introduction of a new production technique at Richmond Road…
As a young machine tool setter in the machine shop in the early
1960s I was told one day that a new NC (numerically controlled) machine
known as the Cramic was being delivered specifically to machine the new
solid wings skins for the P.1127. I was to assist in the bedding in,
alignment and machining of the vacuum chucks and thereafter the
machining of the first sets of solid wing skins.
Having been used to setting conventional lathes and milling machines for many years this NC technology was completely new to me. However, I was not too concerned as I was sure some training would be in the pipeline somewhere; not so!
Eventually the Cramic arrived and, along with two large vacuum pumps, was placed in the specifically prepared foundations. The various alignment checks were carried out by Cramic personnel and I was shown by them the manual controls of the machine and cut some test pieces to verify the alignment.
The vacuum chucks arrived and were bolted to the
machine table. I then machined in the faces of the chucks a grid of
slots 3/16 ins wide by .060 ins deep. At the intersection of each slot
a hole was drilled to accept a grub screw. The vacuum pumps were then
connected to the chucks and the machine was ready to go.
When the raw
material arrived it was DTD 5020 in solution-treated condition, in slab
form some 15 ft long, 5 ft wide, 3ins thick and roughly triangular in
shape. My first thoughts were “However much does this cost and if I
scrap it how will I survive?”
The first operation was to get the slab onto the machine table.
Overhead tackle had been set up and I manoeuvred the slab into
position, packed it underneath with off-cut aluminium sheet where
necessary, and pinch clamped it to the table in a stress free state. It
was now ready to be faced flat. With one flat face the slab could be
turned over and sucked down onto the vacuum chucks ready for routing
the egg-box pattern spars and ribs.
All grub screws under the slab were
removed and a sealant was manually placed around the perimeter of the
slab to retain the vacuum. Datum holes were drilled in the waste
material at each end of the slab and from these holes each tape and
cutter combination began its prescribed cutter path.
Finished skin thicknesses varied between 0.2" at the inboard wing root end to .080" at the outboard tip. I machined the first five sets of solid skins. Most of the tapes ran for more than one hour and some had to be run several times at different depth settings. A night shift and additional setter were introduced after the first few sets of skins to cope with production schedules.
The purpose of going to solid wing skins was to reduce the number of
through-skin fixings associated with fabricated skins as these were
resulting in unacceptable integral tank fuel leakage, especially after
heavy landings. From slabs weighing up to 2,000 lbs the finished skins,
in some cases, weighed as little as 140lbs - that's a lot of swarf.
On completion of the machining process the skins were placed on purpose-built trolleys with formers so they could be clamped down to the wing profile and heat treated up to full specification - two birds with one stone.
I think this is a good example of production techniques solving a design problem, albeit in an expensive way. I have great respect for the work of the process engineers involved, John Duncan and Mickey Vaughan, since I don't believe any tapes had to be altered for cutter speed or feed problems, the whole process being so well worked out.