Model J Head
This unit, after the original M head and before the introduction of the variable-speed 2-hp head, was the standard Bridgeport fitting but was then offered as a cheaper alternative to the 2J2. The counterbalanced R8 quill - hard-chrome plated and ground, then lapped to fit the honed spindle bore - had 5-inches travel and was fitted with both hand and power feeds. The hand feed could be applied through either a quick-action lever when drilling - or by a handwheel for fine feeds. The power feed, at the rate of 0.0015, 0.003 and 0.006 inches per spindle revolution, worked in both directions and could be set to trip out automatically; a 0.001" graduated micrometer stop was fitted as standard. A wide range of spindle adaptors was available to mount a variety of tooling.
Whilst the specifications of more recent versions of the head varied, in the past a choice of two 1 h.p. motors, each wired for reverse, was offered (and others, possibly, depending upon the year of production); a 1800 rpm unit that gave (in direct drive and "backgear") spindle speeds of 80, 135, 220, 330, 660, 1100,1800 and 2720 rpm - and a 3600 rpm version that had the effect of doubling each of the aforementioned speeds.
When vertical, the head could be run continuously - but needed modification to the lubrication system if used in the same way horizontally..
Model M Head
This was the head fitted to the first Bridgeport milling machines and had 3.5-inches of quill travel through both fine and quick-action feeds by hand operation only. The spindle could be ordered with a No. 2 Morse, B & S No. 7 or a B-3 taper - and the maximum collet capacity was 0.5 inches. Special tapers were available to order and it is highly likely that a factory already equipped with tooling to a different specification might have ordered its machines so as to be able to mount cutters already in stock. The 0.5 h.p. motor was available as either a 1200 rpm unit - which gave spindle speeds of 275, 425, 700, 1050, 2100 and 4250 rpm - or as a 3600 rpm model with rather higher spindle speeds of 950, 1350, 2200, 3250, 6500 and 12000 rpm. The unit could be mounted either on the front or back of the ram and, if on the latter, fitted to a swivel adaptor that allowed it to be angled in both planes.
Details of the various head mounting adaptors can be found below.