Tracing a complex contour...?

[Kenneth]

Howdy,

I recently saw a very simple design for a stand for a banjo...

I could make one in a flash as it is essentially a plywood box, about 20" X 14" X 8", with a padded hole in the top in which the banjo rests (with its neck going up and slightly tilted back).

The problem is that I can't come up with a good way to determine the proper shape of the hole.

It is a moderately complex shape in that it must accommodate the curve of the resonator, and the varying diameters of the instrument itself. Also, the instrument is rather fragile so doing things like dragging a divider along its surface would be out.

What might be some ways (other than running it through my table saw) that I could get a reasonably accurate "tracing" if the banjo, about 6" from its tailpiece?

Sincere thanks for any help,

[John]

Have a digital camera? Place the banjo on a contrasting surface, with a yardstick or tape measure next to it. Light the banjo and make pictures from directly overhead.

Edit the image in any photo software and fill the area around the banjo with black. Enlarge the image to full size and you have a template.

John

[Kenneth]

Hi John,

I thank you for your interesting suggestion, but if I am understanding you correctly, there would be a difficulty:

I need to get the shape of the instrument's cross-section about 6" from the tailpiece end rather than an outline of the instrument at its widest point.

How could the photo approach you suggest do that?

Thanks again,

[Dave]

I suggest that you use a standard carpente's pencil. Lay the banjo on a suitable size piece of craft paper (the plain brown stuff butchers use). Place the flat side of the pencil next to the banjo and trace the outlne. You will end up with an outlne that is 0.250 wider than the banjo. The rest depends on the tools you have. Post a bit more detail and I will be glad to offer a suggestion. Regards, Dave G.

[Kenneth]

Hi Dave,

If I were trying to make an outline of the whole instrument, the suggestion you offer would be perfect, but...

I need to get the outline "part way up" rather than the complete outline.

Here's a link to the design I want to copy:

when you take a look, I suspect you will see the problem with typical tracing methods.

Sincere thanks for your ideas,

[JeffB]

Any reason a regular contour gauge won't work? Obviously not all at once, but you should be able to capture the shape of the instrument in a few steps.

no.spam. to email

Kenneth wrote:

[DanG]

A contour copier would work. A plastic one should not mar anything:

have had good luck just taking a piece of poster board type cardboard, making a few cuts with scissors, trial and error. Might take 3 attempts to get what you want.

[RicodJour]

Cut a piece of scrap plywood with a cutout that the banjo can easily pass through past your intended cross section line - leave about 1/2" clearance between the banjo and the wood. Then prop up the banjo vertically, slide the bottom of the banjo into the plywood cutout, and prop up the plywood until it is horizontal at the desired level. Then use a compass set at 1" to trace the perimeter of the banjo. The compass points must be perpendicular to the banjo outline at all times as you run it around the perimeter. Remove the banjo, place a piece of tracing paper over the plywood and trace the perimeter line you marked with the compass. Then using the compass on the same setting (don't change it between steps), trace 1" inside the marked perimeter to find the exact perimeter. That will create an exact fit, depending on your skill and tool sets - how much clearance you need is up to you. I'd probably leave about 1/8" clearance and taper the hole to make banjo insertion easier.

As an alternative, you could wrap the banjo base in a layer of paper, then a layer of plastic, tape it up tight, make a box form (could be the holder's side walls depending on what finish) of the desired size, prop up the banjo vertically in the box, then use a can of low- expanding spray foam to fill in the box around the wrapped banjo. Now you can use the foam, cut as required, to create an exact template of whatever section you need, or keep it as the banjo box liner and just make a top piece of wood to fit.

R

[Kenneth]

Hello again,

Many thanks for (both) great ideas...

I should have my stand shortly!

[J. Clarke]

Cheap and dirty way, cardboard, scissors, and tape, keep hacking until you've got the right shape.

Not quite so cheap or dirty, contour gage.

Note that I'm suggesting the plastic one rather than the metal one since you said that the instrument was fragile and I figure the plastic is slightly less likely to mar anything than is the metal one.

The way a draftsman would do it--set up a reference surface and measure at points from that surface. A draftsman would use a surface block and inside caliper most likely, but since you don't have those, a practical way to do this cheap with minimal risk of marring the banjo--joint a three foot 2x4 and sand a dime-store plastic ruler to a point, then measure with the point touching the banjo (don't drag it, just touch, write down the measurement, move it a quarter inch, repeat) with the 2x4 far enough back that you still have numbers on the ruler.

High tech, fancy, and fanciful--get a line laser level (the kind that projects a line on all the walls of a room). Set it so that the line crosses the banjo at an appropriate point. Take a photo from the end (use a slow shutter speed), you'll have part of the contour. Rotate the banjo and repeat until you've got it all. Make sure you have a ruler in the photo for a dimensional reference.

[Nova]

A possibility would be:

1. Place the banjo body in a plastic garbage bag to protect the instrument.

1. Appropriately position the body of the instrument is a properly sized box using styrofoam spacers.

2. Use an expanding foam to fill the box making a mold of the body to the required depth/height.

[Kenneth]

Hi Jack (and others),

I am moving in the expanding foam direction because it would provide excellent support for the instrument, but I do need a bit of education:

What kind of foam do I want for this sort of thing?

The only foam I know at all is the "hardware store aerosol insulation stuff."

Is there another sort of foam better for this purpose?

Sincere thanks,

[Zz Yzx]

I'll sell you one just like it for ..... say, I dunno.... about $120. Plus shipping..

-T.E. Stoster>

[DanG]

Make sure you get one that is NON expanding or minimum expanding. It will say something like being used for windows and doors. The regular triple expanding stuff could do harm as it generates quite a bit of pressure.

[Nova]

Hi Ken,

I agree with Dan that a minimum expanding foam would be best. "Great Stuff" now makes an aerosol that is designed for use in foaming around windows to prevent bowing the casement. I'd probably also use a cardboard box as it would allow sidewall flex minimizing pressure on the banjo's body.

[Mike O.]

You may want to be careful with foam unless you have some type of material between the instrument and the foam. It may be possible that the foam could react with the various finishes on the instrument.

Mike O.

[Lew Hodgett]

The stuff they sell at the borg is OK for insulation, but lousy for this application.

Do a Google for "urethane foam".

You want the 2 pound stuff.

It is sold as a two (2) part kit and can be shipped UPS.

It will stick to anything so use a parting agent such as clear packing tape, plastic foam, etc.

Last time I used some, got it from an outfit in West Palm Beach, Fl and I'm in SoCal.

It was still my lowest cost supplier.

Have fun.

Lew

[Father Haskell]

6" from the tailpiece would be through the pot. Scribe it with a carpenter's pencil held flat, with a sheet of paper held over the resonator to protect the finish. The radius is huge, so a 3/16" difference won't be noticeable. You'll have padding to fill the gap, anyway.

To get the neck contour, pack plasticine clay around the desired section, again with a sheet of paper in between to protect the finish.

[Kenneth]

Hello again,

You might want to take a look at the link I posted earlier in the thread:

you will see, there is no involvement with the neck, just the tailpiece end of the instrument.

All the best,

[J T]

Fri, Jan 25, 2008, 8:03pm [e-mail] (Kenneth) doth posteth: Howdy, I recently saw a very simple design for a stand for a banjo... I could make one in a flash as it is essentially a plywood box, about

20" X 14" X 8", with a padded hole in the top in which the banjo rests (with its neck going up and slightly tilted back). The problem is that I can't come up with a good way to determine the proper shape of the hole. It is a moderately complex shape in that it must accommodate the curve of the resonator, and the varying diameters of the instrument itself. Also, the instrument is rather fragile so doing things like dragging a divider along its surface would be out.

I saw the link with the picture You're trying to complicate it.. It's got a rounded pocket (I think 'round' is what they mean by "contoured"), to set a banjo in. It's padded. There's no "moderately complex" shape involved, all you have do do is make it deep enough to accept a banjo - just like they did. Personally, if I was gonna make a stand for a banjo, or whatever, I'd make something that'd let it be supported from the bottom, lean it "back" a bit, and support the neck. Unless I wanted something that would go pretty much un-noticed with the banjo out of it, like playing on stage, I doubt I'd want something like that. OK, in real life, I'd just put it back in the case until I wanted to play again; with a case strong enough someone could step on it and not hurt the case OR banjo.. Remind me, got to get working on my banjo again. Ah well.

JOAT

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