I put this together in my build thread and figured i'd share it here too.
I know this is very elementary for some but hopefully it helps some that are getting into it.
I'll break it up into 2 sections- bending and notching
Bending
I'll use a simple "U" shape as an example. Here's what we're going for:
There's 2 ways to do this. One way is working off of chord length to figure out where bends start/stop which requires math and precision. And there's the way I do it which is working from the centerline out, figuring out where your bends start and eating up tube away from the centerline.
First you need to establish where the bend starts in your bender. Take a scrap piece, make a mark on it, and line that mark up with the die or another fixed point on the bender. Make a bend, 90 deg perferrably since it'll be useful throughout your project. Take the bend out of the bender and feel along it to where the tube deforms at the start of the bend. Note the difference between your mark and the real start of bend. This way you know exactly where the start of bend is.
Here's some dimensions on our sample piece. My method involves a little more waste vs the "math" method, but my method also gives you a get out of jail free card as you'll see.
First step is to rough cut a piece of tubing. In this instance I'd use a piece at minimum 61+34+34= 129". I'd probably cut it at 132 or so until you get the process dialed in. You'll be able to sneak up on waste as you get more confident.
Now you're going to make some marks on the tube. First mark is the centerline, 1/2 the length of whatever your rough cut tube ended up being. From there, knowing the centerline radius of your die (6.5" in this example), mark the start of your bends measuring off your centerline (24"). Slide it in the bender (centerline towards you) and make your first bend.
Now here's where your get out of jail free card comes in, and you'll thank yourself for allowing more scrap. Your target from centerline of part to centerline of leg is 30.5"
If you were dialed in on your start of bend measurement and you hit that 30.5" target then move on to the next side. If you're like me and you goofed up, measured to the wrong side of the tube or something you ended up with the wrong length. Lets say you landed at 29" instead of 30.5. Don't worry you're still good. First, figure out what you did wrong (mis measured, start of bend not where you expected etc). Then simply adjust your offset on the second bend. So you need to add 1.5" to your start of bend on the second leg to get back on target. Erase your 24" "start of bend" mark, and re-mark at 25.5" Now make your second bend and you should be right on the money. Again, be sure you are feeding from the centerline out in your bender. You want the die to consume material you will eventually cut to size.
Now its time to cut the legs to length and get to notching. Where you cut the tube will depend on your confidence level in notching and the complexity of the notch. This example is a simple 90 deg notch which is the easiest. Referring to the above picture, a safe start is to cut the tube where the centerline of the intersecting tube will be. If you nail the notch first shot it will end up long, but its much easier to chew the notch away that to scrap the whole thing and re-do. So cut it at 31" from CL of the horizontal tube.
Notching
There's a few ways to do this, holesaw notcher or by hand as I do it. You can hand notch with a cutoff wheel, chop saw, or bandsaw. Lay your tube down so that you are looking perpendicular to the intersecting tube. Make 2 straight cuts as shown by the dotted lines in this picture. You'll cut off the highlighted triangle on both sides. Note the offset from centerline, and despite what logic tells you, don't cut 45's. I don't take any measurements, just say to myself "knock 30% off on either side at a little less than a 45". Just knock the corners off, don't get too crazy. Youll dial it in later
And you'll end up with something that looks like this
Lay your intersecting tube in it and you'll be pretty suprised how close it already is with only 2 straight cuts.
It may be good enough to weld as is. But you may need to dial it in a bit. Just hit the tips with a flap disk here
After doing that it should fit real good. But you've reduced the wall thickness of the tube at your weld at the points of the notch. So come back with the flap disc and grind down here til you are back to full wall thickness
Check the fit/measurements. You're likely to be a little long still. If you're less than 1/4" long then I usually just eat away at the profile with the flap disk. If you're 1/2" or more long I knock the corners off again and repeat the process.
Granted this is a simple example, when you get into notches that are other than 90 degrees, and especially compound notches you'll want to leave a bit of extra to your rough cut in order to dial your notch in. If you practice enough you'll at least be able to keep pace with a holesaw notcher on 90 deg notches
