The outside of the edge will be the length of the center piece plus double the width of the board. Example would be on th 14" side using 3" wide edging, would be 14" plus 6" for 20". When I started out this seemed complicated for some reason as well. As usual our minds over complicate things.

 

Monte or others, Wanna go into how to take care of rabeted inside edges to receive inserts such as pictures on compound mitered frames? Cut rabets first and then the frame, the rabets turn out slanted when assembled. cut miters first and rabets are puzzling. And do you use a specially gigged shooting board to dress corners of a compound mitered frame stock to dress the edges for a more precise fit. Please expand on this thread for me also. Thanks!

 

I am definitely not an expert on this.

Here is what I do:

1. Cut the pieces extra long.
2. Cut a miter on the end of piece 1
3. Cut the correct miter on the end of piece 2
4. Position piece 1 correctly on the picture (or wood)
5. Use piece 2 to mark where I need to cut the miter on piece 1.

I usually make the cut in step 5 a little long and then sneak up on the exact length through multiple trimmings.

As I said above I am certainly not an expert on this.

 

Jesse, Have you rabeted pieces and cut compound miters; or which ever way its done to end up with a flat rabet to install a picture, mat or panel from behind the frame? I'v sucessfully done flat rabeted frames just as you have described cutting the rabet on long stock, then cutting pieces 1,2,3,4 long and mitering back. You are right on.

 

I have never figured out the math for compound miters. I lay the pieces flat (2-d).

 

I will have to go home and cut it then I can figure out how to explain it.

 

You can start by cutting a miter on the end of your molding or "frame" piece. Line this mitered corner up on the outside corner of your piece. Once you have done this, mark the inside corner of the next miter. Continue all the way around until your done.

If you dont want to do that, make a miter on one end, then measure the horizontal distance from the inside corner of the miter to the outside corner of the miter. You can use this number to add onto the length of your piece to make the next cut.

 

At the moment I'm trying to do flat (2D) miters. If I want to make a picture frame, the inside dimensions of which would be 36" by 24" there has to be a way to calcuate this.

 

"I can't make the frame pieces to the exact dimensions of the piece (12 by 14). Because when I make the 45 degree angle cut everything will be too short. If I make it too long it won't fit together."

I'd check your miter cuts with an accurate miter gauge because if your width is 12 the
inside miter measurement will be 12 by definition. If I understand you correctly.

 

Purrmaster - Measure the backside of your frame pieces from the outside edge to the rabbet, then double it, then add it to 36"....do the same for the 24" side. Lets say your frame pieces measures 1-1/4 from the outside edge to the rabbet. That would be 2-1/2" when doubled. Overall outside dimensions would be 38-1/2" x 26-1/2"

 

Maybe pictures would help. Are you trying to cat a miter and have a rabbet that the piece will fit into?

I'm sorry, I'm just having a difficult time seeing this in my noggin.

 

Purrmaster: Why worry about doing the math to calculate the outside dimensions? If one side of what you're framing is 36", just make two marks on your frame piece 36 inches apart, and cut your 45's outward from those two marks.

(Hint: I always cut a tad long because it's easy to trim a hair off. Adding a hair…. not so easy.)

 

Assuming I did the image upload right, the above images should give you an idea of what I'm trying to accomplish. And failing miserably at. I finally found my digital camera.

The first image shows the piece I want to make a mitered frame around. The second image shows me trying to accomplish this with regular 45 degree miters. The third image shows my original attempt to do it with compound miters.

 

I'm going to see if I can remedy the answer. What Monte Pittman said was correct. If you want the inside of the frame to be 8 inches in length then, with a board of 2 inch in width, you would add the width times 2. so with a 2 inch wide board the out side of your frame will be 12 inches.

However, if you want to account for the rabbit that you set the picture into then you have to do a bit more math. If with the same board you cut in a 1/8 rabbit then the space that the picture will be resting in will be the length of the inside edge plus a 1/4 inch (that's twice the rabbit size) making the total space for the picture 8 1/4 inches In the picture below on the right side, that's a 1/8 indicating the size of the rabbit.

if you wanted the space for the picture 8 inches, then you would make your frame 1/4 smaller. so instead of 12 inches you have 11 3/4.

Now my math mathematical to this is the following. (you will have to used this equation for both sides of the picture frame.

(length of picture) + 2[(width of board)-(width of rabbit)]= (outer length of frame)
or
length of picture) + 2(width of board)-2(width of rabbit)= (outer length of frame)

in a prettier version with
picture length=P
board width=B
rabbit width=R
Total=X
P+2B-2R=X
or
P+2(B-R)=X

and to apply that to the situation i was talking about earlier with
picture length=8
board width=2
rabbit width=1/8
Total=X

8+2(2-1/8)=X
8+2(1 7/8)=X
8+3 3/4=X
11 3/4=X
or
8+2(2)-2(1/8)=X
8+4-1/4=X
11 3/4=X

But honestly, if you have the picture in front of you it's a hell of a lot easier to go by eye and pencil marks.
Hope I helped
~Pat

 

I know this is a long ago thread.. But you have just helped me out a great deal..(i hope you see this Pat)

 

Pat that was one of the most helpful comments I have seen. As a new wood worker this process has always confused me. What you and Monte just said clears up a lot of head scratching I have been doing lately. Thanks a lot!

 

no problem. It also helped me clarify the process in my head and double check that I had it correct. miters are just one of those things that can ruin a whole project (if the project is a frame for something)

 

But honestly, if you have the picture in front of you it's a hell of a lot easier to go by eye and pencil marks.
Hope I helped
~Pat

100% Agree Pat.
When I have a new guy in the shop I would explain just what you did above for a basic understanding. Then, I would tell them to put the calculator away and don't use it again.

Avoid math like the plague if you can. It quite often only invites an opportunity for error.
Unless absolutely necessary, always use 'heel' (inside)measurements, not 'toe'(outside) measurements.
By using heel measurements, you avoid inconsistencies in material width, or rabbet width. 1/64 error x2= Too big, or too small when using outside (toe) dimensions.
Another point…don't take simple inside measurements and cut all four pieces and expect then to fit 'properly', most often, they won't.
'Chase' the miter's (as I would call it) around what you are trying to frame. Cut one, fit the next, manually mark the next miter, cut and repeat.

Purrmaster
Somehow, your making this more complicated than it needs to be. Practice the more simple flat miter's before you move into rabbets and compound miter's.
Btw….what you show in the 3rd pic isn't a compound miter, just a taller version of a flat miter.

Cut a miter on one end of your material. Sit it up against the board, and use the off cut to align your frame piece properly. Mark the 'heel' or inside of the next miter…cut(as Charlie said, cut a hair big), then use the off cut again on that cut to check accuracy.
Using the off cut will also give you an indication as to wether your miter (table) saw is adjusted to a true 45.
Correct any angle deficiencies before proceeding.

Important point:
Take your time and don't get frustrated. Many seasoned vets struggle with the accuracy end of exactly what your trying to accomplish.

 

And while we're at it, another point always worth mentioning:

Once you get your basic lengths correct, if your miters don't fit perfectly, don't automatically assume your miters are not an accurate 45 degrees. The more common problem, in my experience, is that opposing sides are not identical in length. Assuming what you are framing is perfectly square, the opposite sides of your frame have to be exactly the same length or your corners won't fit together right.

 

In the case of this particular project I'm not planning on cutting any rabbets. I just want to those pieces to fit around the piece (it's the top of a box, actually) to hide the end grain.

But I would like to make a picture frame in the future so advice on how to do that is also extremely welcome. My mother has several pictures that don't fit into standard frames so I figured I'd make her some custom frames. I've got a router table (and, obviously, a router) if that helps.

 

I have better dimensions now. I measured it.

It's 13 and 1/4 inches by 13 and 1/8 inches. 337 millimeters by 332 millimeters.

I should add that I am a total moron when it comes to even simple mathematics and geometry. So forgive my head scratching.

 

Yes and apology accepted

: ))

 

Pat, That was a great explanation! Even as math challenged as I am; I got it!

 

I'm glad to hear that. I was afraid of information overload

 

What Charlie said about making sure opposite sides are the exact same length is critical. A little off leads to nothing fitting right.

 

I think I'm probably saying the obvious here, but does it really matter. Surely the expansion/contraction across that slab of timber will split the mitres apart fairly quickly anyway ?
Cheers
Arron