Some jobs just feel like they should be done later… In this case it was cutting the ports in the cylinders and valve chests – you have spent so long getting a nice accurate bore, and now you are about to cut holes in it! The scope for disaster is all there – it reminds me of being given the job of fitting car aerials to new Audi’s as an apprentice in the ’70s – somehow attacking a brand new top-of-the-range motor with a power drill was just a very scary process!
So firstly I marked out the ports on the mating faces of the castings, on a surface block to provide a check when machining (the old addage – “measure twice, cut once”) then converted all the dimensions from the drawing into coordinate dimensions from the centreline base of the cylinder, as I had decided to use the mill’s DRO in “jig boring mode”.
I can’t tell you how many times I checked these dimensions, as on the drawings they are marked from top and bottom, but we needed them all as offsets. ….and it payed off, I found a mistake or two!
The next step was to start machining, I fixed one angleplate to the mill table (accuratly across it) and then set the DRO zero here, then mounted each of the castings between this and another angle plate and set to with the slot drills – tense moments!