I am putting up a mantle over my existing firplace. The wall around the firplace makes an angle with the rest of the walls. The angles are about 45 degrees. This approximation, however, is not a good enough measurement, hence my first question is:
How do I measure an existing wall angle accurately?
Once I have accurate figures, I need to join a few pieces of wood at these angles. I would like to dove-tail these joints, but I do not know how to make dove-tail joints at angles other than 90 degrees. So my second question is:
How do I make dove-tail joints at angles other than 90 degrees?
You will probably be disappointed in your efforts to accurately measure the angle. It is best, in my experience, to use a Sliding Bevel to capture the angle in question. Be sure to take the flatness, or lack thereof, of the wall into account when capturing the angle. It has been my experience that the angle between two walls is not the same angle you will measure over the six inches on either side of the corner. I have used two yard sticks and a C clamp successfully(transferring the angle to a sliding square of course). Building a mock up is a useful exercise. This is especially true for the top surface which has no flexibility over that angle.
Simple geometric construction techniques can be use to divide the angle for a mitred joint. A butt joint can be made straight from the Sliding Bevel by setting your mitre saw’s angle to the on you captured.
As for dovetail joints, do you plan to hand cut it, or will you be using a router-based dove-tail jig? I assume you want the dovetails visible from the front of the fireplace. That is, the tail member would span your fireplace and the pin members would make ~45 degree bends and eventually end at the walls. This sounds like a tough cut, as you also have to get the length correct as well. The layout of the dovetails, if you cut by hand, will be quite difficult. I don’t think you’ll be able to use a marking gauge to layout the location of the shoulder line for the outside edge of the tails, so you’d have to use a square from the edges. Make a mistake and it’s right there at eye-level. Also, since the pins have to slide inbetween the tails with little slop, the cut for the tails has to be quite accurate.
Would you consider using box-joints instead? They look good from either face and are a sight bit easier to cut.
The Sliding Bevel idea is probably going to work really well. I am going to purchase one as soon as I get finish posting. After doing a quick search on Google for, “Box-Joints,” they will probably do fine. Is a sliding bevel the same thing as a T-bevel?
If you really want to go nuts with precision, you can buy a digital protractor – Bosch makes one for $120 or so.
I’d really avoid doing dovetails on an angle unless you’re a jointing zen master. But you could fake it, more-or-less, if you like the look by using a butterfly keyed joint. The only picture I could find (http://www.loggia.com/vignette/images/113a.gif) shows it being used for a table-top, but you can use the same technique on an angle joint. Basically, you make a dovetail cut into the pieces to be joined, cut a matching key, usually out of a different colored wood, and drive it into place. Then you sand it flush.
If you use a straight bit instead of a dovetail to cut the grooves, then the key is a lot easier to make, but maybe not as decorative.
Hey Chefguy, I have no beef with that. My woodworking instruction is of a definite English bent, as most of my books are English, and they pretty much perfected it when the Americas were still chopping wood with flint tools. Please note that I chose a Marples Sliding Bevel, as I’m quite fond of Marples tools(I could shave with my chisels if needed).
Also, I’m quite lousy at expressing myself in words when it comes to such physical things at joinery. It took forever for me to write that post. So, I didn’t see your post before posting mine.
Guys, I can’t thank you enough for your input. I am far from where I need to be Carpentry-wise, but with the quick response from you all, I am able take this project in the proper directions.
Sure, no contention intended, just clarification of my answer for *Ficer67. Some carpenters call the sliding bevel an “angle dangle”, if you can imagine.
