In the first part of this project I described how the patterns for the oil pump body castings, K-102/2, and the cambox pump plunger, K-152, were made.
The heat-treated LM-25 aluminium castings, cast by AJD Foundries, turned up in the first week of January and so the two main components, shown below,
were ready for final machining.
Machining the Cambox Oil Pump Body K-102/2
The first step was to reverse engineer the existing pump body to determine key dimensions. The next step was to decide the order in which the various machining operations would be done.
The milling machine with the DRO can conveniently be used as a CMM (Coordinate Measuring Machine) to determine the coordinates of the holes.
An existing spare cambox was set up with the camshaft bearing and a short length of 5/8” diameter spindle through the centre, the other end of which was securely held in the collet chuck on the milling machine.
The cambox was then clamped to the table and the DRO set to (0,0). The order of events is important to ensure that the origin is now located at the centre of the camshaft.
A pointer is now inserted into the collet chuck that will be used to position the table at each screw hole in turn to determine their coordinates.
First, the centre is checked
by rotating the steel bar and then a 3/16" BSW threaded rod
with a pointed end is screwed into each hole in turn and the table aligned with
the pointer in the collet chuck (a magnifying glass is useful here)
and the coordinates read directly from the DRO.
Two points to note: the cambox has been rotated during the initial setup to give the same x coordinate for the 2 top screw holes (those are the ones on the left side in the above picture) and, interestingly, although the holes appear to be symmetrically positioned, they are not! I checked 2 more camboxes from other engines, the K-102/2 pump body and a K-102/3 cambox end cover to confirm this. The coordinates for these must all be known before machining starts to avoid disturbing the subsequent setup on the milling machine.
One of the issues in machining castings is that no reference
surface exists until the first machining operation has been completed – it is
just a lump of metal with an irregular shaped surface.
When I made the patterns for the castings, I incorporated a
short length of nominal ½” diameter “spindle” into the casting to help in the
initial setup. The front face of the pump body – the face with the 5 screw
holes which interfaces with the cambox, would be the reference face but the
first machining operation needs to include not only the face itself but also the
5 holes and the main ½” diameter hole for the plunger to ensure that they are orthogonal
to the mating face.
The pump body casting can now be held in a 4-jaw chuck (necessary because of its irregular shape) and orientated appropriately and on-centre - this is where the dummy spindle comes in useful. This part of the setup is the most time consuming to ensure that all the holes will be in the correct places and the face that is to be machined is orientated correctly and not skewed.
The machining itself is quite straightforward – the main face followed by the 5 screw holes are machined first
Followed by the central hole for the pump plunger.
The plunger hole has a flat bottom and a new slot drill rather that a regular drill is used for the final cut to ensure a good surface finish.
The slots that connect the 2 oil outlets on the cambox (either side of the cam chamber) to the angled feed hole (not yet machined) into the pump have also been machined. It is convenient that the 4-jaw chuck holding the pump body is mounted on the dividing head as this makes orientating the body to machine the slots much easier.
Recesses for the screw heads are machined when the body is removed from the 4-jaw chuck.
The holes/threads for the plunger locating pin (K-156) and the oil return to the tank are machined next. The threads are, respectively, ¼” BSCY and 1/8” BSP. The body is clamped to an angle plate and, again, carefully set up to ensure both holes are in the correct place.
The hole for the oil return also passes through the pumping chamber in the main plunger hole to the angled section that feeds oil from the cambox.
While the casting is on the angle plate, the ¼" BSCY threaded hole for the valve lifter cable adjuster can be machined.
The final machining operation is the angled drilling (a #22 drill) that connects the slots to the vertical hole that brings the oil to the pumping chamber. Again, this requires a very careful setup to ensure the hole connects properly with the vertical hole. Before the hole is drilled, a slot drill (that produces a flat end) and small centre drill are used to start the twist drill in the slotted part.
Machining of the pump body is now complete although I will do a bit of cosmetic finishing to the casting in due course.
Machining the Pump Plunger K152
The initial machining of the pump plunger that was undertaken by Venture Precision Engineering produced short lengths of 13mm diameter shaft in O1 tool steel with the all-important sinusoidal groove for the reciprocating motion of the plunger.
The first step was to chop off the excess material and finish the outside diameter to size so that it is a close fit in the pump body. The plunger was then set up in a chuck on the rotary indexer and the flat section that opens and closes the ports at the end of the pump was machined.
There are 2 very important points to note about the
machining of this flat: firstly, the flat must have a specific orientation with respect to the groove and, secondly, the depth of cut is 0.2” -
leaving 0.3” of “solid part”. Why are these details important? Because both control the timing of the opening and closing of the ports (ie the holes) to ensure that oil is "sucked into" and "pushed out of" the pumping chamber.
The final piece of machining is the 1/8” wide slot that mates with the Oldham coupling.
The orientation of this slot is orthogonal to the extremes of the pump stroke. This is from the viewpoint of structural strength to avoid a thin section rather than the fluid dynamics of the pump.
Machining of both parts is now complete. The picture below shows the original pump components on the left and the new ones on the right.
There is still the Plunger Locating Pin (K-156) to be machined and, of course, testing – does it actually work?
Watch this space…..
.....and finally..... I have been corresponding with Jeff Frankcombe from Tasmania about various things Velocette and he was able to add some useful information about the AJS V-Twin record breaker that ended up in Tasmania.
The picture below shows Trevor Jowett who was the AJS dealer that brought the V-Twin to Tasmania
and the 2nd picture is from Old Bike Australasia about a group of riders that toured Tasmania in the 50s where they came across the AJS.
Thank You Jeff for passing this on.
There is more information on the history of this bike for those that haven't read my previous blogs on the topic.
