Setting the SEL cutting stop

Jimsworkshop.net

PRO Member
Call me lame, but I just now discovered that I have been cutting all my segments TOO LONG because of the way I was setting the stop.image.jpg

As you can see I was setting the distance between the blade and the stop to the SEL. That results in the segment being cut too long by a factor of 1.035 for the 12 segs/ring shown.

Anyone know a better way to set the stop?
 

mfisher

Super Moderator
Staff member
One approach I use for setting SEL is:
- On a scrap piece of .125" thick wood I cut the end at the angle of my segment. (5degrees for 36 segment). Then using a digital caliper I set it to the desired SEL. From there I mark the wood.
- I place the wood along the set fence on my cutting sled till the mark is at the edge (outside) of the saw blade. I move the stop up to the stick.
- Then cut a test piece and measure. Then do any fine tuning to achieve the SEL.

SEL should be measured using the miter angle, not 90 degrees from the blade to the stop.
 

GrahamJ46

PRO Member
I export the spreadsheet from SP or WTP into an Excel spreadsheet where I calculate the setup blocks down to 1/32" to set the space between the blade and the stop. I don't have the measure anything and it works every time! If I had a 1/64" setup block I could use that if necessary but I doubt that kind of accuracy is justified.
 

Ken Sherwin

PRO Member
You are measuring the stop along the adjacent side of a 15 degree triangle but the SEL is measured along the hypotenuse. 1/COS(15 deg) = 1.035, exactly your error. I have a DRO on my fence where I fasten my stop block so I have exactly the same problem. I want to edit the order of the cutting table and do other edits too so I export the cutting table to Excel. I make another column that converts the SEL column (which comes in as text due to the inch character (BOOO !) and multiply by the COS of the angle I'm cutting. Since I typically do 12-sided rings, I use COS(15) or 0.966 and things work out perfectly.

Make sense?


PS: I also calibrate the zero on my fence DRO to the fence side of the blade to account for the blade width.
 
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GrahamJ46

PRO Member
I do the same thing in my Excel spreadsheet so that I have the correct distance to the blade. So I don't have to keep copying the spreadsheet I wrote a VBA program that takes care of all the conversions needed, including text to numbers and dimensions. For a safe cutting length the min length has 3" added and rounded up to the next whole number. Each time I have a new export from SP I just call up the VBA program and everything is done in a fraction of a second. SP is easy to work with but WTP is more complex. The export from SP also automatically creates a cut list. At least it did but there is a bug I need to fix. The cut list feature uses a second spreadsheet from SP. I optimize by BW and SEL , Group by BW and then export. Although parsing the second spreadsheet is tricky it contains all of the info to create a cut list. Alternatively it is quick and easy to create the cut list manually from the converted first spreadsheet. I have attached a jpg of the converted spreadsheet ready to take to the shop.
 

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Mason Platt

PRO Member
I’m new to segmented turning and getting used to the software. But in regards to the OP question. If you are cutting the SEL based on 90 degrees to the blade and taking into account blade kerf. Won’t the error made by cutting at 90 degrees vs angle of the sled be the same for every segment. So in the end the diameter of each ring may be wider than you had planned but you can turn it down to desired thickness on the lathe Or an I completely missing something critically important?
 

Ken Sherwin

PRO Member
It depends on just how close you chose the theoretical wall thickness . A 10" dia ring will be 10.35" so the ID would also be too big. If you wanted both a precise OD and a precise wall thickness, you run the risk of making another funnel. (We've all made them, haven't we?) I make my starting walls a little bit thick just in case my rings are not as concentric as perhaps they could be and it makes the rings easier to glue. Nevertheless, I compensate for the cosine error just because its the right thing to do.
 

Mason Platt

PRO Member
So to simplify, does anyone know the formula to use to compensate for the error. Without having to put it into excel(which I don’t have). But I do have a calculator.
 

Ken Sherwin

PRO Member
1. Find the angle of the cut. Divide 180 by the number of segments. A 12-segment ring will have an angle of 15 degrees.
2. Find the correction, which is the cosine of the angle. COS(15) = 0.966
3. Multiply each SEL by the correction.
4. Actually measure the first segment to make sure you calculated correctly. Trust me that this is the most important step,
 

mfisher

Super Moderator
Staff member
Here is a link to a good discussion on SEL

Bob explains the software is using cord length instead of arc length
 
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