SEL versus actual blade to stop distance

[Donald Lewis]

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. Your programs simplify everything about segmented turning except this one item. This might be a good addition to future version of the applications you produce. This would help prevent having to cut a bunch of extra segments trying to make the ones for your project have the correct SEL.

 

[Lloyd Johnson]

I actually think this is a resonable request and I can see where it would be a good help. If I needed an SEL of 3/4” for a 12-segment ring, I calculate the lineal measurement at .528” which means I would move my thin strip device to be next to the blade when I set the distance at .528” and then move the device a couple inches before the blade before the final lockdown. In theory, the segment cut from this setting should yield and SEL of 3/4”.

This appears to be the correct formula:
Stop measurement = (SEL / 2) / tan(cutting angle / 2)

In this case, with a SEL of 3/4" and a cutting angle of 15 degrees:

Stop measurement = (0.75 / 2) / tan(15 / 2) = 0.528"

Easy, peasy - if my trigonometry is correct. I know as much about trigonometry as I do rocket surgery, though.

 

[Donald Lewis]

Good morning Lloyd. I am including URL's for several YouTube videos on the subject of SEL versus actual blade distance to the stop.

Title: Digital Saw Stop for Segmented Turning
Many of these stops have a direct measurement to the blade contrary to the angled distance necessary. What you describe seems kind of cumbersome. I have seen a couple of articles on this poblem that the articles call cosine error. A direct 90 degree to the blade would make this an accurate cut an go process without having to cut a bunch of test pieces. I have only been segmented turning for a few months and I already have a big pile of wasted wood segments. A lot of what you are doing to save material in your applications is being wasted cutting test pieces. I recently joined the AAW Segmented Woodturners chapter and found a thread on one of their forums discussing this problem. I still think this would be worthwhile to persue.

 

[Jimsworkshop.net]

What I did:
1) Export the WTP sheet to a .csv file.
2) Add a column next to the SEL column labeled "Corrected SEL" with this formula: =G3*COS(RADIANS(K3))
where G is the column that has SEL and K is the column with the miter angle.

Then I use the "corrected SEL" value to set my stop.

 

[JonathanKlein]

It is interesting to see that your formula and mine are very different but return the same result. I like your formula a lot better, though. An SEL of 3/4” yields a lineal measurement of .528” using both formulas.

I’ll also look at adding it to Woodturner PRO as I want to make several changes but I have to jump through some expensive hoops because 3rd party tools that were used 22 years ago are no longer supported by Visual Studio so I have to take a different approach.

Thanks for posting your formula.

Lloyd

I have a similar question - I naively thought that the SEL * number of segments would be the linear distance needed (plus I guess a little slop for the kerf) but I see that isn't the case from your formula above. So until the update this I guess is best handled via excel.

 

[Lloyd Johnson]

When cutting segments in the economy mode (as opposed to the grain match mode), you flip the board over after each cut. This means that a board cut into 12 segments (for example) will have six long SELs and six short SELs and 5 saw kerfs. I always calculate the minimum length that a board must be to get the number of segments from it. I do this because you might have a nice remnant of an exotic wood and this lets you know if you can use it or not. You always have to add enough to the board length to account for safety and extras, if wanted.

In the grain match, you don’t flip the board after cuts so you have 12 long SELs and 11 saw kerfs. Woodturner PRO calculates both economy and grain match but Segment PRO only calculates economy since grain match is not a standard procedure.

 

[JonathanKlein]

When cutting segments in the economy mode (as opposed to the grain match mode), you flip the board over after each cut. This means that a board cut into 12 segments (for example) will have six long SELs and six short SELs and 5 saw kerfs. I always calculate the minimum length that a board must be to get the number of segments from it. I do this because you might have a nice remnant of an exotic wood and this lets you know if you can use it or not. You always have to add enough to the board length to account for safety and extras, if wanted.

In the grain match, you don’t flip the board after cuts so you have 12 long SELs and 11 saw kerfs. Woodturner PRO calculates both economy and grain match but Segment PRO only calculates economy since grain match is not a standard procedure.

Okay - it would be helpful to not only have the SEL but the distance to set up the stop block - that is what I originally thought was meant by SEL and hence the math wasn't working out but found in the tutorials that SEL was not the stop block distance but rather the long edge of the segment. I will use excel for now but hopefully you can just add this column.

 

[JonathanKlein]

I actually think this is a resonable request and I can see where it would be a good help. If I needed an SEL of 3/4” for a 12-segment ring, I calculate the lineal measurement at .528” which means I would move my thin strip device to be next to the blade when I set the distance at .528” and then move the device a couple inches before the blade before the final lockdown. In theory, the segment cut from this setting should yield and SEL of 3/4”.

This appears to be the correct formula:
Stop measurement = (SEL / 2) / tan(cutting angle / 2)

In this case, with a SEL of 3/4" and a cutting angle of 15 degrees:

Stop measurement = (0.75 / 2) / tan(15 / 2) = 0.528"

Easy, peasy - if my trigonometry is correct. I know as much about trigonometry as I do rocket surgery, though.

the above formula isn't working for me - in excel i get .13858 - am i doing something wrong?

 

[mfisher]

the above formula isn't working for me - in excel i get .13858 - am i doing something wrong?

I came up with the same number.

Will see if Lloyd can explain.

 

[JonathanKlein]

Lloyd.
I was working in excel and I am not coming up with 0.528

(0.75 / 2) = 0.375
tan(15/2) = 2.706014

0/375 / 2.706014 = 0.13858

in Excel I just typed in =TAN(15/2). Hopefully that is the proper use of the tangent function.
=(0/75/2)/tan(15/2)

Lloyd, are you using degrees or radians? in excel tan, cos, etc. are expecting the input in terms of radians, not degrees. but 15 is based on the number of degrees in your example and that seems odd to be 'swapping' from degrees into radians without calling the radians(15/2)

 

[JonathanKlein]

What I did:
1) Export the WTP sheet to a .csv file.
2) Add a column next to the SEL column labeled "Corrected SEL" with this formula: =G3*COS(RADIANS(K3))
where G is the column that has SEL and K is the column with the miter angle.

Then I use the "corrected SEL" value to set my stop.

View attachment 4856

Sorry but I still am not getting the number - so the SEL is .75. there are 12 segments meaning the cutting angle is I believe 15 degrees. 15 degrees in radians is .261799, cos(.261799) = .96 and that times .75 is .724. if we use 30 degrees we get closer at .649 but still not .528. In your formula of G3*cos(radians(k3)) I'm assuming k3=360/12/2 or 15 degrees. and that g3 was the original SEL of .75. what is the magic I'm missing

 

[Jimsworkshop.net]

I was in error. It should be as shown above.
Or in Excel (since excel uses radians): Stop Distance = SEl * COS(RADIANS(MiterAngle/2))

 

[JonathanKlein]

View attachment 4939I was in error. It should be as shown above.
Or in Excel (since excel uses radians): Stop Distance = SEl * COS(RADIANS(MiterAngle/2))

Cool. Thank you. Now the formula works

 

[Jim Morgan]

View attachment 4939I was in error. It should be as shown above.
Or in Excel (since excel uses radians): Stop Distance = SEl * COS(RADIANS(MiterAngle/2))

Your math is correct. Keep in mind, however, that the change is proportionally the same for all rings - if you don't apply the correction, all of your rings will be slightly oversized. Unless you are making a piece to some specific dimension, it won't matter.

That being said, I always figure that precision and accuracy are of paramount importance in preparing segments, so I always apply the correction in my work.

 

[JYALBERT]

I actually think this is a resonable request and I can see where it would be a good help. If I needed an SEL of 3/4” for a 12-segment ring, I calculate the lineal measurement at .528” which means I would move my thin strip device to be next to the blade when I set the distance at .528” and then move the device a couple inches before the blade before the final lockdown. In theory, the segment cut from this setting should yield and SEL of 3/4”.

This appears to be the correct formula:
Stop measurement = (SEL / 2) / tan(cutting angle / 2)

In this case, with a SEL of 3/4" and a cutting angle of 15 degrees:

Stop measurement = (0.75 / 2) / tan(15 / 2) = 0.528"

Easy, peasy - if my trigonometry is correct. I know as much about trigonometry as I do rocket surgery, though.

Lloyd, I have had the same issue. I would multiply Pi x Diameter and divide by segment count, e.g. a 12 segment, 8" in ring would require 3.1416 * 8 = 25.133" and divided by 12 = 2.09" which always ended up with the ring too large when I'd set the stop 2.09" away from the blade. I get the visual explanation of the distance because of the trigonometry...we measured a 90 degree distance to the blade but are cutting at the 75 degree angle (15 degrees off 90). I never really had a problem with it; you just have to turn away more wood. However, I do like exactness to some extent. I calculate the formula you provided and it isn't coming out? Actual segment length of .75"/2 = .375" the tangent of 15 degrees divided by 2 is the tangent(7.5). The tangent of 7.5 degrees = .13165. Divide .375" by .13165 and I don't anywhere near .528", I get 2.849"? What am I missing?

 

[mfisher]

Lloyd, I have had the same issue. I would multiply Pi x Diameter and divide by segment count, e.g. a 12 segment, 8" in ring would require 3.1416 * 8 = 25.133" and divided by 12 = 2.09" which always ended up with the ring too large when I'd set the stop 2.09" away from the blade. I get the visual explanation of the distance because of the trigonometry...we measured a 90 degree distance to the blade but are cutting at the 75 degree angle (15 degrees off 90). I never really had a problem with it; you just have to turn away more wood. However, I do like exactness to some extent. I calculate the formula you provided and it isn't coming out? Actual segment length of .75"/2 = .375" the tangent of 15 degrees divided by 2 is the tangent(7.5). The tangent of 7.5 degrees = .13165. Divide .375" by .13165 and I don't anywhere near .528", I get 2.849"? What am I missing?

I have always measured the distance from the blade along the fence angle, not at 90 degrees. The software has given me good results.

Basically, will set my digital calipers at the SEL distance. then I will take a scrap piece of stock, miter the end and then mark the SEL on it. I then lay the stock along the fence (set at the miter angle needed) and adjust the stop block.

I also tend to slightly overbuild my rings. If I want a 6" ring I will have the ring at 6.125 and turn it down to 6", This gives me a slight wiggle room in case I do not get the ring completely dead center in glue up.

I like the idea of measuring at 90 degrees to the blade rather than along the fence line (angle).

 

[JYALBERT]

I have always measured the distance from the blade along the fence angle, not at 90 degrees. The software has given me good results.

Basically, will set my digital calipers at the SEL distance. then I will take a scrap piece of stock, miter the end and then mark the SEL on it. I then lay the stock along the fence (set at the miter angle needed) and adjust the stop block.

I also tend to slightly overbuild my rings. If I want a 6" ring I will have the ring at 6.125 and turn it down to 6", This gives me a slight wiggle room in case I do not get the ring completely dead center in glue up.

I like the idea of measuring at 90 degrees to the blade rather than along the fence line (angle).

I have always measured the distance from the blade along the fence angle, not at 90 degrees. The software has given me good results.

Basically, will set my digital calipers at the SEL distance. then I will take a scrap piece of stock, miter the end and then mark the SEL on it. I then lay the stock along the fence (set at the miter angle needed) and adjust the stop block.

I also tend to slightly overbuild my rings. If I want a 6" ring I will have the ring at 6.125 and turn it down to 6", This gives me a slight wiggle room in case I do not get the ring completely dead center in glue up.

I like the idea of measuring at 90 degrees to the blade rather than along the fence line (angle).

I get it! But I might have a build on your method? Use stock you are using and cut the first angle on fence 1. (It's that first cut you make on the stock.) Then, you mark your SEL length from the point of the just cut line on the outside of the stock (long point). Move the stock to fence 2 and slide along the fence past the blade like you were going to free-hand cut a segment. Set the blade on the cut line and place the stop flat at the end of the "to be cut segment". Slide the stop back and lock down and it's set. My stop expands into the miter slot enough to not lose the setting but be able to slide it. Two points of failure: measuring where the line goes and setting the blade on it. I like this better than trying to use the trigonometry to determine the stop block position. But having the software make that calculation would certainly be an enhancement. Regardless, thanks for a new take on how to solve the problem. In reality, is it that big a problem? I really don't think so.

 

[mfisher]

I get it! But I might have a build on your method? Use stock you are using and cut the first angle on fence 1. (It's that first cut you make on the stock.) Then, you mark your SEL length from the point of the just cut line on the outside of the stock (long point). Move the stock to fence 2 and slide along the fence past the blade like you were going to free-hand cut a segment. Set the blade on the cut line and place the stop flat at the end of the "to be cut segment". Slide the stop back and lock down and it's set. My stop expands into the miter slot enough to not lose the setting but be able to slide it. Two points of failure: measuring where the line goes and setting the blade on it. I like this better than trying to use the trigonometry to determine the stop block position. But having the software make that calculation would certainly be an enhancement. Regardless, thanks for a new take on how to solve the problem. In reality, is it that big a problem? I really don't think so.

I agree. Would not be a bad enhancement.
Yet it is not a big deal if you measure along the fence angle line instead of 90 degree to the blade.
Been using WTP for years.

Like I have mentioned before I tend to give myself 0.125 to 0.25" buffer. I prefer to turn down to desired diameter than take a chance that the ring ends up too small after rounding. One reason I don't get hung up to much on SEL down to the thousandths or hundredths.

 

[Lloyd Johnson]

Here are my thoughts about the lineal measurement...

Jim’s drawing (very helpful, btw) shows both measurements in use. First (and the method that has been used since the first segment was cut on a table saw), you mark the SEL on the board that is on the angled fence, slide it so that the mark is at the edge of the blade and set the stop at that point. Second, you calculate the lineal measurement from the SEL and cutting angle and use a measuring tool to set the stop that distance from the blade.

Both methods place the stop at the exact same place. One is not more accurate than the other and one is not faster than the other.

Here’s the problem - very few of my thousands of users are looking for a lineal calculation. Fewer yet have any understanding of what it is or how or why to use it. If the Summary suddenly shows up with a new column that makes no sense to them, they will find my phone number or email address which is on every page of the website and expect an explanation of what it is, how it works and why they should use it in the first place.

Yes, I could make a video tutorial that explains it all but users would have to make the effort to follow a link and watch the tutorial or contact me instead. That tutorial would also need to address where to get a suitable measuring device or how to build your own - each of which would spur additional requests for support. I would consider doing this if it was a better mouse trap or could make a noticeable difference in the practice or enjoyment of segmenting but that is simply not the case.

Thankfully, adding the formula using Excel is trivial so those that want to use this method have an easy way getting it. The Cutting Summary that already has too many columns will stay as it is.