Marking curves

Discussion in 'Carpenters' Talk' started by -chippy_john, Jul 8, 2008.

  1. -chippy_john

    -chippy_john New Member

    I had to make some curved firring pieces today for a lead roof on a bay window (think Chinese Pagoda shapeish).

    1250 mm in length with a 60mm deflection from straight at the centre. I ended up marking a pattern piece freehand before cutting with a jigsaw and cleaning it up with a spokeshave. It all turned out fine but is there a better way to mark out something like this?

    Normally for curves beyond the compass of my compass I would use a batten with a nail through one end and a pencil the other (or a piece of string) but with an approximate diameter of about 8m that didn't really seem practical.
  2. The Dormouse

    The Dormouse New Member

    I use a cheap set of kiddies plastic curves & circles from WHS but I haven't looked there recently.
  3. Wayne K

    Wayne K New Member

  4. Mr Mike

    Mr Mike New Member

    I would've done similar John, but probably would've bent something like some spare parting bead and pinned that to the pattern piece to get/mark out the curve. Then if I didn't have access to a bandsaw, I would've cut with jigsaw as you did.......and then cleaned up with a compass plane.....(which I have but doesn't get used enough !)
  5. -chippy_john

    -chippy_john New Member

    Cheers guys;

    Dormouse, are you talking about some sort of bendy plastic gizmo? It's certainly an idea, although you would still have to create the curve yourself.

    Not sure about the spreadsheet Wayne,it's an interesting link but will it mark out a curve for me?

    Thanks Mike, so far it looks like a hand bodge is the only way to go. Would have been better on a band saw,(if they'd been ordered earlier,) but I'm probably cheaper and quicker than the joinery shop anyway.
  6. cabinet

    cabinet New Member

    Out of curiosity I have just had a look at the Excel link offered by Wayne this gives a radius of 3.285m for a cord length of 1250 and a height of 60. I double checked this with my AutoCAD programme and yes 3.285 it is correct. If you know the radius then a nail, piece of string and a pencil you can mark out the radius on the timber in question.

    Or you could use a length of chain or make a set of trammels from a length of timber, to mark out the radius once calculated by Wayne,s link

    Wayne thank you for a very useful link I for one are most appreciative
  7. -chippy_john

    -chippy_john New Member

    I did appreciate Wayne's link and I'll certainly remember it for the future when with some prior warning I could make use of it, however this particular job was thrown at me on site, "just cut some firrings for this roof will you". I needed something right there and then and was really just wondering what others would have done.
  8. cabinet

    cabinet New Member

    You might want to put this in your site note book (you know the one you keep in your tool box with your pocket calculator, that your record allsorts of info in just in case it comes in useful)

    L = ½ the length of the chord (span) connecting the two ends of the arc;

    R= Deflection squared plus length or cord squared divided twice the deflection.

    R= D² + L ² ÷ 2 D

    In your example

    D = 60 therefore 60² = 3600

    L = 1250 ½ =625 therefore 625² = 390625

    Twice D = 120

    R= 3600 + 390625 ÷ 120 = 3285.208
  9. -chippy_john

    -chippy_john New Member

    Cheers cabinet, duly noted, just need some non-stretchy string now.
  10. cabinet

    cabinet New Member

    I use a cheap set of kiddies plastic curves & circles
    from WHS but I haven't looked there recently.

    Do children’s geometry sets contain a french curve that will be able to mark a radius of 3.285m over 1.250m ?
  11. cabinet

    cabinet New Member

    Cheers cabinet, duly noted, just need some
    non-stretchy string now.

    Pleased to be of assistance; some times us cabinet makers who are as old as young conker trees have our uses.

    non-stretchy string to go with you site note book!

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