ARMOUR WELDING ON THE
CHURCHILL TANK MARK VII
The inception of the Churchill tank dates from the opening stages of the
war, when a tank was required for coping with the Siegfried Line defences.
Preliminary designs were in course of preparation when France was overrun and
England was faced with the possibility of invasion. The Churchill tank as it
finally appeared, therefore, was designed to fight in the English countryside.
The Churchill tank Mark 1 was armed with a 2‑pdr. gun, a coaxial
7.92 mm. Besa machine gun in the turret and a 3 in. howitzer in the hull by the
side of the driver. In the Mark II tank the howitzer was replaced by a second
7.92 mm. Besa machine gun, the two machine guns being retained on all
subsequent Marks. The turret was cast and the hull was of riveted double‑skin
construction with 3.5 in. frontal armour and 2.5 in. side armour, both
superimposed on a 0.5 in. mild‑steel or high‑tensile‑steel
lining. Experience in the early stages of the North African campaign led to the
substitution of a 6‑pdr. gun for the 2‑pdr. and later, for the
European campaign, the tank with which this bulletin is primarily concerned, the
" heavy” Churchill Mark V11 was developed, mounting a 75 mm. gun. This
tank has a composite cast and plate turret of welded construction and a single‑skin
welded hull with frontal armour on a 6 in. basis and side‑armour of 3.75
in. All armour plate is to I.T. 80 specification and the turret casting to I.T.
90 specification.
The Churchill Mark VII weighs‑nearly 40 tons and measures 24 ft. 2
in. long, 10 ft. 10 in. wide over air‑inlet louvers; 9 ft. 5 in. over
sandguards and 9 ft. 0 in. high over the No. 9 aerial base. The air‑inlet
louvres are detachable to bring the overall width of the vehicle within railway
loading gauge.
As with all previous Marks, it
is manned by a crew of five, consisting of a commander, gunner and loader in
the turret and a driver and machine-gunner in the hull. The engine is a Bedford
Twin 12‑cylinder, horizontally opposed unit developing 340 b.h.p. at
2,200 r.p.m. It drives through a David Brown, four‑speed constant‑mesh,
gearbox, giving a maximum road speed of 31 m.p.h. Steering is on the controlled
differential system. The suspension is peculiar to the Churchill, and consists
of eleven spring bogies on each side of the tank, engaging the lower run of the
track by means of flanged wheels of comparatively small diameter. Side panniers
are incorporated in the tank hull and the bogies are bolted underneath the
panniers; the tops of the panniers being fitted with skid rails to carry the returning
top halves of the tracks. Damage to one bogie does not materially upset the
suspension, and the damaged bogie can be replaced without disturbing any other
part.
The Churchill tank has gained an excellent reputation in the fields in
which it has been employed, and it was widely used for the mounting of certain
of the special devices required for the European invasion. The designer of the
Churchill tank was Vauxhall Motors Ltd., who assumed their responsibilities in
July 1940. A welded Churchill Mark V11 is illustrated in Fig.
1, and a Mark VIII, which is similar except that it is armed with a 95 mm
howitzer, in Fig. 2.
THE WELDED CHURCHILL
When the Churchill was initially designed the
possibility of welded construction was considered seriously but little was
then known of the performance of heavy welded armour construction in battle
and immediate production was a paramount necessity. It was decided that the
risk could not then be taken. Nevertheless this did not preclude experimental
development, and the first welded mock‑up in heavy armour plate was
submitted to firing trial in July 1940,with highly satisfactory results. This
mock‑up was built by Babcock & Wilcox Ltd., who were appointed
technical parents on the welded development by Vauxhall Motors Ltd. In November
1940; Babcock & Wilcox Ltd. were given an order for an experimental
Churchill Mark IV welded hull and, an order for a second, similar hull in
January 1941.
It was about this time that
the decision was made to arm the Churchill with a 6‑pdr. gun and as this
required a larger turret than the cast one then in use, it was decided that
this change provided a suitable opportunity for introducing welded construction
in production. Accordingly, in May 1941, Babcock & Wilcox Ltd. were asked
to construct a welded turret for firing trial, and subsequently 21 further
experimental turrets were ordered from a number of representative firms and
using plates supplied by several different steel makers. The firing trial on
the Babcock & Wilcox turret took place in July 1941, and the results were
so favourable that a production order for 100 turrets was placed with Babcock
& Wilcox Ltd. forthwith (August 1941). Firing trials continued to be
conducted on the other experimental turrets as they became available, and the
results confirmed the wisdom of the decision to adopt welded construction.
At this time a discussion
was proceeding as to the necessity for postheating welded armour structures in order
to relieve welding stresses and temper the heat‑affected zones; and
advantage was taken of the turret programme to investigate the matter. The
original Babcock & Wilcox experimental turret had been postheated, so the
first of the production turrets ordered from this firm was made to precisely
the same procedure but without postheating and, in October 1941, was submitted
to an exactly similar firing trial. The comparative results showed that
postheating was unnecessary, but to confirm the point, five of the 21 further
experimental turrets were fired-at in the non‑postheated condition, and
the results not only supported the original conclusion but disclosed the danger
that subsequent postheating might actually degrade the ballistic quality of the
armour plates.
At the same period, a parallel investigation was in progress on
the experimental hulls. The first of these hulls was approaching completion and
was postheated, but in the case of the second hull postheating was omitted. The
two hulls were built into tanks and subjected to 1,500 miles rough‑usage
running, after which the non‑postheated hull was stripped and submitted
to firing trial in July 1942. These trials confirmed the conclusion that
postheating was unnecessary, and provided ample evidence that the welding of
heavy tank hulls was sound policy. To put the matter beyond doubt, a Mark IV
riveted hull was submitted to a similar firing trial, and although it behaved
well for a riveted structure, the correctness of the decision to weld heavy
armour was unchallenged.
In all this early work the joints were pre-coated
with 25:20 electrodes, a practice that it was possible later to discontinue. In
the meantime, a comparison between riveted and welded construction had been
made from the production angle. An analysis was made by the Department of Tank
Design, in collaboration with the contractors concerned between the current
production data on riveted Mark IV hulls, built by The Whessoe Foundry, &
Engineering Co. Ltd., Darlington, and Metropolitan‑Cammell Carriage and
Wagon Co. Ltd., Old Park Works, Wednesbury; and the welded prototypes built by
Babcock & Wilcox Ltd.
In a report prepared by the Welding and Gas
Cutting Branch in March 1942, it was concluded that welding would save 20 per
cent. in man‑hours and 4 per cent. in weight, as compared with riveting
but required 50 per-cent more floor space in the fabricator's works. This additional floor space would be largely
counterbalanced by a reduction in space required at the plate maker's works,
due to the greatly reduced amount of drilling required. The additional floor
space was, therefore, not regarded as a serious drawback in the circumstances
but the savings in labour and material were vital considerations. Subsequent
experience showed that a greater saving in welding man‑hours was effected
than had been estimated and the allowance for floor space had been over‑generous.
By the
time the foregoing investigations were completed, the demand had arisen for the
"heavy" Churchill, and it was decided that the Mark VII tank should
be of welded construction. To assist in obtaining the necessary production the
contract was extended to certain other firms, including The Whessoe Foundry
& Engineering Co. Ltd., Newton Chambers & Co. Ltd., and the Gloucester
Railway Carriage & Wagon Co. Ltd.
As a result of further development work,
Babcock & Wilcox Ltd. abandoned the practice of pre-coating and adopted a
particular technique of horizontal vertical welding that came to be known as
“flat‑bead” welding. This method was employed for all the experimental
structures with which they had been concerned and, on the basis of the firing
trials, was accepted by the Department of Tank Design for production. At a later date The Whessoe Foundry &
Engineering Co. Ltd. introduced a gravity welding technique on the Churchill
hull, as far as the jigs and manipulators would permit and they were
subsequently followed, with modifications, by Gloucester Railway & Carriage
Wagon Co. Ltd. and Newton Chambers & Co. Ltd. Gravity welding with large‑gauge
electrodes (i.e. five sixteenths and three eights in. diameter) was by then the
accepted practice for all other types of tank construction and offered certain
advantages, economic and otherwise.
As already mentioned, in designing the turret
for the Mark VII vehicle to take the 75‑mm. gun, a composite cast and
welded construction was decided upon. For the main structure of a turret a
casting possesses the advantages over a welded structure in that the thickness
of sections can be varied as desired, and turret can be contoured more efficiently
so that weight is saved. On the other hand, if the roof and floor are cast
integral with the turret structure, they would have to be thicker than necessary
so as to avoid extreme changes of section in the casting. The construction most
economical was therefore a cast turret with rolled roof and floor plates welded-in
and this was the construction adopted for the Mark VII turret.
MANUFACTURING PROCEDURE
When the additional contractors were introduced
they adopted jigs and manipulators to Babcock & Wilcox designs, and
followed the manufacturing procedure adopted by that firm. Subsequently, in
certain details, each contractor developed methods or equipment to suit his own
convenience and the adoption of gravity welding has, of course, involved an
important departure in procedure. The retention of existing equipment, which
was the obviously correct course from the standpoint of maintaining production,
imposed a certain limitation in exploiting gravity welding fully. We shall,
therefore, describe the manufacturing procedure as carried out at the works of
Babcock & Wilcox Ltd. referring to departures made by other contractors as
they arise in the course of the description.
PLATE PREPARATION
All contractors receive
plate ready-profiled, mainly by gas cutting but all edges to be welded have to
be dressed before the plates are assembled. The dressing operation is carried
out by portable grinders, flame de-scaling and wire brushing,
HULL
For purposes of manufacture the
hull is divided into the following sub-assemblies: floor, side panniers, nose
and roof. The rear end is built up and two outer idler plates added during the
main assembly and the engine‑compartment roof forms a separate sub‑assembly.
FLOOR
The floor sub‑assembly
consists of a 25 mm. plate under driver and fighting compartments, a 20 mm.
plate under the engine compartment, 2.25‑in. side members and four cross
members, the whole being built up in a trunnion‑mounted jig. When the
tank hull comes to be assembled it is lined up on longitudinal and transverse
centre lines passing through the axis of rotation of the turret and in building
the floor, the first operation is to place the bottom plates on the jig, line
them up, and mark longitudinal and transverse centre lines passing through a
hole drilled on the centre point of the turret. This operation is shown in Fig. 3. The plates are then clamped and the transverse
butt-joint between welded.
In the next stage, the side
members are introduced and welded, as shown in Fig. 4.
The outside welds for these members have to be interrupted where they pass
beneath the clamps, and are completed after the assembly has been removed from
the jig. To avoid this, the Gloucester Railway Carriage & Wagon Co. Ltd.
merely tack the assembly in the jig and then transfer it to a manipulator,
depicted in Fig. 5, which enables the welds to be
deposited complete in the gravity position.
Finally, the four cross
members, which are mild‑steel fabrications, are welded in position (see Fig. 6), tapping pads are welded around the inspection
holes, inspection covers fitted, and sundry minor items added.
SIDE PANNIERS
The first stage in the construction
of a side pannier is the welding of the nose as a separate sub‑assembly;
an operation which is carried out in the jigs seen in Fig.
7. At Whessoe Foundry & Engineering Co. Ltd. these jigs are not used,
the nose being built up as described later in this paragraph. The pannier
itself is built up in a rotary manipulator which is shown partly plated in Fig. 8, and with the nose added in Fig. 9. At the Whessoe works, the two nose plates are
placed temporarily in position on the pannier against a template and tacked,
after which they are removed and the inside weld is deposited. The nose is then
returned to the pannier assembly for welding in position; the outside weld of
the nose being deposited in the manipulator. This procedure enables both inside
and outside welds on the nose to be laid down in the gravity position whereas
the Babcock jigs are designed for horizontal‑vertical welding.
After the main carcase of the
pannier has been welded, bulkhead plates are fixed and a number of ferrules,
bosses and other attachments are located by simple fixtures and welded (see Fig. 10). Amongst these fixtures are pairs of key
plates for the reception of special devices. The distance between these plates
has to be held to an accuracy of 0.020 in. and each contractor has developed
his own jig attachment for fitting them. Each design depends on a plate with
machined faces for setting the key plates to exact distance but, as will be
seen from Figs. 11‑14, the methods of clamping the key plates differs
considerably.
Fig. 11 also shows
the type of fixture used by Newton Chambers &Co. Ltd. for welding the
pannier‑door hinge brackets. This is a precision operation, and the
methods adopted are of some interest. The machined taper of the door opening
is placed face upwards, checked for out-of-round caused by the welding of the
top and bottom plates and adjusted by grinding until the door beds down to the
required depth and clearance limits. The
outside faces and the side plates, in the vicinity of the hinge location, are
now ground level and the hinge assembly, consisting of two straps extending
over the door, and four hinge blocks
seating on the side plate, is threaded on a master pin and laid in position.
Shims are then placed under the door straps to bring them to correct
height, and the straps are fastened to the door by set screws. Shims are now
fitted between the hinge blocks and the side plate until the blocks bed firmly
and swing freely on the hinge pin.
The clamping fixture seen in Fig. 12 is now placed in position and is secured by being
hooked under the pannier top plate on one side, and bolted to the bogie
suspension holes on the other. Hinge blocks and straps are now welded, the
master pin removed and two permanent pins fitted. Final adjustment of the door
does not take place until the hull is nearly completed, thus avoiding the
danger of the hinges being strained by the door violently opening when the hull
is tilted during assembly. (see also Fig. 13 and Fig. 14)
As already mentioned, skid rails are attached to the pannier tops to
carry the return run of the track. In view of the heavy wear to which these
rails are subjected, their attachment created something of a problem.
Originally they were secured to the pannier tops by bolts with countersunk
heads, but the heads wore away with the track abrasion and the rails came
adrift before their useful life was ended. Furthermore, the nuts; being on the
inside of the pannier, could not easily be reached when it was desired to remove
the rails. A proposal was then made to weld the rails to the panniers, but the
rails were made of silico‑manganese steel which was difficult to weld, inclined
to be brittle and could develop cracks. Eventually this welding problem was overcome
by pre-coating the rails with 25:20 electrodes and by welding only around the
ends, leaving the centre of the rail free instead of employing staggered
intermittent welds, as had previously been done.
NOSE
The main nose assembly consists of visor, glacis, and nose plates, and
the inner idler‑support brackets. It is built up and welded in a trunnion
jig as seen in Fig. 15, after which it is removed
from the jig, and a towing eye and certain other fittings are added. Those
contractors who have adopted gravity welding take advantage of this trunnion
jig to deposit the heavy transverse welds on this assembly in the gravity
position. Accurate fitting is required for the visor door and because the hinge
straps are welded to the door, care must be taken to prevent the pull of the
welds from throwing the assembly out of register. Babcock & Wilcox Ltd.
overcame this difficulty by welding the straps to the door in a block jig,
whereas Whessoe Foundry & Engineering Co. Ltd. and Gloucester Railway,
Carriage and Wagon Co. Ltd. fit the door to the visor plate before the nose is
assembled, as shown in Fig. 16. Babcock & Wilcox
Ltd. and Newton Chambers & Co. Ltd. fit the visor door after the hull has
been completed.
ROOF PLATE AND ENGINE‑COMPARTMENT ROOF
These assemblies call for little comment. The roof‑plate sub‑assembly
jig is seen plated up on the right of Fig. 17 and a completed
roof plate is in the centre. Originally, the frames for the roof doors were
bolted‑on units, as are seen stacked along the front of Fig. 17 but more recently, the hinges and splash strips
have been welded directly to the roof. The engine‑compartment roof is a
simple framework carrying four, access doors. It is built up in a jig similar
to that used for the roof plate.
MAIN HULL ASSEMBLY
The hulls are assembled in rotary manipulators. Babcock
& Wilcox Ltd. are equipped with twelve, Newton Chambers & Co. Ltd. with
ten (Figs. 18 and 19),
and Whessoe Foundry & Engineering Co. Ltd. and Gloucester Railway Carriage
& Wagon Co. Ltd. with seven each.
Operations are commenced by seating the floor assembly
in the lower half of the manipulator and following it with the panniers which
are lined up fore and aft on the transverse centre line marked on the floor
assembly and set to gauge distance apart along the longitudinal centre line.
This stage is seen in Fig. 20. The main nose is then
fitted (Fig. 21) and the entire assembly is lined up
with a straight edge and plumb bob, then held with screw jacks extending
between the panniers as seen in Fig. 22. This stage
of the assembly is then welded. Fig. 23 depicts a
hull in this stage at Gloucester Railway Carriage & Wagon Co. Ltd. and
shows the use by that firm of a screw jack to give an outward preset to the
inner idler brackets. Fig. 24, taken at the same
works, shows a hull in this stage positioned for gravity welding.
Fabrication continues by insertion of the upper side
members, followed by the bulkheads. In connection with the upper side members,
an interesting departure is made by Newton Chambers & Co. Ltd. The members
are laid on the floor in a locating fixture which holds them in their exact
relative positions, then the top half of the manipulator cage is lowered over
them and they are bolted to the cage as seen in Fig. 25.
Thus, when the cage is closed after the insertion of the panniers, the upper
side members are offered to the assembly in their correct relative positions.
Incidentally, this procedure includes these structures in the first‑stage
assembly.
The assembly of the rear end of the hull is an
operation calling for considerable accuracy, as the alignment of the drive is
dependant on it. The structure comprises an inner and outer pair of drive
plates connected by end plates and the apertures in these drive plates must be
truly concentric as well as on an axis at right angles to the centre line of
the hull. To do this, a jig frame is built up, locating on the bogie bracket
holes, and carrying the drive plates by their apertures on spigots. This frame
is seen in course of construction in Fig. 26. A
cross beam is laid inside the rear ends of the panniers and is bolted to them
through the bolt-holes for the rear bogie brackets. A longitudinal frame is
then bolted to this beam, giving the stage shown in Fig.
26. Side members to which the drive plates have already been fixed are then
bolted to this frame, as seen in Fig .27. Fitted
bolts are used throughout to ensure that all parts of the jig frame register
accurately. Brackets attached to the jig frame provide seatings for the rear
plates which are now fitted as shown in Fig. 28.
After the rear end has been welded, the jig frame is removed and the rear sloping
plate inserted in position and welded.
Variants of the foregoing procedure have been
introduced by Newton Chambers & Co. Ltd. and by the Whessoe Foundry &
Engineering Co. Ltd. Newton Chambers builds up the jig frame, complete with
drive plates, independently of the hull, as in Fig. 29,
then offer it to the hull as a unit. At the Whessoe Works, the rear unit is
completed to the rear plates, as seen in Fig. 30, with rear sloping plate being laid in
position before the rear unit is offered up. This operation is completed before
the manipulator is closed and before any main welding is carried out on the
hull. These variants are stated to save several hours of plating time.
Concurrently, with the fitting of the rear end, the roof plate is fitted as
depicted in Fig. 31. All parts of the structure
added since the first‑stage welding are now welded up.
The final operation in the manipulator is the fitting of the outer idler
plates. These must be in accurate alignment with the inner idler plates, and
they are jigged as shown in Fig. 32. Each outer
plate is held in register with the corresponding inner plate by a dumbbell-shaped fixture having rectangular spigots at
the ends that engage the jaws of the plates. Slotted distance-bars are also
laid over the plates to hold them parallel. Fig. 33
shows the same stage at the Whessoe
Works. The manipulator in this instance being positioned for gravity welding on
the hull in accordance with the
procedure adopted by that firm.
The hull is now removed from the manipulator to receive the finishing
operations (see Fig. 34). Amongst these, an
interesting item is the fixing of the gearbox and engine bearing pads, which
have to be positioned accurately with regard to both one another and to the
centre line of the final drive. At the Whessoe Works this is carried out by the
jig shown in Fig. 35 which not only locates the pads
in correct alignment with the final drive, but ensures that they are in correct
vertical register with one another, thus allowing for any warping or unevenness
of the floor.
Turret
As already stated, the turret is a casting with welded top and bottom
plates. Operations are commenced by buffing the bottom plate and then aligning
it to receive the traverse‑gear bracket which will be welded in place
next. The plate is then marked out for centre lines and laid in position in the
turret casting which has been previously buffed to receive it. To locate the
plate, a fixture is registered on the back and sides of the gun‑mounting
aperture and carries a trammel by which the hole in the bottom plate is set
truly central. The plate is then clamped and welded. Fig.
36 shows the plate ready for welding. Certain reinforcing members are then
set on the inside of the bottom plate and welded.
The top of the turret consists of two plates which are connected by a
transverse weld across the centre of the turret. Precautions must be taken to
prevent the plates bellying downwards after welding. After buffing and drilling
the plates are fitted over a restraining support, as seen in Fig. 37, which gives an upward preset of three-eighths
of an inch when the plates are clamped down. The loader's door-drain-channel,
cupola ring, aerial bases and smoke‑mortar boss are then added to
complete the turret which finally appears as seen in Fig.
38.
Turrets are also made by the Whessoe Foundry & Engineering Co. Ltd.,
a group of turrets under construction at these works being seen in Fig. 39. The most important difference in procedure as
compared with that at Babcock & Wilcox Ltd. lies in the treatment of the
top plates which are clamped to a faceplate as seen in Fig.
40 and welded before they are offered to the casting.
Sandguards
Figs. 41 and 42
show the manufacture of sandguards by the Gloucester Welding Co. Ltd. (branch
of Thos. Ash & Co. Ltd., Birmingham) who are subcontractors to the Gloucester
Railway Carriage and Wagon Co. Ltd. The guards are of mild‑steel sheet
and are assembled in sections using wooden Jigs, then spot welded as seen in Fig. 41. The completed sections are then fitted
together on the steel formers seen in Fig. 42 and
the construction completed by arc welding.
Final Assembly
Of the four main contractors already mentioned, Newton Chambers &
Co. Ltd. and the Gloucester Railway Carriage & Wagon Co. Ltd. produce
completed tanks and views of the final assembly lines at these two works are
given in Fig. 43 and Fig. 44.
The other contractors send their hulls and turrets to Vauxhall Motors Ltd.,
Charles Roberts & Co. Ltd., and Dennis Bros. Ltd. for final assembly.
Abbreviations key:
B&W: Babcock
& Wilcox
GRC&W Gloucester
Railway Carriage & Wagon
NC Newton
Chambers
VM Vauxhall
Motors
WF Whessoe Foundry
& Engineering
END