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If you continue to have problems with the wood splitting, see if you can pick up some white oak - it has a much more interlocking grain, a lot harder to split. I was lucky to have some left over from a boat keel/stem project. Red oak has a linear/tubular grain, white oak would be better.Fun fact with red oak - if you have a straight grain length of it, dip one end in water, and blow in the other end, bubbles will come out in the water! The tubular grain is a lot like a bundle of straws. Yes, this really works!Or, if you are more comfy with metalwork, you could always make the formers out of some big chunks of steel! (Just kidding - doubt it would work that well, it would tend to squish the copper with the hammering rather than giving/rebounding like the wood does).
I usually make the former for the forward flange (around the boiler tube) wider to avoid the breakage problem. There is no limit other than the amount of wood used. The side former needs to be the correct width, of course, but the interior shape is mostly unimportant. I have never had any problem with the side former, but it could be made sturdier by creating a smaller cutout.Gene
I've got a couple of options on mallets. I have a rubber mallet, a wood mallet, a plastic mallet, and a brass hammer. Then we get to the ball peen hammer and your basic claw hammer, plus a small sledgehammer. I'm sure something in there will work!
Alec recommends using a large 16oz flat-faced planishing hammer. This is one of those hammers with a large flat steel face that is used for shaping sheet metals. Looking at the pictures in the book I suspect he is using a couple of the hammers that you find in the cheap car body repair hammer sets as I also spotted a cross-face hammer being used for the internal curves.When years ago I made my two boilers I followed Alec's recommendations and it worked for me JoP.S. Alec does say do not use a wooden hammer as the copper will quickly destroy it