Ooops.

So, everything set to go. WCS origin set at top left rear corner (more about that later....). What could possibly go wrong etc etc.

First, rough out and finish the large hole in the middle:

This morning (Sunday) I loaded the code into the controller, did some last minute checks and hit the green button. The big hole appeared as planned. Next - the first drilling operation (M3 clearance drill). Copied this across to the controller, carefully forgot to actually load it into the program space and hit the green button. This reruns the program that was last used.....

I was all fingers and thumbs this morning so I forgot to start the video. Pity, as it would have shown beyond any doubt that the jaws / teeth of a keyless chuck are well capable of machining loominum, with or (very rapidly) without a 3.5mm drill in place. It was a hell of a crash but nothing got irreparably damaged and no mud was made. 

It had been trying to machine the large hole (again), this time with a 16mm keyless chuck and 3.5mm drill instead of a stubby end mill holder with a 10mm end mill. Obviously the total stickout of the chuck and drill are somewhat greater than for the end mill.

Bit of a scuff on the edge of the large hole where the body of the chuck tried and failed to burnish a large chamfer, so the best recovery plan seemed to be to turn the work upside down on the fixture and have another go. After all, the fixings are symmetrical. Start again....

After that recovery, everything seemed to go well enough. Luckily I checked the tool offsets before proceeding again, as I found that the drill lengths were all to cock.

2D Profiling the middle M3 fixings:

Roughing out the external profile:

Counterbore the four M4 fixings:

Chamfer / edge break:

Finished:

So finally, I have the cover plate machined. Still held to the remaining stock with the tabs I added in Fusion CAM. But the fit isn't quite as perfect as I'd have hoped for given that all the CAD and CAM work was done in Fusion - no manual operations or guesswork involved. WTF??

Looking more closely, there's the best part of 1mm of misalignment between the mating edges where the cover and bracket come together. If I assembled them with their fixing screws (I haven't tapped the M3 and M4 fixing holes yet), that offset would be plain to see. And the chamfer on the big hole is variable around the circumference and the chamfer down the "bottom" edge is almost non-existent.

I've tried to line up the fillets and taper profiles here. The misalignment is "readily evident". Bloody annoying, not least as it's almost certainly my own stupid fault.

There's a misalignment of the best part of a mm at the bottom:

 And a similar gap at the top:

So what happened? It's not rocket science although it caught me out:

• I'd dimensioned the stock body as exactly 4" (101.6mm), whereas it's actually 101.36mm, ie 0.24mm undersize. The main bracket body is 101.82mm, ie 0.22mm oversize. These are both pretty good tolerances for extrusions but that's 0.46mm difference straight away.
• Stupidly, I'd chopped and changed on the WCS origin front: 
• For the main body, I started out with the origin at the top back left on the basis that the stock was pretty close to nominal (actually 0.2mm oversize).
• When I turned the body over to machine the top side, I transferred the datum to that side by using the central large bore, given that it's probably the main feature.
• Then, when it came to the cover plate, I randomly chose to use the top back left for its WCS origin, figuring that it didn't matter as long as I aligned the rear face of the stock with it correctly.
• Swapping over the datum origin when I flipped the main body probably wasn't an issue - in fact it was probably the recommended method. But touching off on the rear left (top) corner was dumb. 
• And much of the problem arose because I flipped the cover stock over after my crash. As I set my datum on the rear edge, any difference between the modelled stock and the actual stock would now appear as a misalignment.
• I should probably have:
• Ensured that the main body stock was truly centred eg defined the origin on the stock centre line, not the top corner.
• Defined the WCS origin as the mid point of the large hole and positioned the centre line of the actual stock on that point. That way, the cover will be mid positioned.

A couple of other things:

• The side pockets for the central M3 fixings have a little nib in the middle. The 3mm end mill did a nice job creating the pocket but didn't clear out the floor across the whole area. That's presumably the result of using a profile toolpath, rather than a clearing operation.
• The M3 and M4 clearance holes did not even break through. Clearly some problem with the drill lengths.

What to do? This is the plan:

• Make another cover blank with fixing holes to suit the existing fixture.
• Define my WCS origin (particularly the Y offset) to