Before toolholder experts start throwing up their hands that I am discussing side lock endmill holders, the fact remains it is likely still the most popular toolholder is use today (rivaled by the ER collet chuck), therefore including them in our research is both relevant and responsible.
When measuring runout on a side lock holder once, we noticed extreme runout. We had measured both the bore of the holder and the shank of the endmill, but this runout was TWICE what it should have been based on those measurements. AND, it was perpendicular to the screw hole, not inline as we expected. What we found was the flat on the endmill was ground so the bevel of the screw made contact with the angles of the flat. We get what they were going for here, a more accurate taper to taper contact, but when we applied prussian blue and reassembled we found uneven contact points.
The tightening of the screw made contact on the center line as intended as shown by the lower arrow, but the twisting and downward force of the screw cause the tool to pivot in the bore. The end of the toolholder bore acted as a fulcrum.
Two of the four endmill brands tested had these narrower flats. Amazing what you find when you measure stuff.
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