TOP COMPONENT DATUMS

The top component started life as a large block of cast iron. As can be seen from the sectional view, this is where 3D printing came into its own.

It was decided to use the datum at the virtual point A.

The datum is at the coincident point where the spark plug and valve axes cross. It is useful in that it gives a reference for all the external faces. The point doesn’t actually exist in the section, so had to figure out how to use the point as a measurement datum.

The drawing gives the angles, widths and distances from the datum point front and back.

Front and rear elevations of the cylinder block.

As can be seen from the photos of each end of the cylinder block, there is a large core plug with a screwed on aluminium plate.

These core plugs were in an undefined position. It was decided that if centred the core plugs on virtual point X, and instead of a hole they became a protruding 20mm diameter boss on each end of the block, these cylinders could be used to hold the block in a collet at one end, and on a centre at the other. The centre of these bosses could also become datum point X, so that the tool could be touched on the outside of the bosses, and then moved the required distance to locate each of the faces.

Having the cylinders on the ends of the block, means that one end can be held in the dividing head of the milling machine to rotate it.

Another problem was a fairly sophisticated dividing head had a 40-tooth worm, and it has a direct dividing device on the end with 60 teeth. Neither of these would give the 10° increments required. The answer was to make a new dividing plate with 36 teeth (10° per division).

Using the 10° increments, it was possible to divide to each of the faces without having to use the worm division, and without having to count holes, both of which can cause mistakes. The worm was left engaged for rigidity.

The photo shows the new 36 space dividing plate to allow direct divisions in 10° increments.

MACHINING OPERATIONS

This is the first step in machining the material for the top of the cylinder block.

The block is 290mm long, 83mm wide and 60mm thick.

The weight is 11 kg. There will not be a lot left at the finish!

This is the first trial setup of the top cast iron block in the milling machine. It is held at both ends by the protruding 20mm diameter bosses. At one end, it is located in the collet of the dividing head.

Some security brackets will be added at this end when the machining begins.

At the nearest end the block is held by the centre in the middle of the boss.

Locating the position of all the faces of the component was done by touching the tool on the outside of the bosses and then moving it the required distance away to each face.

For the first machining operation, it was decided to carry out the riskiest operation first; the drilling of the waterways on the exhaust side of the top block. These waterways extend the full length of the block.

On the inlet side, hardly any water space exists around the port and fortunately, there is little need for any on the model. On the exhaust side it is different. There are large water spaces at C and D.

These can only be replicated by drilling the largest diameter holes that will not break into the exhaust ports, along the full length of the block. Holes could then be drilled between the cylinders vertically, to communicate with the major water spaces and outlet points.

The important function of the waterways around the cylinder is to keep the exhaust side of the cylinder block cool.

Here the longitudinal waterway is being drilled 10.5” deep, using a ¼” home-made silver steel D-bit. The component is set up on an angle plate and bolted to it, after being packed up with precision blocks and feeler gauges. It was started with a carbide centring drill, then drilled slightly over 1” into the block with a 5.9mm drill. The hole was then opened out with an end mill. That provided a start for the ¼” D-bit.

The D-bit had to be sharpened several times and was  withdrawn every 0.025”, and the chips blown out with an airline. The card disk is to help keep dust out of the spindle and collet. Rate of progress is about 1” per 30 minutes.

The drill emerged on target after 5 hours!

Part one here.  Part two here.  Part  three here.  Part four here. Part five Part six   Part seven

Bentley series starts here.