How to Make an Arched Casing
From contributor L:
If you have access to a CNC, have a male/female mold made for the form. It needs to be very accurate and have the ability to flex the outer part a little to ensure a tight glueup. You can use your shaper and a sliding table jig to cut finger joints. Put ears on the parts for clamping and cut them away after. If the profiled edges are a near match to a router bit (bits) you can do it on a router table. Or you can just order the molding from someone that makes them all the time. (Cheaper and easier!) To refine the routed shape to a perfect match, you can make a scratch stock fairly easily.
From contributor T:
Not to continue pounding the CNC drum, but... Put the shout out to some of the CNC people in your area and it shouldn't be too much of a problem.
From contributor K:
I think contributor H's answer is the easiest, especially in paint grade. But I generally have a tough time finding a pleasing grain arrangement when I do segments. I would be inclined to want to do the glue-up you’d like to avoid. I gather you have some experience with this, but if not, there’s plenty of info on this site, and any stair guy (ideally, any woodworker) should be able to help with details like forms, selection of material, ripping/stacking/clamping strategies, glues, cauls, etc.
You could also, since it is a casing and will be fastened to the wall, bricklay your lumber, so you have a basic arc shape, and then cut the arc. Although you have an easy time matching grain, it sits horizontal, which may or may not be a good thing.
Alternatively (and even more complex, but fewer laminations): Make the arc out of MDF, subtracting enough from the inside and outside for the profiles. Use this arc as the form. Oak veneer on the MDF. Glue/clamp your laminations directly to the arc – you could probably do the inside and the outside at the same time, since you've only got to build up an inch or so (?). Fair the curve(s), flush the top, rout the profiles. You still have the problem with grain, but it looks more like a solid nosing on a table or countertop, which is not foreign to most. Way more complex, but another way to skin it, if you don’t like a ton of laminations.
If you don’t have a lot of experience with curves and laminations, I say go for it! This is a good basic project which will serve as a foundation for more complex work in the future.
From contributor P:
I think the best final product will be the result of doing a nice clean lamination. It is quite straightforward for this application. I would try to cut all the lams from one piece of straight grained or rift red oak. You will glue them up in the sequence you cut them. Rift gives a much better grain match than flat sawn.
You only need to make one form. The form will be convex, or the inside rad. Two sheets of 3/4 ply glued together with the inside rad or the casing. Cover this edge with masking tape to keep the laminations from sticking to the form.
Pre-plan where all your clamps will be and how you plan on holding everything together. I do it with the form vertically. The trick to getting a tight join is having one or several caul strips to help distribute the clamp load. These can be extra lams that are not glued, or, as I prefer, something a little stiffer. You could get away with Titebond, but a plastic resin would be better if it is light in color. Unibond 800 is good. After 24 hours, pull it out of the clamps. Joint one face, run the other side through the planer to size.
It sounds as if you are going to just use a router to run the profile. This is fine. Remember, the outer radius lams edges are under tension, and they will want to tear out very easily.
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