Here's an old school corner clamp used for welding 90* tubing and angle iron. This design has been around for ages. I didn't see it on the site so I thought I'd do a quick instructable and snap a few pictures. While a corner clamp can be bought, good ones can cost up to $100 or more. The cheap ones run $20, but as in all things in life- YGWYPF. The corner clamp made in this instructable is from scrap materials, is dead accurate, costs less that $20 and will last forever.
Step 1: Gather and Cut Materials
I used 3/16" angle iron and flat bar to construct the corner clamp. Any angle iron will do, but I'd suggest using the heavy duty stuff. The heavier weight steel tends to warp less and since you'll have this tool for a long time, make it out of the good stuff. Here I have two 10" pieces of angle. The flat bar is cut at a 45* angle at either end. The lengths are roughly 6" on the short end and 15" on the short end. But really all you need to do is space them out so the form stays rigid later.
Step 2: Basic Layout of the Clamp
Here's the basic layout of the clamp. The top or mouth of the clamp is open. I like to keep the mouth roughly 3/4" open. This allows the material to be brought right into the corner, but still leaves space to weld. Also, making it less than 1" will allow more versatility in how the clamp is used. If the opening were greater than 1" then 1" tubing would go through the clamp at the corner when welding a post. This will become more clear in the photos later in the instructable.
This is the topside view. Remember not to weld on this side! I repeat: Don't weld on this side! All of the welding is done from the bottom. This allows both the inside and outside of this clamp to be used without the interference of welds.
Step 3: Clamp It Up and Check for Square
After cleaning the metal with a flap wheel, assemble the corner clamp by using four C-clamps. Check the inside corner with a framing square for accuracy. There should be no wiggling movement of the square when properly adjusted to 90*.
Step 4: Start Welding Up the Clamp From the Back
Once you have the assembly where you want it, flip it over and start welding. Notice that I welded one side completely and then allowed it to cool. One side can be welded completely because it's the relationship between the two pieces of angle iron that is important. After welding the one side completely, I flipped it over and checked for square again. Once I was happy, I tacked the opposite side. After it was tacked, I flipped it over again and checked for square. It's important to note that the metal needs to cool sufficiently. Don't go full on and simply weld the whole corner clamp up. Allow the form to cool and check it with the framing square. Keep the c-clamps on the assembly until the clamp is complete.
Even though I've made these clamps before, after checking for square with one side welded and one side tacked, I found the corner clamp was off a bit. I ended up cutting the tacks and re-clamping. The second time around I got it nice and square.
Step 5: Welding Completed
After the welding was complete I checked for square using a different framing square. Sometimes framing squares get tweaked and are no longer square. Better to check with more than one if more than one is around. Notice I also trimmed the corners where a small triangle of the longer lower brace was over the edge. After the welding was completed I simply touched up with the flap wheel to get it all smooth.
(Note: See comments down below for checking the accuracy of a framing square, or search on youtube. Thanks for the tip, DavidF15!)
What to do if the clamp/jig didn't come out perfectly square or has gone out of square?
If it doesn't come out perfectly square, or after a few years and falling off the bench it has gone out of square... Fear not! First, identify the low spots. Then simply add a tack weld at any low spot. After the tack is in place, insert a straight piece of metal and check with a framing square. Use a hand file to file down the tack weld until the fixture is back at 90*.
Step 6: Using the Clamp
The corner clamp can be used on the outside and inside. This is why no welding is done on the front or face of the clamp. In the photos I show some examples of how the clamp can be used. Obviously I didn't add any c-clamps, but if I were actually welding these pieces together they would be clamped down to the frame. In one photo it can be seen how a vertical post could be added. It also becomes apparent why the corner opening in the clamp can't be too large. If it were, then smaller tubing and angle iron would just fall through when welding a post. A note of caution if using the outside of the clamp/jig. Be aware of the natural radius (curve) on the inside corner of the angle iron. Sometimes this radius can interfere with the material being clamped, so be sure to check for this before welding up your project.
Well, that's it! Super easy, right? You now have an ultra sturdy, accurate clamp for all of your welding projects. I hope you've enjoyed this instructable. Thanks for viewing!