Expanded z axis, new spindle

Over the last weekend, I upgraded my Z-axis.

I replaced the stock spindle with a more powerful DeWalt DW660, which included replacing the stock mounts with DW 660 mounts. I'd bought the DeWalt spindle and the mounts a while back; if I had to buy again today I'd buy a quiet cut spindle, so that I could control the spindle's speed from software.  

While I was at it, I also replaced the stock 200mm Makerslide with one of the 500mm sections from my original Shapeoko 2 kit.  I don't need the extra height now, but when I upgrade the X axis I will.

Both upgrades are working fine, but I'm still having problems with the cutting depth. When I try to carve a circle 1/2 an inch deep I only get a circle about 3/8 of an inch deep.  If I reduce the depth per pass from .028 inches per pass to .014 inches per pass it goes a bit deeper.  If I support the table under the spindle it goes even deeper.

I think there is too much flexibility in the current setup.  Right now there are two big sources of the flexibility.  

The first is that the 25mm by 50mm extrusions that make up the table have too much flex in them, given that they span 1800mm.  I'm going to deal with this by using some of the 25mm by 50mm extrusions as supports under the table instead of as the table surface. I'm also going to start using 25mm by 75mm extrusions for the table surface.  The 25mm by 75mm extrusions are a lot stiffer in 75mm axis (with a moment of inertia of 37cm⁴ vs 12cm⁴), and a bunch stiffer in the 25mm axis (3.1cm⁴ vs 4.5cm⁴) as well.  They also manage to be a little bit cheaper per surface area than the 25mm by 50mm extrusions because you don't need as much hardware to hook them up to the table.