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Thread: Parabolic curves for solar collectors/hot dog cookers

  1. #1 Guest

    Default Parabolic curves for solar collectors/hot dog cookers

    Do any of you mathematician/solar energy guru's out there have a dxf or ? file of a true parabolic curve that is copyright free so the rest of us can make hot dog cookers or solar water heater parts? tnx in advance ko

  2. #2
    Gerald D Guest


    How to construct your own parabolic curve for your cooker:

    1. Decide how far you you want the focal point (F) to be away from the reflector directly behind it (R)
    2. Draw a baseline (B) that is 2 times FR away from the focal point. Therefore BF = 2 X FR, or put in another way, the reflector is halfway between the focal point and the baseline.

    (This is the most convenient (for reflectors) definition of a parabola: THE REFLECTOR IS THE SAME DISTANCE FROM THE FOCAL POINT AS IT IS FROM THE BASELINE)

    3. Now draw a construction line (C) parallel to the baseline, but at a distance (towards F) of slightly more than the original BR. Let’s call this distance BC.
    4. Draw a circle with radius BC around the focal point. Note that it is BC, not FC, although the centre is on F.
    5. The parallel construction line and the circle will intersect at two points that are on the parabola.
    6. Carry on with a couple more constuction lines and circles, each time finding the intersections when the parallel distance equals the radius.

    I am not going to get involved with a discussion on drawing a curve throught the points and how the ShopBot will give a true resolution – it has been discussed elsewhere on this forum from time to time. However, for cookers, short straight lines are probably OK.


  3. #3
    Join Date
    Mar 1999
    Bob Neitzke 360-795-3621, 275 Columbia St Cathlamet WA 98612


    now here is an interesting project !

    With my 3d-CAD one can place a curve/arc/parabolic
    and then spin it to get either a solid or surface.
    Given GD's formula(s) one could construct the curve with points and then pass a b-spline thru them to get the size/scale desired also.
    It'll depend on the parabolic function capabilities of whatever CAD you use.

    Of course the toolpath to dig out the whole thing is the real problem.

    One "could" pass sets of parallel "cutting planes " thru the parabolic surface to get the "top view" circles. And once those circles are defined, then do a "copy parallel" of the circles, inward, to get concentric "cutting circles.

    A bit labor intensive but you would have the opportunity to select ...
    1. plane seperation to suit tool size
    2. Circle offset to suite tool size

    Final pass would then just be the original circles.

    As you can see I'm always attempting ot get CAD to create the toolpaths without having to employ an expensive and usually expensive post processing package/"add on" to the CAD.

    Given success on the CAD function ...
    I'm not sure how you will be "surfacing" the final cut and finished part to achieve reflectivity an heat dissapation.

    I have heard that "fluid/liquid" when SPUN will create a parabolic shape.
    So, maybe one could spin a puddle of thin epoxy or other curable fluid until it hardens and THEN do some sort of reflective coating.

    This is gonna take some effort so I wouldnt run out and buy those HotDogs just yet :-)

  4. #4 Guest


    I'd always heard that spinning a fluid makes a mess.

  5. #5
    Gerald D Guest


    Bob, I can see you salivating already!

    Shouldn't a 2D parabola be sufficient for the sausage of a burger - maybe even better!. The sausage is a a thin cylinder which can pass through the focal point perpendicular to the sketch above.

    If you cut two plywood supports to fit in between the baseline and the reflector, space them one hotdog length apart, and put a sheet of shiny stuff on the curved surface, you could be cooking in about an hour if the sun is shining!

    Another useless statistic that is cluttering my mind: The sun's radiation is about a kilowatt per square yard (on a surface perpendicular to the rays, here on earth, if no clouds/pollution in the way, if sun has risen to a reasonable height, etc. etc.)

  6. #6
    Join Date
    Mar 1999
    Bob Neitzke 360-795-3621, 275 Columbia St Cathlamet WA 98612


    You'll need a decent speed control for the fluid spinning "trick".

    BTW, I've sworn OFF hotdogs !

    I'm WITH YOU but lets not loose track of the original problem posed by KO.

    Certainly PLANAR parabolas are easier to construct. Like back in highschool with a band saw, when we still remembered some algebra :-)

    Obviously we SB users will NOT be doing mass production of Parabolic cookers. Therefore the easiest thing to do is to head for the nearest military scrap yard and buy a "used" antenna.

  7. #7 Guest


    Here's a little something to play with.

    It is set up to demonstrate the Swept curve pocket in Vector 9.0. There is an Excel spread sheet with formulae included that you can use to play with the parabolic shape. If you do not have version 9.0, you could copy-paste the offset-spline curve about the Z axis with multiple copies to create an Orange section type of cut or use the Skin function to create passes in a radial direction.

    It should be obvious from the graphics that it is quite easy to develop the "waterline" tool path with Vector 9.0. We also have put some pictures of parts cut with this function on the Vectorcam web site.

  8. #8
    Gerald D Guest


    Bob, that is one of the things that I envy the most about you Americans - the quality of stock, and the convenience, of the "nearest military scrap yard"!

  9. #9
    Join Date
    Mar 1999
    Bob Neitzke 360-795-3621, 275 Columbia St Cathlamet WA 98612


    Fred is definately on the right track !
    I like his web example.

    He's done what I would have with my CAD's curve and surfacing functions.

    But we still dont have a tool path,... do we ?
    I havent gotten into Vector usage.

    Unless Vector does a nice bunch of concentric, circular toolpath "THINGS" then it's not much use, except for the drawing and conceptulization ( like my CAD too ).
    I'm not taking a shot here. ok :-)
    Vector looks like it has some kewel functions.

    I'm just back to my original notion of passing planes thru the parabolic surface that Fred or whoever has created to help generate the toolpath(S).

    Gerald, yep ! we DO have lotsa scrap !
    if we havent ground it all up and used it as aggregate in some new project.
    But seriously folks, the last time I was actually at a military scrap yard was 25 yrs ago.

    Hey, wait a minute ...
    isnt the JUNK YARD WARS cable tv program originating from the UK ???

  10. #10 Guest


    It's a tool path, compensated for a 1 inch diameter full radius cutter. (I put photos of some wooden parts that I cut on the Vector 9 page. Note: they are not round)

    You can generate a planar slice by breaking the swept curve into sections the thickness of your material(copy-paste a line in X-Z plane, multiple copies and break intersect) and then generating the rings. Each one can be a separate tool path & they can all be arranged on a flat sheet with locating/alignment holes so that it is easy to stack them up and glue together the complete assembly from sheet stock.

    I'm considering making this a class project for our next Shopbot-Vector training class.

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