Indestructible Corner Clamp (Jig) for Welding Projects




Introduction: Indestructible Corner Clamp (Jig) for Welding Projects

About: Hello, my name is Kevin. I like to tinker in the workshop transforming the ideas I've engineered in my head into reality. My professional background is in automotive mechanical restoration and I run a tiny...

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!

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43 Discussions

when sourcing the angle iron, make sure the internal corner is nice and sharp, some has rounded internal corners.

great idea... I can't think why I never made one before!

1 reply

I need to build one of these. My last project used the magnetic corner type of tool but you really can't be very accurate. I managed but could have been better. Using a clamp and the jig almost guarantees you will not mess it up!

1 reply

They are very handy to have around, for sure! I use them for woodworking projects too. Of course angles always should be checked and rechecked, but generally speaking the jig takes probably a good 98-99% out of fussing with loose clamps and a table. Also, a machinist could clean up any variation with a mill, but for my needs (and I suspect most here) a 98-99% accuracy works just fine. I hope you find them useful in your shop! Thanks for the comment!

Thanks for the simple and clear instructable. It's very timely for me because I'm just getting into welding and need some useful projects for practice.

1 reply

As always, simple ideas are the best. Thanks for posting this.

1 reply

Thanks for reading it! Glad you found it useful!

If you suspect your framing square isn't right, they are easy to check. If you have a straightedge and a pencil--you draw a "square" line, flip the square over, and check the if line agrees with the square. You can correct them with a center punch--if the angle is less than 90, sketch a line along the crotch mitre you make a few indentations on the inside half of the line to spread the inside, or if too open, make indentations on the outer half of the mitre line until it closes up.

3 replies

Thanks for the tip! I just youtubed the topic and found exactly the process you describe. Appreciate the comment!

Thanks to both of you for the Instructable and for the tip for correcting a framing square.

Here's a link to save someone the hassle of searching:

Glad you enjoyed the instructable! Thanks for posting up the link!

Really great idea! Thanks for sharing it mate.

1 reply

Thanks for viewing! Glad you found it worthwhile!

I knew there were a few missing jigs in the shop (storage unit) that im working out of rt now. It's perfect timing for me to see this, since i recently started doing a little fabrication and arc/flux core wire welding, in the past couple few months. Looks easy enough to create pretty quickly. Thanks for posting! Aloha!

1 reply

Thanks for viewing! I honestly couldn't believe this wasn't already in the massive Instructable library! At least, I couldn't find it. I'm sure this was published in PM probably 50 years ago. This is a super easy project to complete, my main suggestion is simply to take plenty of time and allow the fixture to cool between steps. Doing so will ensure a perfectly accurate clamp/jig. Glad you enjoyed the instructable!


Very good idea; but as allways, there is some room for improvment: If you weld the outside of the angle-iron to the flat stockinstead of the inside, you have no hassle with the inside radius of the angles. There would be a sharp corner between the angle-iron and the flat stock if you let it stick out ~ 2" over the angle-iron.

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Hi Albert. Yes, you are correct in that one must be aware of the radius of the inside of angle iron. For anyone unaware, angle iron doesn't have a perfectly square inside corner, rather there is a slight radius, or curve. There is a good reason however, why I don't have the angle iron flipped over the other way. If the angle iron on the clamp/jig were on the outside (square part to the outside) this would limit the ability of the clamp to be placed inside the framework of whatever is being constructed. Whereas the clamp as designed in the instructable here, the outside corner of the project can be placed on inside of the jig, so it doesn't matter how large or small the project is. To think of this another way, if a very small square was being formed (let's say a 4" x 4" square) in the clamp in this instructable using the outside of the jig would only work for one corner. As soon as the second corner were attempted, the 10" length of the angle iron in this case would interfere with the construction of the 4" x 4" square. But since the inside of the jig can be used without interference of the natural radius of the angle iron, this is no problem because the 10" angle just runs 6" outside of the 4" x 4" square. That is why I oriented the angle iron in the manner in which is photographed above. The bottom line is, simply be aware that there is a radius on the inside of angle iron when clamping. Thanks for bringing this to light. Unless of course, I've missed interpreted what you're saying, Albert. In that case, please elaborate/clarify.

My Dad taught me to weld when I was a kid but I just got my own setup about a year ago I wish I had or could even remember all the clamps, fixtures and benders he had in his shop. I'm now keeping an eye on welding tutorials for handy tips and techniques. Thanks for this Kevin, I can absolutely see the value in this fixture!