Radius Dish on a Pedestal drill experiment.

Timbuck

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I was pottering around this afternoon on my new Tenor jig and tidying up the workshop..When Ding! an Idea came into my head on how to make a radius dish on a Pedestal drill...I've seen all sorts of other methods used ( including the two Guys who suspended a router on a 20 foot length of chain from the roof)...Anyway! so I dug out an old 4 1/2 inch sanding disc and made this experimental set up...the workpiece is a disc of MDF I had laying around and I mounted it on a board with a pivot, on the Drilling table tilted the table at 5 degree's and proceeded to rotate the MDF disc by hand and fed the sander into the workpiece...and "waddya know" in a sort of way It worked!...what amazed me was the speed at which the radius was formed....and the finish is spot on.
I haven't done any calculations yet on what is required to produce say a 22 foot Radius..but it make sense that with a 10 inch sanding disc it's possible to produce a 20 inch wide radius dish.
This is just an experiment and it seems to work ok. so now i'll go back to the drawing board and design a better set up with more accurate tooling...Here is a couple of pics so you can fathom out how it works.

PICT6357.jpg

PICT6358.jpg
 
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You are a credit to thinking tinkerers everywhere! Wonderful.
 
Well thats thats it then ..I'm convinced it works..when you think about it..it has to work.. all you need is a sanding disc and a pedestal drill with an angle ajustable type table..(but you can shim the angle)...
I calculate that with a 10" sanding disc to produce a "20 inch diameter dish" with a 20' parabola you need to set the table angle at 1 deg 10'..and go down at the centre point to .200".
To produce a 10' parabola ..Table angle 2 deg 28'..and go down at centre to.428" ..So now i've put all those complicated methods with CNC machines and router cradles and form gauges on the back burner...Oh yes! and don't forget to wear a dust mask and have the vac running all the time ..I'm off now to buy a sheet of MDF to make my first real one...I'll report back later.
 
I've just drawn out on CAD a couple of radii skids - I need to make my sanding dishes again. Far too technical for me Ken. My mate will make a couple of templates for me on his CNC router and I have all sorts of plans for them... yes, I am investigating the world of CNC and if my efforts today prove successful, will be putting dovetails back on tenors, concerts and baritones and may be able to provide mold kits for the Martin style sopranos...
 
I've just drawn out on CAD a couple of radii skids - I need to make my sanding dishes again. Far too technical for me Ken. My mate will make a couple of templates for me on his CNC router and I have all sorts of plans for them... yes, I am investigating the world of CNC and if my efforts today prove successful, will be putting dovetails back on tenors, concerts and baritones and may be able to provide mold kits for the Martin style sopranos...

Any cnc system you're looking at, in particular?
 
That learning curve is a bridge too far. As long as I can create the dxf files I have a man who can do the rest - well three separate people actually - one for inlay, one for one off and one for production...
 
Timbuck:

Would you go through the calculations on how you arrived at the table angles for us less gifted builders.

You refer to the shape as a parabola. Is the resulting shape parabolic or a circular arc?

Your design results in a lot less stuff in the way of jigs to be stored when not in use. It will however produce a lot more powder fine dust than a router that throws out mostly fine wood chips. Your method would work best by moving the drill press outside once the weather gets better.

I have just completed building a radius dish jig based on Dominator's design so I probably will stay with what I've got so I don't have to explain to my wife why the jig I spent good money to build isn't good enough.


Konala
 
My learned Son "Mike" has informed me that it is "not" a Parabola ..So it's a circular arc..or a concave dish or whatever:eek:..To work out the Angle of the table you divide the sanding disc radius by the desired dish radius which will give you the Sine of the Angle for the Table setting (I then use Sine tables to find the angle), but some calculators have that function...Multiply the Sine of the angle by the sanding disc diameter to give you the depth at the centre...You don't realy need the depth, just keep going down into the MDF untill the sander makes full contact.

Here is the calc's for a ten foot rad...Using a 10" sanding disc.
angles.jpg
 
My learned Son "Mike" has informed me that it is "not" a Parabola ..So it's a circular arc..or a concave dish or whatever:eek:..To work out the Angle of the table you divide the sanding disc radius by the desired dish radius which will give you the Sine of the Angle for the Table setting (I then use Sine tables to find the angle), but some calculators have that function...Multiply the Sine of the angle by the sanding disc diameter to give you the depth at the centre...You don't realy need the depth, just keep going down into the MDF untill the sander makes full contact.

Here is the calc's for a ten foot rad...Using a 10" sanding disc.
angles.jpg


:worship::worship:

Wow. That's why I paid a guy $40 to cnc one for me. I have the utmost respect for your talent and genious. I could never have figured that out.

Steve
 
Putting my theory to the test...first I made a 10" sander disc ..loaded it with 60 grit sandpaper.and mounted it in the drill chuck.
Strange how you think some thing will work on the drawing board and when you start to build, problems arise..Tilting the table angle became a slight problem as the column of the drill got in the way..so I swung the set up round 90 degrees to the front and shimmed the angle...
like this ..(The pivot board is fastened to the drill table with woodscrews and washers from beneath)
PICT6408.jpg

Then I mounted my 20" MDF Dish blank on the pivot pin, put the drill into a low gear and lowered the disc down to the workpiece and rotated it with my hand (as it starts sanding the action of the spinning sander rotates the workpiece automaticly) The trick is to get just the right pressure ..so I set the stop on the drill so that the sander is just engaging nicely with the MDF.
Here it is in action (just click on the image)...The actual forming of the radius only took about 5 min's once the set up was done.

You can stop anytime and check how the dish is forming.
PICT6394.jpg

And here is the finished Dish..Not quite large enough for a Guitar but ideal for a Uke.
PICT6405.jpg
 
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:worship::worship:

Wow. That's why I paid a guy $40 to cnc one for me. I have the utmost respect for your talent and genious. I could never have figured that out.

Steve

I paid $36 to have a 20x20 done on the cnc and that included the MDF. I have the utmost respect for you Timbuck. I'd never even try to figure it out. I'm glad my son has a head for math because his dad can't do much more than read a tape measure.

Andrew
 
Rethink

Use the rule of “similar triangles” and the math gets easier. No trig.

r = radius of sanding disc in inches
R = desired radius of dish in inches
d = depth of cut in inches
D = diameter of sanding disc in inches

Then:

r / R = d / D

Solving for d:

d = r / R * D

i.e. 5 / 120 * 10 = 0.417

If you want a 15-foot radius dish and you're using a 10" sanding disc:

d = 5 / 180 * 10
d = 0.278"

If you leave your drill press table level and shim the board that the disc you're making is mounted on, the same ratio will apply (it's similar triangles "all the way down"). Let's say your pivot pin is mounted 12 inches from the end of the board. For a 15-foot radius you'll need a shim at the centerline of the pivot pin 0.333" thick.

To help visualize what's going on, remember your geometry. The intersection of a plane and a sphere is always a circle. The circle here is the edge of your sanding disc.
 
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Thanks for this - I was hopeless at math at school so all this was beyond me. However I took a leap of faith and made 2 new go-bar decks and 2 new sanding dishes. I just don't know how it works but it does and I can now get on with more building as I slowly upgrade all my old tools.
 
It's certainly dished, but I'm not quite convinced that what you get is actually a circular arc (or even parabolic). I'll have to sit down with a calculator for a couple hours sometime to figure out the 3-dimensional trigonometry. Don't suppose it really makes a difference, though. And it sure beats swinging a router on a 20-foot pendulum.

If I understand this correctly, all the action is on the perimeter of the disc. What if, instead, you used something that's meant to cut with the edge--like a dado or circular saw blade (either abrasive or fine-toothed)? Might go quicker and make sawdust more than sanding dust. Then use the sanding disc to remove the saw marks.

Dan

C'mon, Bolly! Let's fire up the cuatro!
 
It's certainly dished, but I'm not quite convinced that what you get is actually a circular arc (or even parabolic). I'll have to sit down with a calculator for a couple hours sometime to figure out the 3-dimensional trigonometry. Don't suppose it really makes a difference, though. And it sure beats swinging a router on a 20-foot pendulum.

If I understand this correctly, all the action is on the perimeter of the disc. What if, instead, you used something that's meant to cut with the edge--like a dado or circular saw blade (either abrasive or fine-toothed)? Might go quicker and make sawdust more than sanding dust. Then use the sanding disc to remove the saw marks.


Actually, when you consider the waist area, the abrasive path is at least a few inches wide.
 
It's been over 2 years now since I made a couple of dishes with this method, and started this thread at the same time...I'd be interested to know if anybody else tried it this way..and how did it go?
 
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