Question
I am a cabinetmaker who is looking at getting into making trim for residential customers. I know that upper end houses sometimes have arched trim that needs to be made for the windows and doors. As far as I know, that solid wood has to be steam bent and then run on a curved moulder. I am wondering how you make templates for the arched pieces. I am guessing that you have to make templates for each window and door because each arch may be a little bit different.
How do you make your templates? Do you use some kind of transparent material and just trace the upper portion of the window? Also, if anybody has any information about steam bending wood and making the arched trim, I would appreciate it.
Forum Responses
(Architectural Woodworking Forum)
From contributor M:
I think you are misguided with the steam bending. Either laminate thin strips around a form (they make clamping table for this) or saw the blanks out of wide lumber. The only thing you should template are elliptical casings. Radius trim can be figured mathematically or, if it is a half round, by the diameter.
If cut and sequenced properly, the thin strips can literally look like a solid piece. You can build your own bending forms, or buy one of the many adjustable forms available. Some are very reasonably priced and functional.
You are likely to have some issues, though, with maple; we have experienced small, black lines that are visible at each glue joint in maple.
Regarding templates, I use either Kraft paper or corrugated cardboard. Cardboard is preferred. The above poster was correct in saying that really the only need for a template is elliptical arches (half of an oval) or, of course, an oval window.
A half round window is easy; it is half of a circle. Measure the inside dimension at the widest point, add your reveal (typically 1/8"), multiply your reveal by 2, and that is your overall inside dimension. Now divide by 2; that is the inside dimension radius. That is the information you need. Once you know the radius, you can make a form, even out of a sheet of plywood if need be. Make yourself a large compass and set your radius and draw it out. Now screw on blocks of wood and bend the strips around these blocks. You will want to lay down plastic so the strips do not stick to your plywood or blocks. Prepare to get messy. There will be a lot of glue that oozes out.
To measure a segmental arch (eyebrow type arch), it is also quite simple. At a job site I will tack in a small finish nail at each end of the arched jamb extension. Then I stretch a piece of mason's line (string) from one nail to the other. Then I measure up from the string to the highest point of the arch. Then I measure across the overall width of the window. The height is called the height; the width is called the chord.
Once you have the height information and the chord information, you can calculate the radius. A segmental arch is really just a segment or part of a bigger circle. I have an Excel spreadsheet I wrote that calculates the radius if you know the chord and the Height.
Curved casings and radius moldings and stuff were traditionally cut from solid boards and put together as annular segments. This can often be done quicker than it takes to build a bending form and re-sawing all the stock. (Yes, it takes a little more work in the field).
On casings that require shapes that can't be done on the adjustable form, we cut male and female forms from particleboard on the CNC router. We use the clamp rack to squeeze the strips between the inner and outer form. Make one of the form halves very stiff and the other a bit on the flexy side so you can get really tight glue lines. Once the glue has cured, scrape the excess and run the blank through the wide belt to flatten and bring to about the right thickness. Put the appropriate knives in the head for the Stegherr arch molding shaper; adjust for width and thickness and run. If the customer is willing to pay for a backout, run that first. The same knives are used for the curved casing as we use on the Weinig molder, but we have to put them in a slightly smaller diameter head for the Stegherr. The arched molding shaper just follows the curve of the wood, so no fixturing is normally needed. Profiles on curved edges are run on a shaper using a molder head or on the CNC router. Curved crown molding is usually run on a heavy tilt shaper using a molding head. If the crown is not a true radius it can be run on the CNC, with more setup fixturing they can be run on the tilt shaper. Moldings run on the CNC take up valuable machine time and require some additional sanding due to the multiple pass machining.
The time (cost) in making curved crown mostly goes into making the blank and not the actual machining. The blanks are brick laid from layers that have been joined by dovetail keys to hold them tight. Setup for molding production can be done on the cheap for small quantities, but to be able to provide the full range, you will need to spend some money on equipment and learning. You can use a knife service and a William & Hussey molder if time is not a consideration. The W&H is agonizingly slow, but can usually get there. Deeper profiles require multiple passes with less and less feed roller contact each pass. Cheap, rubber feed roll planers can also get there, but have drawbacks for any sort of production use. A heavy shaper can do a lot. You can jig it to do curved casing and put a single wheel power feed on it to improve quality. A heavy tilt shaper can do even more. Feeds make shapers much safer and produce higher quality moldings. It’s possible to hand grind molding knives, but much better to use a profile grinder.
