Seatstay mitre angle

[ltwiens38]

Hi everyone,

This one has been stumping me for a while now.

I use a fixture that holds my seatstays at the correct angle from each other and then setup my mill to cut the seat tube mitre into them. I measure the SS/ST angle off a full scale drawing then adjust my mill accordingly. For example, if I measure a 46.4 degree angle, I'll set the mill angle to 36 degrees and adjust my fixture by the remaining 10.4 to get to 46.4 degrees. I double check my angle on the drawing, then my mill and then my fixture. It's all good. When I take the SS's back to fit onto the frame, they're off. BY nearly 10 DEGREES! I can't figure out why they're off.

Does anyone know what is going on? I've tried keeping the fixture level and setting the mill to the full 46.4 degrees and that doesn't make a difference. I have some experience framing homes and I know that angles in hips and valleys get screwy when sheeting roofs - is that what's going on here?

Now I know why I should have paid more attention in math class in HS.

Thanks,

Lyle Wiens

[crumpton]

At 46.4* I'm guessing you got this from the acute side(SS to ST)? but you are attacking the cut from the obtuse?

If this is the case set your cut to 43.6* (90-46.4=43.6)

[ltwiens38]

Thanks Nick! I think you're on to something.

Lyle Wiens

[crumpton]

Hey, I'm only following. It was Euclid that was on to something...