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Forming a blister

To form a blister for your accelerator foot a simple process is described below.

Having decided how large you want your blister make a pair of female moulds by screwing together two pieces of 22 or 25mm ply or MDF. Birch ply is the best as it is usually free of the voids found in marine and WBP. Cut out the shape with a jig-saw and finish the inner edges with a surform or rasp and glasspaper. Make the line as smooth as possible as this will determine the edge line of your blister.

Make a male plug form using the same thickness of material and form it to the bulge you want. The deeper you make the blister the more you are stretching the metal and if it has a flat face as shown in the picture below

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most of the stretching will be in the raised wall..

I used a piece of 25mm birch-ply for the plug and determined the depth using a piece of 15mm MDF.

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Assemble the female moulds with the sheet in between the two and clamp as firmly as possible ( locating screws outside of the sheet edges help in lining up )

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Put the plug in place and, having put the underside of the mould on a really solid base, hit with a weighty mallet or hammer. (If you have a workshop press you can save your hammer arm for the tea-drinking)

When you have started the process of stretching the wall you should get to the point where the plug is below the surface of the female moulds. Put the depth blank in place and continue hitting till it is flush with the surface of the mould. Undo the clamps and there you are.

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Avoid putting the blister too close to the edge of the sheet as the metal will pucker if it can. See arrow on pics 1&4. This shows a pucker.The main work that the clamps should be doing is preventing this puckering.

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R.A.Chethandle

Q Did you anneal the aluminium as the recommended 1050A14 is half hard?
A I didn't anneal as I was only going a short depth and I wanted the flat area of the blister hard rather than annealed and so more vulnerable to dents. I used the material supplied by Phil.I have no idea what grade it is. Generally most stockholders carry half-hard unless you order specially (and pay the premium!)

Q Did you radius the edge of the female mould to get a rounded edge and more material to form the blister?
A I didn't radius as I wanted a crisp line. Variations are possible.


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