Instructables
Picture of Easier Precision Miters on a Radial Arm Saw
A nicely fitting miter joint is very rewarding.  Radial arm saws allow the user to swing the saw arm 45 degrees left or right to cut miters.  It seems like a good system until you begin to use it for something precise, like a picture frame.  Swinging the arm to the right allows a good view of the material and works pretty well.

Step 1: The problems appear

Picture of The problems appear
When you need to swing the arm to the left there is no table  remaining for material support to the left of the blade.  The operator cannot see any marking lines on the material and he may need to reach under the motor to secure the material against the fence.

Step 2: Make a miter guide

Picture of Make a miter guide
The miter guide shown here is not original with me, but the basic idea came from a how-to magazine several decades ago.  This one is a little small, but adequate for smaller picture frames, etc.   I used a rectangle of 3/8 inch plywood.   Two cleats of 3/8 inch plywood are glued to the rectangle so they are exactly square to one another, but 45 degrees off of the direction of the sawblade's direction of  travel. 

Step 3: To use it

Picture of To use it
Make the back edge of the miter guide even with the back edge of the saw table and clamp it to the table.  Raise the sawblade off of the table a little so you do not saw your guide in two. 

Step 4: Another way to use it

Picture of Another way to use it
Move the guide to the side for even more clearance between your hand and the motor.  Here the guide is resting against the fence.  For proper use, the fence should not be higher than the thickness of the material used to make the guide.

Step 5: A bigger, more substantial guide

Picture of A bigger, more substantial guide
This guide is positioned and used just like the view in the last step, except that the fence will need to be removed to make space for it.  It is made from 3/4 inch plywood or chipboard.  The rectangular base for this guide is 13 1/2 inches by 30 inches.  13 1/2 inches is the distance between my saw's blade and the left edge of the table.  The fences on the guide are about 3 inches high.  The black arrow indicates the direction of the blade travel.  The red arrows indicate approximately where to clamp the guide to the saw table. 

Because of the way this guide works, the left member of a miter joint is cut from the right side of the guide, and vice versa. 

If you wish, one of the fences could be made slightly adjustable, just in case your miters are not quite perfect the first time you use the guide. 

(I would have shown a photo of the guide I made and have used, but my guide has disappeared and I cannot find it, so I made this illustration with Google Sketch-Up.)

Step 6: Own a table saw?

Picture of Own a table saw?
This is the same guide, but adapted for a table saw.  Notice the graphic shows a bar of steel for the table saw's miter gauge slot screwed to the underside of the guide.  If loss of cutting thickness is an issue, the rectangular base could be made of metal an eighth of an inch thick.  The bar of steel for the miter gauge slot is usually 3/8 x 3/4 inch.  Place it so the lower edge of the guide (as shown in the graphic) is very close to the blade.  Be careful to make the bar of steel parallel to the edge of the guide.
 
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Ak-492 years ago
Just a heads up... Not sure if your radial arm qualifies, check:
http://www.cpsc.gov/cpscpub/prerel/prhtml01/01031.html

The safety of a Free GUARD is a great thing!!!

Ak-49
Phil B (author)  Ak-492 years ago
Thanks. Someone else mentioned that. I checked the site with my serial number and my saw is too old to qualify. "They" want me to cut the electrical cord and send the saw motor to them, which destroys a perfectly good saw. I have used that saw 40 years and never even scratched myself, other than while changing the blade. The video that illustrates the alleged danger shows someone with his hand firmly planted in the path of the blade. No one with any sense would ever do that. That is one of the first things covered in the operator's manual.
rimar20004 years ago
Yes, Phil, miter cutting is frustrating. One is sure that the cut is correct, until one try to connect all parts...

Your method is perfect, the parts MUST connect at 90 degrees, even if the cutting angle isn't exactly 45 º.
Phil B (author)  rimar20004 years ago
I like to say the first three corners will always fit.  It is the fourth corner that is always the problem.  But, that is just a piece of humor and not really accurate.  The beauty of a miter guide like I have shown is that, as you said, so long as the guide provides a 90 degree corner, the cuts can be 44 and 46 degrees, but they still make a nice tight 90 degree corner.  Thank you for your comment.
HAHAHA, it's true!  I always went out winning 3 to 1, then!

zieak4 years ago
Thank you!  I get frustrated with mitre cuts.  I just finished trim on 12 windows... The only drawback to this is when you're trying to cut a long or heavy piece it is more difficult to support.  But I'm sure i can rig up something!
Phil B (author)  zieak4 years ago
If I were mitering long, heavy pieces I would move my saw out to the middle of my garage floor so I had plenty of room.  Then I would get my hands on some supports so the pieces I am cutting are very well supported at the same level as the surface of the miter guide.  Roller systems, commercial or home-built, have been very popular in years past.  They look a lot like roller systems you see in package shipping depots.  I have used sawhorses and clamped a 1 x 4 vertically to each of the sawhorses so the pieces I am cutting can rest on the tops of the 1 x 4's.  Thank you for your comment.