SOME THOUGHTS ABOUT BEATING TIN BOTTOMS TO A RESONATING SHAPE.
FIRSTLY BEFORE YOU BEGIN
Select a tin less than 10cms deep and at least 20cms wide. It is hard to move a hammer inside deep tins.
Old tins are better than new tins because the quality of the metal and thickness declines as time goes by.
Round tins are the easiest.
Some tins already have shapes molded into the bottom when manufactured. These moldings can be a help or a nuisance. A flat bottomed tin with a ridge around the edge is best to start with.
When you have selected a tin, take the lid off and hang in loosely by its side with one hand and give it a sharp tap on the bottom with an end of a finger on the other hand. If it gives a bit of a ring when tapped, it means the tin shows promise. If it gives a dull thud , you have a challenge. If you have a tuner try to measure the note of the tin as a starting point.
The simple tools needed are:-
Small ball head hammer.
A full newspaper as a pad to beat on.
Some sticky tape
Short length of wire about 2mm thick.
A soft wood block about 20cms long and about 10x10cms wide.
Piece of metal pipe about about 20cms long and 10cms wide.
SECONDLY CONSIDER A RESONATOR'S ACTION
A resonator is a spring, it moves up and down with the twitching of the string. When you have finished beating the tin you should be able to lay the tin on a bench bottom side up and press in the middle with you fingers and see the middle go down and return to position when pressure is released.
Arching the bottom of the tin in one curve from the side up to the middle and down again to the other side does not work very well. It is a rigid curve. A pliable curve is needed.
What does work best is to to go up from the sides to form a ridge then down to make a trough and up again in the middle to make a low dome. It is the ridge and trough that make the spring because the inner dome is a rigid arch.
THIRDLY CONSIDER THE TIN ON THE NECK
The instrument neck will go through the tin just underneath the bottom and if you want to use a straight piece of wood for the neck, the tin cannot be too high in the middle because the action will be too high and it cannot be too low in the trough or the inside bottom of the tin will touch the neck.
READY TO BEAT THE TIN
To start put the tin on the full newspaper on a solid bench. Beat around the inside edge of the bottom with a small ball hammer. Go a few times around . It is the start of the ridge.
Next beat around a bit further in and again a bit further in until have reached the half way point of the width of the ridge. You should now have an even curve from the edge of the tin to the top of the ridge.
Next make a circle of wire the same size as the inside edge of the ridge where the ridge will start to become the trough. Sticky tape the wire ring to the outside bottom of the tin in correct position to make the inner edge of the ridge. Return the tin to the newspaper. Beat around next to the out side of the wire. Keep beating until the second side of the ridge is completed.
Next take the piece of metal piper about 10cms across and place it on a solid bench. Place the outside bottom of the tin on the pipe so that it is in the middle. Put the fingers of one hand inside the tin and hold it firmly on the pipe while you beat around the inside edge of the pipe. Go around and around with the hammer lots of times until the tin is beaten into a dome shape inside the pipe.
Lastly take the block of softwood. or a piece of harder wood with some padding on top and stand it on end on a solid bench. Place the tin so the inside bottom of the tin rests on the top on the block. Holding the tin with one hand so that the remaining flat section of the tin rests on the block, beat around and around until the trough is formed. Accurate hammer work is need where the trough meets the outer ridge and the inner dome.
Look at the close up photos of tins that are posted on this site and you will see what what to do better than ten thousand words. Best of luck to you.
TESTING THE TIN
The first and most exciting test is to hold the tin loosely by its side and give the inner dome a sharp tap with finger nail or pencil. If it rings and sustains you have success.
Put the tin on a solid bench and with two finger separated the width of a bridge fingers press down slowly but firmly on the dome and watch the action of the metal. If it dents you have a problem. I will not explain possible solutions here.
With a tuner, try to measure the note of the tin when it is tapped. This is a clue for deciding on scale length, tuning and strings for the instrument. Sometimes a bit more reshaping is a good idea but I will not explain here because it is intuition which comes with practice.
The most horrible test is to put the lid on the tin and with thumb and fingers squeeze down on the ridge and see if it wants to dent inwards. Horror trip if it does dent or creases. It is the worst problem. I assure you if it happens once it will happen again. It must be solved or he tin discarded. There are possible solutions but it usually turns out best to give up on the tin.
Hold the tin up and look with one eye across the bottom of the tin so that the nearest edge and to far edge are aligned and see how far the dome rises above the edge. It should not stand more than about 3mm above the edge. This is not for sound quality but so you have enough margin to put a low bridge on top of the dome and end up with a string action not too high above the neck when it is fitted.
COMPLETING THE INSTRUMENT.
Beating the tin is the easy part of making these instruments sing. The finesse and challenge is deciding on scale length, tuning and and string weights to make the particular tin perform to its best. In the course of experimenting I have put instruments together pulled them apart to re beat the tin, changed necks to a different scale, changed strings and tunings several times and still not been very happy. Sometimes I am happy with an instrument and it stands around for some months until I pick it up again and have a listen and wonder what I was thinking when I put it like that and I change it again because it has become obvious what I should have done. I can only say that a huge variety of tone colours are available from different tins and different set ups and it can be hard to decide which way to go.
I have a dream. Factory guitars die and rain forests trees get very old. Tins become the world's musical pleasure. But there are not enough old tins to do it and an enterprising club of handmade instrument makers decide to somehow produce a small range of good quality metal tins already shaped and ready to use just like a cigar box. And it is all done by hand in a quaint little factory and everyone in the club and everyone else live happily ever after. Is such a dream still possible in this best of all possible worlds. I suspect not. Argh.
Anyone else happy to chase my dream, I will run with you, I cannot catch it by myself.
Seriously, I am convinced that this design of tin instrument has a good future and should be developed collectively before some corporate does it. I am strongly for the common good.