Page 1 of 6 123 ... LastLast
Results 1 to 10 of 58

Thread: How to make a Radius Dish

  1. #1
    Join Date
    Jun 2009
    Location
    Between Bordeaux & the Atlantic. S.W.France
    Posts
    159

    Default How to make a Radius Dish

    Radius dishes are fairly expensive to buy and very messy to make if you make them, as most do, from MDF using a router or sander and dedicated jigs. Here is another method of making a dish which is cheap, clean and easy and you donít even need a router so you donít need to make jigs. The only disadvantage I can think of is that it involves a little math, although if you have a CAD programme you can even avoid that. I should mention that this is not my idea. I saw something similar a long time ago on another forum but I canít remember where or who posted it, otherwise Iíd give him credit.

    What this method involves is forcing a thin sheet of MDF into a spherical dish by screwing it down to a heavier rigid base with a series of spacers between the two. Hereís a drawing of a section through the dish showing what I mean.


    Using the formulae you can decide how many spacers you think you need and at what radii (r) and calculate the thickness they need to be using the second formula. If you have some strips of wood that are close to the required thicknesses, you can use the first formula to calculate at what radius to place them.
    If you have a CAD programme you just draw it out and the programme will tell you how thick to make the spacers or where to place them.

    So having done the math, I prepared the materials and this is what they looked like.
    A piece of 1ľĒ particle board (offcut from a kitchen worktop). 16Ē diameter seems to me to be big enough for any size of ukulele.
    A piece of ľĒ MDF
    Strips of wood for spacers
    Woodscrews


    I taped the strips together and marked them at 1Ē intervals, then drilled clearance holes for the screws in the centre of each section.


    I then sawed the strips down the middle (the brass rod was to line them up, I didnít saw through that!).

  2. #2
    Join Date
    Jun 2009
    Location
    Between Bordeaux & the Atlantic. S.W.France
    Posts
    159

    Default

    The strips were then sawn into separate pieces.


    The base board was then marked out for the positions of the spacers (12 on each circle seemed to be enough) and pilot holes drilled for the screws.


    I stuck the spacers down using thin CA (Zap)


    Here they are all glued in place. Note that the inside edge of the spacer is placed on the circle where the MDF disc will make contact when it is screwed down.

  3. #3
    Join Date
    Jun 2009
    Location
    Between Bordeaux & the Atlantic. S.W.France
    Posts
    159

    Default

    Clearance holes for the screws are drilled and countersunk in the MDF disc. Make sure the screw heads are below the surface. I then put a blob of glue on each spacer, positioned the disc and screwed it down, starting from the centre. Here it is, finished with a piece of wood placed on it to shown the curve.


    If you were worried about the dish flexing under pressure from go-bars, for instance, a few blobs of strategically placed ‘Bondo’ (car body filler) before screwing the disc down should make the whole thing solid. Just make sure you get it screwed down before the Bondo hardens!

    Some people have made a similar dish by using one spacer in the form of a ring around the outside edge and one screw in the centre of the disc, but this doesn’t give you a spherical form. This drawing shows the difference. The red shape is circular. The green shape is a spline curve which is tightest in the middle and flattens out towards the ends.


    This shape can be used if you want to make tops or backs that are only curved in one direction and instead of a dish you make a ‘trough thingy’. In fact it’s the logical shape to make for that sort of top or back, as it’s the shape the wood takes naturally if you flex it. I made this one for an acoustic bass guitar with a deep curve on one side, for the back, and a shallower one on the other for the front. The intermediate spacers were just pushed into place and glued to make the whole thing more rigid.

  4. #4
    Join Date
    Dec 2009
    Posts
    19

    Default

    Nice photos.
    John

  5. #5
    Join Date
    Aug 2009
    Location
    Lansing, Michigan
    Posts
    1,572

    Default

    Very interesting, and certainly a technique I'd not seen before! Thanks for sharing it with us.

  6. #6
    Join Date
    Dec 2007
    Location
    Roseville, CA
    Posts
    1,827

    Default

    Nice photo documentation indeed. And the CAD images are great too. A little more time consuming than other methods but much cleaner and healthier than having all that mdf dust in the shop. Thanks for sharing.
    --------------------------------------
    Dominator

    Dominator Ukulele Tabs
    Dominator on MySpace
    Dominator on Youtube
    --------------------------------------
    Practice Makes Practice Perfect

  7. #7
    Join Date
    Nov 2009
    Location
    Smyrna, Georgia
    Posts
    2,196

    Default

    Very nice tutorial and great photos and drawings. Concerning whether you get a spherical shape when you use a ring, would you get a parabola instead? I presume the purpose is to focus the sound, so a parabola may actually work better ( more likely about the same) if you build it with the focal point in the right place. I have no idea where the right place is for a ukulele's sound to be focused. I am planing on a flat bottom for my first build, but think your method will be the one I use for my first disc. Thanks.

  8. #8
    Join Date
    Aug 2008
    Location
    Wales, UK
    Posts
    5,055

    Default

    That flat build will cause you problems for sure...

  9. #9
    Join Date
    Jun 2009
    Location
    Between Bordeaux & the Atlantic. S.W.France
    Posts
    159

    Default

    My pleasure. Sharing is what makes great forums.

    I'm not sure myself what the difference is between a spline curve and a parabola, except that I seem to remember that a parabola is what you get when you take an off-centre vertical slice through a cone. I wouldn't worry too much about 'focussing the sound'. I dont think that's what domed or arched fronts and backs are all about. The problem with a circular dish made with an annular spacer around the edge and one screw in the middle is that the shape changes at different positions on the dish. So, if you stick sandpaper to it and rub your braces on the surface to profile them you're not going to get the shape you want.

    Difficult to describe but in this drawing, the brace is the same shape as the dish.


    But if you move the brace, it doesn't fit the dish any more.


    So if you're making a domed front or back, rather than an arched one, a spherical dish is better as the shape is the same wherever you are on the dish.

    One of the problems with making arched tops or backs is that, for the braces to fit the arch properly, they have to be sanded by holding them in the position they will be in once installed, and rubbing longitudinally in the trough (if you see what I mean).

  10. #10
    Join Date
    Jun 2009
    Location
    Between Bordeaux & the Atlantic. S.W.France
    Posts
    159

    Default

    Quote Originally Posted by Pete Howlett View Post
    That flat build will cause you problems for sure...
    It might be helpful to tell him why Pete

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •