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Drummond M-Type Lathe 1921-1924
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With B-Type production having stopped during 1919, Drummond began work on a pilot batch of a new model during 1919-1920 - these lathes carrying Serial numbers from 1 to around 131. With production arrangements made, in 1921 Drummond were again ready to launch an improved version of their smallest lathe, this time designated as the M-Type. However, this first version, to be heavily revised in in late 1924, was to have the shortest run of any model (save perhaps the original produced in 1902 and first modified in 1904). The aim of the new alterations was to improved rigidity and resistance to tool chatter and the main changes focused on the headstock where a greater mass of metal was used, the futile over-arm brace removed and the flexible and poorly supported system of using two flat surfaces simply bolted together (by which means the headstock could be rotated slightly) was abandoned. In its place was a positive location with a tongue on the base of the headstock located against a matching vertical surface on the bed - the whole assembly clamped down firmly with four bolts. Larger spindle bearings were employed, the adjustment of which reverted to an earlier Drummond design where a large ring, screwed onto the end of the bearing itself, pulled it into the tapered headstock housing and provided a means of accurately adjusting the clearance. The design was actually more complex than at first appeared and required careful manipulation of a "drilled oil bolt" that acted, via a wedge, to expand the bearing fully into its housing.  As a final touch, a proper ball-bearing thrust race was engineered to fit against the inside left-hand end of the spindle. As an aid to heavier rates of metal removal, a long-needed, three-step flat-belt drive replaced the by-now hopelessly inadequate small-diameter round "gut" drive previously used . The spindle nose was again, as in 1912, upgraded and fitted with a 1" x 12 t.p.i. thread backed by a 11/8" register designed to improve the rigidity of screwed attachments. The backgear ratio was changed from 20:60T to 22:66T and engaged by rotating an eccentric shaft instead of sliding into place. In place of  having to reach for a spanner to disengage the 66t bull wheel from its embrace with the belt pulley, a cleverly designed "ramp" (that acted as a cam) was machined into a plate on the front of the gear. If everything was adjusted to slide easily, a swift push by a gnarled thumb against a knurled-edge knob was sufficient to persuaded the close-fitting engagement pin out of mesh with the pulley-flange. To engage direct drive the backgear was swung away and the knob merely moved slightly in the other direction - when a spring on the pin returned it automatically to engage with the pulley. The writer spent some time using one of the lathes and the speed with which backgear could be engaged and disengaged was a delight. The No. 1 Morse taper tailstock was also further increased in strength and the barrel diameter increased from 3/4" to 7/8". The previous rather crude screw-in barrel clamp gave way to a much more elegant and efficient "cotter" or "internally-split" locking device and some machines (but not, surprisingly, all) were given a hollow barrel to solve the problem of how to remove centres and chucks.
While the T-slotted cross slide casting was unchanged, the top slide was all-new: its feed screw was off-set to the side in order to provide a longer and slower-wearing nut and allow the tailstock to come right up to the right-hand face of the cross slide; its base was given an improved (though still hard-to-read) degree-graduated scale on an inserted brass plate; the early-pattern clog-heel toolpost was retained. The cross-slide feed screw handle was modified to have two finger grips and given a zeroing micrometer dial. Although the changewheels remained unchanged their studs, instead of being fitted with slow-to-remove screw-on knurled-edge finger nuts, were given a new design of (patented) push-in "spring" retainer  - a design that was to be used until the end of production. On previous models the changewheel banjo arm had always been retained by a crude through-bolt that ran up against the mounting boss, a design that made it difficult to get enough grip to stop the arm being moved sideways when heavy cuts were taken. This was now supplemented by casting in a curved arm at the back through which passed a stud and nut to ensure that the arm could be firmly locked in place. The flywheel was balanced and ran on a ball race, as did the "pitman" driving link,  both helping the operator to maintain a good speed with less effort.
M-Type Serial Numbers - from the Works official records:
Introduced in 1921 and further developed in late 1924 in which form it continued unchanged until production was taken up by Myford in 1941/3. Some pre-production examples were manufactured during 1919/1920 - though how these were numbered is not known. Also lost in the mists of time is when the digits were stamped: this could have been upon completion of a build, or immediately before dispatch - in which case unsold, obsolete models might have carried numbers from a sequence applied to newer version.
1919 - 1920 Serials 1 to 66
1920 -          Serials 67 to 131
1920 - 1922 Serials 132 to 1121
1922 - 1923 Serials 1122 to 1319
1923 - 1925 Serials 1320 to 1649 "A" suffix applied from No. 1405A--but at random
1925 - 1926 Serials 1450 to 1887
1926 - 1927 Serials 1897A to 2226A
1927 - 1929 Serials 2227A to 2685A
1929 - 1931 Serials  2686A to 2949A
1931 - 1933 Serials 2950A to 3345A
1933 - 1935 Serials 3346A to 3675A
1935 - 1937 Serials 3676A to 4071A
1937 - 1939 Serials 4072A to 4203A
1939 - 1941 Serials 4204A to 4401A
1941 - 1942 Serials 4402A to 4467A
1941 - 1942 Serials 4468A to 4592A (Separate page in ledger)
1943 -          Serials 4534A to 4592A
A pattern machine (presumably a sample) was dispatched to Myford Engineering on March 25th, 1943 and the final Drummond production M-type delivered to the Admiralty on June 30th of the same year..

1921--1924 Drummond on the maker's treadle stand with ball-bearing flywheel

1921-1924 Drummond M Type with overhead drive powering the gear-cutting attachment