Now the problem of contacting a toroidal cutter with the facet of a triangle.

We need to find the equation of the plane which contains the triangle. If the triangle is defined by three points **p1** = (x1, y1, z1), **p2** = (x2, y2, z2), **p3 **= (x3, y3, z3) then a surface normal will be perpendicular to both **p1**-**p2** and **p1**-**p3:**

N = (NX, NY, NZ)** =** (**p1**-**p3**)x(**p1**-**p2**)

and it can be normalized to unit length by setting

n = (nx, ny, nz)** = N** / sqrt(NX^2 + NY^2 + NZ^2)

I've drawn a surface normal in red in the picture above.

Now the plane containing the triangle is given by

a x + b y + c z + d = 0

where (a, b, c) = (nx, ny, nz) and d can be found by noting that any of the points p1, p2, or p3 must satisfy the equation:

d = - nx*x1 -ny*y1 -nz*z1

the plane is going to make an angle theta with any vertical line, where theta = asin(c)

Then we need a new figure:

We start out at **ei** = (xe, ye, zi) , the point on the plane that intersects the line along which we're dropping the cutter (green dot above). It needs to satisfy the equation for the plane, so

zi** ** = - (d + a xe + b ye) / c

*Note: *c=0 is a special case which needs to be handled separately.

from here we need to climb up to the correct ze, and that's done by first summing the green distance, then the red one, and then dropping down by r:

ze = - (d + a xe + b ye) / c + (R-r)/tan(theta) + r/sin(theta) - r

Now we have the cutter in contact with the plane, but we still need to check that the cutter contact point (CC-point, the blue dot above) is within the triangle facet and not some other point on the plane. To do that we need (x,y,z): If we start at the red point **e =** (xe, ye, ze) we get to he CC point by travelling upwards to the yellow point, and then along the normal down to the CC point:

**CC** = **e + **( (R-r)*tan(theta)+r )**k** - ( (R-r)/cos(theta) +r )**n**

where **k**=(0, 0, 1) is a unit vector along the positive z-axis.

All of this seems to work as evidenced by the top picture where a toroidal cutter is brought into contact with a facet. The CC point is indicated with a green dot, and the CL (cutter location, or (xe, ye, ze)) point with a red dot.

Now we need to check if the CC point lies within the triangle, but that will have to wait until the next post... (Mr. Todd has some thoughts on this)