Making Radius Trim On the Jobsite

Making Radius Trim On the Jobsite


Lately, my crew and I are installing a lot of flat stock casing in the homes that we trim out—which means I also have to make more and more radius casing to finish the tops of windows and doors that have arched heads. Besides the sizes of the openings always being different, some openings require a full semi-circular arch, while others require just a segmental arch. And occasionally we make radius casing that’s stained and top coated with fine finish. That’s when it’s even more important to get a perfect fit and a good grain match, too!

Like a lot of finish carpenters, I’ve struggled to figure out a full-proof system for cutting radius trim on the jobsite. With the help of Gary Katz—and especially Todd Murdock, I’ve developed a dependable approach and have battle-tested it to be sure it’s accurate. Here’s what works best for me and my crew.

Measure the opening

The first step is simply to see what you’re working with. Measure the opening carefully. Measuring the width is easy; just make sure you do it right where the leg jambs intersect the head jamb. This measurement is called the “chord length.” To measure the rise, I screw or clamp a straight edge horizontally at the intersection of the jamb legs and the head jamb. Then I mark the center of the jambs on my straight edge and measure up, at that location, to the inside dimension (ID) of the head jamb.

The ID of the jamb is what all of the casework must be registered from. If you want, you can also mark the ID of the jamb legs right on your straight edge so that you can easily transfer the chord length onto your work table.

The arched opening I’ll be using as an example in this article measures 36-in. wide at the ID of the jambs and has a rise of 7 in.

Believe it or not, with just these two simple measurements and a construction calculator, I can easily determine everything else I need to know to create the radius head casing. But before I get into the calculations, I want to review the terminology used for describing parts of a circle so that we’re all visualizing the same thing.

Arc: A portion of the circumference of a circle or other curve.

Arc Angle (arc measure): The angle formed by the arc endpoints and the center of the circle.

Arc Length: The distance along the curved line making up the arc.

Chord: A line segment joining two points on a circle or curve.

Circumference: The distance around the perimeter of a circle.

Radius: The distance from the center point of a circle to its perimeter; equal to one half of the diameter.

Using my Construction Master Pro calculator, I start by entering the chord length; punching 36 in. into the calculator and then pressing the RUN key (don’t forget to press the INCH key after you enter a measurement!). Next I enter 7 in. and press the RISE key. To solve for the Radius, you must first press the CONV key to access the calculator’s secondary key function, and then press the ARC key (note the Radius label above the Arc key). The radius in this example is 26 5/8 in. Next, press the ARC key again to find the Arc Angle—85° in this case. Pressing the ARC key a third time will even give me the Arc Length—39 1/2 in. While the Arc Length isn’t something I specifically need to know for this job, it’s useful information that can be valuable for other types of radius work.

If you’re using BuildCalc, all of the keystrokes are the same, but you’ll notice that after you press the CONV key, the label on the Arc key will automatically switch to “Radius.” AND…when you press the Arc button the second time, the screen will switch to an enhanced ‘Arc Layout’ page. It’s like a virtual note pad. You can even email or print the results right from the App!

Don’t wait to start jotting down the results from your calculations—I always do that right near my worktable. Trust me, you can’t remember all the figures and you’ll need to reference them over and over again. A written list helps me stay organized and avoid silly memory mistakes.

Make a story pole

With the head jamb radius established, I can now determine the inside radius and outside radius of the head casing. In this example I’ll be using 4-in. wide flat casing with a 1/4-in. reveal. Knowing the reveal dimension is important! If you’ve ever had to fight hard to get even reveals while installing radius trim, there’s a good chance the reveal you were trying to achieve was never accounted for when the trim was milled.

For this type of layout I’ve learned it’s best to make a story pole. Gary Katz is getting older and repeats himself a lot. One thing he never stops talking about is the stuff he’s learned from other carpenters, especially Jed Dixon, the stair builder who taught him to always make a story pole. Now I do the same thing whenever I have to lay out anything with more than two dimensions!

I start with a piece of scrap and make a clean square-cut on one end. Then I hook that end with my tape and make a mark for the jamb radius. Next I add a 1/4 in. for the reveal between the inside of the jamb and the IR (Inside Radius) of the casing, then I make another mark 4 in. further to define the OR (Outside Radius) of the casing.

This story pole is absolutely essential and critical to maintaining precision and repeatability—something that cannot be achieved by pulling out a tape measure every time a measurement must be set. As you’ll see in this article, I return to the story pole repeatedly to set dimensions.

Make a pencil trammel

If you’ve ever used trammel points to help layout and draw your work, then you know how precise they can be—as long as you keep your pencil very sharp, and as long as you adjust the points precisely!

That is why I always use the story pole to set my trammel points. I attach my trammels to a sanded piece of 1/2-in. x 1 1/4-in. stock that’s just a few inches longer than the Outside Radius of the casing.

Make a full-size drawing

I start with a full-size drawing on a large piece of plywood—big enough to use as a worktable and router table. First, I strike a center line, and install a spacer block. The spacer block needs to be the same thickness as your casing material to elevate the router trammel arm and maintain a level plane. We’ll get to that soon.

Next, I mark lines that represent the inside edges of the jamb legs, being careful to lay them out equally from the centerline. Then I drill an 1/8-in. hole on the center line, right into the spacer block, establishing the center point for all of my radii.

That’s when I turn to my story pole the first time.

By simply registering the steel pivot point at the end of the story pole, and setting the pencil point at the jamb radius mark, I can maintain a much higher degree of accuracy than trying to set the trammel points to a tape measure that can’t be held still. I use the same technique for setting the trammel points for the IR and OR of the casing, too, and that’s not all…

Don’t forget to sharpen the pencil until it’s a needle! That’s the only way to maintain consistent accuracy.

Trammels are fantastic tools. They’re easy to setup, easy to swing, and you can still find antique iron trammels on eBay! I place the steel point into the pivot point on the centerline…

…then I swing the pencil for each arc, starting with the IR of the jamb, then the IR of the casing. Each time I adjust the pencil trammel to a new radius, I use the story pole to be sure it’s dead on.

The last arc is the OR of the casing.

Anyone with a sharp eye probably noticed that I have a sheet of 1/4-in. plywood clamped on top of the backboard. This is a set I use at JLC LIVE!, so we can draw those arcs over and over on a clean sheet. Otherwise the backboard, with all the router grooves cut in it, becomes really distracting.

Make a router trammel

Once again, the story pole helps me layout one last critical component for this radius casing project—a router trammel arm. Yes, you need to make two trammel arms—one for your sliding trammel points, and one for your router.

I use a piece of 1×6 that’s around 12 inches longer than my story pole and strike a centerline down the middle lengthwise.

Next, leaving enough room from the end of the board to comfortably mount the base of the router, I’ll strike a perpendicular line plotting the center of the router cutter.

I like to use a 1/2-in. upcut spiral router bit to cut the actual casing. I prefer the half inch cutter because it fits so nicely into a 5/8-in. template guide, which I find to be a little more interchangeable between my different routers—that’s the template guide I use on all my hinge and lock templates, too. But you could use any size bit and any size template guide.

Just remember this: The OUTSIDE of the cutter will be cutting the IR of the casing; and the INSIDE of the cutter will be cutting the OR of the casing. To establish the INSIDE and OUTSIDE of my cutter on the story pole, because I use a 1/2-in. router bit, I measure and mark 1/4 in. on each side of the center-of-cutter line.

To maintain as much precision as possible, I use a sharp nail set to strike a pilot hole at the center of the cutter, which helps guide my 5/8-in. drill bit.

Now it’s back to the story pole! Holding the butt end of the story pole even with the outer line (the OUTSIDE of the cutter)…

…I transfer the IR of the casing on the story pole to the router trammel.

Next, holding the butt end of the story pole even with inner line (the INSIDE of the cutter)…

…I transfer the OR casing mark to the router trammel.

Finally, using that same sharp nail set (or an awl if you’ve got one!), mark and drill the intersections precisely.

Because I use a 5/8-in. template guide, I drill a 5/8-in. hole at the center of the cutter for the template guide—I want a snug fit.

And because I use a #10 washer-head screw to secure the trammel arm to the center block, I drill a 3/16-in. hole at each of the two other intersections (the pivot points for the IR and OR of the casing).

How many pieces make up the blank?

For me, the number of pieces required to make up the blank is determined by the width of stock I have on hand, and the size of the radius; for stain-grade jobs, I like to maintain as much grain continuity as possible—I want to avoid the appearance of any horizontal or level grain, which ruins the look of the radius trim!

Picking the number of pieces requires a bit of common carpenter-sense, but not a lot of mental gymnastics. I just hold a few pieces of stock over my full-size drawing until I’ve determined the proper number of pieces. If you have enough time to plan ahead, do a quick Sketchup drawing of the opening—that’s a sure-fire way to determine everything you need to know about the layout, and that’s the method I used for this article. I chose to use three pieces or segments to make up the radius casing.

Determining the lengths and miter angles

There are several ways to determine the lengths of each segment, and the precise miter angles for the segments, too. And no, you don’t have to cut a bunch of small pieces and experiment until you get the right angles! When it comes to this task, construction calculators really earn their keep.

Since I’ve decided to use three pieces to construct the blank for my casing, I simply need to divide the arc angle of the head jamb into three equal segments. By entering the jamb radius and the new arc angle into the calculator I can work backwards to find the new chord length!

Start by entering 26 5/8 in. (Jamb Radius) into the calculator followed by the CONV key and then the ARC key. Next, enter 85 (Jamb Arc Angle) divide it by 3 (number of desired segments) and then press the ARC key to store the result as the new arc angle. Now press the ARC button again. The result is the new arc length—not something we really need. What we’re after is the freakin’ short point-to-short point measurement, from miter to miter. To get that measurement, just press the ARC button one last time. That’s the new CHORD length!

Of course, you’ll want to know the angle of the miters, too. That’s simple: just divide the new arc angle (28.33) by 2. The miter angle is 14.17˚. Yes, construction calculators are awesome, but I’d probably set my miter saw at 14˚ and call it good.


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