Thanks Benni

I've got a little more progress made of the frame.
I mounted the frame in the vice on the mill on some parallels and then using an edge-finder I zeroed on the bottom of the frame (actually I zeroed off the jaw of the vice) and then zeroed off one side of the frame blank. I then moved in to the center of the frame and zeroed there. This is because of the way I have marked the dimensions on the plans, working from the center of the frame rather than from one edge.
Then I put a little center drill ( I really must get some proper spotting drills ) in the chuck and center drilled all the holes using the coords off my plans.

Then I set about drilling all the thru holes, and all was going well until a little "plink" sound. Grrrrr, I broke a 1.5mm drill halfway thru leaving a small segment of drlllbit stuck fast.


I finished off drilling the rest of the thru holes with a fresh bit, and bored out the larger blind bores for the cylinder pivots and for the ball-races for the crankshaft. Then I turned the blank over and bored out the larger blind bores on that side, then very carefully drilled the failed 1.5mm hole from the other side until I heard and felt the drillbit start to catch the stuck fragment.
I then spent an hour or more digging it out using various methods with not much success until I tried using a pointed V carbide burr. I then very gradually fed the burr into the stuck drillbit until I had bored most of the way through the stuck bit, then I took the remainder of the broken drillbit, ground the broken end flat and then I applied a little persuasion with a hammer and the stuck bit popped right out.
Then I took a couple of pieces of 6mm steel rod for the next step.

These 6mm rods fit into the blind bores for the ballraces, and are used along with the 45degree angle I made earlier to mount the frame vertically in the mill vice as shown, the two HSS lathe bits are there to raise the frame up enough to allow me to get the right angle

Then (once the vice was tightened up) I touched off the 6mm rod with a 16mm endmill, zeroed the DRO and then raised up the Z-axis by 38.87mm (according to my plans the distance from the center of the crankshaft to this edge of the frame is 41.87mm, subtract half of the 6mm rod brings you to 38.87mm) and zeroed again.

Then it was just a case of raising the cutter up and then nibbling my way back down to zero. Then I loosened the vice, rotate the frame and align using the 45degree angle and tighten the vice. Then nibble away the 2nd corner.

I did think about shaping more of these upper arms while having it in this position, but I have some long holes to drill, and want to do that before spending any more efforts, just in case I totally mess up the long boring. The shaping of the outsides of these arms are not critical, so the shaping can wait til later.
Next up will be boring these 4 long steam passages between 38-46mm approx deep, then if nothing snaps or gets stuck, I will be shaping the outer edges of the frame.
One handy tip I have learnt from snapping the 1.5mm drillbit, when I started using the new bit I began rubbing a pencil up the flutes. This seemed to stop the ali welding itself into the flutes, and I re-applied the pencil a couple more times per 12mm deep hole, this seemed to work really well, good old carbon lubrication.
Tim