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  1. #1
    Join Date
    May 2009
    Location
    UK
    Posts
    1

    Default Pattern for making a cone/frustum from sheet metal

    Hi,

    I am hoping to make a frustum (a cone with its end chopped off) which will act as an amplifier/horn for an noise instrument I am building. I will cut a flat shape from sheet metal, before bending and welding it to form the frustum.

    I have good technical support from the workshop in which I'm working, but I do not know how to draw the pattern to cut from the sheet. I've searched the net, but to no avail. I can imagine it's fairly simple once you know the math behind it, so if someone could show/tell me how to work this out it would be really great.

    The smaller opening of the shape will be 30cm dia.; the larger opening 100cm in dia.; and the length 100cm,

    Many thanks,

    Phil

    (P.S.: I am certainly no expect, so please be gentle!).

  2. #2
    Join Date
    Jan 2004
    Location
    Mackay North Qld
    Posts
    6,446

    Default

    Hi
    First up and welcome to the forum.Lets us know how you get on.

    100 CM,s -that is 1 metre diameter - Its a fair sized cone but the principals the same, small or large.

    I would be marking directly to the sheet. For something that big I would make up a set of trammels.. ie a stick with with points that can be be moved along the stick to allow use as a large scriber compass.

    Basically you need to mark out the base ie 100 cms,
    halfway along mark a perpendicular line upwards for a height 100cm.

    • At the top of the perpendicular line mark out 15 cms each side so that line is parallel to the base line.
    • Run a line from each side from the base to the top picking up the end points of the parallel line. When this is done ,you should have a traingle 1metre at the base and 2 sides the same length that apex above the one metre mark.
    • The apex point is where you swing your radius from. Using a flexible tape mark out the circumference along the radius you have marked. Maths says it should be 3 metres 141.6mms long.Centre that circumference either side of your vertical apex line . Divide your radius up into equal sections .Something of that size say by 24 .
    • Your top radius is laid out the same and should have the same no of divisions as the bottom.

    I would do a couple of practices on paper first.

    Grahame

  3. #3
    Join Date
    Oct 2006
    Location
    Tallahassee FL USA
    Age
    82
    Posts
    372

    Default

    What Grahame said, but verify your definition of "length." His procedure will produce a frustrum with the stated length perpendicular between the planes of the top and bottom. If the defined "length" is along the sloping sides, a different kettle of fish, and a slightly different procedure.

    A full-size paper pattern will be unwieldy. Thank Goodness for metric. Divide all measurements by 10, for a scale-model test.

    If this is the most complicated part of the instrument, and if precision isn't too critical, make this part first, and make the other parts to suit it.

    Cheers,
    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  4. #4
    Join Date
    Jan 2004
    Location
    Mackay North Qld
    Posts
    6,446

    Default

    Hi Joe
    Of course your right,thank you for the reminder."True length""-the sloping sides.
    The term is true length and it is what I should have mentioned


    To make this clear , one must visualise the cone sitting on its broad base.There is a difference in length to what we see in 2D ie flat drawing from the central vertical line and the real length or "true length" which in reality lays on the incline.We use the central vertical line to establish the height from base to frustum which in this case was 100cms or 1 metre.

    The line joining the large diameter to the small diameter is the true length in this case.

    Also I have had a rush of excrement to the brain re trammel heads.
    If you have too much cash by all means spend the $80Au or $90Au to buy a pair ,but if you are in poverty like the rest of us may I suggest the el cheapo model made from 50 x 25 box section.

    One cuts out one of the 50 faces and drill say a 12mm hole (1/2" if that takes your fancy) and insert a a 12piece of all thread. Tackweld the all thread into the hole and drill a hole to suit a hardened concrete Nail.Drill the hole so its flush up against the other uncut 50mm face of box.

    Thats your scriber.A washer and nut to lock it against the box face and on to the lock mechanism for the timber arm.Its just another hole ,say to suit a mm bolt and nut. Insert bolt with nut screwed on through hole and tack weld.
    Make two of these and them cut a length of timber (lumber for Joe) to suit the length of your arc-something around 1.6mts ,I guess.

    I have attached a diagram.There,s no nut drawn ,but there's enough detail to get the picture.

    Grahame

  5. #5
    Join Date
    Oct 2006
    Location
    Tallahassee FL USA
    Age
    82
    Posts
    372

    Default

    Ouch! $80 for trammel points?

    http://www.mcmaster.com/#

    I entered "trammel" in the search box. I have a set like Figure A. I've used them for swings up to about 10 feet, with furring strip for the beam. (Two-person job, IIRC).

    For less heroic layouts, I use a strip of mylar about 1" wide, length less than my two arms span, and a push pin for the pivot. Another push pin to punch the radius hole, to accept the pin or a pencil. Hold taut for swinging. For fine adjustment, punch a new hole nearby, and mark which hole is right. When there are enough holes to create confusion, replace the strip. Or, "erase" the previous holes with masking tape, to punch new holes.

    And thanks for the reminder about "true length" nomenclature, Grahame.

    Cheers,
    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

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