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 Post subject: Auto fillet info (needs formula help though)Posted: Tue Aug 18, 2009 12:46 pm

Joined: Tue Jan 01, 2008 7:30 pm
Location: Texas, USA
Hello everyone. As most people who use cad know a 1mm fillet on a 90 degree angle looks a lot shallower than a 1mm fillet on, say, a 60 degree angle. I just spent an hour or two compiling data and I can't find any formula to solve it. What I did was make a cube of 1mm^3, auto fillet one edge by .5mm and take the perimiter on one of the two faces that have the curve on it, documented the angle, fillet size, perimeter of one of the faces that has a curved corner and a few more things. Then I altered my sketch to make it a rhombic prism (so degrees could be 80, 100, 80, 100) and repeated the cycle. My info is below.

Angle auto filleted by .5mm
perimeter (P) of a face with a curved corner
Fillet that makes it the same P as 90 degrees
Difference between the the fillet sizes

90
3.785
.500
0

85
3.738
.410
-.09

80
3.681
.337
-.163

60
3.315
.157
-.343

50
3.007
.1065
-.3945

I didn't do any above 90 degrees because I figured they would just be the inverse. I am not sure how accurate this is, their might be 1 or 2 numbers that are one or two off becuase Alibre Xpress rounds to the nearest thousandth but any help possible would be a great contribution.
-Tanner

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 Post subject: Re: Auto fillet info (needs formula help though)Posted: Tue Aug 18, 2009 1:43 pm

Joined: Fri Feb 08, 2008 1:47 am
Location: near Utrecht, Netherlands
This is pretty basic math. You're just looking at part of a circle.

For perimeter P, radius r and angle a:
P=pi*r*(180-a)/180

This can be rewritten to a formula that tells us the fillet radius Rn we should use on an edge with angle a to get the same perimeter on an edge with angle 90 filleted by radius R0:

Rn=90*R0/(180-a)

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 Post subject: Re: Auto fillet info (needs formula help though)Posted: Tue Aug 18, 2009 3:07 pm

Joined: Tue Jan 01, 2008 7:30 pm
Location: Texas, USA
D'oh! Thanks for the equation though I feel stupid now. Well, that's summer

_________________
"I discovered the triangle one day while shaving. I trimmed my beard like the intersection of three circles and noticed how I could unfog a square in the bathroom mirror by rubbing my beard circularly against the glass."-Franz Reuleaux

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