Question: The problem of dividing section by a divider

Kyle Jo

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Sep 6, 2019
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Thread starter #1
Hello. How's it going well?

Im in trouble in dividing section on the ring
I have a ring(diameter:: 16mm). And I want to
divide into 8 section.

So i calculated like this.
16×3.14÷8 = 6.28
and adjust my divider to 6.28 and
divided the ring. But it was so different.
It's big difference like about 5mm??

Could you guys get me to know what problem is

or
share your ways to divide the ring??

Thank you.
 
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#2
Hard to know exactly where to start without knowing if the ring is already fully formed or in a yet to be formed 'flat' state (a photo is always helpful).

Rather than dividing each segment by a certain number of millimeters, try this. Working from the outside edge of the ring, with an extra fine point permanent marker (something that is easily removed with acetone or fingernail polish remover, or just by buffing away), looking a the ring straight on from the side, carefully find the centerline of the ring, and place small point marks top and bottom exactly on the out side edge exactly at the centerline. Once the ring has been marked this way, the rest is easy. Working on the side of the ring, between each of the two marks find the exact center point, and mark to divide the ring into quadrants, then between each of those marks to divide the ring into 8ths, then between each 8th to divide the ring into 16ths. If you've done this on both sides, you should be able to then lightly draw reference lines across the surface of the ring, dividing the ring into exactly 16 equal sections.

If you're daring this will also work perfectly with a divider, but you're going to leave permanent marks (scratches) that will need to be engraved over (away) or removed with buffing.
 
Last edited:

colinskelly

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#4
Hello. I've come across this a while ago. Measuring the distance between the points of the divider will only be accurate on a flat surface because it is a straight line between the tips. That same distance travels around the circumference of the outside of the ring. The distance between the tips of the divider always remains a straight line from point to point. If you divide the ring into 2 sections, the dividers would measure the diameter of the ring (8mm in your example). It is easier to see the circumference is much longer than the diameter.

Rogers segmented circle is the easiest method I've found with minimal tools. A watch face is also something I've used to divide a ring up.
 

Big-Un

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#5
Here is a 16 segment circle. Just set your ring on the center then using a marker on the outside mark off every other segment.
Roger, I have the set of "segmented" circles like the one posted here only the entire set, divided into many different sections. When I get back home I'll try and locate it and post it. I may be off a little, but I think it was listed on this forum way back when.
 

mitch

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#6
i wrap a thin (1/8" wide) strip of white Post-It correction tape around the band, mark where the ends overlap, peel it off and stick it to my bench top. then i divide the circumference by however many sections i need, put the tape back on the band and mark the metal.
 

Big-Un

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#10
i wrap a thin (1/8" wide) strip of white Post-It correction tape around the band, mark where the ends overlap, peel it off and stick it to my bench top. then i divide the circumference by however many sections i need, put the tape back on the band and mark the metal.
That sounds way too easy! Wish I had thought of it, I tend to make things too complicated when a simple solution is there for the taking with just a little forethought.
 

papart1

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#14
In the piping trade pertaining to what Mitch is talking about he used small tape.......etc as what is called a Wrap-a-Round to measure round stuff, works on anything round,oval,moon and some rombus's
 

dhall

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#15
No question, the "wrap it around to find the length, then lay it out flat and mark divisions, then re-wrap and mark" method is fast and accurate. I use it, too. The real problem in using dividers on a ring or curved surface - you are actually marking the chord length, not the arc length. The arc will always be longer than the chord; you're guaranteed to fail if you are calculating your segment length by dividing the circumference, unless you've got a way to determine chord length.

Attached is a chart from the Machinery Handbook in which you can figure out chord lengths and arc lengths of some divisions. Not all divisions are possible with this chart, because it doesn't really address our use, dividing a ring into X-number of segments. Rather, it relies on a starting point of knowing the interior angle (center angle) whose rays would mark the start and end points of a given arc and/or chord, in whole degree increments - not helpful if you want to divide into 7 segments, for example. The dimensions listed in the chart are for a radius of 1 (doesn't matter if you use millimeters, inches, miles or nanometers - stick with your unit of measure and it'll work). Multiply the number from the chart by the radius of your ring and it will work. For example, if you wanted 10 divisions, you'd go to the 36 degree angle line on the chart. Let's say your ring has an O.D. of 22 mm. The radius of your ring would be 11, so you multiply the number corresponding to a chord with an angle of 36 degrees (0.61803) by 11 (0.61803 x 11 = 6.79833 mm), and that's the distance between the points of your divider that will exactly divide the ring into 10 equal segments.

Easy? Hey, I started by saying I use the other method. For those of you wired to want to know another way, here it is. Because the chart list is only for whole number degree center angles you'd have to interpolate for anything else.

Another (faster) way is to use CAD. Draw a circle, draw an inscribed polygon of the correct number of divisions you want for your ring/circle (sharing the same center point), and measure the length of one edge of the polygon. It takes about 30 seconds to do this. Set your dividers and you're ready to go. You can get a free version of a very powerful CAD program that will allow you to do what I just described. Rhino is the name of the software and McNeel & Associates is the developer. www.rhino3d.com It's a full version, with a limitation on time/number of saves. I have no connection to the company, I don't benefit in any way, etc., etc., but if you've been wondering about CAD, it doesn't get much cheaper than free, and they have a lot of tutorials on their site to help get you going.

Best regards,
Doug
 

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TallGary

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#16
To make this easier...
Print the PDF
Select the template you need for your chosen number of segments.
Place the ring on the template - centered for even spacing, off-center for graduated spacing.
Mark the ring with your selected spacing.

Hope this helps.

Regards,
Gary
 

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