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 the spinning carbide. Feed rate determines that contact time, and contact time determines whether heat accumulates past the point where wood begins to char.

For a complete overview of router bit burning mechanisms, see why router bits burn wood.

Contact Time and Thermal Accumulation

When a router bit spinning at 20,000 RPM encounters a wood fiber, the interaction happens in milliseconds. The carbide edge strikes the fiber, compresses it briefly, cuts through, and moves on. During that contact, friction generates heat. Whether that heat causes burning depends on how long the interaction lasts and whether the wood can conduct the heat away before the next carbide flute arrives.

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

Slow down to half an inch per second. Now each fiber spends 100 milliseconds in contact with the hot carbide. Double the contact time means double the heat transfer into that specific location. The surrounding wood can only conduct heat away at a fixed rate - determined by wood species density and moisture content. The heat input now exceeds the dissipation rate. Temperature rises. At 400 degrees Fahrenheit, the wood chars.

The relationship isn't linear because heat dissipation isn't instant. Wood has relatively poor thermal conductivity compared to metals. Heat entering a wood fiber takes time to spread into adjacent fibers and the surrounding air. During that lag, if more heat keeps entering through continued bit contact, the temperature at the contact point climbs rapidly.

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 the wood can possibly conduct it away. The contact point quickly reaches charring temperature, then continues heating past that into deeper pyrolysis. The scorch mark you see after a pause is wood that spent sustained time above 400 degrees.

Material Removal Rate

Feed rate affects more than just contact time - it also determines how much material the router bit removes with each revolution.

A router spinning at 22,000 RPM turns 366 times per second. If you move that router through wood at one inch per second, each revolution of the bit advances 0.0027 inches into fresh material. The cutting edge takes a bite of just under three thousandths of an inch per flute.

Slow your feed to half an inch per second and each flute now takes a bite of 0.0014 inches - half as much. The bit spends twice as long in the wood removing half the material per pass. Less material removal per revolution means each carbide flute is working less efficiently. Reduced efficiency means more friction. More friction generates more heat.

This seems counterintuitive. Slower feed should mean easier cutting, right? But router bits are designed to work at specific material removal rates. Too little material per flute and the carbide edge isn't slicing cleanly - it's rubbing and burnishing rather than cutting. That rubbing creates tremendous friction without producing chips to carry heat away.

Fast feed rates keep the bit cutting rather than rubbing. Each carbide flute encounters enough material to slice a proper chip. The chip formation itself is efficient - the wood separates cleanly with minimal energy wasted to heat. The chips also carry heat away from the cutting zone as they eject from the flute gullets. Effective chip evacuation means the heat source (the cutting edge) operates cooler.

There's a ceiling to this, naturally. Feed too fast and the bit can't cut the material faster than you're presenting it. The bit deflects, bends slightly under load, and you get chatter marks. But up to that ceiling, faster feeds generally mean cooler cuts because the bit works efficiently within its design parameters.

Speed and Feed Interaction

Feed rate can't be considered in isolation from router speed. The two variables work together to determine the actual bite per tooth that the cutting edge experiences.

At 22,000 RPM with a half-inch per second feed, a two-flute bit takes a 0.0014-inch bite per tooth. Reduce router speed to 16,000 RPM with the same feed and bite per tooth increases to 0.0019 inches. Same feed rate, different router speed, different material removal efficiency.

This is why larger diameter bits need slower router speeds. A two-inch raised panel bit has cutting edges traveling at much higher linear speeds than a half-inch bit at the same RPM. Router bit speed vs diameter physics shows that tip speed increases proportionally with diameter. Higher tip speeds mean each point on the cutting edge spends less time in contact with any given fiber, which sounds good for heat control. But the increased surface speed also means more friction per unit time at the contact point.

The solution is to slow the router RPM for large bits while maintaining feed rate. This brings tip speed down to reasonable ranges while keeping bite per tooth in the efficient cutting zone. A two-inch bit might run at 12,000 RPM instead of 22,000, and you'd feed it at similar linear speed to a smaller bit. The result is efficient cutting without excessive heat.

Feed rate has to be adjusted for router speed, bit diameter, and the number of flutes. A four-flute bit can handle slower feeds than a two-flute bit at the same RPM because each flute encounters fresh material more frequently. The bite per tooth stays reasonable even though you're moving the router more slowly through the cut.

Material-Specific Feed Considerations

Different woods tolerate different feed rates because their density and structure determine how they respond to cutting forces and how quickly they conduct heat away.

Dense hardwoods like maple, cherry, and walnut allow faster feeds because their tight fiber structure resists deflection under the cutting force. The bit can move quickly through the material without causing chatter or surface imperfections. The dense wood also conducts heat better than softer species, helping dissipate the friction heat that cutting generates. You can push hard through oak and maintain clean cuts at feed rates that would cause problems in softer woods.

Softwoods present a different challenge. Pine, fir, and cedar have lower density and more variation between early wood and late wood growth rings. Fast feeds can cause tearout where the bit encounters sudden density changes. But slow feeds in softwoods create their own problems - namely resin melting. Pine contains resin pockets throughout its structure. Slow feed rates mean extended contact time, which generates enough heat to liquefy that resin. The liquid resin immediately adheres to the carbide cutting edge, creating buildup that increases friction and makes burning worse. Understanding what resin buildup does to cutting edges explains why softwoods often burn even at moderate feeds.

The solution with resinous softwoods is to keep feeds relatively fast - fast enough that contact time stays short and resin doesn't have time to melt significantly. The balance is narrower than with hardwoods. Too slow and you get burning from resin buildup. Too fast and you get tearout from the soft, variable-density wood structure.

Plywood demands special consideration. The alternating grain direction of the plies means the bit encounters cross-grain and long-grain in rapid succession. Feed rates that work for solid wood can cause spelling in plywood face veneers. But slow feeds in plywood almost guarantee burning because of the glue content. Phenolic and urea-formaldehyde adhesives melt at lower temperatures than wood chars. Slow feeds give the glue time to melt and stick to the bit, creating the gummy buildup that makes carbide tips dull faster in plywood. The feed rate has to be quick enough to minimize glue contact time while not so fast that face veneer tears out.

MDF burns at nearly any feed rate because its entire structure is wood fiber suspended in adhesive binder. Every cubic millimeter contains glue. There's no feed rate that avoids prolonged contact with adhesive - it's unavoidable. MDF work requires sharp bits, proper speeds, and acceptance that some burning is inherent to the material.

Feed Direction and Climb Cutting

The direction you feed the router relative to bit rotation affects both cutting efficiency and burning tendency. Conventional routing feeds against the bit rotation - pushing the router left to right when working an edge facing you. Climb cutting feeds with the bit rotation - right to left on that same edge.

Conventional routing has the bit pulling itself into the wood. The cutting edge enters from the thin side of the chip and exits at full depth. This provides good control because you're fighting the bit's tendency to grab the workpiece. But it also means the bit spends slightly more time in contact with each fiber because it has to work its way into the full depth of cut.

Climb cutting reverses this. The bit enters at full depth and exits thin. Each carbide flute takes an immediate full bite of material and shears it off cleanly. The cutting is more efficient, generates less friction, and produces less heat. Contact time per fiber is minimized because the bit isn't gradually working into the cut - it takes full depth immediately and moves on.

The tradeoff is control. Climb cutting has the bit pulling the router along the workpiece. It wants to run away from you. On handheld routing, this makes climb cutting dangerous unless you're prepared for the force. On router tables with proper setup, climb cutting can reduce burning significantly because of the reduced contact time and more efficient cutting action.

The physics of end grain routing heat generation shows that end grain sometimes benefits from climb cutting because the immediate full-depth bite helps prevent the crushing and compression that makes end grain burn so readily.

The Repositioning Problem

Actual routing operations involve more than just smooth linear motion. You have to reposition hands, adjust stance, reach around clamps, navigate corners. Every time forward motion slows or stops, contact time increases and burning risk spikes.

Inside corners present the worst case. The bit must remove maximum material in minimum space while you're changing direction with the router. Forward motion slows as you navigate the turn. Contact time increases. The wood heats past charring temperature. You get dark burn marks specifically in the corner even though the straight sections cut cleanly.

Hand repositioning creates similar problems. You're moving along smoothly, feeding at a good pace, then you need to shift your grip to reach the rest of the cut. The router slows or stops briefly during that shift. The bit continues spinning at full speed against now-stationary wood. Burn mark.

Experienced woodworkers develop techniques to minimize these pauses - planning grip positions before starting the cut, using both hands to maintain momentum through position changes, routing in shorter sections that don't require mid-cut repositioning. But the fundamental issue remains: any moment where feed rate drops to zero while the bit spins creates excessive contact time and burning.

Template routing with bearing-guided bits compounds this because the bearing adds a second variable. If bearing friction is high, it resists smooth motion and causes the router to drag rather than glide. That drag reduces feed rate, increases contact time, and promotes burning even when you're actively trying to maintain smooth motion.

Feed Rate vs Surface Finish

There's a balance between feed rate and surface quality that affects burning independently of pure heat generation. Very fast feeds leave visible marks - not burn marks, but mill marks from the individual bites each carbide flute takes out of the wood.

At extremely fast feeds, you can see the scalloped pattern where each flute passed through. The wood between flute paths doesn't get cut away completely. This rougher surface has more exposed edge area per square inch. More edge area means more locations where the bit makes contact. More contact points mean more total friction across the surface even though contact time per point is brief.

The roughness also affects how cleanly subsequent sanding works. A rough-cut surface with deep mill marks requires aggressive sanding to flatten. That sanding removes material that could have been left if the routing surface finish was smoother to begin with.

Slowing the feed produces a smoother surface with less visible flute marks. The scallop pattern becomes so shallow it's barely detectable. But slower feeds increase burning risk through extended contact time. The balance is finding the feed rate that produces acceptable surface finish without creating so much contact time that burning becomes problematic.

This balance shifts based on router speed and bit quality. Higher RPM means more flute passes per inch at the same feed rate, which naturally produces smoother surface. A sharp bit cuts cleaner peaks and valleys in the scallop pattern compared to a dull bit that tears and compresses fibers. Sharp bits can tolerate slower feeds without burning because they cut efficiently even during extended contact. Dull bits burn at almost any feed rate because the poor cutting efficiency generates excess heat regardless.

The Feedback Loop

Feed rate problems tend to compound rather than remain isolated issues. Slow feed causes burning. Burning creates resin deposits or charred material on the carbide edge. Deposits reduce cutting efficiency. Reduced efficiency requires even slower feeds to prevent chatter. Slower feeds create more burning. The cycle continues.

Breaking this cycle requires recognizing it early. The first hint of burn marks is a signal to increase feed rate, not decrease it. The instinct when cutting becomes difficult is to slow down and "take it easy" on the tool. But with routers, slowing down usually makes burning worse because contact time is the primary driver of heat accumulation.

The exception is when chatter marks appear alongside burning. Chatter indicates you're already at or past the maximum feed rate the bit can handle. Slowing down might be necessary, but the burning suggests the bit is dull, the router speed is wrong for the bit diameter, or the material has properties that make it prone to heat accumulation regardless of feed. Those are the variables to address rather than simply adjusting feed rate.

Understanding what feed rate does to thermal accumulation means recognizing that the time wood spends touching hot carbide matters more than almost any other single variable in router burning. Everything else - bit sharpness, router speed, material properties, bearing condition - affects the system, but contact time is where those factors translate into actual heat buildup that causes charring.

FAQ

What is the ideal feed rate for routing? Feed rate varies by material, bit diameter, and router speed. Dense hardwoods tolerate 1-2 inches per second with small bits. Larger bits need slower feeds - perhaps half an inch per second - but router speed should decrease proportionally to maintain efficient cutting.

Why does slowing down cause more burning? Slower feed increases contact time between wood and the spinning bit. Extended contact allows heat to accumulate faster than the wood can conduct it away. Each point on the cut line spends more time touching hot carbide, raising temperature past the charring threshold.

Does climb cutting reduce burning? Climb cutting can reduce burning because the bit takes an immediate full-depth bite and exits quickly, minimizing contact time per fiber. The cutting is more efficient with less rubbing friction. However, climb cutting is harder to control and can be dangerous with handheld routers.

How do I maintain consistent feed rate? Plan hand positions before starting the cut. Use both hands on the router. Route in shorter sections that don't require mid-cut repositioning. Practice smooth motion without pauses. On router tables, use featherboards to maintain consistent workpiece pressure against the fence.

Why do corners burn more than straight sections? Corners require direction changes that slow forward motion. The router must remove maximum material in minimum space while navigating the turn. Reduced feed rate means increased contact time, allowing heat to accumulate past the charring point.

Does feed rate matter with sharp bits? Sharp bits are more forgiving of feed rate variation because they cut efficiently even during extended contact. But extremely slow feeds will burn even with sharp carbide because contact time becomes excessive regardless of cutting efficiency.

What feed rate works for MDF? MDF burns at most feed rates because its entire structure contains adhesive binder. Use the fastest feed that doesn't cause chatter or surface tearout. Keep bits sharp and clean. Accept that some burning is inherent to MDF's glue content.

Should I speed up or slow down when I see burning? Speed up if the bit is cutting without chatter. Burning indicates excessive contact time. Slow down only if you see chatter marks or surface tearout alongside burning, which suggests you're at the maximum feed rate and the burning has a different cause.