Today I assembled the base, columns and Jig, intending to mill a flat top to the whole assembly.
To my surprise and dismay, it was apparent that one of the columns was one mm out of position. Dont know how that happened. A typo in the CAD drawing or CNC program? It could not be a zeroing issue, because all 3 columns on that side would be out of position.
What to do?
I decided to turn the 2.5mm holes in the column base into slots 3.5mm long. The bolting position on the jig was exactly correct, so I used that to zero the position, found a 2.5mm end mill, and gingerly milled the slot. holding the column upside down in the milling vice.
Fortunately, that seems to have worked. The columns are now all correctly located. The tops of the columns are the crucial plane and position, and they seem good. I doubt that the slots will be an issue in the finished engine. They are invisible under the column feet. If necessary, I will make a couple of locating pins and drill them in position right through the base from underneath. I doubt that will be required.
After that, I did take a milling skim off the column tops, to create a dead flat plane, to which to bolt the cylinder bases, when I have made them.
Not tomorrow though. I am off to Ballarat Victoria to a swap meet on the aerodrome. In previous years there have been approximately a thousand stalls. Some are shed clean outs, some commercial vendors and dealers, and lots of ancient cars and machinery and parts. The best stalls are the ones selling used tools. I seem to come home after each meet with a heavy pack full of tools and materials, and a lighter wallet. But it is always interesting and fun.