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You should easily be able to mill aluminum with that setup. The videos I put up the machine in them has 5 full inches of Z that are usable. Yours sounds to be 2 inches shorter, much more rigid. Make sure your dewalt is 90 from the surface in both directions, maybe you are off at a slight angle and that is causing your problem. You can shim the top with a few wraps of tape.
Have you milled wood successfully yet? Can you snug up any of the center assembly a bit more? How big is your build in the x and Y direction? Can you cut more towards one corner instead of the center? How long is the part of your bit that sticks out when fully inserted?

I see people with 8″ Z and of course they will have an issue, but yours should be fine. Before we had the rigid middle z part you could easily move the z-axis around and it still milled aluminum just fine, it is easily 4 times more rigid now if not more. Yes a more rigid machine is better but I’m not sure that is your problem.

Single flute will also help by taking more load per bite of the cutting edge. If you take too small of a bite you actually work harden the material and it gets worse.

The universal might not be the best addition yet, not for aluminum. I love that mount but it does reduce rigidity by moving the tool out a bit further and reducing the contact area with the z mounts. If you want to do aluminum you need to keep everything at a maximum.

As for camar0’s part. He took it down and removed all his accounts for some reason. Another user put it back on thingiverse, I can’t imagine he would be okay with that. The part itself, I have not tried it. It does increase the contact area with the rails, but it adds more rolling resistance and mass, and I’m not convinced of the geometery. I’m sure someone else will chime in on this but I really want to hear from someone that didn’t start with his parts and added them later. When it first came out I asked him and a few of the first people how much better it made the machine. None of them had tried it stock first, even Camar0, yet they all said it made it better. How could they know? I guess this one is a sore spot with me.

If you flex your z axis by pushing on the tip of your cutting tool, look closely at where the deflection happens. I have been working on a new center assembly to try and beef things up a bit but it is starting to come down to conduit flex.

If I make the roller motor mounts by milling them myself I plan on making a very small machine that is extremely short and just feed a strip of aluminum in and cut them 1 at a time as fast as possible.