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Drummond B-Type Power Cross Feed
Admiralty Model (B.S.)
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The Mk. 1 "B.S. Type" was introduced for Admiralty use in 1916 and included a range of modifications to improve its ease of use on board ships as they corkscrewed around in heavy seas. It was closely based on the standard "B-Type" machine but fitted with a power-feed apron with both longitudinal and cross feeds driven from a shaft fitted below the leadscrew  and geared to its left-hand end.   
In order to make the assembly work correctly the simple full nut that connected leadscrew to apron  was replaced by a proper double clasp nut. A saddle traverse by hand-operated rack and pinion was also fitted to the B.S. and a tumbler reverse for the leadscrew drive attached to the slotted bracket on the headstock front - a bracket that normally carried a stud to mount the extra changewheel required for left-hand screwcutting. When the tumble-reverse unit became available on the B.S. it also entered the accessory lists for the standard machine - however, the full B.S. specification was only available for Drummond B and M Types, and Myford M Type, destined for naval use; it was never offered on the civilian market nor, so as far as is known, was ever the subject of an advertising booklet. On the Mk. 1 BS the power-feed selector arm worked across the front of the apron whilst on much more common Mk. 2 it was positioned to hang downwards and indent into slots machined into the underneath face.

Very rare Mk. 1 BS Type with the power-feed quadrant selector arm working across the face of the Apron. The headstock of this lathe was the type with the strengthening loop over the top.

Mk. 2 BS Type
The bottom edge of the distinctly different apron of the B.S. Type carried a  spring-loaded quadrant selector with indent positions for hand or power cross feed. The ordinary B had a solid nut on the leadscrew but the fitting of power sliding and surfacing meant that a split nut, that could be disengaged, was required; this was operated by a tapered lever positioned at the top right of the apron.
The spring-loaded operating knob that engaged the cross feed was of an identical design (and something of a Drummond trademark) to that used for the engagement and disengagement of  the leadscrew clasp nuts on later versions of the lathe.

Power was supplied to the apron by a separate shaft, gear-driven from the left-hand end of the leadscrew.
Another view of this assembly here

Below, left and right. The power-feed mechanism was contained within a two-part, cast-iron housing the inner section of which was secured to the front of the saddle by just two slot-headed set screws.

A cut-out on the edge of the "cam plate" (at 8 o'clock, above) carrying the power-feed transfer gears interlocked with the leadscrew clasp-nut cam to prevent both being engaged at once.

Left and above: the final drive to the cross-feed screw was by a gear connected to the square rod that slid through the upper drive gear.