Replies

1. 
   ring | Apr 19, 2006 04:30pm | #1

   jackplane,

   The geometry is no big deal but I'm not gonna try to explain it here.  Lay out a square of 775 each side.  Measure in 227 from each corner and that will give you the points of your octagon.  Each side of the 8 sides should be = 321.

   DR

1. 
   jackplane | Apr 19, 2006 05:13pm | #2

   DR,

    yep, that's what I got. Thanks. Sometimes coffee helps only so much. 

2. 
   douglas2cats | Apr 20, 2006 03:28am | #3

   My normal tendency is to mess with the math to get a numerical answer. I replied that way several months ago to a similar thread. Someone else on that thread came up with a much simpler method IMHO. Use a trammel or beam compass to draw a 775mm diam circle centered on your MDF. Then put an accurate 45 deg. or combo square on the corners and put a line that's tangent to (just touches) the circle.

   If you build it he will come.

3. 
   AlanWS | Apr 20, 2006 11:09pm | #4

   If you have a lot to cut:

   Cut squares 30.5" on a side. Put one on a crosscut sled with a corner over the kerf. It's exactly in the correct position when two diagonally opposite corners are each 15.25" (half the side length) over from the kerf. It might be helpful to draw a line there first. Attach a couple of blocks to the sled with double sided tape, touching the sides of the square furthest from the blade. Now, with no measuring or marking, put each square against the blocks and cut off the corners.

1. 
   Daggs | Apr 21, 2006 12:52am | #5

   How to lay out an octagon any size octagon:

    

   From a square of side length X, find the length of the radius that circumscribes the square.  This is the distance from the middle of the square to a corner.  Call it Y

   From the corner of the square, draw a circle of radius Y and mark where it intersects the sides of the square.  Do this at all four corners.

   Draw lines from mark to mark, across the corner.  You'll end up with an octagon inscribed in the square. 

   The math to calculate the length of the side of the octagon escapes me at the moment.

4. 
   eddiefromAustralia | Apr 21, 2006 01:59am | #6

   Jack,

   Rather than repost - here's an old link asking the same question.

   The drawing that I've put up there is the way that I was trained to do it.  If accuracy's not critical, I'd use a tape to transfer the measurement

   http://forums.taunton.com/fw-knots/messages?msg=9961.11

    

   Cheers,

   eddie

5. 
   Dave45 | Apr 21, 2006 04:05pm | #7

   AutoCad makes this stuff easy - lol.

   
6. 
   skycowboypaul | Apr 22, 2006 04:27am | #8

   When I have to do octagons I go here and enter 1 measurement, neatest little thing you ever saw.  It's an octagon calculator.

   http://www.members.cox.net/ultimate_poker_table_top/Octagon%20Layout%20Calculator.html