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Q about board lengths

cnsranch

Jerry Prinds
I don't have enough experience to answer this question on my own.....
SP gives you a board length to start, and I understand that's the minimum length to get the number of pieces you need for the ring. In my first bowl I added around 10% to that length but still came up short in some rings.
Does the length given in SP include the kerf cuts needed?
My next project will be a relatively simple painted bowl, but with 48 segments to a ring. So, when SP calls for, say, a 20" lenth of wood, what length do I really need to start with to be safe, without having too much waste left over?
 
When comparing rings between Woodturner Pro and Segment Pro I show (1 test) SP was about 1/2 inch longer. Adding 10% might not be enough to allow for all segments plus sufficient left to be safe cutting. I don't know if the calculated board length takes into account the first cut when you are establishing the end angle. For example, a 7.75 outer diameter ring with 24 1 inch segments calls for approx board length of 22.6. These are all approximate numbers but the 10% only give you 2 1/4 inch for your initial end cut plus safety amount for the last cut. In this case I would start off with a board at least 26 inches long. I don't want to waste wood but I've been caught in the past and have had to glue a scrap on the end of the board to cut that final segment.
 
Lloyd will have to answer the math part.

I did a quick comparison between WTP and SP. the board length required was basically the same in both programs. Could you post the SP project plan so we can take a look? I will compare it to what WTP comes up with.

Stuart. I have never had a problem with WTP board length (bl) being short.
SP seems to give a slightly longer bl than wtp.
 
Stuart,
I used Woodturner PRO extensively to verify that the Segment PRO results were the same and the only difference I can find is that Woodturner PRO adds a saw kerf at both ends of the board when one of those kerfs is not necessary.

One possibility is that you are not taking the inside diameter into consideration. Since the board is flipped after every cut, the long segment edge length as well as the short segment edge length have to be considered. The math for this is actually quite easy. You simply have to add the long SEL to the short SEL together and multiply that time 1/2 of the number of segments. To that you have to add the saw kerf width adjusted for the tangent of the angle times the number of segments minus 1.

The way I verified the calculations was to draw a Segment PRO wall profile that was a straight profile with a radius of 5" and set the wall width to 1". I then looked for a row in the summary that had outside and inside diameters of exactly 10" and 8". I then build a Woodturner PRO ring with the same diameters. This is the best way to show the comparison between the two.

Lloyd
 
I always like to add several inches to the board length. In case of a bad cut, or a piece goes flying and I can't find it. I also cut some extra pieces, just in case I need them
 
Good advice Bob. I do the same thing , (most of the time). It always helps to have a few spare segments, especially if your working with thin segments. Depending on the wood species I have experienced wood tear out when cutting the segments on the table saw. Zebra wood comes to mind.
 
Lloyd, thanks for the way you explained the length. I've never given it much thought because I always add several inches onto whatever WP calculates just to be certain I'm not trying to cut a segment with just a small bit left.
 
Both WP and SP allow for saw kerf. As others have said, you must add to allow enough wood to hold the piece for the final cuts and for a few extra segments. I usually eyeball it but add at least 4-6" depending on the SEL you are cutting. Better to have 2" left than 2" short. Your time is worth more than a few inches of wood. I also find I can make the final cuts on the segeasy sled with shorter pieces than other sleds or a miter saw. I do us clamps on the segeasy sled.
 
I leave my boards as long as I can within reason and cut the rings with the widest rings first, this way I can rip down the left over to use in rings that are narrower.
 
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