August 2019 47
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Laser Cut Tank
from scratch without any machinery.
I will concentrate on describing the
process of producing the working drawings
as this took several years before being
able to acquire and house the machinery.
Some work from the back of an envelope
while others use pencil, paper and T-square
or even a 2D CAD package. We did very
little engineering drawing at Birmingham
University and I proved to be no
artist. There were 2D CAD packages
available for home use but at that
time they were very unfriendly. They
also wanted measurements for each
line that I did not have. Some had 3D
facilities but these mainly consisted
of declaring spheres, cubes etc. and
combining them.
Parametric design was available in
3D CAD as represented by AutoCAD
or Solid Works but not aff ordable
for the home user. About this time,
I discovered the old Alibre. It was
the only aff ordable package for
home use that followed parametric design
allowing me to concentrate on the shape
and relative spacing of the object and add
measurements later.
The means of obtaining the basic
dimensions proved to be “interesting”,
in the Chinese sense, either measure the
Bovington exhibit, scale photos or the
Engineer three view illustration. Physical
measurement would be tedious and lead to
accumulating of errors. Photos are seldom
fully side on and suff er from perspective
even if they are. That left the three view
illustration from Engineer.
The three view Engineer illustration
was a work of art, very detailed but not an
engineering drawing following modern
conventions, fi g. 1. It is eff ectively an
assembly drawing that is very precise in
shape and detail but understanding the
section planes proved to be a minefi eld.
Overall, I had only two precise dimensions
(the horizontal distance between idler
and sprocket centres and total height)
from the blueprint cards. The track plate
drawing gave two views that were helped
by an illustration in old report held by Tank
Museum. The overall eff ect was inscrutable
and trips to Bovington revealed that alldetail of the tracks and running gear
was hidden behind the side armour
panels.
The fi rst problem was the shape of
the track frame side panels. They had
subtle changes in direction and partial
circles at each end. This did not favour
building the shape from scratch in
the Alibre Part module or in 2D CAD.
The Alibre concept is to create the 3D
model and internally export this to
the Drawing module to produce the
normal three view engineering drawing.
However, a scan could be imported into
the 2D CAD, traced, tidied and exported
as a DXF. David Jupp, the technical expert at
Mintronics, said that I could go in the back
way by importing the DXF into the Drawing
module. It could be tested by Alibre for
inconsistencies then cut & pasted into the
Alibre Part module workspace.
Those with the sharper eyes will see that
the elevation view from Engineer shows a
section through the upper track run to the
engine bay. Those lines were estimated
and added in the 2D CAD. The result was
a closed fi gure outline showing the track
frame side panel shape together with all
the rivet, axle and mud chute holes. It
only needed the magic dimension 1046
mm declared for the horizontal distance
between Idler and Sprocket axle centres
and it was done. The fi les for inner and
outer track frames, fi g. 2, were exported
as DXF and the disc sent to a laser cutting
fi rm. They produced a precision result far
better than anything I could have marked
out and machined.
Next up was the running gear: Tracks,
road wheels, Sprockets, idlers, axles and
bearing mounts. Here the data came from
the Tank Museum’s miniature blueprints,
fi g. 3a. However, these diagrams were
really assembly drawings albeit withExported Drawing of the Outer Track Frame for Laser Cutting.Fig.2
Fig.3a
Heavy Road Roller Assembly from the Blueprint. Tank Museum. The Heavy Road Roller Axle Assembly remodelled in Alibre.
Fig.3b
Fig.3c
The Drawing of the Heavy Road Rollers
produced automatically by AlibreFig.4b
Individual Track Link. Tank Museum.