Chip Load for Drilling: The Number Most Machinists Never Calculate

Ask a milling machinist what chip load they're running and you'll usually get a number. Ask a drill press operator the same question and you'll often get a blank stare. Chip load is treated as a milling concept in most shops, but it's just as relevant to drilling — maybe more so, since the consequences of ignoring it are a broken drill in a blind hole or a part that needs to be scrapped.

Here's what chip load means in drilling, how to calculate it, and how to use it to make better decisions about feeds and speeds.

What Chip Load Actually Means in Drilling

Chip load is the thickness of material removed by each cutting edge per revolution. In drilling, a standard jobber drill has two cutting lips, so each lip removes half the total feed per revolution. That per-edge slice of material is the chip load.

Too low a chip load — feeding too slowly for the RPM — and the cutting edge rubs instead of cuts. Rubbing generates heat without removing material efficiently, which dulls the cutting edge faster and can work-harden stainless and similar materials before the edge can bite in. Too high a chip load and you're overloading the cutting edge, risking chip packing in the flutes, and generating more force than the bit or the setup can handle.

The right chip load produces a chip that curls cleanly and evacuates well. It's the regime where the drill is actually cutting, not rubbing or smashing.

The Formula

Chip load per tooth (cutting lip) in drilling:

Where IPR is inches per revolution — the axial advance of the drill per full spindle rotation.

If you're working in inches per minute (IPM) on a CNC:

For manual drill press work where you're controlling feed by feel, the calculation still matters — you're just using it to set a target IPR and then developing the feel for what that feed rate actually is on your machine.

Reference Chip Loads by Material

These are starting points for standard HSS jobber drills. Adjust based on results:

• Mild steel (1018, A36): 0.002–0.004" per tooth at typical diameters. Larger drills (over 1/2") can push toward 0.005".
• Alloy steel (4140, 4340): 0.001–0.003" per tooth. The higher the hardness, the lower the chip load target.
• 304/316 Stainless: 0.001–0.002" per tooth. Stainless wants adequate chip load to stay below the work-hardening threshold — going too light is a common mistake.
• Aluminum: 0.003–0.008" per tooth. Aluminum can take aggressive chip loads. The bigger concern is chip evacuation — large, curled chips need to exit the flute before the next revolution.
• Cast iron: 0.002–0.004" per tooth. Similar to mild steel.
• Brass: 0.002–0.004" per tooth. Remember that the bigger issue with brass is geometry (positive rake causing dig-in), not chip load.

A Worked Example

You're drilling 1/2" holes in 1018 steel. Target SFM: 90.

Target chip load for mild steel with a 1/2" drill: 0.003" per tooth.

On a CNC, you'd program 700 RPM and 4.2 IPM. On a manual drill press, 700 RPM means you're at whatever belt step produces that speed, and you're trying to feed at a rate that feels like you're getting a clean cutting chip without forcing the quill.

Reading the Chips

The chip coming off a drilling operation tells you whether your chip load is in the right zone.

Correct chip load:

Short, curled chips in steel — sometimes called "6 o'clock curls." Thin ribbons in aluminum. The color should be silver to straw in steel. No blue color unless you're doing a heavy finishing operation without coolant.

Too low (rubbing):

Powdery, dusty chips or very fine, dark chips. Blue color at the tip. The bit feels like it's sliding rather than biting. In stainless, this is especially bad — you're hardening the surface ahead of the cutting edge on every pass.

Too high (overload):

Long, stringy chips that pack in the flutes. Heavy cutting sounds. The quill wants to be pushed rather than feed smoothly. Breakthrough behavior is violent. The drill wanders at entry because the edge can't handle the load.

Why This Matters More Than You Think

Most drill wear in manual shop environments isn't from running too fast — it's from feeding too slowly at too high an RPM. The machinist slows down the feed rate because it "feels safer," but a slow feed at high RPM is exactly the combination that generates rubbing instead of cutting. The drill dulls in minutes instead of hours.

Calculating chip load and feeding at the right rate is how you protect your tooling. A drill running at the correct chip load will last dramatically longer than the same drill running at a rubbing chip load, even if the rubbing scenario "felt" more controlled.

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