These tappet covers were rather damaged after a number of years of use and what looks like some rough treatment – looked like a pair of mole grips had been used in the past to remove them. So, the first stage was to face the top surface in the lathe and run the tool along the edge of the fins to remove the nicks. The top cooling fins were all drilled with a different number of holes in each, these upper fins were also machined off in the lathe.
The next stage was to machine the 6 faces of the nut and the simplest way to do this is to use a rotary table and an end mill.
The first task was to mount each nut in turn central onto the rotary table. However, I don’t have a 3-jaw self-centering chuck that fixes to the table.
My solution was to turn an adaptor that would fit snuggly into the 2MT hole (Warco HV4 rotary table) in the centre of the table and then locate the tappet cover in the upper face.
I machined an offcut of aluminium that happened to have a hole through the centre – this hole is of no significance.
This adaptor does not hold the part in place, it’s only purpose is to align the tappet centre with the centre of the table.
The adaptor placed into the 2MT hole – a slightly snug fit to ensure that it centres.
This adaptor has a recess into which the tappet cover fits.
The snug fit to the table and then to the part to be machined results in a good repeatable alignment of the centres. However, I still need to clamp this down so that I can then machine the nut faces.
The whole process of figuring out the best way to machine something with the tools you have at hand is part of the model engineering experience. Sometimes it can take hours to figure out the best method and so I have created a gallery just for workholding.
A T-nut and clamp is used to lock the nut to the table, note the use of just a single clamp and so I had to machine 3 of the nut surfaces before moving the clamp around the table so that I could access the other 3 nut faces.
Note the small shim of aluminium to protect the upper surface. The part being machined was quite a distance above the rotary table and so not as stable. Therefore best to make smaller cuts with the end mill to avoid rotating the part out of position or even worse dislodging it completely.
One of the nuts straight after the machining of the 6 faces.
The result was rather pleasing, now just another 3 to go – this is when the time put into machining the centering adaptor paid back three times over.
The final 4 tappet covers with the hexagonal nut profiles machined.