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  1. #1
    Join Date
    Dec 2018
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    NSW
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    586

    Default Notching RHS to a cylinder- how to calculate angles

    I'm building a BBQ, the big, obnoxious, American kind- AKA an Offset smoker.

    I'm using a cylinder of approx 530mm dia. I don't own a spring caliper, so 530mm is as best as I could get with the tools I have.

    The frame is a truncated isosceles triangle which intersects the cylinder at a point (a chord of the circle).

    Is there an easier (maybe mathematical) way to calculate rough angles to allow a bevel to be cut then scalloped , say, for an edge of the RHS to meet a given chord of said circle?


    I reverted to the old cardboard cutout, and transposed the shape after laying out the design on my welding table with chalk, compass and ruler. Worked, but a lot of messing about.
    20190721_154233.jpg

    Pic of the project. Disregard the plumb line which is clearly not plumb. Also disregard the dirt floor fab area I'm working in. It's a bit too big for inside my little shed.

  2. #2
    Join Date
    Dec 2018
    Location
    NSW
    Posts
    586

    Default

    ... also disregard the fact that gravity clearly doesn't apply in my workshop.
    Explains all the missing 10mm sockets...


    Hindsight being 20/20... I could have worked out radius / diameter of cylinder using its circumference.

  3. #3
    Join Date
    Oct 2011
    Location
    Norwood-ish, Adelaide
    Age
    59
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    6,542

    Default

    The angle of bevel will be the tangent of the circle where the leg touches. The tangent is at 90 degrees to a radial line, so using some trig if you can work out the angle of a radial line to the contact point, the tangent angle should be easy.

    Michael

  4. #4
    Join Date
    Jun 2007
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    sydney ( st marys )
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    64
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    4,887

    Default

    If you are making more just stick with the template.

  5. #5
    Join Date
    Dec 2018
    Location
    NSW
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    586

    Default

    I'm going to brush up on my trig, and bust out the year 9 math skills...
    I don't know why, but this job bamboozled me. The whole time as I was man-handling these lumps of RHS on and off the bench, I was thinking ' there has to be a way to at least get the cut close first time'.

    I'll need to work out some examples, but in the instance pictured above, the cylinder is not centered on the apex of the triangle. Not sure how this will affect the calculations steps, but in the end the fun is in working it out.

    The next one I build will be a bit more balcony friendly. 100L as opposed to 400L water capacity

  6. #6
    Join Date
    Aug 2008
    Location
    near Rockhampton
    Posts
    6,216

    Default

    I generally just draw stuff in one of the free CAD programs and measure from there.
    Gold, the colour of choice for the discerning person.

  7. #7
    Join Date
    Jul 2011
    Location
    Adelaide
    Posts
    837

    Default

    Quote Originally Posted by Michael G View Post
    The angle of bevel will be the tangent of the circle where the leg touches. The tangent is at 90 degrees to a radial line, so using some trig if you can work out the angle of a radial line to the contact point, the tangent angle should be easy.

    Michael
    It is only a tangent if the leg lies on a radial line which is not necessarily the case.

    Edit. I see what you mean now. You are correct.

  8. #8
    Join Date
    Jul 2019
    Location
    Caboolture
    Age
    47
    Posts
    28

    Default

    place the leg against the cylinder where you want it
    place a marker in the middle of a washer
    Run washer around outside of cylinder while pen marks the leg.
    Should give you the profile to cut.

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