Bearing Friction vs Cutting Friction in Template Routing

Template routing relies on a bearing mounted to the router bit to follow a pattern while the cutting edges shape matching profiles. When everything works correctly, the bearing spins freely on its own axis while the bit rotates inside it - two independent rotation systems. The bearing generates almost no heat. The cutting edges generate normal friction. Everything stays cool.

But bearings don't stay perfect. They accumulate sawdust, develop rough spots, lose lubrication. When a bearing stops spinning freely, it creates its own heat source that operates independently of the cutting action and preheats the entire system before any wood gets cut.

How Bearings Degrade

Sawdust infiltration is the primary pathway. Routing produces fine particles that settle everywhere, including into the gap between inner and outer bearing races. Dust mixes with bearing lubricant, creating an abrasive paste that gradually grinds away smooth metal surfaces. Rolling friction that was originally very low increases as surface roughness develops.

Wood resin compounds the problem. Resin vapor from cutting condenses on the cooler bearing surfaces, becoming an adhesive that causes balls to stick rather than roll. What started as rolling friction becomes partial sliding friction - and sliding friction generates 10 to 20 times more heat than rolling friction for equivalent load.

Impact damage from striking template edges creates flat spots on ball bearings and races. Corrosion from workshop humidity pits bearing surfaces. Seal failure exposes internals to contamination. Each mechanism feeds the others - rougher surfaces trap more dust, which creates more roughness.

The progression feels gradual at first. Slightly stiffer but still functional. Then noticeably rough - you can feel grit when spinning it by hand. Then significant resistance. Finally, complete seizure - the outer race locked to the bit body, dragging across the template at router speed.

The Preheating Effect

Heat generated at the bearing doesn't stay localized. It conducts through the steel bit body to the cutting edges. The temperature gradient drives conduction rate - a bearing at 300 to 400 degrees from friction while the cutting edge starts at ambient creates rapid heat flow.

Bottom-bearing flush-trim bits suffer worst. The bearing sits immediately adjacent to cutting edges, so heat transfer is nearly instantaneous. Top-bearing pattern bits have the full bit body length between bearing and cutting edges, providing some thermal isolation.

If bearing friction has preheated the bit body to 200 degrees, and cutting normally raises carbide temperature to 300 degrees, the combined temperature reaches 500 - well above wood's charring threshold. The bearing raised the baseline, requiring less additional cutting heat to cause burning. You get scorch marks even with proper feed rate, sharp edges, and appropriate router speed because the bearing friction is supplying heat the cutting action didn't need to contribute.

The Multiple-Pass Problem

Template routing often means making multiple identical parts. The thermal behavior changes across sequential pieces as heat accumulates in the system.

First part: cold system, clean cuts. Second part: warm bearing from the first cut, higher baseline temperature. By the fifth or sixth part, the bearing is hot, the template is warm, and the bit body has absorbed heat from multiple cycles. Each additional part adds heat faster than cooling removes it. Parts that route cleanly individually show burning when routed sequentially.

This thermal accumulation explains why production routing burns more than single-piece work even with identical technique. Professional shops running production often direct compressed air at bearings between cuts to prevent the ramp-up.

Bearing lubrication degrades with each thermal cycle too. Heat melts or thins whatever lubrication remains. Thin lubricant provides less protection, allowing more metal-to-metal contact. Higher friction degrades lubrication further. After multiple thermal cycles, lubrication may be completely lost.

Template Material Matters

The material the bearing rides against affects friction at the contact interface.

Hardboard - dense, smooth, resistant to bearing indentation. Moderate friction even with partially degraded bearings. A reliable default.

MDF - softer, more porous. Bearing pressure can slightly indent it, creating resistance. The softer material generates more dust at the contact point, contributing to bearing contamination.

Phenolic laminate - hard, smooth plastic surface creating minimal friction. Doesn't generate dust. Doesn't compress under bearing pressure. Bearing heat stays low even with partially degraded bearings. The best template material for bearing longevity.

Plywood - varies by quality. Baltic birch provides a good surface. Lower-grade plywood with surface voids creates problems - the bearing drops into depressions and climbs out, damaging itself. Glue lines create friction variations as the bearing alternates between wood and harder adhesive contact.

Diagnosing Bearing vs Cutting Problems

Burning that appears immediately when the bit contacts wood - before significant cutting has occurred - suggests bearing problems. Pure cutting heat takes time to build; instant burning indicates a different source.

Burning that progresses across sequential parts without technique changes points to thermal accumulation. The feed rate hasn't changed, but heat is building faster than it dissipates.

The most direct test: after a cut, note if the bearing is too hot to touch comfortably - above roughly 140 degrees. Functional bearings produce modest warmth. Problem bearings produce real heat. Darkened or melted streaks on the template surface indicate extreme bearing friction.

Spinning the bearing by hand isolates the variable completely. Smooth rotation with fingertip pressure means the bearing is fine. Gritty, rough, or resistant rotation means the bearing is contributing heat that your cutting technique can't compensate for.