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Segment Length Calculation

blackhorse

blackhorse

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I probably have not thought it through enough, but I really don't understand why some say so much precision in calculating segment length is required. Since much of the OD is turned away anyway, isn't "D/# of segments" good enough?

Joe
 
Joe, really for the OD you are probably correct. You end up with a larger OD using the simple method. Which you need to account for in your layout. But it should be (usually) too tough.

Here you might run into issues is the inside, since the simple method can result in a situation where the inside is larger then designed.
 
Hi Joe,
When you calulate diameter using the simple method of D X Pi and then divide by the number pieces you get measurements for tip to tip of the ring. You can drawn the ring out flat, input the segments and you will see that the ring is reduced at the ring point. It could be 1/8" in a 6'' ring and a 1/4" in a 12" ring. The lost is also determined by the number of pieces. The more the pieces the less the size lost. All this does not really matter. If you allow enough. Given a catch or lots of splinters you will lose wood anyway. The inter dimensions can be quite accurate by taking the sizes off your drawing. You can pretty well turn from the inside out. I was taught this method from Curt Theobald, one of the grue's of segmenting. Showing a drawing that I use as a plan for segmented turning. Just print out on 1/4" graph paper and make changes if needed. The drawing will show how wide to make a segment. GT
 

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Joe,

There's absolutely nothing wrong with calculating segment lengths this way: (D x Pi)/ # of segs; especially if you are using at least 8 segments. The key to having enough wall thickness to play with, is what's referred to as "padding". That's the cushion you create by pretending that your finished wall thickness is going to be thicker than you actually desire. In George's drawing, you will notice that he has drawn his segment widths (inside of vessel to outside), much thicker than needed. This "padding" allows more flexibility during the actual shaping of the vessel. It also waste more wood, but the little bit of wasted wood is nothing compared to your wasted time if you run out of material in order to create the desired shape.

Have Fun,
Malcolm
 
Thanks, guys. You confirmed exactly what I thought. I just completed a walnut dish with bloodwood and maple segmented ring. Now on to a 6" fully (except for the base) segmented bowl. Pretty exciting!

Thanks again.

Joe
 
Ok Joe,
Sounds like you are on your way. Posting 3 circles segmented with a top view. You can see what effect an 8 pc circle has on a 6" circle. Looks like it's about .060 unless I missed something. GT
 

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Although the "Long Side Calculation" is important to achieve the size you desire. The precision required is when the angled cuts are made and then prepared for glueing.

Walt
 
Walt said:
The precision required is when the angled cuts are made and then prepared for glueing.

Walt
Click to expand...

Thanks again, George. Quite illustrative!

Walt, speaking of miter cuts, as a beginner I'm finding that just a few hand passes on sandpaer after cutting gives me better results than machine sanding because I haven't made the proper sanding jigs, as Malcolm recommends. Without the jigs and a proper disc sander, I'm making my cuts LESS accurate.

Joe
 
I havent got my segments perfect yet either, but close. I just cut them on my sled, glue up 2 halves of each layer, and then hit it on the belt sander real quick. Works perfect so far. :)

Anywhere I can get a program for segments? Having trouble processing the layers in my head. :confused:
 
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