As a caveat, the points or tails of the molding do not often coincide with the molding slope. You must measure the slope of the molding from the molding’s backside regardless of the length of the molding’s tails.

Conclusion

The math behind the calculation of the compound angles can be a bit confusing for ordinary woodworkers and beginners. The formulae can be very daunting as well. But learning how to calculate compound miter angles would mean you are raising your level of woodworking skills and knowledge a notch higher. 

The most common mistake when calculating the compound angles lies in using the wrong corner angle. Remember that the corner angle must be lesser than 100°. The only instances when the corner angle is larger than 90° would be when you are dealing with a three-sided box or any wall corner that comes with a sharp edge.

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