What Feed Rate Does to Heat Buildup

The difference between a clean router cut and a scorched edge often comes down to seconds. Not the total time of the cut, but the fractional seconds that any individual wood fiber spends in contact with spinning carbide. Feed rate determines that contact time, and contact time determines whether heat accumulates past the point where wood begins to char.

Why Slower Burns Hotter

Move the router through a cut at one inch per second. Each point along the cut line experiences contact with the spinning bit for perhaps 50 milliseconds. The wood fiber heats during contact, then begins cooling as the bit moves away. If heat dissipates faster than it accumulated, no burning.

Slow to half an inch per second. Now each fiber spends 100 milliseconds in contact - double the time, double the heat transfer into that specific location. Wood has relatively poor thermal conductivity. Heat entering a fiber takes time to spread into adjacent fibers and air. During that lag, if more heat keeps arriving through continued bit contact, temperature climbs rapidly past the 400-degree charring threshold.

This is why pausing mid-cut creates the darkest burn marks. Stop moving while the bit continues spinning and contact time goes from milliseconds to full seconds. The carbide edge returns to the same location with every revolution, adding heat faster than wood can conduct it away. The scorch mark from a mid-cut pause is wood that spent sustained time above 400 degrees.

The Counterintuitive Fix

The instinct when cutting becomes difficult is to slow down and "take it easy" on the tool. With routers, that instinct makes burning worse.

Faster feeds keep each fiber's contact time short. The bit encounters the fiber briefly, cuts it, and moves on before heat accumulates. Chips carry heat away from the cutting zone as they eject. Effective chip evacuation means the cutting edge runs cooler.

There's a ceiling. Push too fast and the bit deflects under load, creating chatter marks - washboard ripples where the bit flexes and makes intermittent contact. But up to that ceiling, faster feeds mean cooler cuts. The first sign of burn marks is usually a signal to speed up, not slow down.

Material Removal Efficiency

Feed rate affects more than contact time. It determines how much material the bit removes per revolution, which changes cutting efficiency in ways that matter for heat.

At 22,000 RPM, each revolution advances through the wood by an amount determined by feed rate. At one inch per second: each flute takes a bite of roughly three thousandths of an inch. At half an inch per second: one and a half thousandths per flute.

Router bits are designed to work at specific material removal rates. Too little material per flute and the carbide isn't slicing - it's rubbing and burnishing. Rubbing creates tremendous friction without producing chips. No chips means no heat carried away from the cutting zone.

The interaction with router speed and diameter matters here. A two-inch panel bit needs slower RPM to keep tip velocity reasonable. But if you reduce RPM while also reducing feed rate, bite per tooth drops into the rubbing range. The solution: reduce RPM for large bits while maintaining feed rate, keeping the bite per tooth in the efficient cutting zone.

What the Wood Demands

Dense hardwoods tolerate faster feeds because their tight fiber structure resists deflection under cutting force. Less deflection means you can push quicker without chatter. The dense wood also conducts heat better than softer species, helping dissipate friction heat.

Resinous softwoods need a narrow feed window. Too slow and extended contact time melts resin that builds up on the cutting edge, creating the feedback loop where gummy edges cut less efficiently, generating more heat, melting more resin. Too fast and the soft, variable-density structure tears out instead of cutting cleanly.

Plywood demands fast feeds to minimize contact time with the adhesive between plies. The glue melts at lower temperatures than wood chars. Slow feeds give it time to liquefy and stick to the bit, building up the gummy layer that makes carbide dull faster in plywood. But face veneer spalling limits how fast you can push.

MDF burns at nearly any feed rate because its entire structure is wood fiber suspended in adhesive binder. There's no feed rate that avoids prolonged contact with glue. Sharp bits and acceptance of inherent material limitations matter more than feed rate optimization.

The Repositioning Problem

Actual routing involves more than smooth linear motion. Hand repositioning. Stance adjustment. Navigating around clamps. Changing direction at corners. Every time forward motion slows or stops, contact time spikes.

Inside corners are the worst case. The bit removes maximum material in minimum space while you're changing direction. Forward motion slows. Contact time increases. Dark burn marks appear specifically in the corner while straight sections cut clean.

Template routing with bearing-guided bits compounds this because bearing friction resists smooth motion. A rough bearing causes the router to drag rather than glide, reducing feed rate and increasing contact time even when you're actively trying to maintain smooth movement.

Feed Rate vs Surface Finish

Very fast feeds leave visible mill marks - the scalloped pattern where each flute passed. Slowing the feed produces smoother surfaces with shallower scallops. The balance: feed rate that produces acceptable finish without enough contact time to cause burning.

This balance shifts with router speed and bit condition. Higher RPM means more flute passes per inch at the same feed, naturally producing smoother surfaces. A sharp bit cuts cleaner scallops than a dull one. Sharp bits can tolerate slower feeds without burning because efficient cutting generates less heat even during extended contact. Dull bits burn at almost any feed rate because poor cutting efficiency creates excess heat regardless.

The feedback loop works here too. Slow feed causes burning. Burning creates resin deposits on the carbide. Deposits reduce cutting efficiency. Reduced efficiency seems to call for even slower feeds. Slower feeds create more burning. Breaking the cycle means recognizing that contact time - the fraction of a second each fiber spends touching hot carbide - matters more than almost any other single variable in router burning.