Corner clamps for a frame, What is this for?
 

I was making a frame to put around a mirror in my daughters room last night.
using 1"x 3" pine around a 60" x 18" mirror.

When it came time to glue up the frame, I could not find the strap clamp I
usually use.
Digging through the tool box I came across some corner clamps I inherited
but have never used.

One of the clamps has a guide for a saw blade that would run right down the
45 degree cut where the corners meet.
The other 3 clamps do not have this feature.

I'm guessing by clamping the two pieces of wood together and running a saw
through this guide it would make a perfectly fitting joint?????

I had used my power miter saw to cut the 45's on the frame and using the
clamps, they did not align very well, not a problem in this project as the
frame will be painted so I just filled the imperfections in the joint.

I'm going to examine this closer when I get home tonight.

1. I used the 45 degree stops on the miter saw and as I haven't fine tuned
them I doubt they were accurate right out of the box.

2. I'm also not sure that the clamps are exactly 90 degrees???

3. What's the better method of cutting 45's?
A - adjust the stops on my miter saw so they are exactly 45 degrees.
B - Cut all the 45's and then put them in the clamp with the saw blade
guide and run a saw blade through which would even off the two faces so they
came together. I guess I would have to repeat this for all 4 corners...

I plan on making a few picture frames and would like to get a nice miter on
the corners.


Thanks

Sr