I'm attempting to build a program in PV which will help me plan and design
some woodturning projects, appropriately using the LATHE construction in PV
(as well as my wood lathe). This particular technique is called "inside-out
turning" where four identically square blanks or sticks are packed together
and turned on the lathe. Next, they are unbundled, and individually rotated
by the same amount, then packed back together.  If the rotation is 180
degrees, then what was on the outside, is now on the inside and the new
outside can be turned.

My problem is in the learning curve to use the lathe construction. What I
want is essentially a curve (any curve) on the inside of a cylinder.  I
don't care about the outside, except the wall thickness should be
substantial to eliminate any corners on the square object from which the
lathe is differenced. I am currently not understanding the shape of the
interior of the POV doc on lathe and  bezier_spline object.

Can someone clarify the interior shape or suggest an alternate form of a
hollow cylinder which has curves (coves and beads in woodturning lexicon) on
the inside?


I tried the following which has the same spline scaled, and differenced,
which sorta works, but the wall thickness isn't quite enough and I get
artifacts from the corners of the bundle of sticks where the curve is
extreme.


/* In the following I subtracted 1 from all the X coordinates so the orange
pair were on the Y axis, but otherwise the same as the documentation.*/

/* set the control points to be used */
#declare Red_Point    = <1.00, 1.00>;
#declare Orange_Point = <0.00, 1.50>;
#declare Green_Point  = <1.00, 3.50>;
#declare Blue_Point   = <0.50, 4.00>;
#declare Green_Point2 = <1.00, 4.50>;
#declare Orange_Point2= <0.00, 6.50>;
#declare Red_Point2   = <1.00, 7.00>;

difference {
  lathe {
    bezier_spline
    8,
    Red_Point, Orange_Point, Green_Point, Blue_Point
    Blue_Point, Green_Point2, Orange_Point2, Red_Point2
    scale <6, 1, 1>
    pigment { Cyan }
    }

  lathe {
    bezier_spline
    8,
    Red_Point, Orange_Point, Green_Point, Blue_Point
    Blue_Point, Green_Point2, Orange_Point2, Red_Point2
    scale <3.0, 1, 1>
    pigment { Yellow }
    }
}




Stephen

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