Is this as good as it gets, or, should I start troubleshooting? Simple, 45° line to 7/16" depth to check both X and Y travel. 1/8" end mill used. Ignore the threaded insert divot. 
Im not an expert but that looks like it has a lot of wiggle in the play and not even a straight line.
You can do better than that. 0.003" is a good target tolerance. Check out the tuning video and then try the circle, square, triangle test.
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circle, square, triangle test
Whar? hehe
You can draw them in Easel.
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Of all the tuning steps I’ve taken - squaring the frame, tightening the belts, adjusting the eccentric nuts, adjusting the grbl shield voltage pots - it’s the eccentric nut adjustment that seems to be the biggest factor. There seems to be a reasonably sized zone of belt tension that’s not too hard to dial in, but I found that one of my eccentric nuts was set too tightly, and that was causing major binding problems. Once I got those all sorted, the machine performed much better.
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Ok, watch out for bound up eccentric nuts, check. or maybe society would be better served if we didn’t untie them?
(Sorry, I couldn’t resist)
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Or maybe we just need to change from briefs to boxers.
Both axes move freely, by hand, with no noticeable binding. I’m thinking (hoping) it’s just a fine-tuning issue with the motor currents.
did you do the entire 7/16 in one pass or did you take multiple passes?
What was your feeds and speeds?
Ive seen this sorta thing if I go too deep too fast with the default spindle
I believe I just used the Easel settings for maple.
Ran the CST test after readjusting all eccentrics. Looks better, but, I’ll reserve judgement until I can do this in some poplar, which I have, and, is harder than PVC foam.
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That looks way better.
Do you have a digital caliper or a dial indicator?
Yes to both.
Bump.
Also, here is the test in cast, clear acrylic. There is chipping at the upper edges. 1/8" ball-end, two-flute, 0.020" per pass, 20 ipm.
You might be able to increase the IPM to get less chatter.
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Try a slightly shallower cut and high spindle RPM as well. The 300 watt spindle, just by its nature, doesn’t really have a whole lot of torque and can get bogged down pretty easily. I’ve been milling 1/4" clear acrylic at like 12-14,000 RPM, 35 IPM, and .010" cut depth. Everything’s coming out quite nicely. I’m still seeing a little bit of chatter evidence on the edges, but no chipping, so I’ll probably go 40 IPM on my next cut to see what happens.
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Better. This was with Easel defaults (0.020", 25 ipm, full RPM) with an 1/8", two-flute, straight bit, down to 0.080" depth.
Interesting features at lower-left of the circle, and. left corners of triangle
Luckily, this material is just drops from parts I use in good volume (I have tons of them).
looks like your straight flute is chattering against the edges of the cut… do you have access to a fishtail or a single flute up cut bit?
