How do you set the stop when you know S.E.L?

For us newbies, SEL (segment edge length) is the measured length of the long edge of the trapezoid that makes up each segment you cut. For the purposes of simplicity, this edge determines the outside diameter of each ring. (Put away your slide rules boys, I have to keep it simple and limit the trigonometry or my brain gets all ‘splodey.)
 
Bob Beaupre said:
That's pretty much the same as my process also.
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Let me know if I'm understanding all of this information. I invested a considerable amount of money in a stop-block with a digital linier read-out. From what I am reading, it appears that the SEL can't be accurately measured 90 degrees from the blade to the stop-block using my digital read-out. Instead, the Segment Pro SEL must be either mathematically adjusted for the segment angle or measured and marked 'by eye' for each segment and SEL along the length of the board.

The alternative to using errors introduced by hand-eye marking and cutting is to employ mathematical equations in a spreadsheet to adjust the SEL from 90 degree equivalent to the SEL based on the angle of the segment. I'm wondering why Segment Pro and other similar programs do not have a column on the cut-sheet that makes adjustments to SEL based on the cut-angle, thus allowing for direct 90 degree measurements between blade and stop-block.
 
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Dennis Castle said:
Let me know if I'm understanding all of this information. I invested a considerable amount of money in a stop-block with a digital linier read-out. From what I am reading, it appears that the SEL can't be accurately measured 90 degrees from the blade to the stop-block using my digital read-out. Instead, the Segment Pro SEL must be either mathematically adjusted for the segment angle or measured and marked 'by eye' for each segment and SEL along the length of the board.

The alternative to using errors introduced by hand-eye marking and cutting is to employ mathematical equations in a spreadsheet to adjust the SEL from 90 degree equivalent to the SEL based on the angle of the segment. I'm wondering why Segment Pro and other similar programs do not have a column on the cut-sheet that makes adjustments to SEL based on the cut-angle, thus allowing for direct 90 degree measurements between blade and stop-block.
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You are correct but consider this. For a long, long time now, to cut any board to the length you want it, you would cut the first end to the desired angle, measure exactly along the board and mark it at the desired length, cut it where you marked, then measure the result to make sure you did it correctly. Right? SEL is the length exactly along the board and you can measure, mark, cut, and check your segment just like any other board.

Now you (and I and some others) want to measure, not along the board but at some other angle and wonder why a correction in the cut length has to be made. The fancy digital readouts we have now didn't exist for hobbyists when these programs were written and many guys still don't have them. I always export my results to Excel so I can sort them and change font size on a whim so it's easy to toss in that 3.4% divider to fix the 15 degree error that I'M making by measuring at an angle to the segment.

To check your segment once you cut it, you still need to know what the real part is supposed to measure. I still check my work occasionally to be sure nothing has slipped so I have both in my shop sheet. My neighbor who still uses the measure, mark, cut, check method doesn't have to deal with some correction factor that doesn't apply to him.
 
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Ken Sherwin said:
You are correct but consider this. For a long, long time now, to cut any board to the length you want it, you would cut the first end to the desired angle, measure exactly along the board and mark it at the desired length, cut it where you marked, then measure the result to make sure you did it correctly. Right? SEL is the length exactly along the board and you can measure, mark, cut, and check your segment just like any other board.

Now you (and I and some others) want to measure, not along the board but at some other angle and wonder why a correction in the cut length has to be made. The fancy digital readouts we have now didn't exist for hobbyists when these programs were written and many guys still don't have them. I always export my results to Excel so I can sort them and change font size on a whim so it's easy to toss in that 3.4% divider to fix the 15 degree error that I'M making by measuring at an angle to the segment.

To check your segment once you cut it, you still need to know what the real part is supposed to measure. I still check my work occasionally to be sure nothing has slipped so I have both in my shop sheet. My neighbor who still uses the measure, mark, cut, check method doesn't have to deal with some correction factor that doesn't apply to him.
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Thanks for your reply to my post. I am new to segmented turning and I enjoy the attention-to-detail, knowledge, and experience required to pursue perfection in a segmented vessel.

After purchasing a digital saw-stop, horizontal segment machine, several segmented bowl computer programs, books, and upgrading my table saw, it's just hard to accept that the segmented programs are not able to perform a mathematical calculation for converting an angled SEL measurement to an SEL that can be measured at 90 degrees between the blade and the saw-stop.

I understand Lloyd's comments about complications and confusion in adjusting the SEL on his cut-sheet, but at the end of all this pursuit for perfection, we are evidently back to "sneaking up" on a measurement. If you would be willing to provide me with your Excel formula for converting SEL on my Excel spreadsheet to allow for measurement at 90 degrees, it would be greatly appreciated. I just don't want to pursue all of this detail along the way and then fall back at the end to "sneaking up" on each SEL cut.
 
Dennis Castle said:
Thanks for your reply to my post. I am new to segmented turning and I enjoy the attention-to-detail, knowledge, and experience required to pursue perfection in a segmented vessel.

After purchasing a digital saw-stop, horizontal segment machine, several segmented bowl computer programs, books, and upgrading my table saw, it's just hard to accept that the segmented programs are not able to perform a mathematical calculation for converting an angled SEL measurement to an SEL that can be measured at 90 degrees between the blade and the saw-stop.
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imo, segmented software formulas were written based on how the majority of people measured SEL.
This thread may be of interest to you

SEL versus actual blade to stop distance

Have you ever considered adding the actual distance between the blade and the stop used on the saw to get the correct SEL for the angle the of the segment being cut. I have watched many videos of people using the SEL to set the saw stop. This gives a segment that is actually longer than the SEL...
woodturnerpro.com

in Excel you can use the formula of (since excel uses radians):
Stop Distance = SEl * COS(RADIANS(MiterAngle/2))


Dennis Castle said:
I understand Lloyd's comments about complications and confusion in adjusting the SEL on his cut-sheet, but at the end of all this pursuit for perfection, we are evidently back to "sneaking up" on a measurement. If you would be willing to provide me with your Excel formula for converting SEL on my Excel spreadsheet to allow for measurement at 90 degrees, it would be greatly appreciated. I just don't want to pursue all of this detail along the way and then fall back at the end to "sneaking up" on each SEL cut.
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in Excel (since excel uses radians):
Stop Distance = SEl * COS(RADIANS(MiterAngle/2))
 
Dennis Castle said:
If you would be willing to provide me with your Excel formula for converting SEL on my Excel spreadsheet to allow for measurement at 90 degrees, it would be greatly appreciated. I just don't want to pursue all of this detail along the way and then fall back at the end to "sneaking up" on each SEL cut.
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Certainly.

Important part:

CutAngle = 180 degrees / number of segments in a ring
StopSetting = SEL * COS(CutAngle)

Nearly useless blather:
1) Don't forget to keep your degrees and radians straight.

2). This assumes you are cutting your segments symmetrically. For example, both sides of the segment are close to 15 degrees. I mention this because it is possible and sometimes even fun to cut one side at 0 degrees and the other side at 30 degrees, or even at -10 and 40 degrees for a 12 segment ring. It all works as long as the included angle is 30.00 degrees. You can get weird grain patterns but that's why you offset the angles like that. I did some just to see what happened but did not pay attention to the cosine errors and did not care about resulting ring diameter. I was just playing.

3) The reality is that how close your real segments are to the theoretical cut length isn't all that critical. A 3% error in actual length results in a 3% difference in the OD of that ring but it gets turned dorn to round then down some more to shape anyway. I can't glue rings perfectly concentric consistently anyway so I make up for that with a little bit of extra wall thickness.

4) The cosine error in measuring is dependent on the cut angle or number of segments in the ring and rapidly fades in insignificance.
12 segments = 3.4% error,
16 segments = 1.9% error,
24 segments = 0.86% error.

etc.

5) The critical things that must be right are:
- Getting the total included angle on every segment both repeatable and dead accurate.
- Getting both gluing faces of every ring dead flat.
- If you are making a pattern in your piece, getting the correct species of wood in the correct place in every ring and getting every ring oriented correctly relative to its neighbors.

Nothing else really matters to the thousandth. Sometimes it's fun to chase those zeros to the right of the decimal just to prove to myself that I still can but all I'm really controlling is the thickness of the shavings on the floor.
 
Ken:
I installed your adjustment formula into the Seg-Pro Excel cutsheet and a 24 segment ring with cut-angle @ 5.5 degrees showed an adjustment in SEL from .5550 to .3933, a reduction of .1617 or 1.6%? The true cut-angle of 7.5 degrees was adjusted on the cut-sheet to 5.5 degrees for open segments so I used 5.5 degrees in the formula.

So setting a stop at 90 degrees to the blade at .3933 should provide a measurable SEL with a reading of .5550. If this sounds right, all I need to do now is understand whether these figures need to be converted from degrees to radians or visa-versa.

I realize that I am spending time and energy on somthing that could "work out in the wash", but since I have this equipment and time on my hands, why not? Thanks again.
 
Dennis Castle said:
Ken:
I installed your adjustment formula into the Seg-Pro Excel cutsheet and a 24 segment ring with cut-angle @ 5.5 degrees showed an adjustment in SEL from .5550 to .3933, a reduction of .1617 or 1.6%? The true cut-angle of 7.5 degrees was adjusted on the cut-sheet to 5.5 degrees for open segments so I used 5.5 degrees in the formula.

So setting a stop at 90 degrees to the blade at .3933 should provide a measurable SEL with a reading of .5550. If this sounds right, all I need to do now is understand whether these figures need to be converted from degrees to radians or visa-versa.

I realize that I am spending time and energy on somthing that could "work out in the wash", but since I have this equipment and time on my hands, why not? Thanks again.
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If your cut angle is 5.5 degrees, the cosine of 5.5 degrees is 0.9954. Therefore your 0.5550 SEL should be reduced to 0.5524, a difference of 0.00255. I suspect the surface finish left by the saw blade is rougher than that.

The cosine of 5.5 radians is 0.7086. Multiplying 0.5550 by 0.7086 gives 0.3933. That's where you got your .3933 number but I haven't found where .1617 came from.

Does this help?
 
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They sure do. I wasn't paying much attention when I was "copying" a ceramic vase I liked the shape of and I assumed the rings would be about 3/4" thick. When all 25 rings were stacked up and turned, the piece was about 2" too tall because I never checked my assumption. Now I check the accumulated height as I'm gluing rings and if I'm running a little tall, I can thin the next ring to bring it back. Sometimes chasing the little deviations is fun. Other times, close is good. All part of the hobby.
 
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