Cutting compound angles, tearing my hair out!

[samuel99]

Hello! I'm in the process of renovating the stair railing at home. I've found myself in a situation I don't understand. I'm supposed to "simply" miter two boards together, sized 34*95. The angle between the two boards in the horizontal plane is 122 degrees, and the stairs are inclined at 30 degrees.

One might think you could just miter in both planes at half of the angles, that is, tilt the blade (180-122)/2=29 degrees and miter 30/2=15 degrees but no, it did NOT work.

So I was googling and found https://jansson.us/jcompound.html but I don't understand how to input my values?



In my world, this is what I'm supposed to do:


So the evening ended with me sitting on the stairs for three hours, googling, and attempting to saw 10 times, only to become more and more frustrated. Any help is VERY appreciated!

 

[Mellanbarn]

Impressive miter saw if you can cut 95mm standing with the blade tilted. Have you thought that through correctly? (or maybe I'm misunderstanding) 🤔
When I think about it in my head, you shouldn't split the 30 degrees in 2. But I've never done what you're trying to do, so it could be completely off.

Edit:
So with the board lying flat, 30 degrees miter and tilt the blade 29

 

[samuel99]

Impressive miter saw if you can cut 95mm standing with a tilted blade. Have you got that right? (or maybe I misunderstand) 🤔
When I reason in my head, you shouldn't split the 30 degrees in 2. But I've never done what you want to do, so it could be completely wrong.

Edit:
So with the board laying flat, 30 degrees miter and tilt the blade 29

Yes, it's a bit confusing, I barely know anymore myself. But the boards are cut laying flat. So the miter is to cut the slope of the stairs, and the blade tilt is to cut the angle in the horizontal plane.

 

[samuel99]

Edit:
Thus with a lying board, 30 degrees gives and tilt the blade 29

I will try this when I get home. But I have a feeling that it's not that simple as the angles become different when cutting diagonally in two directions simultaneously.

The simplest would of course have been to make a "butt joint", but it doesn't feel as fun.

 

[Mellanbarn]

My reasoning is based on the fact that the cross-section should become vertical when the board is mounted at a 30-degree angle.

 

[samuel99]

My reasoning is based on the idea that the cross-section should be vertical when the board is mounted at a 30-degree angle.

I tried your suggestion but it turned out wrong.

But now it's solved anyway, I did it like this

 

[Mellanbarn]

Glad you found a solution.👍 I missed that one of the boards should be horizontal, I thought the staircase continued after the angle. 🤥