I need to produce some frames for 40x48 inch 3/16 thick mirrors. The moulding is 3 ¾ wide and will be v-nailed and glued. Will those corners be strong enough to hold the mirror or should I also add corner angles or mending plates?
Either will work. Some commercial mirrors use triangles of Masonite at the corners: it's probably cheaper, just doesn't look good from the rear. We cover our backs with a dustcover anyway so it doesn't matter.
Mark: We back the entire frame with masonite - cut approx 1.5" smaller than outsdie dimension of the frame and screwed down every few inches. This adds some weight, but adds plenty of strength and fixes the problem of dustcovers/corner angles. It's probably over-doing it, but these will last a LONG time without any concerns about cracked corners.
Use finishing washers on the screw heads and it adds a clean look.
A 3/16" thick, 40" x 48" large mirror is not heavy enough to crack open a 3 3/4" wide molding frame if properly joined with glue alone (accurate mitres, clean surfaces and good pressure clamps). But proper joining is a different story and, just for your peace of mind, you can use mending plates too. If you fear they may be objectionable, you always can use a little flat, sharpen wood chisel to sink them in. Prepare the recessing bed for the mending plates before gluing the frame.
One trick that is surprisingly effective is the
addition of a countersunk angle plate = a mending
plate made at a right angle. Since the plate is
within the wood, and screwed to the wood, it makes
a very strong corner.