Built along lines of the original light Swiss 'Geneva' pattern watchmakers' lathe - as distinct from the heavier American Webster Whitcombe (WW) models - the B.T.M. watchmakers' lathe had a round bed with the flat element at the top rather than at the (more usual) back. It was intended for, and widely used by, precision industries and the armed services for the manufacture and repair of mechanical instruments. It is unlikely that the lathes were built by B.T.M themselves, but rather designed and developed by them with manufacture farmed out to the Royal Ordnance, a Government organisation who have numerous factories dotted around the country side producing not only munitions but all kinds of high-precision mechanisms. The numbers built must have been considerable for the model continued to be available - in boxes marked "Manufactured by ROF" (ROF referring to Royal Ordnance Factory) - in a black crackle finish until the early 1950s with sales handled by, amongst others, the well known machinery dealer E. H. Jones of Edgware Road, The Hyde, London N.W.9.
With a headstock that looks to have been an exact copy of that used on the 1920s and 1930s G.Boley watchmakers' lathe, the lathe had a maximum through-collet capacity of 4.8 mm (and partial of 5.4 mm), with the 8 mm bore headstock spindle running in plain bronze bearings with a stated running clearance of 0.00025". A spring-loaded pin was fitted as standard to engage in a ring of indexing holes in the front face of the headstock pulley. The lathe was always supplied in the maker's well-made lockable wooden box complete, unlike those from many competing makers, with lid-mounted, felt-padded holding blocks. A very generous level of equipment was included that enabled the lathe to be pressed into service immediately. Items included: a straight bed; a bed with a gap; compound screw-feed slide rest (pictured in detail at the bottom of this page); headstock with hollow spindle and 3-step pulley; two tailstocks - one with a securing lug and one without; a T-rest holder with wide and narrow Tees; box chuck; self-centring 3-jaw precision ring-scroll chuck; 5 step chucks (collets); a set of plain collets to take 1.5, 2.1, 3.2, 3.6, 4.0, 4.2, 4.4, 4.8, 5.2 and 5.4 mm diameters; a set of cone collets to take 0.8, 1.0, 1.2, 1.8, 2.4, 2.6 2.8 and 3.0 mm diameters; a driving chuck with carrier; a pulley runner for tailstock with two pulleys; two plain and six hollow centres, two carriers and a centre ejector with vulcanite knob and a driving belt. Available as extras were an electric motor and drive pulley, a round foot-stand for bench mounting and a lever-action tailstock.
One interesting aspect of the B.T.M. was the very robust, well-supported compound slide assembly that more closely resembled the design of those fitted to a precision instrument makers' lathe. It was carried on a large casting that formed a pseudo apron (though which the cross-feed screw carrier passed) and had an unusually long (split) hole through the base by which means it was clamped to the bed.
For the military market it is thought that the lathe was finished in dark green and for the civilian (apart from the crackle-black mounting post) in nickel plate or polished steel. According to reports by contemporary users, many of these lathes were returned from Reserve Stores after the war (together with masses of other unused, high-precision equipment) and sold off. However, they were not cheap and in 1946, in one of only two known public advertisements, were offered at £45 : 10s ; 0d for the fully-equipped version in a box and £26 : 4s : 6d for the basic model..