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Measuring Distance on Circle Circumfrence

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Forum topic by Jim Crockett (USN Retired) posted 836 days ago 1349 views 0 times favorited 10 replies Add to Favorites Watch
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Jim Crockett (USN Retired)

852 posts in 2332 days


836 days ago

I am planning a holder/organizer to hold all of my forstner bits. I want to make it circular with hole spacing so that each bit will have 1/4” clearance from both sides. Have all of the measurements completed but began wondering how I am going to measure point-to-point distances (to 1/32”) on a circle’s circumfrence. Sure, I can use a rule and measure straight-line distance and that will probably be close enough, but there must be a good way to measure along the circle’s edge.

Any thoughts, suggestions, or sarcastic remarks?

-- A veteran is someone who, at one point in his/her life, wrote a blank check made payable to "The United States of America," for an amount of "up to and including his/her life".


10 replies so far

View crank49's profile

crank49

3338 posts in 1570 days


#1 posted 836 days ago

Use a compass.
Swing an Arc at the required length from hole to hole.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View kizerpea's profile

kizerpea

746 posts in 967 days


#2 posted 836 days ago

yes there is a…. cloth tape used for sewing an making clothes..1st u need to know the dia of the circle u want to make…example 20”dia circle…20×3.14 =62.8 in around the circle…that may b right i have been up to long to think clear.

-- IF YOUR NOT MAKING DUST...YOU ARE COLLECTING IT! SOUTH CAROLINA.

View Doss's profile

Doss

779 posts in 864 days


#3 posted 836 days ago

Is there any way you can post up a sketch of the holder and the measurement you’re trying to figure out? I can’t seem to visualize what you are talking about… but’s it’s going to be simple math when I see how you envision it.

-- "Well, at least we can still use it as firewood... maybe." - Doss

View a1Jim's profile

a1Jim

112000 posts in 2176 days


#4 posted 836 days ago

Since a circle has 360 degrees I would use the compass to walk it around the circumference and divide it up into equal measurements until you get what you want. I guess I would start by dividing it in half.

-- http://artisticwoodstudio.com Custom furniture

View dustbunny's profile

dustbunny

1149 posts in 1894 days


#5 posted 836 days ago

Google Sketchup.
Fast, easy, and accurate.
Free download.
Do the layout and print a template 1:1 scale.

Lisa

-- Imagination rules the world. ~ Napoleon Bonaparte ~ http://quiltedwood.com

View Jeremy Greiner's profile

Jeremy Greiner

568 posts in 1371 days


#6 posted 836 days ago

Woo hoo math time!
Measuring from center point of the bit, to the center point of the next bit will be just as accurate, realistically this is how circles are produced on computers, just with a ton more points.

The key is making sure that it’s always equal distant from a primary center point, the center of the circle.

You need to calculate out the radius of the circle, to do that .. measure the size of all your forstner bits together, and add 1/4’’ for each forstner bit.

For example:
2’’ + 1 1/2’’ + 1’’ + 3/4’’ + 1/2’’ + 1/4’’ = 6’’ + (1/4’’ per bit) = 7 1/2’‘

This is the circumference of your circle, from that you can calculate the radius by taking the circumference dividing it by PI, then dividing it by 2

7.5 / 3.14 = 2.388535031847134 / 2 = 1.194267515923567 (you can round to the nearest 32nd, 16th or 8th if you want, just always round UP, if you round down your could find your circle too small)

Rounding to the nearest 16th would be 1 and 4/16ths or 1 1/4’‘

Our radius will be 1 1/4’’ .. now that we know this, draw out a circle with a 1 1/4’’ radius either using a string, or a compass.


With the circle drawn out pick an arbitrary point on the circle, this will be your start point. (A on the picture)
From your start point, take your ruler and measure out the distance to your next point.

The distance from 1 point to the other, is half of the width of bit 1 + half the width of bit 2 + 1/4’‘

Here is a list of distances, based on the bits above.
1’’ + 3/4’’ + 1/4’’ = 2’’
3/4’’ + 1/2’’ + 1/4’’ = 1 1/2’‘
1/2’’ + 3/8’’ + 1/4’’ = 1 1/8’‘
3/8’’ + 1/4’’ + 1/4’’ = 7/8’‘
1/4’’ + 1/8’’ + 1/4’’ = 5/8’‘

So after I just pick out point A, I measure 2’’ from A on the circle, this is point B, then measure 1 1/2’’ from point B along the circle etc.. repeat until all dots are are drawn.

Here is a final picture of all the dots.

Once you have the dots, those are the center points for your drill bits you just have to drill them.

Hope this helps, if you have any questions feel free to let me know :)

-jeremy

-- Easy to use end grain cutting board designer: http://www.1024studios.com/cuttingboard.html

View Tootles's profile

Tootles

681 posts in 1101 days


#7 posted 836 days ago

I agree with Lisa – Sketchup.

But when working with circles for applicatiions like this, it is worth increasing the number of segments in the circumference of the circle. The standard is 24 so the endpoints of the segment lines are 15° apart. All you do is select the circle tool and immediately type in a bigger number – 60 will give you 6° between segment end points, 180 will give you 2° between segment endpoints etc.

-- I may have lost my marbles, but I still have my love of woodworking

View Tootles's profile

Tootles

681 posts in 1101 days


#8 posted 836 days ago

Careful Jeremy – the straight line distance betwee A & B is 2” in your photo above, but if the circle radius is 1 1/4”, then the segment length is 2 5/16”.

What this means is that the radius calculated by Jeremy may be a liitle small. It would take a bit of trial and error to work it all out.

-- I may have lost my marbles, but I still have my love of woodworking

View Jeremy Greiner's profile

Jeremy Greiner

568 posts in 1371 days


#9 posted 836 days ago

It’s not a little small, it’s actually a little big due to rounding.

Here is a picture of the ring with the forstner bits on them.

If you drill the holes with the forstner bit you intend to put in that hole, centered on the dot you made, the spacing is pretty close to perfect.

-- Easy to use end grain cutting board designer: http://www.1024studios.com/cuttingboard.html

View Tootles's profile

Tootles

681 posts in 1101 days


#10 posted 836 days ago

Okay, you’re right. I’ve done it mathematically using angles they way I calculated the segment length. It does work out a little bigger, not smaller. Best of all, the emphasis does also turn out to be on “little”. Good Job Jeremy

-- I may have lost my marbles, but I still have my love of woodworking

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