How do I quarter a circle using a compass?

Discussion in 'Carpenters' Talk' started by dvddvd, Jul 5, 2021.

  1. dvddvd

    dvddvd Well-Known Member

    I have some large metal rings and want to mark and drill each one in 4 equal places around the edge.

    They are 650mm diameter so that makes the circumference 2040mm, but how do i work out the measurement if im using a compass to mark each place to drill?

    Of course dividing the circumference by 4 only gives the outside edge measurement between each mark not if im using a compass?
  2. Hans_25

    Hans_25 Screwfix Select

    Set the compass to 460mm and walk it around the circumference, it should divide it into 4. Let me know if it works and I'll tell you how I worked it out ;-).
  3. Astramax

    Astramax Super Member

    What happened to degrees!
    Abrickie and I-Man like this.
  4. I-Man

    I-Man Screwfix Select

    Don't quite get why do need to use a compass?
  5. Astramax

    Astramax Super Member

    North, South, East and West.
  6. Hans_25

    Hans_25 Screwfix Select

    Today is a no news day
    Abrickie and I-Man like this.
  7. I-Man

    I-Man Screwfix Select

    Never Eat Shredded Wheat is still my go to
    Astramax likes this.
  8. dvddvd

    dvddvd Well-Known Member

    Just thought it would be easier? Then I dont have to find the centre etc
  9. Astramax

    Astramax Super Member

    Do a drawing and use that as a template.
  10. Astramax

    Astramax Super Member

    Look for a 360 degree protractor.
  11. CGN

    CGN Screwfix Select

    Visualise it as a square/right angled triangles. You can then use trigonometry
  12. metrokitchens

    metrokitchens Screwfix Select

    Bit of string 510mm long is your friend. Use the string as a measure to mark drill points. Job done and no maths involved
    Peterdevon likes this.
  13. dvddvd

    dvddvd Well-Known Member

    thanks all
  14. RolandK

    RolandK Screwfix Select

    Tape some paper together and put the ring on the paper and draw round it. Cut the paper circle out, fold it into quarters, put it back on the ring and mark the drilling points.
  15. FlyByNight

    FlyByNight Screwfix Select

    With a pure pair of compasses.

    Draw round the ring on a flat sheet of paper or ply.

    About 30-35% or the way up the circle draw a straight line from two points on the circumference.

    Using just your compasses, bisect that line, and then join the two point used to create the bisector and extend it out to the circumference at both ends. That will be a diameter.

    Bisect the diameter and again the bisecting line will be a diameter at right angles to the first, giving two more points.

    You then have four equi-spaced points on teh circumference.

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