Hole becomes oversized (drilling)

?Likely causes

• Runout when mounting the drill, or spindle playAgreed by 2+ makers^{[1][2][3][4][5]}
• Left and right cutting edges are uneven (large lip height difference, asymmetric point angle, eccentric chisel edge)Agreed by 2+ makers^{[6][7][2][4][5]}
• Insufficient drill rigidity (long flute length, thin HSS drill)Agreed by 2+ makers^[6][5][2]
• Chip clogging (the deeper part of the hole widens)Agreed by 2+ makers^[3][1][8]
• Feed is low (the hole mouth widens)One maker^[3]
• Feed is excessive and cutting load is largeOne maker^[2]
• For indexable drills, insufficient coolant or insufficient toughness of the peripheral insertOne maker^[1]

✓Fixes

Cutting conditions

• If the deeper part widens, increase the feed to break the chips and reduce the step amountAgreed by 2+ makers^[3][8]
• Review the feed per revolution (guideline: drill diameter × 0.02)One maker^[2]

Tool

• When regrinding, keep the lip height difference to 0.02-0.03 mm or less and eliminate chisel edge eccentricityAgreed by 2+ makers^{[7][2][4][9][8]}
• Increase rigidity with a carbide drill, a short flute drill, or a drill with a large web thicknessAgreed by 2+ makers^[2][6][5]

Setup

• Keep mounting runout to about 0.02 mm or less (select the holder, check the spindle, and measure at each mounting)Agreed by 2+ makers^{[10][11][8][4][1]}

Coolant

• Increase the cutting fluid pressure and flow, and clear clogged filters or oil holesAgreed by 2+ makers^[8][1]

Where sources differ: Guideline for hole oversize: about 1% of the diameter for HSS drills, 20-40 μm for carbide drills (Mitsubishi e-learning).

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