Model Engineers’ Workshop – August 2019

48 http://www.model-engineer.co.uk Model Engineers’ Workshop

some useful measurements, but the
blueprinting had suffered with age and
smudging and there was at least one
conflict in the measurements between
drawings. The individual parts were
recreated in Alibre, fig. 3b, using the
measurements from the blueprints.
They were then fitted into the virtual
track frames to check against the photos
and adapt for modelling purposes. A
problem was found in the idler adjuster
bracket. Unusually two different views
were available for this part but some of
the dimensions shown differed. While
the running gear parts for the heavy and
medium tanks were identical, the photos
suggest that this bracket may differ
slightly hence the conflict was indicated.
Modelling the part in Alibre showed
where minor adjustments would solve
the problem and the relevant dimensions
were easily changed.
Once the 3D model parts were
complete, it was easy to transfer the
data internally, straight to the Drawing
module. Opening the Drawing module
showed the standard view layout 2D
working drawings, fig. 3c. These can be
dimensioned automatically but was done
manually for convenience of placing.
The original tracks may be pictured as
two engineering or welded chains with bent
link extensions on both sides, with a track
foot plate riveted across 2 side by side links,
fi g. 4a. Shows the basic drawing of the
Track Link and Roller Assy from the Tank
Museum information pack. It took some
interpretation! Figure 4b was discovered in
another Tank Museum report and provided
more useful information on the shape. The
fi nal design used was simplifi ed as shown
in the version produced in Alibre, fi g. 4c.
Figure 4d Shows a reproduction track link
assembly produced sometime aft er WW2.
Again, sharp eyed readers will see that the
links are back to front when compared with
fi g. 4b thus highlighting the diffi culty in
interpreting the original drawing in fi g. 4a.
Initially the sprocket design appeared
straight forward. The blueprint and
Engineer illustration showed that it had
23 teeth and the “chain” pitch was 38 mm
at 1/5th scale but it spanned 2 teeth. The
originals were probably standard sizes in
1916, alas no longer. There is a well known
bodge where you drill roller size holes

round the sprocket

PCD, cut down to

just above the PCD

and approximate

the shoulders by

fi ling. The idea

was modelled in

Alibre and the tooth

shoulder radius

found by scribing

an arc from the

roller centre two

teeth away. The 3D

virtual sprocket was

converted to a 2D

drawing in Alibre,

fi g. 5, and sent to

my cousin, Stephen

Day, in New Zealand

who was able to have

them laser cut for free.

The fi nal layout for the Track Frame and

Running Gear is shown at fi g. 6. Note that

the outer Frame Panel has been removed.

The engine and transmission have yet

to be thought out in detail for production,

but some virtual modelling has already

been done. There were concerns about size

of engine (a 31 cc Honda GX31 strimmer)

fi tting into hull interior space. The

engine was laboriously measured, and all

protuberances noted then fed into Alibre

as a series of parts to give an acceptably

accurate assembly. This assembly was then

Fig.4c

Individual Track Link reproduced in Alibre.

Reproduction Link at Bovington.

Fig.4d Fig.5a

Sprocket Construction Sketch from Alibre.

The Drawing sent to New Zealand.

Fig.5b

Alibre Mock-Up of the Track Frame and Running Gear with the outer Frame Panel removed.

Fig.6