A worn-out bristle brush can easily be recycled into a useful tool called a whip burnisher or rotary hammer to improve or even eliminate minor imperfections and surface porosity in castings and other metal surfaces.
The first step is to use the jeweler’s saw to cut off the worn-out bristle brush. This will leave just the shaft of the former brush wheel — a piece of mild “soft” steel about an inch and a half long. This type of steel works well because it’s not hardened or tempered. It’s strong, but you can still bend it easily without breaking it.
Use the coarse grinding wheel on the bench grinder to round the end steel piece. Remove coarse grinding marks with the medium wheel, then insert the steel into the flex-shaft handpiece and spin it against 320, 400 and then 600 grit sandpaper sticks to make a smooth, rounded end. Finally, polish the rounded end with tripoli and rouge.
The last step is to make a slight bend about 6-7 mm from the end you have rounded and polished. The further you bend the stock, the more aggressive the burnisher will be when you use it. You can adjust the tool to suit your preferences as you use it.
To use the whip burnisher, insert it in the flex-shaft handpiece and spin it at medium speed. Bring a metal surface against the spinning end, and it will hammer the surface and smooth out imperfections, such as porosity. A touch of burr lubricant will improve the performance of the whip burnisher.
Lightly sand the surface of your piece with 400 and 600 grit sandpaper after burnishing it, then polish as usual.
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