Reverse segment calculator

Rather than doing calculations I find it easier to make full size drawings. What is the advantage of your way of making rings? I see what I believe to be disadvantages. Trapezoids are more efficient in wood use and there is not much end grain to contend with.

Ray

Full sized drawings are easy with trapezoids. Not so much with reverse segments because they don't intersect the center of the ring. But even with a trapezoid approach all one needs is the length of the outer edge. Much easier to just use a segment calculator and set my jig accordingly.

To me, here isn't any particular advantage to one way versus the other. It's a design decision. I like regular segmented turnings but reverse segment vessels have visual "movement". The goal is to create an artistically pleasing piece, not just a functional bowl. A trapezoidal ring may use a bit less wood than a reversed segment, but not that much really. End grain isn't that muhc of an issue - I'm still mostly turning side grain. I'll post a photo later showing the effect and what the rings look like after they're trimmed on the bandsaw & lathe. It makes more sense when you see the destination...

...Kevin

OK, I take back what I said about end grain. Good catch Ray.

Over in a.b.p.w I posted a couple new shots. One is a bowl I finished the other day using the reverse segment method, and the other is the glue-up that I posted yesterday. I must have done them slightly differently, as the glue-up clearly is going to yield a lot of end grain, whereas the bowl has side grain. Go figure.

This is the first time I've tried doing the reverse segments, so there's a learning curve. I'll have to go out to the garage and cut some more segments and play with them to see what I did differently. I suspect I didn't 'reverse' the segments on the bowl - that is, the angle faced toward the center rather than the outside, and on the glue-up I did reverse them.

I'll take some shots of my experimenting and post the results...

...Kevin

I added a reverse-segment calculator to my web site:

The math looks like this:

# angles $a = 180 / $nseg; $a2 = 360 / $nseg;

# ID triangle $b = $ir * dcos ($a); $ie = $ir * dsin ($a); $ie2 = $ie * 2;

# outer radius cutoff for equations below if ($a2 >= 90) { $rc = $or*2; } else { $wc = $ie * dtan ($a2); $rc = $b + $wc; }

if ($or < $rc) { # a thin ring. width is limited by the perpendicular radius

$w = $or - $b; $l = $ie2 + $w / dsin ($a2); $cutlen = $l; if ($nseg < 4) { $cutlen += $w * abs (dcos ($a2)); }

} else { # a thick ring. width is limited by the intersection

$lo = sqrt ($or * $or - $b * $b); $w = ($lo - $ie) * dsin ($a2); $l = $lo + $ie; $cutlen = $l; }

...Kevin1

If what you are looking for is the pinwheel effect you might consider cutting like for trapezoids but making one angle 90 degrees and doubling the angle for the other end. I have been doing segmented bowls for about a year. Some of my projects at

I think that's what I ended up doing on the bowl I posted, where the side grain was showing rather than the end grain. It tightened up nicely on itself and doesn't have the end grain exposed.

When I did the other one, you'll notice the cross brace in the middle - it kept the segments from all sliding in. Exactly right though - it is a pain to clamp when they're glued up that way.

...Kevin

"Great. I see that he implemented both sets of equations along with the test for when to use which set. (And they appear to work. It is always nice to see things when they work.)

I see two red circles and two blue circles. The red circles are the inner and outer radii. One of the blue circles shows the inner edge of the segments. However the purpose of the other blue circle is a mystery to me.

What is the purpose of the second blue circle?"

I wasn't sure so figured I'd ask here...

Your finished bowl looks nice. Sometimes photography does not do justice to subtle detail. How did the end grain piece turn out?

Ray

Most of the "top" segmented turners I've talked with say to avoid cross-grain.

It's always a hard one - seems like there's always some contrary expansion that goes on no matter what you do. :-) Glad those are stable.

Thanks Ray. The end grain piece is the lid to the bowl, the darn day job has prevented me from finishing it! (Not that I'm complaining - plenty of guys out of work that would love such "troubles".)

I'll post a nicer photo of the bowl when I get the lid done. I think it'll be interesting to see how seasonal humidity changes affect it...

...Kevin

Always a good idea. If the pieces are narrow you may get away with it. We'll see on the piece I did. If the segment isn't too wide there won't be much movement. One thing I usually do also is to anchor the pieces with a ring of solid wood, (ex: the ring of cherry around the top of my bowl).

On Mon, 12 Dec 2011 18:19:06 -0600, Ray wrote (in message ):

I posted over there, here's it is again: the outer blue circle is where the math changes, and you start to get end grain tear-out issues. For an OD bigger than the blue circle, you're cutting into end grain no matter which way the ring is turning (much like solid-blank bowl turning). For OD smaller than the blue circle, if you mount the ring the right way you're always turning "downhill".

For an OD the same as the blue circle, the OD is tangent to the edge of one segment at exactly the spot where it intersects the adacent segment.

The inner blue circle is how big your clamping block needs to be.