How do I calculate a compound Miter?

[Iplayfarmer]

I drew this up in AutoDesk Inventor...

Perfect!

I wish I could afford the price of a good AutoCad type program like that and the time to learn how to use it well.

 

[Spiveyman]

Pooh_Bear - sweet drawing program. I was trying inadequately to do it in MS Word! Although it was only 1:00 AM.

The 30° cut is the same as I was looking at, however I was trying to keep the legs square to the ground to help with cutting the "feet" end of the legs. As you rotate those legs both angles will change. The position of the leg relative to the box and the ground can stay the same but spinning it will change both angles, won't it? (By spinning I mean like a log would spin with a logger running on it in the water - best way I can think of to make sure you understand what dimension I'm referring to.) If the legs were round it would probably be easier to deal with as he will need to lay the thing flat on an edge to mark and cut the angles.

I'm not familiar with an angle grinder, does that work the same way a mitre saw works but for metals?

 

[Iplayfarmer]

I'm not familiar with an angle grinder, does that work the same way a mitre saw works but for metals?

No. I'm still in the rudimentary stages of acquiring metal working tools.

This is an Angle Grinder

It's what I use to cut out metals. I've done some cutting with a recipricating saw and with cutting rods in my welder, but it seems with either of those I spend as much time grinding with my angle grider as I ever spent cutting.

I just put a thin "skinny wheel" in it and cut away. It's pretty precise and gives me a finish edge that usually doesn't require any more finesse.

 

[GT2]

If I'm understanding correctly; Why dont you just cut them 60d and then v notch them so they will fit the corners of the center tube?

If you had done it my way you would be done by now!


Just joking, I relise your just trying to understand the process.

 

[Spiveyman]

This has been bugging me all day. If you use a round leg, there's no "compounding" of the angle. In looking through the posts, GuglioLS already said that. Also, you said you kind of mocked one up and it turned out to be 35°. If you used the same angles as in Pooh_Bear's drawing, (30° & 40°), but rotated the leg a bit you'd get that 35° angle. That's the thing with the square leg, there's an infinite number of top/side angles that would position the leg where you wanted it, but each one would be rotated differently along the length of the leg. You have to figure out what orientation you want the square leg. That's where Pooh's drawing program comes in handy. You can rotate that sucker and pick what you like.

 

[Iplayfarmer]

This has been bugging me all day. If you use a round leg, there's no "compounding" of the angle. In looking through the posts, GuglioLS already said that. Also, you said you kind of mocked one up and it turned out to be 35ｰ. If you used the same angles as in Pooh_Bear's drawing, (30ｰ & 40ｰ), but rotated the leg a bit you'd get that 35ｰ angle. That's the thing with the square leg, there's an infinite number of top/side angles that would position the leg where you wanted it, but each one would be rotated differently along the length of the leg. You have to figure out what orientation you want the square leg. That's where Pooh's drawing program comes in handy. You can rotate that sucker and pick what you like.

You're right that there's an infinite number of angles. In my perfect little world the sides of the leg would be plumb up and down. That should limit the number of angles to 1. I'm wondering if there's some ratio between the angles that has to be followed.

For now, I'm probably going to take Pooh Bear's angles just to get the thing made, but I've got to spend some more time looking into california's framing and roofing tables. What I'm trying to do is exactly what you'd have to do to make the ridgeline beam of a roof at the end of a house where the 30 degree roof meets at the end of the house with another 30 degree slope running perpendicular to the first.

I've got an old framing square at home. I'll have to scrape the paint, spackle, and concrete off of it and see if it tells me how to do this.

 

[California]

... That's the thing with the square leg, there's an infinite number of top/side angles that would position the leg where you wanted it, but each one would be rotated differently along the length of the leg. You have to figure out what orientation you want the square leg. That's where Pooh's drawing program comes in handy.

Still thinking like a carpenter - the rotation of the leg (around its axis) is important only if the upper corners of those legs have to meet a sheet metal cone that is slipped down over all three legs.

You need Pooh's drawing program to tell you the angles for that.

But as a git-er-done solution I would eyeball an approximate non compound angle on a scrap of wood, try it in place and re-cut if needed, then cut the steel legs. By non-compound I mean the longest surface terminates with a straight-across cut. You might draw a circle on the floor and mark 3 target points on the perimeter, then cut to hit those points. A guy I used to work alongside had an expression for that kind of solution: 'Good enough for the girls I go with!'

Edit - I see you just posted. That corner rafter is called a hip rafter.

It just occurred to me that a new quality framing square comes with a pamphlet explaining the tables.

Edit some more: I Googled on 'tables on framing square hip rafter'. The first several hits were all useful. I think the excerpt from the Wagner book might be the best.

 

[Spiveyman]

In my perfect little world the sides of the leg would be plumb up and down.

That's what I figured, but I'll have to say California's "git-er-done" mentality sounds very appealing right now. I just want to see a picture of that thing completed so my brain can have some rest. Between that and discussing Bernouille's principles related to dam building my noggin is plum tuckered!

 

[Iplayfarmer]

I'm just glad to know that I wasn't the only one laying awake in the middle of the night wondering how to do this.

 

[hazmat]

Nothing new to add about solving the geometry, but if you want to model in 3D try google sketchup the price is right (free) & it is pretty intuitive.