Printing head of dot printer

Claim 1

A printing head for a dot printer, said printing head comprising: (a) a cover; (b) a guide holder mounted on said cover; (c) a needle guide surface on said guide holder, said needle guide surface having: (i) a groove formed therein the width of which is a little greater than the diameter of the working ends of the needles and (ii) a plurality of needle support holes formed in the bottom of said groove and having a diameter which is a little greater than the corresponding needle diameter; (d) a plurality of needles disposed in said guide holder and slidably projecting through said holes, each of said needles normally being disposed in a first position in which it projects through one of said holes by a first amount which is less than the depth of said groove; (e) electromagnetic means for driving said plurality of needles from said first position to a second position in which they project through said holes by a second amount which is greater than the depth of said groove; (f) mechanical means for returning said plurality of needles from said second position to said first position after each actuation thereof; and (g) spacer means disposed between said cover and said guide holder, said spacer means including: (i) a removable gauge spacer the thickness s of which is less than the stroke of said needles and (ii) a plurality of removable spacer shims of thickness s', said removable gauge spacer and said removable spacer shims being disposed between said cover and said guide holder such that, when said removable gauge spacer and said removable shims are in position and said needles are in their first position, the working ends of said needles are beneath said needle guide surface by the distance s, which distance is sufficient so that said needles will disengage completely from a printing ribbon passing over said needle guide surface; in the absence of said removable gauge spacer, the working ends of said needles are coincident with said needle guide surface; and, when said removable gauge spacer and one of said plurality of removable spacer shims is removed, the working ends of said needles protrude beyond said needle guide surface by the distance s', which distance is sufficient so that working ends of said needles which have been thickened by repeated use can be ground off and said needles ground down until their working ends are once more coincident with said needle guide surface, after which said removable spacer guide can be replaced, restoring the printing head to its operative condition.