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Apart from their collet capacity, the MD3 and MD4 Horizontal bench Milling Machines were identical: the MD3 accepted 3C collets with a maximum capacity of 1/2" whilst the MD4 was able to take 4C collets with a through bore of up to 3/4". The miller consisted, in effect, of the base section and enclosed headstock from the company's precision bench lathe and, like those machines, could be had with either a plain or ball bearing supported spindle The 6-speed drive system and its controls were identical to the lathe with twin V belts driving the spindle from an under-slung 2-speed electric motor and countershaft unit. The outer of two concentrically-mounted electrical-control levers on the left-hand side of the body selected  forwards, stop or reverse with the inner giving the high or low speed range and stop; other speeds were selected by rearranging the belts on the countershaft; the stands offered for the miller were very similar to those for the lathes with a choice of both enclosed and open versions.
Manufactured from the: "finest grade, heat-treated and seasoned alloy iron" the head and base had hand-scraped surfaces to ensure perfect mating; the spindle was made from the best quality ball-bearing steel and hardened and ground both internally and externally; the spilt and adjustable plain bearings were of close-grained grey iron with a high graphite content, tapered on their outside and lapped to a mirror finish on the inside. The makers claimed that this construction was superior to the usual 3°- 45º steel bearing as: "it is mechanically impossible for a 3°- 45º bearing to wear equally - practice has proven this". The speed range of the plain bearing spindle was 180 to 2000 rpm whilst the optional ball-bearing spindle ran from 230 to 3900 rpm. The twin V pulley was balanced and the spindle end thrust adjustable against a ball race; each bearing was lubricated through an external oil cup fitted with a wick feed to ensure that only oil, not dirt, was drawn up.
The spindle could be prevented from turning by a convenient spring-loaded (lockable) plunger mounted on top of the headstock and (most usefully for a miller) carried in addition to draw-in collets a standard external Hardinge-style tapered nose with quick-release fitting for mounting chucks, faceplates and other attachments.
With hand-scraped ways and positive locks, the table,  had a working surface of 12" x 31/4" and carried a central T slot that could both locate accessories and be used, together with the two outer standard T slots, for clamping work. The longitudinal travel was 57/8", the traverse 33/8" and the vertical 53/4"; zeroing micrometer dials with tapered faces were fitted to all three axes of movement and the 1/4" square handles for all table controls were interchangeable. In its originally-advertised form the miller was supplied complete with a dividing head carrying a 60-notch index plate (others were available to order), a tailstock and centre to match the dividing head, a draw-in sleeve tube to hold collets, a swivel-base vise and the necessary handles and wrenches. As an option, but not illustrated in the company's sales literature, was an overarm support for the spindle, presumably like that used on the later No. 5 machines.
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Continued
Using the same base, table and knee from the MD3/MD4 machines, the BB2V miller had a pre-loaded, ball-bearing spindle vertical head that mounted directly in place of the horizontal unit. The spindle accepted the Hardinge 2VD collet with a maximum capacity of 1/2". In that the components were interchangeable, and restricted only by the difficulty of changing over the endless V belts, it would have been possible to convert one machine (MD3/MD4 and BB2V) to the other.
Originally advertised as a "Cataract" the machine was supplied complete on a 6-speed, 40" x 30" underdrive stand with a speed range of either 300 to 3250 rpm or 300 to 5000 rpm. A 2-speed general Electric motor appears to have been the favoured installation with (one early versions) the electrical control levers mounted concentrically on a single shaft and marked Reverse, Stop, Forward and Low Stop High. Later versions, labelled "Hardinge", were of an almost identical specification but with the two switch levers on separate shafts and, strangely, the table's longitudinal travel reduced by 7/8". 

A superb example of the wonderful little Hardinge BB2V

Drive on this version of the BB2V was provided by a 2-speed forward/reverse General Electric motor and a simple 3-speed countershaft.

Continued:
A direct descendent of the original No. 5, the MD5 was a much heavier and more robust machine than the MD3 and MD4. In the early 1930s it was the only Hardinge miller able to be fitted to a compact cast-iron stand, the other machines requiring much larger lathe-like cabinets - though, as an option, that type of stand was available as well. The 24" x 6" (working surface 20" x 6") table carried three ordinary 7/16" T slots; the longitudinal travel; was 133/4" and the traverse 6" and the vertical 7". The 5C-collet fitting 7/8"-diameter spindle was made from ball-bearing grade steel and hardened and ground; it ran in 3 degree taper bearings of phosphor bronze that were adjustable for wear.
Manufactured in hardened steel, the overarm was  ground and lapped to a perfect fit with in the casting. The cutter arbor was 7 inches long and 7/8" in diameter and the distance between arbor and overarm 4 inches.  The largest of the 4-step spindle pulleys was 6.5 inches in diameter and took a 13/8"-wide belt.
With the cast stand the machine weighed 825 lbs and occupied, allowing for the table's longitudinal travel, a floor space of 45" x 43".