A spiral notch (question from a non-woodworker)

[Kiteman]

Hi,

I have a problem - I need to cut a spiral notch in a disc of plywood. I haven't decided on the exact dimensions yet (I don't know the limits of the materials yet), but the notch will be roughly 3mm wide and deep, in a disc roughly 200-250mm across and will spiral down to a smallest diameter of roughly 50mm.

Obviously, the best tool for the job is a router, but I don't own one. I'm not keen on buying one (I'm unlikely to need it again), so I need (somebody's) advice: what other options are there for cutting the notch?

The relevant power tools I already own are a hand-held drill and a rotary tool (a Woolworth's version of a Dremel) - is there a technique that employs these (maybe a special bit that is much cheaper than a new router), or do I have to re-learn what few chiselling skills I picked up in woodwork lessons 25 years ago?

Thank you in advance for your help and advice to a non-woodworker.

K.

 

[Evergreen]

Kiteman

Hmmm. By "notch", do you mean "groove"? On the assumption that you do, you've got a tricky job on your hands if you only want to use basic tools. I'm also guessing that you don't want to try freehanding it with the rotary tool?

I suggest that you draw your spiral on the disc of plywood and then cut the line with a utility knife. You could then widen the cut with the tip of a saw. 3 mm is very narrow for any kind of chisel and you'll have all sorts of problems with the changes in grain direction as you follow the spiral. With any luck, you'll be able to keep the depth of the groove constant by using the layers of the plywood as a guide.

Good luck!

Regards.

 

[Kiteman]

Yes, I did mean "groove".

I didn't think of the layers as a guide - I may try to free-hand it after all.

Thank you for your reply.

 

[Lukey]

Can I just ask what you are going to use this spiral for?

 

[dirtydeeds]

kiteman

i admire your cahones

cutting a 3mm wide spiral into a plywood disk by hand or by router

be prepared for a number of trial pieces

 

[dirtydeeds]

forgot to say

trend will almost certainly do 3mm router cutters

axminster sell 3mm wide japanese chisels

 

[Kiteman]

Can I just ask what you are going to use this spiral for?

Conch shells are made into horns by making a hole at the centre - the spiral chamber then becomes the resonating chamber of the horn, like the tube of a trumpet.

I am trying to mimic that in plywood - one of the discs holding the spiral in its shape will be pierced to allow a brass instrument mouthpiece to be inserted.

My first objective is to prove it can be done, by making something that makes a reasonable sound.

After that I will work on tuning - changing diameter and the number of turns in the spiral to produce an instrument with an actual note, and possibly piercing the disc opposite the mouthpiece to allow a wider range notes to be played.

Well, that's the plan...

 

[Lukey]

Wow...

So will you be cutting this groove into the INSIDE of something or the outside?

 

[mr]

I think at this point a drawing might be useful.
Cheers Mike

 

[Brucio]

Sounds like a job for Scrollsawman...
Two discs; one without spiral, one with spiral, and clag them together.
Bruce

 

[Gill]

Sounds like a job for Scrollsawman...
Two discs; one without spiral, one with spiral, and clag them together.
Bruce

Clag them together? These terms are getting too technical for me!

:lol:

I know what you mean, Bruce, and I agree with you. A scroll saw in the hands of an experienced scroller would do the job nicely.

It's difficult to know how to do it with just the tools Kiteman mentions. You can draw a spiral by wrapping cord around a fixed post, tying a pencil to it, and letting it unwind so that the pencil draws the spiral. I'm wondering if it might be possible to do something similar with a Dremel, although it would have to be mounted in a router base attachment. Any cord would have to be kept away from the whizzy cutting parts of the Dremel too!

Gill

 

[David C]

Fabrication, i.e. gluing three layers together, with the centre layer sawn on a band or scroll saw or even jigsaw sounds favorite to me.

And then why not Coping or fretsaw.

I thought the interior passage/groove of conch shells etc varied in width?

David

 

[Kiteman]

Good grief. I think I've given myself a huge job!

I imagined I'd just be cutting the pieces, cut the groove, slot together and glue.

I may not get away with just the tools I have.

(Looks around) I guess I have to post an image elsewhere online and then link to it?

Oh, and yes, a conch shell is effectively a long cone rolled up. I thought I was making my life easier by leaving the sides parallel...

 

[Kiteman]

OK, I have scanned the few sketches I have made so far (as for most of my projects, the main plans are floating about in my head - I usually document them as I make them so that others can follow them.

The sketch: http://www.flickr.com/photos/76333500@N00/1568562682/

My other projects: http://www.instructables.com/member/Kiteman/

 

[mr]

Doesn't a shell sound due to its walls vibrating? If so aren't you doomed to failure trying to reproduce that in any way that doesnt allow for extremely fine wall thickness and a suitable material? ie not spiral cut mdf. Just thinking aloud really...
Cheers Mike

 

[Kiteman]

It is a fact that the quality of note of a musical instrument depends on the material that vibrates to make the noise.

That is true of instruments like guitars or violins, where the body of the instrument vibrates to generate the sound.

However, in instruments like horns, flutes or recorders, the material that vibrates is the air within - the quality of the note is due primarily to the volume of air vibrating, and secondarily to the shape of the resonating chamber. If they are built to identical internal dimensions, a flute made of PVC will sound the same as a flute made of an exotic hardwood or finest silver.

The conch shell has a low tone because the cavity within is quite long because it is spiralled with the shell. That is what I hope to reproduce.

I have been given advice elsewhere not to cut a groove at all, but to make a double row of panel-pins to hold the spiral in shape while I glue it.

Does that sound acceptable?

 

[SketchUp Guru]

I have a thought about how to cut the spiral groove although I think a router would be the tool to use. My thought is to start with some sort of bobbin at the center that is fixed to the plywood so it can turn. To that bobbin, attach a wire whose other end is fixed to the router. The boobin would need to be made to take the wire so it coils up as you work around the center.

Does that make any sort of sense?

The other idea is to make a turn table that is geared to a carriage that holds a router. The gearing would make the router move laterally as the turntable is rotated. This might be a better way and more easy reproducible.

 

[Evergreen]

Kiteman

I'm not sure what the 3mm wide material is that you're going to roll into a spiral (if I've understood your project correctly). If it's very strong, it'll try to unwind itself and pop out of any restraining panel pins. However, if it's something that accepts the crimp you'll be putting into it, the double row of panel pins might well work. Provided your adhesive is strong enough. If you don't have clamps, I'd use a grab adhesive like Liquid Nails or Wickes Forget Nails High Strength. I reckon it's worth a shot before you try some of our other proposals.

BTW, that instructables site is really interesting!

Regards.

 

[Inspector]

An alternate to cutting a groove would be to form your spiral and place it carefully on the plywood that has a bead of thickened epoxy in the spiral shape. After it sets flip it over and stick the second disc to complete. If the spiral is going to be of multiple wraps, smear the epoxy over the entire surface. Since you are using cardboard for the spiral you might as well skip the plywood altogether and use cardboard for them too.

 

[Kiteman]

I've had a suggestion from somebody else that I should have thought of myself - a double spiral row of panel pins to slot the spiral ply into and hold it long enough for glue to set.

All I've got to do now is make it...

(BTW, it is free to join Instructables, and you don't have to specialise in one set of materials in your makes. Anybody that is active here would also be a welcome addition there, plus there are currently two free competitions running with thousands of pounds worth of prizes)