I personally had a hard time learning how to cut metal miter joints. When I looked for an instructable it was hard to find one to help me. Once I learned how to do the cuts and was able to do it with ease: I made this instructable. It is to help others who had trouble understanding how metal miter joints work like I did (and you get a cool frame out of it, too). I hope it helps :)
1.) Hillman 6-ft x 1-in Hot-Rolled Weldable Steel Solid Angle.
2.)Hillman 11-in x 1-in x 1-in Hot-Rolled Weldable Steel Plain Square Tube
1.) Evolution 15-Amp 14-in Chop Saw
2.) DEWALT 4.5-in 20-Volt Max Cordless Angle Grinder
3.) Kobalt 25-ft Auto Lock Tape Measure
4.) Millermatic 250 MIG welder
5.) "Z 36 grit" and "metal grinding" Grinding wheels
6.) IRWIN 2-in Cast Iron Clamp-On Vise
7.) Lincoln Electric Welding Magnet X4
Step 1: Know Your Angles
We all know simple math (like 2+2=4) miter cuts are just another simple math equation 45+45= 90. On the blueprint, you will see slashes in all four corners: these slashes are our cuts. Each cut will be made at a 45° angle and when put together it makes a 90° angle.
Step 2: Setting Up Your Chop Saw
Make sure your chop saw is on a stable area that is long enough to hold the metal straight (no wobbly/short tables). I used the floor because it is long enough and stable. Next, on your chop saw you have a Miter (seen in picture above). The miter is in charge of making sure your metal stays at a 45°. Adjust your miter to the 45.
Step 3: CUT!
Slide your metal in between the miter and the clamp. With it "upside down" so the angle fit in the angle of the miter to the saw's base (in the picture above). Well doing this you probably noticed it wasn't sturdy when you clamp it... This is when you add the Plain Square Tube. The square tube will fit inside the solid angle steel. (in the picture above). Next, using a lifter or scrap wood, put it under the end of your metal (opposite end of where the saw is) so the metal is perfectly straight all the way across. To start off, cut just the edge. Thus, making your first angle. (REMEBER CLAMP YOUR METAL AS TIGHT AS YOU CAN SO IT DOES NOT WOBBLE OR FLY OUT)
Take your square tube and put it against the miter, putting the angle steel "around it" (refer to picture above). Using the Measuring tape measure the first length. I started with the 11" because it is the top of the frame (blueprint above). Cut where you had marked the 11" at. (HINT: when cutting, go down slowly. If you go to fast the metal will not cut straight. and u will have unequal angles.)
Going back to how you first set up the metal. put your angle steel against the meter, square tube on the top, clamp and cut just a little piece off (don't forget to make sure the metal is leveled the whole time). Flip, measure 14", clamp, cut. Do this for all your sides. (You will have a little bit left over).
Step 4: Grinding (take One)
Looking at your newly cut metals it probably has pieces sticking out or looks un-smooth (example in the picture above). To fix this, you have to use the Grinder to "sand" the edges smooth. If it is not smooth they will not align will result in making a gap (example in the picture above). This gap will make it harder to weld and make the frame look un-equal. Using the Vise, put your new cut metal in horizontally and tighten the clamp. Using the Grinder and the Z 36 Grit Grinding wheel, grind the uneven pieces flat and also grind the top to make it very smooth. Your weld will be in that area so you need to make it smooth so it connects better. Don't use just the corner of the grinder to grind it down (even though it is easiest). Instead, keep the grinder grit area flat against the whole workplace to make it easier to stay flat.
Step 5: Welding Safety Attire
Safety is critical for everything, especially welding. Dealing with high power electric currents can be very dangerous. For beginner welders, I advise getting "Welding For Dummies"( https://www.amazon.com/Welding-Dummies-Steven-Robe...) It is very informational and simple to understand.
Darkening welding helmet https://www.amazon.com/Antra-AH7-X90-0000-Controll...
Welding Jacket https://www.amazon.com/Miller-Electric-2241909-Wel...
Closed-toe shoes https://www.amazon.com/Timberland-PRO-Direct-Six-I...
Welding Gloves https://www.amazon.com/dp/B00547HD0O?aaxitk=pF4hlE...
Long Sleeve All Cotton Shirt https://www.amazon.com/Black-Stallion-FR-Cotton-T-...
Safety glasses https://www.amazon.com/NoCry-Resistant-Wrap-Around...
Step 6: Preparing for Welding
Preparing is as important as doing. My teacher told me "Work on aligning it for 30 minutes or more if you have to. If you can make it line up perfect now, it will look better and be easier later." So if it takes you 30 minutes or more just to prepare it, it is okay. Just stay patient :)
On your welding work area, put your four pieces in the proper order. In the four corners place a Welding Magnet. This is where the "30 mins of adjusting" comes in. The magnets are going to hold the corners together well you weld. MAKE SURE THE METAL ALIGN WITH EACH OTHER. No one likes crooked picture frames, so you might want to get a level ruler or level square. (if you can clamp it to your workspace, if not just work carefully not to bump it.)
Once your frame is in place, time to get the welder ready. (If you already know how to weld, then don't read this part lol). Starting with the gas tank, set your gas between 20 and 30 (make sure to turn your gas on, first time I weld I forgot to and it's very hard, very ugly, and very unstable if you don't). Set your wire speed to 40 and your voltage to between16 and 19. (Picture above for visual help).
Lastly, connect your clamp to your workspace so when you begin welding the connection will be complete.
Step 7: Welding
Now that your work is all ready to go and connected to your workspace: it's time to weld. To start off, put three single welds on each corner (where the red magnets are). This will hold your work together so you can take the magnets and clamps off. Once you have placed the three dots down on every side; take the clamps and magnets off. Now you can put the bead down over the three dots on every corner. Remember to go slow and don't rush your beads.
Step 8: Grinding Pt.2
TIME TO CLEAN UP! Now is the time to clean your welds and smooth it out. Put your newly-welded frame in the Vise, make sure it is secure and fasten the clamp tight. Using your "Metal grinding" grinding wheel and your grinder, grind the welds flat so it is smooth and even with the rest of your metal. If you want to make your frame non-rustique/antique looking you can grind the whole frame. I advise if you do that, grind in circular motions. This will give your frame a more clean look instead of just lines and streaks.
ONCE YOU FINISHED GRINDING, YOU HAVE A NEW, GORGEOUS FRAME AND KNOWLEDGE ON HOW TO DO MITER CUTS!!!!!!