Coping Steel Corners
Flat 90-degree corners without the miters!
Text, photos and video by Tom Hintz
Posted – 2-9-2011
Making square and flat 90-degree corners with steel angle stock can be frustrating which almost guarantees sloppy results. Mitering the corner is one option if you have an abrasive wheel (or similar) saw and when done right results in perfectly square and flat corners. The problem is getting all four of miters just right without spending an afternoon grinding small corrections into the pieces to get the joint right. Of course if you spend much time fitting the joints your chances of grinding away too much material increase dramatically and your target dimensions will be compromised.
The good news is that coping is another corner-making process that you can do with common tools, including a simple jigsaw (also a band saw or hacksaw) and an angle grinder. I prefer a good jigsaw equipped with a metal-cutting blade because it is faster and with a little care can produce better results much faster than I can get with a hand-operated hacksaw.
As easy as the coping process is taking the time to make a few “learning” joints with scrap material before jumping into production of a real project. This is also a good time to become familiar with how to measure and cut you0r project pieces to the right length to end up with the finished dimensions desired.
Coping a joint simply means removing overlapped material to create a flat secure joint that has both pieces on a single plane. In the case of angle material coping can be an easier procedure than cutting perfect miters. In many projects having at flat bottom is necessary to support something that is going to use that flange as a base. The only tricks to coping are to remove only the overlapping material and to remember to allow for the thickness of the steel itself. Forgetting this in one corner is bad enough but make the same mistake in all four corners and a frame that was supposed to be square won’t be.
Mark and Cut
One thing to keep in mind when cutting the raw pieces to be coped is that we usually have to allow for at least one thickness of the metal at each corner. When we cut the notch in the bottom surface that brings the surfaces 90-degrees to it into contact with each other. The hard way to deal with this is to grind or cut a sliver off of the upright side we are cutting the notch in. The easier way is to cut one set of opposing sides short by twice the thickness of the metal. Then we can cope the ends of the longer pieces to accept the shorter sides and still come out with the dimensions we want. This is way easier to show in the video later on this page than in words here.
To cut the notches we just lay the corner pieces together as they will be in the project. Trace a cut line on the piece to be notched, keeping that line as close as possible to the piece that will fit into it. Then remember that because you scribed the line around the piece that will fit into it that line should remain after you cut the notch. To get a proper fit we want to remove the material inside of that line, not on or to the other side of the line. This is where precision and patience pays off. Take your time and get the fit these notches right and welding the corners up later is easy. Once you cope a few corners it gets to be much easier.
After fitting all of the pieces and laying them out to check their fit as an assembly we can move on to welding. Taking the time to do a test fit and tuning up any corners that need it will save lots of time later as well as making it easier to get a finished piece that fits the dimensions that you need.
For most square-cornered projects like the frame built in the accompanying photos and video I use my Bessey Angle Clamps to hold the pieces while I tack them all in place. I also have used my Bessey Magnetic Squares to hold the pieces for tacking. Then I can take the assembly out of the clamps for finish welding or I might do most or all of the welding with the clamps still in place. That decision is based on what is easiest for that project and generates the best results.
Coping a 90-degree joint is very easy once you do it a time or two. While it is used most often on 90-degree corners as in this story you can cope joints of virtually any angle or shape. This is a great way to join material with different shapes to one another and do it seamlessly. The procedure remains the same regardless of the angle or shape.
I think that in many cases coped corners are easier to weld than traditional miters. I know that there will be exceptions to that but for the most part I think beginner metalworkers (and many veterans if my email is at all representative) will find coping one of the easier, more durable corner-making techniques.
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