It would seem to me that if the cutting path errors aren't caused by
unintended table movement, then the wandering path is likely to be caused by
flexing of the endmill.
It's not easy to see the length of the endmill, or just the cutting section
of it, from the photo.
Another significant problem that happens when a small endmill is used to cut
the full width of a slot, is chip evacuation, or lack of it.
With cut chips present in the cutting area, the endmill is frequently
jamming as it's rotating, causing the endmill to try to cut fresh material,
plus the thickness of the chip(s) passing by the flute on the opposite side
of the endmill.
Chip interference can be heard as clicking or crunching sounds as the
endmill is cutting.
Some erratic table movement can probably be detected by placing a finger at
the point where the table dovetails meet.
The two conditions will lead to a wandering cutting path. The limited
rigidity of the minimill may also be a contributing factor.
Using a cutting tool lubricant in a squirt bottle, to constantly flush the
chips out of the cut should improve the results, but the sides of the slot
probably still won't be perfectly straight.
The spiral of the flutes will help carry chips away, but only to a limited
degree (less at lower RPMs). Using a medium viscosity cutting lubricant will
help the chips flow outward from the cut.
The other recommendation of drilling the ends of the slot location with a
rigid drill, followed by material removal with a smaller diameter endmill
would likely be the best procedure, especially with a light duty machine.
Flooding a cutting tool lubricant to flush chips away can obviously be
fairly messy, so coming up with a method to recover and contain the lube
might be worthwhile.
A small endmill isn't going to be slinging much lube, but the flooding
action should be constrained to the table area.
It might be possible to retrofit a gutter system around the edge of the
table, or the workpiece clamping methods might need to be reconsidered, to
allow the small parts to be cut with them sitting in a shallow pan on the
The type of dispenser bottle I prefer for cutting lube is the wash bottle
with 90 degree spout shown near the bottom of this page (but in a 6oz
These types of squeeze bottles doesn't need to be inverted, and the tip
allows very good visibility of the work area.
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