How HSS Drill Resharpening Actually Works on a WinsloMatic

When machinists ask about our resharpening service, the most common question is: what does the machine actually fix? A dull drill feels obviously dull at the machine — it won't cut, it takes more pressure, the hole wanders. But the geometry of what's worn and what's being restored isn't always visible to someone who hasn't worked on a drill grinder.

Here's a complete explanation of what a machine regrind on a WinsloMatic restores, and why that matters for your holes.

The Four Geometry Elements That Wear

A drill bit has four critical geometry elements. All four degrade with use. A hand-touch on a bench grinder may restore one of them inconsistently. A machine regrind restores all four simultaneously, within the original specification.

1. Chisel Edge Width

The chisel edge is the short, flat cutting section at the absolute center of the drill tip — the part that bridges the two main cutting lips. On a new drill, it's narrow. As the drill is used, the center of the tip gets rubbed flat against the workpiece bottom on every drilling operation. The chisel edge widens.

A wide chisel edge is the primary cause of drilling thrust — the downward force required to advance the drill. The chisel edge doesn't cut like the main lips do; it scrapes and pushes. When it's wide, you're pushing harder, your machine is deflecting more, and small drills in cross-drilled setups start breaking. The WinsloMatic restores the chisel edge to its original narrow geometry.

2. Lip Relief Angle

Behind each cutting lip, the face angles away from the cutting edge. This relief angle is what allows the lip to bite into the material rather than rubbing on top of it. Think of it like the angle behind a chisel edge — without clearance behind the cutting line, the tool can't enter the material.

As a drill is used, the relief face behind each lip gets rubbed flat. The relief angle decreases — sometimes to zero or negative. A drill with lost relief generates heat instead of making chips. The cutting edges are still there, they just can't bite. The machine regrind restores the relief angle to specification.

3. Lip Height Equality

Both cutting lips must be the same length, at the same angle, at the same height relative to the drill axis. When they're matched, each lip takes the same cut on every revolution and the drill runs true. When one lip is longer than the other — by even a few thousandths — that lip cuts more material and pushes the drill off-center.

Unequal lips are the primary cause of oversized holes. A drill that should cut 3/8" will consistently run 0.005–0.015" oversize with mismatched lips. Lip height inequality accumulates through hand regrinding, where it's essentially impossible to hold exact symmetry. The WinsloMatic grinds both lips simultaneously using a cam-driven mechanism that references off the drill's own geometry, ensuring equal lips every time.

4. Split Point Geometry

On split-point drills — the four-facet geometry common on better-quality HSS and cobalt bits — there are two additional secondary cutting edges created by the split. These eliminate the chisel edge and reduce thrust dramatically. They also wear, and they wear differently than the main lips.

A worn split point often looks intact at a casual glance because the overall point shape is preserved. But the secondary edges have rounded over, and the bit starts behaving like a standard dull drill — increased thrust, wandering entry. A basic drill grinder that only handles the main lips won't restore split point geometry. The WinsloMatic handles both.

What a Hand Regrind Can and Can't Do

A skilled machinist with a good bench grinder can restore sharp lips and adequate relief on a drill — particularly on larger-diameter bits where there's enough real estate to work with. What's nearly impossible to do by hand is achieve consistent lip height equality. The eye isn't accurate enough, and there's no reference mechanism to hold both lips to the same geometric relationship with the drill axis.

This is why hand-reground drills often cut acceptably but produce holes that are slightly oversized. The sharpening removed the dull material, but the lips are off by a few thousandths. For rough work, this is fine. For close-tolerance drilling, it compounds into problems.

How Many Times Can a Drill Be Resharpened?

An HSS jobber drill can typically be resharpened 5–8 times before it reaches end of life. Each regrind removes a small amount of material from the tip and shortens the overall drill slightly. As the drill shortens, the flute depth near the tip decreases and chip evacuation capacity reduces. A drill that has been reground many times will show a noticeably thicker web (the core of the drill at the tip) — the web naturally thickens as you grind back toward the shank.

Retirement is correct when the web is so thick that thrust requirements become impractical, or when the remaining flute length is insufficient to evacuate chips from the full depth of the holes you're drilling. Short of those conditions, resharpening is almost always more economical than replacement for quality HSS drills.

The Economic Case

A name-brand HSS drill in a common size (3/8", 1/2", 1/4") costs $8–25. Resharpening costs a fraction of that. Over the life of a drill — 5–8 grinds — the total cost per sharp bit is a fraction of repeatedly buying new. Shops that run systematic regrind cycles on their HSS inventory cut their drill spend significantly compared to shops that replace on failure.

The mail-in model at MachinistPost works for shops that don't have a WinsloMatic on site — which is most shops. Ship us a batch, get them back sharp, put them back in service. The per-hole cost of a properly maintained resharpened drill is lower than a new cheap import drill and produces better results.

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