The large casings in old homes were often jack mitered. This is a combination of a butt joint and a miter. The profiled portion was mitered and the flat part was butt jointed. This was a practical consideration, large miters tend to open at the inside corner. Large casings would have really large miters, for instance the miter on a 5" casing--on the diagonal--is about 7 inches long. As two pieces of casing meet in a miter the outside corner of the miter is likely to stay closed because the wood is very stable on the long axis, but minor changes in the width of the casing open up the inside corner of the miter (as both pieces shrink away from the joint). A 0.5 percent change in the width of the two pieces of 5" casing causes about a 1/16" gap at the inside corner of the miter. The jack miter under the same circumstances, will develop half the gap or less, which is barely visible. Here is a mock up of a Victorian casing with a jack miter so you can see the pieces of the assembly:
Jack miters were easy to so with hand saws, but modern power miters saws can't complete these cuts. This irony is interesting, one of the few things easier to do before power tools. The modern casings in most new homes are so small that jack miters are not so important anyway. New door casings are just 3 pieces whereas this Victorian "cabinet head" casing assembly is 15 pieces.
That looks great! I also do a lot of built up mouldings. I really like what you've done with the door casing here. Modern door casing is scrapwood as far as Im concerned. Were jack miters really invented just because of teh eventual shrinkage problem? I always thought it was sort of a design aesthetic.
Thanks, and yes jack miters were a smart way to help keep the joints tight. An interesting development is that this is even more true today, because the butt-jointed portion can be pocket hole screwed together on the back side. This is hidden, or course, but someday it will be how a future remodeler will know that my work is not original.
Thanks, I made all the parts except for the 2-1/4" crown. This was for 5 doorways, so it was not worth having knives ground and having a custom setup charge at a millwork shop for a small run. I made the stepped casing on my table saw, run on edge. Each pass I dropped the blade 3/4" and moved the fence over 1/8". I made a high fence and used featherboards to keep things feeding smoothly. Here is one photo of the set up and a link to the photo album:
Thanks Donald, the photos tell most of the story. Since few carpenters have much experience with these techniques, I wrote an article about this for the Journal of Light Construction (JLC). It will be published next month. If OHJ doesn't mind I can provide a link to the online version when it comes out. Anyway here is a close up of the door casing primed and installed, but before paint so the components can be seen:
Good to know this is of interest. I also use the table saw to prepare stock for milling on the router table. I rip bevels on the edge at the same angle as the profile. This stock removal takes little time and prevents tear-out and bits and motors have much less work to do and last longer. The stock also feeds much smoother and easier producing better results with less sanding. Here is an example from the same project: