In 1995 GCHQ released to the Bletchley Park Trust some 2,000 BTM documents and drawings regarding the Bombe. John Harper, a retired engineer, was encouraged to undertake the building of a replica Bombe to serve as a tribute to the many brilliant people who contributed to the codebreaking work at Bletchley Park.

He had worked at ICL, the successor company to BTM, and this enabled him to contact a number of veterans who had been involved in their manufacture and maintenance, and their advice proved invaluable. He recruited a team of volunteers, which in time grew to more than 60 people, many from an engineering background, and some well-equipped with home workshops. These individuals proved highly innovative in tackling the problems of recreating 1940s equipment in a 21st-century context where many of the materials and techniques were no longer readily available. AutoCad software was used to put some of the drawings on to a computer, enabling 900 new drawings to be completed with 100 new assembly drawings being produced. There were a considerable number of parts to be made, for example, more than 12,000 studs were needed in the commutators and over 18,000 drum brushes. In addition, around 50,000 cable terminations were required on nearly twelve miles of wiring. Around 17,000 screws of varying sizes hold the machine together, but most items weighed only a few ounces (grammes). In total, the Bombe weighed about one ton.

The GCHQ documents related to several different Bombe models and the first task was to copy them all and redraw those for the chosen version using the AutoCad computer tool. These computer-generated drawings were then combined to provide complex assembly drawings, in effect, building a machine on paper!

Other vital information was also obtained from a report prepared by the 6812th US Army Signal Security Detachment, who operated a bay of ten Bombes at Eastcote. Among other important information which helped the rebuild team were photographs which proved invaluable in completing the ‘jigsaw’. Most of the rebuild parts are of new manufacture, and wherever possible, parts were made exactly to the original drawings using the correct material.

Some original parts were obtained from surviving equipment that used the same technology – mainly punched-card equipment used by BTM/Hollerith or Post Office telephony equipment such as relay components, jack contacts and coil formers. In the very rare cases where deviations from the original specification have been unavoidable, these have been recorded in a log as recommended by the Science Museum.

The drums and commutators on the Bombe rebuild
The drums and commutators on the Bombe rebuild

The first part to be constructed was the steel frame and in September 1997 it was installed in the original wartime Bombe Hut 11. In August 1999 some Letchworth Enigma plates were fitted and the rear gate put in position. In May 2000 the main gearbox casting was machined and ready for assembly and the following May the motor and gearbox were running on DC power. Between 2000 and 2002 the cable-forms were constructed by Nortel Networks volunteers using authentic 1940s techniques. In total, they used around 12 miles of PVC insulated wire, and each cableform was wax-string laced, each termination being labelled individually, ready for connection. During the same period, many of the mechanical parts were also made, and in April 2002 the mechanical phase was completed. The rebuild was then moved to Block12

In June 2003 the electrical phase began. Between 2000 and 2005 the manufacture of electrical components was carried out and in January 2004 the Enigma cable assemblies were installed and the commutators mechanically adjusted.

Indicator drums (right), commutators and (bottom right) the start and stop switches on the rebuild
Indicator drums (right), commutators and (bottom right) the start and stop switches on the rebuild

In July 2004 the covers were fitted before removing them later for painting. Again the machine moved, this time to Block B. and in April 2006 the sense relays were in place and the lubrication system was fitted, completing the end of the construction phase. From 2003 onwards, the drum manufacture took place – requiring 180 drums (representing 36 x 5 Enigma wheels, from which three were used each day in the key setting), to fit on three banks of 36 drums, 108 in total, each involving 104 steel brushes on each drum, requiring a total of 11,232 brushes.

The commissioning phase took place between 2006 and 2007. The checking machine then had to be built before the official opening by HRH The Duke of Kent, Chief Patron of the Bletchley Park Trust, in July 2007.

Sponsorship and support

Major sponsorship was received from the British Computer Society, two Berkshire-based companies, Quantel, from Newbury, and the R&D facility at the Nortel Networks offices at Harlow in Essex, while a number of other companies and individuals also contributed, some by providing materials and services. Many wartime veterans also supported the rebuild.

Bombe Rebuild Machine

The target machine was a three-wheel, 36-Enigma version, which had high speed, 104 Siemens-type double sense relays. During the war 69 of this type were built by BTM. The target rebuild machine was modelled partly on No. 297 – Atlanta – used in the US bay at Eastcote and delivered in July 1944.

The Bombe mechanism was not a true cyclometer and could not carry between adjacent commutator segments of the upper drums and at the same time continue sensing. The carry of the target machine takes from 13 character times or points (39-point machine) down to four points (30-point machine), depending on the model. A point is one-26th of the rotation of the top drums or one sense point. The rebuild is a 39 point or 13 point-carry machine, whereas the original Atlanta machine was a 30 point or 4 point-carry machine.

On the rebuilt 39 point Bombe, sensing takes place on 26 steps of the top drums followed by 13 steps of no sensing whilst the carry mechanism operates to step on the middle drums by one position. A further 26 sensings take place followed by 13 more non-sensing whilst a second carry takes place. There are therefore two carries for every three revolutions of the top drums.

Regardless of being a 39 point or 30 point machine, the time that a Bombe takes to sense all 17,576 combinations of drum positions is dependent on the rate of operation of the carry mechanism. The Bombe is designed for 65 carries per minute, which means that a full, uninterrupted run should take 26×26/65 = 10.4 minutes.