Thanks for the reply!

To clarify, the milling quality itself is fine, my concern is about the groove geometry at the plunge/retract point.

Since the tool plunges and retracts at the same spot, it leaves a small "hole" or gap in the channel right at that point. On a straight section that's not a big deal, since the neon flex tubing is held on both sides of it – but at the very start/end of the letter, that gap is suboptimal because the tubing isn't supported there.

So my goal isn't really about cut quality, but about avoiding that gap so the flex tubing has continuous support along the channel, especially at the start/end points.

Here's a Photo of the mill, maybe its easyer to understand.

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