Grind Your Exercises: What a Cutting Edge Really Does

• 1
  Grind Your Exercises to See How a Wedge Cuts
• 2
  Drill Point Angles and What Each One Controls
• 3
  Rake, Relief, and Nose Radius on a Lathe Tool
• 4
  Wheel Choice, Dressing, and Holding the Tool Steady
• 5
  How to Test a Ground Edge Without a Toolroom
• 6
  Grinding Angle Reference by Tool and Material
• 7
  Symptom, Cause, and Fix on a Ground Edge

A ground edge is a wedge. Every angle on it trades strength for sharpness. When you grind your exercises on a bench grinder, you change one angle, cut a test piece, and watch what happens. The feedback loop is short. That is the whole point.

The wedge has three jobs. It must enter the material, lift the chip, and carry heat away from the tip. A drill point with too much relief enters easily but chatters. Too little relief and the heel rubs, so the drill pushes back and the hole wanders. Neither failure shows up in a drawing. Both show up in the first 20 holes.

CNC machines repeat a program, not a good edge. A worn drill still follows the same coordinates, so the error hides until the hole is out of size or the surface tears. Hand grinding skill is what lets you look at a chip and know whether the edge is sharp or just present.

That is why the old masters drilled apprentices on scrap stock for weeks. The exercise is not about making a pretty point. It is about reading the result and correcting the angle that caused it.

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  Sharp is not the same as durableA thin edge cuts cool but chips under interrupted cuts.
• 2
  One change at a timeAdjust a single angle, then test, so you know what moved.
• 3
  Chips tell the storyColor, curl, and length show heat and edge condition.

The point angle sets where the cutting starts. A 118° point is the general-purpose choice for steel and aluminum. A 135° point is flatter and stronger, so it holds up on stainless and hard alloys, though it needs more thrust. A 90° point suits sheet, plastics, and spotting work where you want a shallow entry.

Lip clearance, usually 8° to 12°, is what stops the heel from rubbing. Check it by sighting along the cutting lip: the back edge must sit lower than the front. If the two lips are ground at different lengths, the drill cuts oversize and the point wanders.

Web thinning matters once the drill is resharpened a few times. The web gets thicker as the drill shortens, and a thick web will not start a hole without a pilot. Thinning the web to roughly 0.1 to 0.2 times the drill diameter restores the point's ability to bite.

Chisel edge angle controls how much the center pushes instead of cuts. A long chisel edge means high thrust and a hot tip. Thinning shortens it and drops the thrust noticeably.

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  118° pointGeneral steel and aluminum work.
• 2
  135° pointStainless and hard alloys, stronger tip.
• 3
  8°–12° lip clearanceStops heel rub and heat build-up.
• 4
  Thin the webRestores bite after repeated sharpening.

On a single-point lathe tool, the back rake decides how the chip forms. Positive rake, roughly 8° to 15°, cuts freely and suits aluminum and mild steel. Negative rake, around −5°, puts more material behind the edge and survives interrupted cuts and hard stock.

Side and end relief let the tool feed without dragging. For steel, 5° to 8° is common. For aluminum, 10° to 12° works better because the material springs back after the cut. Too much relief weakens the tip and it will chip on the first hard spot.

Nose radius controls finish and strength. A small radius, 0.4 mm, gives a sharper corner for fine detail but heats up fast. A larger radius, 0.8 to 1.2 mm, spreads the load and improves finish, but it also raises radial cutting force, which matters on slender parts.

Set the tool on center. A tip above center rubs and work-hardens the surface. A tip below center digs in and can pull the part out of the chuck. The old test is to face a scrap piece and look at the pip left in the center.

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  Positive rakeAluminum and mild steel, freer cutting.
• 2
  Negative rakeInterrupted cuts and hard alloys.
• 3
  Nose radiusSmall for detail, large for finish and strength.

The wheel does the cutting, so a glazed wheel will burn a good tool. Aluminum oxide wheels suit high-speed steel. Silicon carbide suits carbide, though most shops grind carbide on a diamond wheel to avoid cobalt leaching and edge chipping.

Dress the wheel before the finish pass, not after. A sharp, open wheel cuts cool and leaves a finer edge. A loaded wheel rubs, heats the tool, and draws the temper out of high-speed steel. That softened tip will not hold an edge no matter how carefully you set the angle.

Keep the tool rest close, within 3 mm of the wheel, and grind on the wheel's face, not the corner. Use light pressure and let the wheel cut. Heavy pressure heats the tip and can crack carbide. Dip the tool in coolant often when grinding high-speed steel.

Check the edge under a loupe or a low-power microscope. Look for a fine, even line along the lip. A shiny, rounded edge means the tool is burnt and will rub instead of cut.

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  Aluminum oxideHigh-speed steel tools.
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  Diamond wheelCarbide, cooler and cleaner edge.
• 3
  Dress before finishingOpen wheel cuts cool and fine.

Drill a test hole in scrap of the same material. If the drill squeals, the lip clearance is too small. If it grabs and the flutes load up, clearance is too large or the point angle is too sharp for the material. Listen and look at the chip.

For a lathe tool, take a light facing cut and read the chip. A chip that comes off blue and brittle means too much heat at the edge, so add relief or reduce speed. A chip that tears means the edge is dull or the rake is wrong for the material.

Measure the hole or the turned diameter. A drill ground with uneven lips cuts oversize. A lathe tool set off center turns a taper. These are geometry errors, and no speed or feed change will fix them.

Note what you changed and what happened. A written log turns random grinding into a repeatable process, which is what the old masters were really passing down.

• 1
  SquealLip clearance too small.
• 2
  GrabbingClearance too large or point too sharp.
• 3
  Oversize holeUneven lips, regrind the point.