Continued from part one here.

The photos below show the beginning of the hand finishing. Here the component is held in the bore by an expanded mandrel. The external areas are being filed and the radii are being blended. Thank goodness for the mighty Dremel and those cheap diamond coated burrs. The second photo show the component after the first stage finishing. It is adequate for the continuing machining operations.

Photo below shows the jacking ring screwed onto screw threads in order to protect them.

The magneto bore is chucked and the internal drain holes are being drilled for the oil that runs down the vertical shaft lubricating the nest of gears. All the oil that runs out of the cam box at the top, runs down the vertical shaft and finds its way out of the bottom to the sump.

The first photo (left) shows more internal oil drainage holes being drilled. The component is held in the chuck and it is ‘set square’ to ensure the holes are drilled correctly. The second photo below shows the component in the fixed steady in the lathe in order for the lower ball race seating to be machined. Note the

brass shim strip protecting the aluminium component from the steel fingers of the steady.

Here the component is upside down having the final drain holes drilled.

An unsealed open ball race sits inside the bore, seen at the top of the component in the orientation in the photo, and the oil passes through it on its way to the sump.

This next photo shows the component finished, with the gears assembled in it.  Bearing sleeves, which are just bushes, are installed  in the magneto bores and these support the centre  gear. A bush has also been pressed into the back

of bore A to support the water pump drive gear.  The flange piece that is fitted on flat face C has a bearing in it which supports the front of the shaft. The water pump shaft fits in the square hole The square hole in the water pump gear wheel has been broached out using a D-Bit.

Next photo is the component assembled with the water pump, and the following photo shows the component mounted on the engine.

As shown previously, the bevel gear at the bottom is driven off the crankshaft bevel drive. Adjusting the jacking ring lifts or lowers the turret assembly to mesh the two bevel gears. This is how it is done on the full-scale engine. The longitudinal movement of the bottom bevel is achieved by shimming the thrust race in the crankshaft and that action moves the whole crankshaft back and forth by a few thou.

Component 1~ In Conclusion: All the datum points are within the component, but there are components where the datums that are needed aren’t within the component or are in an awkward position which can’t be used easily. To illustrate this point, let us look at the manifold (Component 2).

Component 2 ~ The Bentley 3 Litre engine manifold. Using datums external to the component.

The starting point is again a rectangular block. The back is machined and set against a parallel, and on an end stop which can’t be seen in the photo.

Centre lines are marked on the block, and the X and Y scales on the readout are zeroed to this origin. If all dimensions on the drawing have been prepared using this datum, as long as the end location is maintained and the block set against the fixed parallel, all the holes on each face will be maintained in the correct relationship to each other.

The  photo on the left shows the component turned over to machine the carburettor flanges. There is no ‘marking out’. The holes are in the correct relation to the mounting holes underneath. Note: the large squares left on each end of the preliminary block, allow the job to be set up on any face.

With one end set against an end stop, all longitudinal dimensions stay relative to each other on each face. At the end of each square block, centres have been marked in. These are the external datums or centres of rotation for machining the curves on this face. The second photo (left) shows roughing out of the shape of the top of carburettor bosses the stud bosses, and the priming bosses. 

Here the component is mounted in the dividing head on a milling machine, supported on the lower centres pre-drilled in the end squares, in order to machine the curved outer shapes of the manifold.

The whole job can be rotated by hand, under a cutter. This centre results in curved shapes at A, B and C.

Moving the centre of rotation to the upper centre allows the curved bosses at d,e,f and g to be machined. These curved bosses are on the axis of the throttle spindle. It may sound strange to have a throttle spindle in a manifold, but that is how the sloper carburettors 3 litre Bentleys work.There are no butterfies in the main part of the carburretor. The carburettors are bolted to the manifold and their butterflies sit inside the holes, so it is imperative the holes are in line.

Here the part is being rotated by hand under a 5mm ball ended cutter to produce the outer curves of the manifold. The cutter moves longitudinally along the manifold while it is rotated between the lower centres with square ends. This a procedure that would be difficult without the external datum points (centres) being available.

The carburettor butterfly spindle bosses are produced in the same way using the upper datum centres.

With an expanding mandrel in the inlet bores, the job is partially rotated to produce the short section of curve behind the carburettor manifold flange. Not all of this section is accessible to cut the curve, but most of it is. The technique is to machine as much as possible, and then ‘fudge’ the rest by hand.

The photo above shows the rough finished part. The question now is, can the ‘squares’ now be removed? There is another process where the ‘square’s’ datum centres come into their own, and that is for drilling and reaming the carburettor butterfly spindle holes in line.

After being set between centres in the lathe, the manifold is clamped to an angle plate fixed to the cross slide.

The upper datum centre (used to machine the curves on the bosses) has been centre drilled to a larger diameter, as the centre cone section is required when the component is reversed for the drilling of the second spindle hole. The drill can use this hole as a support to drill both spindle holes.

As can be seen, the brass sleeves are already installed in the carburettor manifold. The brass sleeves prevent the aluminium from wearing due to the operation of the butterflies.

This is representative of the full-scale engine. The ‘square ends’ can now be removed.

Component 2~ In Conclusion: The Manifold (Component 2) shows that having external references with flat faces for mounting the part when machining peculiarly shaped components, and which contain datum points for the step-by-step machining processes, makes the job so much easier and more accurate. It just requires more forethought at the planning stage of the job.

Part three here