Cornice / Crown

You try to make the cut right first time in a miter box, with the crown inserted upside down and at the correct angle. Think of the bed of the miter box as the ceiling, and the back wall of the miter box as the wall.
Remember you can open or close the top or bottom of a joint a little by how the trim sits on the wall, and you can relieve the back of a miter a little with a knife.
Walls and ceilings are seldom square, so you need to learn to make the adjustments efficiently.
Inside corners were coped, not mitered.

Here is the process using a chop saw, but it’s the same with a hand box.

We used to use a Lion mitre trimmer for production work to clean up the cuts. 4-5” was about the upper size limit for the crown in the tool.

It was quicker and safer than a chop saw and left very smooth ends like a shooting board if you kept the blades sharp. The tool has been around for probably 100 years. Clones are available from Rockler, Highland Woodworking, and Wood River. Probably others. I have only ever used the original Lion brand. Before the advent of the chop saw, it was how cuts from a hand miter box were adjusted.

I see them on the used market for maybe $50 or so, but they became redundant with slide compound saws and laser sights.

There is one without a handle sitting in my local scrap yard u should probably rescue, but my production days are over.

• This reply was modified 4 years ago by Larry Geib.
• This reply was modified 4 years ago by Larry Geib.
• This reply was modified 4 years ago by Larry Geib.

You can end the wriggling around by pinning or clamping a stop strip on the bed that restricts movement.

Most metal mitre boxes have stops built in as part of the design.

You position the trim against the bed and the back wall, then set the stop. All your cuts will be bedded to the same angle.

If the walls are square, very little touch up is required. The challenge comes when the walls aren’t pitched to the same angle but often that is left to the painter to caulk. In that video, you can see the trim isn’t bedded completely to wall and ceiling..

And that why you should take care to build casework square.

As to tuning a miter box, most have an eccentrically milled pivot, so you can change the position of the saw swing point slightly by turning it. Unless you know what you are doing, it’s just as easy to get it wrong as right, so I suggest you hold off on trying that until you are absolutely sure it is what is causing the problem, and not inadequate technique. It’s very rare that needs to change. I’ve used my box for 45 years without changing it, despite the box spending years in a truck.

Electric boxes seem to need adjustment more often. A guess is because they are aluminum and not cast iron or steel.

• This reply was modified 4 years ago by Larry Geib.

You can glue some 80 or 60 grit paper to bed and/or back fence. ( 3M reposition spray adhesive or rubber cement work and are reversible) That’s pretty common for guys who had wood beds. Varnish or paint your bed and cleanup is easier. My box has a Metal bed surface like a coarse rasp, so I never had that issue. It didn’t occur to me that as your problem.