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Advise on creating a beam with width way curve (Instrument)

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Tetsuaiga

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Hello, sorry if this is posted in the wrong place.

I'm planning to create a fairly simple string instrument. I lack knowledge and specialist tools (willing to buy some within reasno) so any help is appreciated.

What i'd like to know is how do I create a beam with slight curve raising in the centre. The beam would be hardwood roughly 3-4 foot long, around 1 foot wide, 3" deep, exact measurements still need to be worked on.

I'm thinking I could start by creating a rough shape using a saw to take measured chunks out. Then moving on to a hand plane, perhaps chisel at some stage between?.

I have doubts about the results this would produce. How would be best to get a really good curve, it doesnt have to be exact but not noticeable, as I say the curve is only very slight, perhaps 1 inch difference from edge to middle. One idea is to set up a lazer so as I take wood away with the hand plane so I know im at the correct depth all the way along. I think some kind of curved carving tool would be best but I don't know what tool is available for this.


Thanks
 

beech1948

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From a hand tool point of view tyhere are a quite a few issues with what you have described. My problems with it are simply that I do not fully understand the final shape.

A plane is probably not too useful to create a 1 inch "dished" shape as the plane tried to create a flat surface. There is an old Stanley/Record plane with a flexible base which might work both for the inside and the outside shapes.....usual cost is from about £100. Have a look here:-
http://www.oldtools.co.uk/tools/Stanley_Compass_Plane113.html

The other tools you might need for the shaping are some spokeshaves, a gouge chisel, curved rasp and lots of coase/fine sandpaper used with lots of wheatabix.

Another solution might be to get the design drawn up in a CAD programme and then glue up the blank and get it machined on a CNC machine. If lucky you may be able to get a CNC hobbyist to make it for you. Have a look at some of the UK CNC web sites.

Hope this is of some help.

Alan
 

Cheshirechappie

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An old shipbuilding technique for making curved components (lots of those on ships) is to make templates of the curve required. The template for your work might be of thin plywood, cardboard or similar. The material was then worked away by whatever means seemed most suitable, and the template offered at regular intervals to check progress. Mark the high spots from the template, and work away at those. Keep going until all the high spots are gone, and you're just about there.

If the curve you need is regular anomg the length of the beam, you'll only need one template. Use it to mark the profile on each end of the blank before you start, as a help.

If the curve you require is in two planes, then make several templates - say one every foot along the beam, marking the position of each on the side of the beam. Offer them frequently as work progresses.

To remove the bulk of the waste to start with, a large, fairly shallow carving gouge would probably be quickest (unless you've got an adze!), using aggressive cuts to start with, and progressively shallower ones as you get nearer the profile. Once the basic shape is established, but with many shallow scallops from the chisel, go over to a smoothing (or even a block) plane set fairly coarse. Work away the scallops all over, working with the grain to minimise tear-out, and again checking frequently with the templates, until you're within a gnat's whisker of the template profile. Then set the plane fine, and finish to template all over. You could follow up with a cabinetmaker's scraper to smooth everything to a final finish.

If you're not confident with planes, a cabnetmaker's rasp or two may do the same job, followed by coarse/medium/fine sandpaper. Keep checking against template as before.


Edit to add - the template is, of course, a 'negative' of the job. If you want the beam to be convex in width, the template will be concave, so you can offer it to the job and see where it touches (high spot that needs removal) or where light shows (low spot - don't remove any more).
 

Richard T

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Welcome Tetsuaiga,

When you say "a beam with slight curve raising in the centre" do you mean the centre from end to end or side to side ? Or both?
I imagined from side to side on first reading; like a big fret board but on re reading realised I could be wrong.
 

Tetsuaiga

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Thanks for the replies.

To clarify the top of the wood would be a convex curve, lowering at each width edge.

I think the template idea is great, I would probably just need the one. I don't expect it would achieve machine accuracy but I believe it should be good enough. CNC look interesting, perhaps if I were to make a few of these that would be a good option.

The instrument im interesting in modelling is a chinese instrument called a Guzheng, so a type of zither. It will be pretty much the shape of flipped over guitar bridge, but with much less dramatic curve. Also the curve wouldnt reach the base.

Not entirely sure if I should use planer or spokeshave after the first work, these seem to be basically the same as far as I know. I do have a fairly large draw knife which could be used earlier on perhaps.
 

Pete Maddex

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Hi, Tetsuaiga

You should be able to make this with simple hand tools, a spokeshave will do most of the work and a curved block and sand paper for finishing.
Saw cuts to near the line and chiseling out the waste will get the bulk out.
A good way of drawing a curve is to bend a steel rule or a thin strip of wood and get someone run a pencil along it, or you can use small nails to hold it in position if no one is avalable to help.

Pete
 

Cheshirechappie

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Tetsuaiga":rax8wl33 said:
Thanks for the replies.

To clarify the top of the wood would be a convex curve, lowering at each width edge.

I think the template idea is great, I would probably just need the one. I don't expect it would achieve machine accuracy but I believe it should be good enough. CNC look interesting, perhaps if I were to make a few of these that would be a good option.

The instrument im interesting in modelling is a chinese instrument called a Guzheng, so a type of zither. It will be pretty much the shape of flipped over guitar bridge, but with much less dramatic curve. Also the curve wouldnt reach the base.

Not entirely sure if I should use planer or spokeshave after the first work, these seem to be basically the same as far as I know. I do have a fairly large draw knife which could be used earlier on perhaps.

The drawknife sounds good - you can shift a lot of wood with a drawknife, and used with a slicing (angled) cut, you can shave quite fine as well. That's assuming that you're using fairly 'normal' hardwoods - the ulta-hard exotics might be beyond it's capabilities. Take some time sharpening it before you start - sharpness solves many problems with drawknives!
 

Tetsuaiga

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Thanks for all the help.

I bought a cheap spokeshave today and its really good for what I need.

Testing it on a block though I've found an issue. When the shave reaches the end of the wood the rest falls off before the blade so it doesn't want to cut the very end bit much, not without some difficulty. It wouldn't matter if I just saw the additional bit off after and factor that in. However I quite like the idea of using a laser to shine down the wood ensuring I get a good even curve. This wouldn't be possible with a hump at the end of the wood though.

Any suggestions? (Perhaps the template would do and use laser to finish off with sanding)
 

Steve Blackdog

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When you finish, please can you post some photos! You might want to have a look at one of the guitar building forums, as they will have plenty of advice on carving maple caps on Les Pauls. I seem to remember Sawmill Creek has a separate instrument making board. Project guitar is another good forum.

All the best

Steve
 

Tony Spear

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beech1948":2i4sb8hf said:
Strange! According to that site what's being called a "beam" on this thread appears to be a soundbox and not a solid piece of wood (logical for a stringed instrument, even my Autoharp!).

The pictures aren't very good but the curvature seems to be very complex, across the width and from corner to opposite corner over the length! :shock:
 

Tetsuaiga

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Yes im sure it has a soundbox, though it may be quite a thick one soundboard (which will be softwood). Thanks for pointing that out, I may have been going down the wrong path somewhat. I think im planning to create the soundboard by carving rather than bending the wood though.

I'm still working out the design details and trying to get more information gradually. I don't intent to use the chinese scales on it, which shouldnt take too much adjustment, but like how it can be tuned by moving the bridge and altering the pitch by pressing the string down or the other side of the bridge. The projects really a bit of fun more or less to get and idea how hard making a basic instrument is, I wont bother buying proper tonewoods yet.
 

Tetsuaiga

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Yeah I was originally thinking of making a simple lyre.

I've just tried out this google sketchup program its pretty good for desinging things actually. I quite like the challenge of a less simple instrument. If it ever gets completed i'll post it up here..

Looks like the back, sides and top are all seperate pieces, which I guess makes things easier.
 

xy mosian

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That's a nice explantion. But you don't need all the Maths. Just mark the ends of the curve, draw a straight line between them, mark the offset from the line, at half way. and mark lines to join the ends at the central mark to find the angle. If instead of marking the last lines you use two bits of lathe and nail them at the joint you have it. Very much in the way that Sketchup draws curves actually.

xy
 

bugbear

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xy mosian":3mlp6cyh said:
That's a nice explantion. But you don't need all the Maths. Just mark the ends of the curve, draw a straight line between them, mark the offset from the line, at half way. and mark lines to join the ends at the central mark to find the angle. If instead of marking the last lines you use two bits of lathe and nail them at the joint you have it. Very much in the way that Sketchup draws curves actually.

xy
Yes - the maths just proves it works. The technique just uses sticks, and works just fine wether you care about the maths or not.

BugBear
 
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