砣 is a bound form (b.f.) meaning 'sliding weight' (like on a steelyard) or 'stone roller' (used in traditional milling). It appears in compounds such as 秤砣 (chèngtuó, 'steelyard weight') and 碾砣 (niǎntuó, 'stone roller for grinding'). As a bound form, it cannot stand alone as a word; you must use it within a compound.