With metal / metal slides, provide for long guide lengths, in practice respect a length / width ratio greater than 1.5. For the carriage of the column described the ratio is 2 (width = 60mm, length = 120mm). This rule avoids the phenomenon of jamming and decreases the defect of "lace" of the carriages, thus better precision of the guides.
All carriages must be fitted with brass, bronze or PTFE (Teflon) plates to facilitate sliding. Some plates are equipped with adjustment screws to set up the backlash due to wear.
The feed screws must be chosen with a judicious pitch, the ideal is to use threaded rods M6 because the pitch is 1mm, so with one crank turn, your carriage moves 1mm, add a graduated drum 40 divisions (easy to do with the help of a small spare gear) and you have a 0.025mm caliper.
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