# Cone geometry question

• posted
Hi
Im trying to create a cone with the top cut off, im sure there
is a proper name object, I need to draw it in Autocad then cut it
out from paper and assemble it as a 3D model.
I know the diameter top and bottom and the slant length but I cant
figure out how to calculate the radius of the 2 arcs which will wrap
around the 2 circles.
I have uploaded a picture of what I mean.

Bob
• posted
I think it's called a frustrun. Try that in google.
• posted
i mean frustrum. sorry
• posted
or
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• posted
HINT: The radius of the arc for the upper end of the frustum (the term for a truncated cone) will be the distance from the apex (point) of the cone (if there were one) to a point on the circumference. The length of the arc will be the circumference at the top of the frustum. ___
• posted
Hi Paul Thanks for that, it makes perfect sense now, the radius is from the apex, I tried it, works perfect, thanks to Ben and strawberry too.
Bob
• posted
to calc the radius you need to know the angle and the arc length. that is easy create a pline and do a list on each arc. the formula for the radius is then
R= arclength/angle of arc
LouR
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• posted
sorry i mis lead you: the formula for R = .01745 x angle of arc / arc length.
LouR
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• posted
It's much simpler than that. As I posted earlier, the arc radius is simply the distance from the apex of the cone to any point on the curcumference of the top of the frustum. The length of the arc is the circumference at the top of the frustum. ___

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