Hi Russell,

If I understand what you are trying to do with the trim....then the simplest thing to do (if you don't have a compound miter saw) would be to rip some strips of wood on the table saw with the blade set at 10 degrees, then back the piece of trim in the miter saw with the sacrificial strip ripped to 10 degree bevel.  Then, you could cut the outside miters  the regular way.

Why don't you cut both pcs at 5 degree bevel?
That's the way i look at it.

I don't know if it will be a perfect fit but it's worth a try!

Did you bevel the whole back of the trim?   Why?   It will never fit,you need to keep the same thickness of the trim that you are trying to joint.Try what i suggested about cutting yoiur trim at 45 miter and 5 bevel.Or you may need to cut a 3rd pc (transition between the 2 pcs)but usally i do that with cronw molding when the ceilling changes elevationon at a  corner.

If I'm looking at this right, you can't bevel the entire backside length of that front strip. Things just won't line up, as I think you've realized. But if that front is, say, 6 inches wide, and you back-bevel just those middle 6 inches of the strip, with the overhanging corners then hugging back around slightly to meet up with the sides at 45 degrees, something very close to the desired effect should be achievable... I think.

Alternatively, if you could carefully notch into the front face, the same effect could be attained without having to bevel the trim piece at all.

Either way, tricky stuff.   

This might help, http://www.csgnetwork.com/sawmitercalc.html

Could you just make relief cuts in your column to allow you to use unmodified molding?

From the picture, I'm having a hard time distinguishing where the angle is

I'm pretty good with trigonometry, but seriously, from your description, I got no clue what you're actually trying to do.

Russel,
I can't tell from your sketches, I assume that the columns are angled 10 degrees just on the front face and on the outside ends of the cabinet?  The column faces next to the TV and (of course) on the cabinet rear are plumb?  Is that correct? 

It might be a bit of a task blending the trim widths if one piece is sitting at a 10 degree angle and the others are plumb.  They will in effect need to be different widths, probably easiest by ripping a slight amount off the plumb trim at the bottom edge.

If only the front plane of the column is angled out...
your joints will be the usual 45 degrees, but
the front piece of moulding does need to be back beveled 10 degrees +.

Miter your unaltered stock at the usual 45 degrees but measure the lengths
for the lower edge of the miter joint, not the top because you're going to have
to relieve the top/back of the front stick so the piece can fit tight at the bottom.
The top of the actual miter will be a little behind the front face of the column.

I'd probably assemble the pieces first and do the fine tuning of the back bevel
with a small belt sander or rigid sanding block. To make it easier to fine tune
make a heavy hollow ground back bevel on that front piece before assembly.

EDIT

Okay, the above is only good for the pieces that wrap around the column.
I didn't realize you have a stick crossing the similarly angled stretcher but
the process is still similar unless I've missed something else.

For the sticks to fit as mitered joints they haver to be cut as if they are are
identical in shape. I think I read that you've already back beveled the stock?
If so, you going to have to figure out a way to support the molding from it's face.
When you figure that out you'll be back to making all the miters 45 degrees again
and measuring lengths for the bottom of the molding.

Hopefully you have some unaltered stock left. If you do I'd use some fast setting
goo like plumbers two part putty epoxy to mold an impression of the face of the
molding while it is sitting up perfectly square (in back) to the table. Use something
as a release so the epoxy can be popped off the molding. Make a long enough mold
that you can cut it in two so each half can support the back beveled molding securely
while it is being cut.

Remember, for the faces of the molding to fit right they have to both be cut standing
upright regardless of the condition of the backsides. You can't lean one stick forward
and get a good fit. Similarly, if you've already back-beveled a stick already you have
to find a way to keep it from leaning back while it's cut. Also, the very short sticks
on the inside faces of the columns will need a small trimming after assembly since
they will but into a plane that is leaning 10 degrees.

Another EDIT

If you've already back-beveled your stock all you have to do is rip an off-cut from
some other square stock that has that same bevel and stand that against the fence
and push your molding against the off-cut strip so the stock is supported at the normal
orientation.

One challenge will be that the front molding will need to be 'deeper' to account for the 10 degree slope to match up properly with the sides. 


One of two things will happen on the column, it's a matter of making a decision of what look you are after.

A;  the top of the molding profile projection will be narrower than the sides and be difficult to hide on the miter.  If a releif cut was made to be perpendicular to the sides, then the typical 45-45 =90 miter would work and the relief cut would hide any discrpency of the miter and molding being shy on the front-top.  The molding could be cut, glued, and placed on the column all in one step.    This treatment probably wouldn't look noticeable to the viewer since you won't be looking down on it, and you will see the underside more often.

B:  If you want to  have the same profile projection on the top of the molding aroung the column, the bottom of the molding must be 'made up' to allow for the slope of the column.  If you would cut your unaltered moldings at a typical 45 degree miter, then add on a 10 degree backer to the front piece, or put it on the column as a nailer, this should work.  This will look 'heavier' from the seated position.

I don't think a compound miter will work in this situation with the applied trim.  Crown yes, trim no.

Dumb question: can't you just measure the angle in sketchup?

It needs a transition pcs!!!!

Very, very, very nice work!