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Now you can make your fridge magnets match your cupboard knobs. These rare-earth magnetic bases come in three base sizes, 12mm, 16mm, and 20mm in diameter (approximately 1/2", 5/8" and 3/4") to closely match a range of knob bases, and have attraction forces up to 3 lb, 5 lb and 7 lb respectively.
These bases are also useful for fridge magnet tops made from small turnings, carvings, kids' projects, etc. Just drill a hole in the item to be mounted and put a bit of glue in it for mounting.
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See this table to dcecipher fractional inch sizes