geometry

[Rob Platt]

Draw a circle 573mm radius.
Make your 180 then 90 then 45
Then do your 60 by using your radius size from lets say the 90
any of the above can be divided again if necessary
using a plastic school rule with cms on it you can measure the divisions between the radii around the arc and every degree will be 1cm and then whatever portion there of.
Then any number of sides can be constructed
all the best
rob

 

[bugbear]

Draw a circle 573mm radius.
Make your 180 then 90 then 45
Then do your 60 by using your radius size from lets say the 90
any of the above can be divided again if necessary
using a plastic school rule with cms on it you can measure the divisions between the radii around the arc and every degree will be 1cm and then whatever portion there of.
Then any number of sides can be constructed
all the best
rob

Well, that's another approximation - arc length and chord length are not (quite...) proportional.

BugBear

 

[Rob Platt]

where angels fear to tread

 

[Jacob]

Draw a circle 573mm radius.
Make your 180 then 90 then 45
Then do your 60 by using your radius size from lets say the 90
any of the above can be divided again if necessary
using a plastic school rule with cms on it you can measure the divisions between the radii around the arc and every degree will be 1cm and then whatever portion there of.
Then any number of sides can be constructed
all the best
rob

Well, that's another approximation - arc length and chord length are not (quite...) proportional.

BugBear

Arc and chord are not proportional at all, "quite" doesn't come into it Arc varies from pi x half chord to identity when chord approaches zero.

 

[bugbear]

Draw a circle 573mm radius.
Make your 180 then 90 then 45
Then do your 60 by using your radius size from lets say the 90
any of the above can be divided again if necessary
using a plastic school rule with cms on it you can measure the divisions between the radii around the arc and every degree will be 1cm and then whatever portion there of.
Then any number of sides can be constructed
all the best
rob

Well, that's another approximation - arc length and chord length are not (quite...) proportional.

BugBear

Arc and chord are not proportional at all, "quite" doesn't come into it Arc varies from pi x half chord to identity when chord approaches zero.

Better check your school textbook again; for Rob's number they're very close to proportional - but not quite.

BugBear

 

[Droogs]

oooh - Geometry the new sharpening cool

 

[MusicMan]

Much more fun!

 

[Phil Pascoe]

I should have known not to ask how to divide a circle into seven.

 

[Rob Platt]

What your making is effectively a protractor, the lines for 45,90, 180 and 60 are easily made using a pair of compasses, trammel, length of wood nail and pencil or whatever that can be fixed and maintained. 57.3 X pi = 180. So if you have a point on the arc that is lets say at the 90 radius every unit measured away from that point is 1 degree hence my use in this instance of a plastic rule and cms the less measuring you do around the arc the less likely you are to have an error hence the use of multiple radii. its irrelevant what you use as a unit the maths remain the same 1 unit is 1 degree. If you use cms it can be marked onto a standard sheet of ply or what i use is white painted hardboard
so if you want say a 27 sided figure. Divide 360 by 27 = 13.333. from a point where the radius crosses the arc take your plastic rule bend it so it follows the shape of the arc and mark off the 13.333 units. (easy when you use cms/mms). Take you dividers open them from to the two points and go around the circumference of the circle join up the dots and you will have a 27 sided shape. The actual size of which can be adjusted by reducing or extending the radii. It makes sense to me and I hope I`ve explained it so its understandable. Chords are what you are finishing with but what you are doing is taking your measurements around the circumference.
all the best
rob