Rule of thumb method. This is probably close enough for home carpentry.

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Rule of thumb method. This is probably close enough for home carpentry.

ER — 10/23/2006 5:24:00 PM (archived)

But it's not exact. It's how we made hexagons in school: draw a circle, then without changing your compass spread, place the point anywhere on the circumference and draw an arc intersecting the circumference. Move the compass point to the intersection and draw another one, and so on all the way around. You will find your last arc doesn't quite match up with the first, but it will be very close.

The whole thread (37 posts)

Question: how do you find 3 points on a plywood circle 120 degrees apart, without a protractor? (n/t) - RobVg - 10/23/2006 3:22:08 AM
- Without a template, you put your anchor bolts in. Drill out the plate. - Bowser - 10/24/2006 6:34:51 AM
  - Slight addendum. - Bowser - 10/24/2006 3:41:35 PM
    - You are correct. - RobVG - 10/24/2006 4:26:51 PM
- Yet another method - Fox - 10/24/2006 2:50:16 AM
  - Oops... Laurie beat me to it. - Fox - 10/24/2006 2:51:19 AM
    - Laurie beat me to it, too. - podrock - 10/24/2006 4:04:21 AM
- Here you go, have a protractor: - podrock - 10/23/2006 4:35:57 PM
  - Or... - podrock - 10/23/2006 5:53:39 PM
  - Sorry, that's a compass. ;) (n/t) - RobVG - 10/23/2006 4:41:01 PM
    - Without the needle, it's a protractor. :) - podrock - 10/23/2006 5:01:16 PM
- If I remember enough geometry... - alcaray - 10/23/2006 3:33:38 PM
  - Close. It's 2R sin (60 degrees). (n/t) - ER - 10/23/2006 5:16:08 PM
    - I used geometry, not trig... - alcaray - 10/23/2006 8:36:56 PM
      - What is "rad-5"? - ER - 10/23/2006 10:51:36 PM
        
        final answer! - alcaray - 10/24/2006 5:09:43 PM
        
        Correct! - ER - 10/24/2006 6:28:41 PM
- Cut a circle out of paper, exactly the same size as your plywood circle - SDF - 10/23/2006 1:19:47 PM
  - Rule of thumb method. This is probably close enough for home carpentry. - ER - 10/23/2006 5:24:00 PM
    - Geometrically precise. Error comes from application - nick hoffman - 10/25/2006 12:36:46 PM
      - belay my last - it was just a senior moment - ER - 10/25/2006 2:56:07 PM
      - Another job for the CRC. - ER - 10/25/2006 1:33:16 PM
- Yes, there is a way! - ER - 10/23/2006 11:04:03 AM
- So... tell us what you're building! (n/t) - Laurie Street - 10/23/2006 5:18:08 AM
  - Backyard Observatory. - RobVG - 10/23/2006 4:02:47 PM
    - re: Backyard Observatory. - Laurie Street - 10/23/2006 4:10:28 PM
      - Sure. - RobVG - 10/23/2006 4:32:49 PM
- Why can't you use a protractor? - ER - 10/23/2006 4:22:17 AM
  - Yes, there is a way! - ER - 10/23/2006 11:00:51 AM
  - re: Why can't you use a protractor? - Laurie Street - 10/23/2006 5:19:24 AM
    - Trisecting the angle--attention Mary Anne - ER - 10/23/2006 1:16:37 PM
      - While we're at it: Cameron's Hydraulic Tables? - Fox - 10/24/2006 2:35:29 AM
      - CRC Handbooks - Laurie Street - 10/23/2006 4:02:00 PM
      - Pocket Ref - SDG - 10/23/2006 3:34:20 PM
  - Because I don't have one smarty pants. - RobVG - 10/23/2006 4:40:17 AM
- It's impossible - Laurie Street - 10/23/2006 3:35:48 AM
  - Thanks Laurie, your method is close enough. (now where did I put that string?) (n/t) - RobVG - 10/23/2006 3:44:38 AM

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