Bombe drawings

Coinciding with our Hut 11A: The Bombe Breakthrough exhibition, take a closer look at some of the technical drawings and blueprints for Bombe machines featured in the exhibition. These original drawings were selected from several thousand in the collections of Bletchley Park. Beautifully detailed and incredibly precise, they were all created by hand by the engineers at the British Tabulating Machine Company in Letchworth, who were tasked with building the Bombes. The drawings relate to some of the different types of Bombe machine which were developed over the course of World War Two in response to changes in design and operation of Enigma cipher machines. Together they are testament to the enormous effort that went into producing the incredibly complex Bombe codebreaking machines.

This Collections Uncovered album was first published on 28 March 2018.

Detail from Brush Holder Assembly. This whiteprint shows how each Bombe drum holds 104 spring wire brushes arranged in sets of four, with 19 wires per brush. The brushes make and break electrical contact with brass plates as the drums rotate. These wire brushes were manufactured at the Spirella factory in Letchworth as the workers there were accustomed to using wire for making corsets. This particular drum was used for testing for faults on the machine.

Detail from Circuit Breaker Box Assembly. This component delivers an electrical current 26 times during the rotation of each Bombe drum, once for each letter of the alphabet. It ensures the current is only released at exactly the point required and not when the brushes are touching the plastic moulding.

Carry Cam Shaft Assembly. This is part of the mechanism which moves the Bombe drums. Each full revolution of the top row of drums prompts the middle row to turn 1/26th of a rotation. Similarly, each full turn of the middle row causes the bottom row to move 1/26th. The carry mechanism was adapted from the paper feed of a pre-existing tabulating machine.

Detail from Index Bracket Assembly. This component was situated on the right hand side of the Bombe, near to the hand cranking point. If it were necessary to hand crank a Bombe, for example to set the timing of the machine, the index wheel enables the operator to see where they have reached in the Bombe’s running sequence.

Index. Details of components varied between different models of Bombe. This blueprint shows an index wheel with 52 positions rather than the usual 26. Index wheels are also used to monitor the functioning of the machine by displaying the activity of the drive shaft. The wheel rotates once for every cycle of the machine.

Index Assembly. This index wheel has 28 positions and is probably for a High Speed Keen (HSK) Bombe machine. HSK was a 4-drum Bombe developed in 1943 to attack German naval messages sent on the new 4-rotor Enigma machine.

Accelerator Cam. This drawing on semi-translucent paper is an original from which blueprint or whiteprint copies could be made. It shows a component that modifies the movement of the drums of the High Speed Keen, producing a stuttering motion to allow the brushes to work at normal speed on the quicker spindles of this model.

Cross and Connecting Plug Assembly. These plugs are used to connect together the input and output of each column of three Bombe drums. This allows the output of one column to be linked in sequence to the next one without additional cabling.

Auxiliary Bearing Plate – Lower. This part supports the Bombe drive shafts. Prominently featured are the lubrication points, marked ‘oil grooves’. These are vital as the shaft/plate bearings are metal on metal so lubrication is critically important.