I’ve visited probably a dozen small furniture workshops in the past year that run CNC routers. Almost all of them are cutting at feeds and speeds that are 30-50% slower than optimal.

They’re using feed rate calculators from tool manufacturers or copying settings from online forums. The cuts work—parts come out acceptable—but they’re taking twice as long as necessary and prematurely wearing tooling through suboptimal chip load.

We spent three months testing chip load optimization on our CNC router across different bit types, materials, and cut depths. The results were eye-opening and changed how we program every job.

What Chip Load Actually Is

Chip load is the thickness of material each cutting edge removes per revolution. It’s the fundamental variable that determines cut quality, tool life, and feed rate.

Formula: Chip Load = Feed Rate / (RPM × Number of Flutes)

For a 2-flute bit at 18,000 RPM with 3,600 mm/min feed rate: Chip load = 3,600 / (18,000 × 2) = 0.1 mm

That’s the thickness of chip each flute is cutting. Too low and you’re rubbing instead of cutting (generates heat, wears tools fast). Too high and you’re overloading the bit (causes breakage or poor finish).

The trick is finding the sweet spot for each combination of bit type, material, and cut depth.

Standard Recommendations vs. Reality

Manufacturer recommended chip loads for carbide spiral bits in hardwood typically range 0.08-0.15 mm depending on bit diameter and material hardness.

Online calculators and forum recommendations tend toward the conservative end: 0.05-0.10 mm chip loads “to be safe.”

Our testing found optimal chip loads (best combination of cut quality, tool life, and cycle time) were consistently higher:

Victorian Ash (medium hardness):

• 6mm 2-flute upcut spiral: optimal chip load 0.14-0.18 mm
• 12mm 2-flute compression bit: optimal chip load 0.16-0.22 mm

American Oak (harder):

• 6mm 2-flute upcut spiral: optimal chip load 0.12-0.16 mm
• 12mm 2-flute compression bit: optimal chip load 0.14-0.18 mm

MDF (abrasive but soft):

• 6mm 2-flute upcut spiral: optimal chip load 0.10-0.14 mm
• 12mm 2-flute compression bit: optimal chip load 0.12-0.16 mm

In every case, optimal chip loads were at the high end or above manufacturer conservative recommendations.

The Testing Process

We cut 200+ test samples varying chip load systematically while holding other variables constant (bit type, RPM, depth of cut, material).

For each chip load setting we measured:

• Surface finish quality (visual inspection + surface roughness measurement)
• Edge burning (heat damage to timber)
• Chip evacuation (were chips clearing properly or packing in flutes?)
• Tool wear rate (how many linear meters of cutting before edge dulling became visible)
• Cycle time (obviously)

Optimal chip load was defined as the setting that maximized tool life and surface quality while minimizing cycle time. Too conservative wastes time. Too aggressive sacrifices quality or tool life.

What We Found

Conservative chip loads (0.05-0.08 mm) cause problems:

The bits weren’t cutting—they were rubbing. Instead of shearing clean chips, the cutting edges were burnishing the timber surface and generating heat through friction.

This caused:

• Edge burning on hardwoods (especially Victorian Ash which burns easily)
• Faster tool wear (rubbing is more abrasive than cutting)
• Poor chip evacuation (dust instead of chips)
• Longer cycle times (obvious)

Tool life at conservative chip loads averaged 180-220 linear meters before requiring sharpening. Surface quality was acceptable but required sanding.

Optimal chip loads (0.12-0.18 mm depending on material) performed better across all metrics:

Clean shearing cuts with well-formed chips that evacuated efficiently from flutes. Minimal heat generation. Excellent surface finish straight off the CNC.

Tool life at optimal chip loads averaged 320-380 linear meters before sharpening. That’s 60-80% longer tool life than conservative settings.

Surface finish was dramatically better. Most parts needed no sanding or only light 240-grit touch-up on cross-grain areas.

Aggressive chip loads (0.20+ mm) started causing issues:

Tool deflection became visible, especially on smaller diameter bits. Surface finish degraded. Bit breakage risk increased (we broke two bits during aggressive testing—both 6mm diameter at chip loads above 0.22 mm).

The performance curve is real: there’s an optimal zone, and going too far past it creates problems.

RPM Considerations

Most small workshops run CNCs at whatever RPM their router defaults to (typically 18,000-24,000 RPM for trim routers, 18,000 RPM for spindle motors).

We tested varying RPM while maintaining constant chip load to see if lower RPM with proportionally higher feed rates performed differently than high RPM with lower feed rates.

Results: Lower RPM (12,000-16,000) with higher feed rates produced better surface finish and longer tool life than high RPM (22,000-24,000) with lower feed rates, even at identical chip loads.

The reason appears to be heat generation. High RPM generates more frictional heat even with optimal chip load. Lower RPM with faster feed reduces heat accumulation.

Our current standard for hardwood cutting:

• 14,000-16,000 RPM
• Feed rates calculated for 0.14-0.18 mm chip load
• Results in feed rates around 4,000-5,500 mm/min for 2-flute bits

That’s substantially faster than the “safe” 2,400-3,000 mm/min feeds common in online recommendations.

Depth of Cut Impact

Chip load optimization interacts with depth of cut. Deeper cuts generate more chip volume that must evacuate from the flutes.

We found optimal chip loads decreased slightly with deeper cuts:

6mm 2-flute upcut in Victorian Ash:

• 3mm depth of cut: optimal chip load 0.16-0.18 mm
• 6mm depth of cut: optimal chip load 0.14-0.16 mm
• 10mm depth of cut: optimal chip load 0.12-0.14 mm

The relationship isn’t linear, but there’s clear correlation. Deeper cuts need slightly more conservative chip loads to maintain chip evacuation and prevent bit overload.

For full-depth cuts through 18mm material, we typically reduce target chip load by 15-20% compared to shallow profiling cuts.

Tool Geometry Matters

Compression bits (upcut bottom, downcut top) require different optimization than straight upcut spirals.

Compression bits generate opposing chip flow directions which affects evacuation. We found they performed best with slightly lower chip loads (0.12-0.16 mm) compared to upcut spirals (0.14-0.18 mm) in the same material.

Single-flute bits can run higher chip loads than two-flute bits because there’s more flute space for chip evacuation. Our optimal for single-flute 6mm bits was 0.20-0.24 mm chip load.

But single-flute bits cut slower overall (only one cutting edge vs. two) despite higher chip load per flute. We use them mainly for deep slotting operations where chip evacuation is critical, not for general profiling.

The Business Impact

After optimizing feeds and speeds based on actual chip load testing, our average CNC cycle times dropped 35-40% on typical cabinet jobs.

A set of kitchen cabinet doors that previously took 6.5 hours of CNC time now runs in 4.2 hours. That’s an extra 2+ hours of production capacity per day, or roughly 15-20% capacity increase across our bottleneck operation.

Tool costs actually decreased despite higher feed rates because tool life improved. We’re sharpening bits every 350+ linear meters instead of every 200 meters, and getting better surface finish that reduces downstream sanding time.

The total productivity gain—faster cutting plus reduced sanding—is probably 25-30% on CNC-intensive jobs. That’s significant for a small workshop.

How to Optimize Your Setup

Don’t just copy our numbers—different machines, bits, and materials will have different optimal settings. But the process works:

1. Pick representative materials and bits you use regularly
2. Test chip loads systematically (vary feed rate at constant RPM, measure results)
3. Evaluate surface quality, tool wear, and cycle time (not just one metric)
4. Find the chip load that optimizes all three factors
5. Document your results and build a feed/speed library

This takes time. We probably spent 30 hours over three months doing structured testing. But the productivity gains pay back that investment within weeks.

And you learn things that apply broadly. Once you understand what optimal chip load looks and sounds like, you can dial in new bits and materials much faster.

Bottom Line

If you’re running CNC feeds based on conservative online calculators or manufacturer defaults, you’re probably cutting too slowly and wearing tools too fast.

Proper chip load optimization—based on testing with your specific setup—will increase productivity, improve tool life, and deliver better surface quality.

The sweet spot is usually higher than you think, but there are limits. Test methodically, measure results, and build institutional knowledge about what actually works.

Your cycle times will thank you.

Custom furniture workshop focused on efficient production methods and data-driven process optimization.