Drill Point Regrinding Tolerances: What Matters Most

The Three Critical Tolerances in Drill Regrinding

A drill point has multiple geometric parameters — included point angle, lip relief angle, chisel edge angle, lip height, web thickness, and the helix angle of the body flutes. Not all of these are equally important, and trying to specify everything to tight tolerances makes reconditioning expensive and slow. The three that matter most in practice are lip height difference, included angle, and clearance (relief) angle. Get these right and the drill cuts well. Get them wrong and you'll see the consequences immediately in hole quality and tool life.

Everything else being equal, a drill with perfect geometry on these three parameters will outperform a "sharp" drill with poor geometry on them — even if the improperly ground drill has a better edge surface finish. Geometry is more important than finish in determining how the drill enters the material, how load is balanced between the two lips, and how efficiently it converts torque and thrust into chips.

Lip Height Difference: The Oversized Hole Problem

Lip height difference is the measurement between the two cutting lips at equal distances from the drill axis. In a perfectly ground drill, both lips are at exactly the same height — they share the cutting load equally and present the same geometry to the work. When one lip is higher than the other, it cuts deeper on every revolution, taking a heavier chip, and the hole diameter is larger than the drill diameter by approximately twice the lip height difference.

Example: a 1/2" drill with 0.003" lip height difference drills holes approximately 0.506" in diameter rather than 0.500". That 0.006" oversize might be within tolerance for a clearance hole but is unacceptable for a bore that will be reamed to close tolerance, a press fit, or any feature where the pre-drill diameter affects the final machined size.

A quality CNC drill grinder holds lip height difference to 0.001" or better consistently. Manual regrinding on a bench grinder produces lip height differences of 0.003 to 0.010" — wide enough to cause measurable hole oversize on every diameter. This is one of the clearest performance advantages of machine regrinding over manual regrinding, and one of the easiest to quantify: just measure the holes the drill produces before and after reconditioning.

Acceptable tolerance: 0.001" maximum lip height difference for precision work; 0.002" for general production; anything over 0.003" will produce measurable hole oversize and should be rejected.

Included Angle Tolerance

The included point angle of a standard jobber drill is 118 degrees (some applications use 135-degree points). The two halves of this angle — the angle from each lip to the drill axis — must be equal. If one half is 62 degrees and the other is 56 degrees, the total included angle is still 118, but the asymmetric geometry loads one lip more heavily, mimicking the effect of lip height difference in terms of unequal cutting load distribution.

Tolerance on included angle should be ±1 degree total — 0.5 degrees per half. At tighter tolerances, the geometry is effectively indistinguishable from ideal. At ±2 degrees, performance degradation becomes measurable. At ±3 degrees or more, you've changed the cutting geometry meaningfully enough that the drill behaves differently from its intended specification.

Checking included angle requires either an optical comparator or a purpose-built drill point gauge. Neither is complex to use, but neither is something you can assess by eye. Shops that accept reconditioned drills without any geometry verification are accepting unknown quality — which may be acceptable for non-critical applications but is a problem anywhere hole quality matters.

Relief (Clearance) Angle: Rubbing vs Cutting

The relief angle behind each cutting lip provides the clearance that allows the lip to cut rather than rub against the previously cut surface. Too little relief and the drill rubs rather than cuts — generating heat, slowing penetration, and wearing the drill rapidly. Too much relief and the lip is weakened, prone to chipping and edge breakdown under cutting loads.

Standard relief angles for HSS jobber drills run 8 to 12 degrees at the outer corner, varying toward higher angles at the center (where the cutting speed is lower). The variation with radius is important — a uniform relief angle optimized for the outer lip diameter provides too little relief at small diameters, where lower peripheral speed means each point on the lip stays in contact with the work longer per revolution.

For reconditioning purposes, acceptable relief is 8 to 15 degrees at the outer corner. Under 8 degrees produces rubbing; over 15 degrees produces fragile edges. A grinder set up correctly produces this consistently without individual measurement on every drill. The check is a simple functional one: does the drill cut freely at the expected thrust force? A drill with insufficient relief requires noticeably higher thrust than a properly ground equivalent.

Helix angle — the angle of the flutes — cannot be changed in reconditioning (it's ground into the drill body and can only be reduced by removing material, which is not reconditioning). It's worth knowing the helix angle when selecting drills for purchase, but it's not a reconditioning variable. What reconditioning can restore is the correct relief and lip geometry at the tip — and done correctly, the drill performs as if it were new from a geometry standpoint.