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, creating two independent rotation systems. But bearings don't stay perfect. They accumulate debris, 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.

For broader context on router bit burning, see why router bits burn wood.

Ideal Bearing Operation

A properly functioning template bearing has an outer race that contacts the template and an inner race fixed to the bit body or shank. Between these races sit ball bearings or roller bearings that allow the outer race to rotate while the inner race remains stationary relative to the spinning bit.

When the outer race contacts a template surface, friction between template and bearing race causes the race to rotate. The ball bearings between inner and outer races allow this rotation with minimal resistance. The internal friction in a good bearing comes only from the ball bearings rolling against the race surfaces and the minimal lubricant shear between metal surfaces. This rolling friction is quite low - typically requiring just a few ounces of force to overcome.

The bit spins at router speed - perhaps 20,000 RPM. The bearing outer race rotates at the speed determined by how fast you move the router along the template - perhaps 6 inches per second. These are completely independent rotations. The bit spins continuously at high speed. The bearing races rotate much slower, in the opposite direction, as determined by template geometry and feed rate.

The heat generated by ideal bearing operation is minimal. Rolling friction creates some warmth in the bearing races and balls, but the amount is small compared to cutting friction. The bearing might warm slightly during extended use but shouldn't get hot enough to noticeably affect bit temperature.

This separation of bit rotation from bearing rotation means cutting heat and bearing heat are independent sources. Cutting generates heat at the carbide edges. Bearing friction generates heat at the bearing-template interface. In an ideal setup, cutting heat dominates and bearing heat is negligible.

Bearing Degradation Mechanisms

Template routing bearings experience conditions that cause gradual performance degradation. Understanding these mechanisms explains why bearing friction increases over time and eventually creates significant heating.

Sawdust infiltration is the primary degradation pathway. Wood routing produces fine wood particles that become airborne and settle on all nearby surfaces. Some of this dust inevitably reaches the bearing. The dust enters the gap between inner and outer races. Once inside, dust particles mix with the bearing lubricant, creating an abrasive paste.

This dust-lubricant mixture acts like lapping compound. It gradually grinds away the smooth metal surfaces of the races and balls. The rolling friction that was originally very low increases as surface roughness develops. Rough bearing surfaces require more force to rotate. More force means more friction. More friction generates more heat.

The progression is gradual at first. The bearing feels slightly stiffer but still rotates freely. Over time, as more dust accumulates and surface damage increases, the bearing becomes noticeably rough. You can feel resistance when spinning it by hand. Under routing conditions with load applied, this resistance translates to significant friction and heat.

Resin from the wood being cut contributes to bearing problems beyond simple dust accumulation. Wood resin melts during cutting and becomes airborne as vapor or fine mist. This resin vapor condenses on cooler surfaces near the cutting zone. The bearing, being somewhat cooler than the cutting edge, collects this condensed resin.

Resin deposits on bearing surfaces act as adhesive, causing the balls to stick to races rather than rolling freely. What started as rolling friction becomes partial sliding friction as balls skip and drag instead of rolling smoothly. Sliding friction generates much more heat than rolling friction - typically 10-20 times more for equivalent load.

Impact damage occurs when bearings strike hard surfaces or workpiece edges. Template routing involves the bearing hitting template edges at the start of cuts and riding across surface irregularities during the cut. Each impact creates tiny dents or flat spots on the ball bearings and races. These damage points create high spots that must be forced past during rotation, increasing friction.

Corrosion from moisture in the wood or shop environment can pit bearing surfaces. Workshop humidity causes steel bearing components to develop surface rust if they're not protected by adequate lubrication. Even light surface corrosion creates rough spots that increase friction dramatically. Cutting pressure-treated or wet wood accelerates corrosion through direct moisture contact with bearing surfaces.

Bearing seal failure in sealed bearings exposes the internal components to contamination. Sealed bearings have rubber or metal shields that keep lubricant in and contaminants out. Impact loads or simply wear over time can damage these seals. Once the seal fails, all the degradation mechanisms accelerate because contaminants have free access to bearing internals.

Sliding vs Rolling Friction Differences

The difference between rolling and sliding friction explains why seized or damaged bearings generate so much more heat than functional ones. Understanding these friction types clarifies the magnitude of heating that occurs when bearings fail.

Rolling friction occurs when a ball or cylinder rolls across a surface. The contact point is instantaneously stationary relative to the surface - there's no relative motion at the contact. As the ball rolls, different portions come into contact sequentially, but each contact point experiences no sliding. This type of friction is very low because there's minimal surface shearing.

Sliding friction occurs when surfaces move relative to each other while maintaining contact. The molecules at one surface must slide past molecules of the other surface. This shearing action generates substantial heat. Sliding friction coefficients are typically 0.3-0.6 for metal-on-metal contact, compared to 0.001-0.002 for rolling friction.

A functioning template bearing operates almost entirely on rolling friction. The balls roll against the races. The outer race rotates relative to the template through rolling contact at the bearing-template interface. Heat generation is minimal.

A seized bearing forces all motion into sliding friction. The outer race no longer rotates independently. Instead, it's locked to the bit body and drags across the template surface as the bit spins. The contact pressure between bearing and template combined with sliding motion at router speeds generates tremendous friction heat - hundreds of times more than a functioning bearing.

Partially seized bearings fall between these extremes. The balls might roll intermittently but also drag and skip. The outer race might rotate somewhat but with high resistance. The friction is greater than pure rolling but less than complete seizing. These partially failed bearings generate enough heat to cause problems while not being obviously seized when tested by hand at low speeds.

The heat from sliding friction concentrates at the contact point between bearing and template. Temperatures at this interface can reach several hundred degrees during routing with a seized bearing. This heat conducts into the bit body, raising carbide edge temperature before cutting even begins.

Heat Conduction Through Bit Body

Heat generated at the bearing doesn't stay localized - it conducts through the steel or carbide bit body to the cutting edges. The conduction path and rate determine how much bearing friction affects cutting performance.

Wood router bits typically have steel bodies with carbide tips brazed on. Steel conducts heat moderately well - better than wood but not as well as copper or aluminum. Heat entering the bit body at the bearing location spreads throughout the steel through conduction.

The temperature gradient drives conduction rate. Large temperature differences cause rapid heat flow. The bearing might reach 300-400 degrees from friction while the cutting edge starts at ambient temperature. This steep gradient causes rapid heat conduction along the bit body toward the cooler cutting end.

The bit body's geometry affects conduction. Solid bits with no flutes or voids conduct heat efficiently along their length. Bits with deep flute gullets have less cross-sectional area for heat flow, slowing conduction but also reducing total heat capacity. The balance depends on specific bit design.

Distance from bearing to cutting edge matters. Top-bearing pattern bits have the bearing separated from cutting edges by the bit body length. Heat must conduct the full length before affecting the carbide. Bottom-bearing bits have the bearing directly adjacent to cutting edges. Heat transfer is nearly immediate because the conduction path is very short.

The carbide tips receive conducted heat from the steel body and add their own cutting heat. If bearing friction has preheated the bit body to 200 degrees, and cutting normally raises carbide temperature to 300 degrees, the total temperature reaches 500 degrees - well above wood charring temperature. The bearing heat raises baseline temperature, requiring less additional cutting heat to cause burning.

Solid carbide bits conduct heat better than carbide-tipped steel bits because carbide thermal conductivity is higher than steel. This seems like it would make bearing problems worse by conducting bearing heat faster to cutting edges. Actually, the better conduction helps dissipate heat more efficiently throughout the bit mass, potentially reducing peak temperatures compared to bits with poor conduction.

The bit chuck - where the bit mounts in the router collet - acts as a heat sink. Heat can conduct from the bit into the massive collet and router body, removing it from the cutting system. But this heat sink only helps for portions of the bit in contact with the collet. Heat generated near the cutting edges or bearing doesn't effectively dissipate through the collet because the conduction path is too long.

Template Material Effects

The material used for routing templates affects bearing friction and heat generation. Different materials create different interface conditions at the bearing-template contact point.

Hardboard (tempered Masonite) is a common template material. The dense, smooth surface creates relatively low friction against bearing surfaces. Hardboard is hard enough to resist bearing indentation under normal routing pressure. The smooth surface doesn't abrade bearings significantly. Heat generation at the bearing-hardboard interface stays moderate even with partially degraded bearings.

MDF templates have a softer, more porous surface than hardboard. Bearing pressure can slightly indent MDF, creating resistance to smooth bearing rotation. The softer material also generates more dust at the bearing contact point as material compresses and abrades away. This dust contributes to bearing contamination. MDF's lower density provides less thermal mass to absorb and dissipate bearing friction heat, so surface temperatures rise faster.

Plywood templates vary based on ply quality. Smooth, void-free Baltic birch provides a good bearing surface similar to hardboard. Lower-grade plywood with surface voids creates problems. When the bearing rolls across a void, it suddenly drops into the depression and must climb out. These impact events damage bearings while also creating jerky motion that affects cutting quality. The glue lines in plywood are also harder than surrounding wood, creating friction variations as the bearing alternates between wood and glue contact.

Phenolic laminate (like countertop material) makes excellent templates. The hard, smooth plastic surface creates minimal friction. Phenolic doesn't generate dust or debris at the bearing contact. The material doesn't compress under bearing pressure. Template edges stay crisp through repeated use. Bearing heat generation stays low because the phenolic surface maintains low friction even with partially degraded bearings.

Acrylic (Plexiglas) templates provide visual access to see workpiece position but have mixed performance as bearing surfaces. The plastic is smooth and doesn't generate dust, which is good. But acrylic is relatively soft and can be indented by bearing pressure, especially at high temperatures. The indentations create rough spots that increase friction. Acrylic also has poor dimensional stability with temperature changes, potentially warping during routing sessions.

Metal templates - aluminum or steel - create the highest bearing friction and fastest bearing wear. Metal-on-metal contact without lubrication generates substantial heat even with functional bearings. The hard metal surface doesn't give way under bearing pressure, creating high contact stresses. Metal's excellent thermal conductivity rapidly carries heat away from the contact point but the heat generation rate is so high that surface temperatures still rise substantially.

Bearing Load and Pressure

The force pressing the bearing against the template affects friction and heat generation. Understanding load effects explains why technique variations cause different burning tendencies.

Light bearing pressure - just enough to maintain template contact - creates minimal contact area between bearing and template. Small contact area means low total friction force even if friction coefficient is moderate. Heat generation stays low because the total energy dissipated through friction is proportional to force times distance.

Heavy bearing pressure - pushing the router firmly against the template - increases contact area and deformation at the bearing-template interface. More contact area means more friction and more heat. The increased pressure can also deform template material slightly, creating resistance to bearing rotation beyond simple surface friction.

Variable pressure during routing creates inconsistent heat generation and burning patterns. Sections of the cut where the operator pushed hard show more burning than sections with lighter pressure. This inconsistency makes it difficult to identify bearing friction as the problem because burning appears intermittent rather than constant.

The bearing contact point moves across the template surface as routing progresses. In straight cuts, the same portion of the bearing contacts the template continuously. The heat generated at that location accumulates in both the bearing and template material. Extended straight cuts can cause localized overheating even with functional bearings because heat input exceeds dissipation rate.

Curved cuts continuously change which portion of the bearing contacts the template. The heat generation distributes around the bearing circumference rather than concentrating in one location. This distribution helps manage total bearing temperature, potentially reducing burning compared to straight cuts of equivalent length.

Inside curves load the bearing differently than outside curves. Following an inside curve - routing a concave profile - presses the bearing outward against the template. Following an outside curve - convex profile - presses the bearing inward. The load direction changes affect how forces distribute through the bearing races and balls, potentially affecting friction and heat generation.

Multiple-Pass Thermal Accumulation

Template routing often involves making multiple identical parts using the same template. The thermal behavior changes across sequential parts as heat accumulates in the system.

The first part routes through a cold system. Bearing and template are at room temperature. Cutting generates heat but starts from a low baseline. Burning may not occur if conditions are reasonable.

The second part encounters a warm bearing from the first part's cutting. The bearing hasn't fully cooled between parts. The template is also slightly warm at contact points. Both start warmer than ambient. The additional heat from cutting the second part builds on this warm baseline. Temperatures rise higher than the first part. Burning becomes more likely.

By the fifth or sixth part, the bearing is hot, the template is warm, and the bit body has absorbed heat from multiple cutting cycles. Each additional part adds heat faster than cooling removes it. The system temperature ramps up progressively. Parts that route cleanly individually show burning when routed sequentially without cooling intervals.

This thermal accumulation explains why production routing shows more burning than single-piece work even with identical technique. The continuous operation doesn't allow system cooling. Professional shops running production routing often implement forced cooling - compressed air directed at bearings and bits between cuts - to prevent thermal accumulation.

The accumulated heat affects the workpiece too. Routing warm wood requires less additional heat to reach charring temperature. A board warmed to 100 degrees from lying near the routing operation needs only 300 degrees more temperature rise to char instead of 350 degrees from room temperature. This seemingly small difference can be enough to tip marginal cutting conditions into burning.

Bearing lubrication degradation accelerates with thermal cycling. The heat from cutting melts or thins whatever lubrication remains in the bearing. Thin lubricant provides less protection, allowing more metal-to-metal contact and higher friction. Higher friction generates more heat, further degrading lubrication. After multiple thermal cycles, bearing lubrication may be completely lost, creating severe friction and heat.

Bearing Position Effects

Whether the bearing sits above or below the cutting edges changes how bearing friction affects the system. Top-bearing and bottom-bearing bits have different thermal characteristics.

Top-bearing bits (also called overhang or pattern bits) have the bearing mounted on the bit shank above the cutting edges. The template sits on top of the workpiece. The bearing rides on the template while cutting edges shape the workpiece beneath. Heat generated by bearing friction must conduct down through the bit body to reach cutting edges. The distance provides some thermal isolation.

The workpiece blocks heat radiation from cutting edges upward toward the bearing. The wood between cutting and bearing acts as thermal insulation. This separation means cutting heat and bearing heat remain somewhat independent longer than with bottom-bearing arrangements.

Top-bearing bits accumulate dust and debris on the bearing more readily because chips from cutting are thrown upward toward the bearing by bit rotation. The chip stream directly strikes the bearing, contaminating it faster than if chips moved away from the bearing.

Bottom-bearing bits (also called flush-trim bits) have the bearing below the cutting edges. The workpiece sits on top of the template. The bearing rides on the template edge beneath the workpiece. Heat generated by bearing friction immediately affects cutting edges because the conduction path is very short.

The close proximity of bearing to cutting edges means bearing problems cause burning faster with bottom-bearing bits. A partially seized bearing might not noticeably affect top-bearing routing but causes immediate burning with bottom-bearing bits.

Chip evacuation sends debris away from the bearing in bottom-bearing configurations. The bit rotation throws chips upward, away from the bearing below. This reduces bearing contamination compared to top-bearing bits. However, the cutting heat radiates downward directly toward the bearing, potentially raising bearing temperature through radiation even without bearing friction problems.

Mid-bearing bits have the bearing between two cutting sections - cutters above and below the bearing. These bits use the bearing as a diameter reference while cutting on both sides simultaneously. The bearing sits in perhaps the worst possible location for heat management. It receives conducted heat from cutting edges both above and below. It also has minimal exposure to cooling air because it's surrounded by hot bit body.

Bearing Replacement Timing

Knowing when bearing friction has become problematic helps identify when burning is bearing-related versus cutting-related. The symptoms of bearing degradation progress through recognizable stages.

New bearings spin with almost no perceptible resistance. You can spin the outer race with fingertip pressure and it rotates smoothly with no rough spots or sticking. The rotation continues freely until friction gradually slows it to a stop.

Early degradation shows as slight stiffness. The bearing still spins freely but requires more force to rotate. You notice resistance when spinning by hand but it's still smooth - no rough spots or grabbing. This stage may not cause noticeable burning because the increased friction isn't severe enough to generate significant heat during routing.

Moderate degradation creates roughness. You feel distinct rough spots as the bearing rotates. The outer race catches momentarily then releases as it turns. Spinning the bearing by hand feels gritty or crunchy. At this stage, bearing friction begins causing burning during routing because the rough spots generate heat through impact and sliding friction.

Severe degradation results in significant resistance to rotation. The bearing requires substantial force to turn. It may bind intermittently and require forceful rotation to break free. The bearing feels extremely rough and may make grinding noises when rotated. Routing with bearings in this condition causes immediate burning because bearing friction exceeds cutting friction.

Complete seizure locks the bearing races together. The outer race won't rotate at all regardless of force applied. The bearing must be pried off the bit because it won't spin free. A completely seized bearing drags across the template during routing, generating extreme heat that causes severe burning and potential template damage.

The progression from new to seized typically takes weeks to months depending on use intensity and maintenance practices. Light hobbyist use might allow a bearing to last years. Heavy production use might degrade bearings in days or weeks. Environmental factors - dust levels, humidity, wood resin content - strongly affect degradation rate.

Lubrication and Contamination

Bearing lubrication degrades over time through multiple mechanisms that accelerate during template routing. Understanding lubrication behavior explains bearing friction progression.

New bearings come with factory lubrication - typically light grease. The grease separates metal surfaces, providing a thin film that prevents metal-to-metal contact. As long as this film remains intact, friction stays low because surfaces slide on lubricant rather than contacting directly.

Heat from cutting and bearing friction causes grease to thin. The lubricant's viscosity decreases at higher temperatures, reducing its ability to maintain separation between surfaces. Very hot bearings may have lubricant thin to nearly water-like consistency, providing minimal protection.

Dust contamination mixes with lubricant, creating an abrasive paste. Wood dust particles suspended in grease act like grinding compound. Each rotation drags this abrasive mixture across bearing surfaces, wearing away material and increasing surface roughness. More roughness generates more friction, more heat, and faster degradation.

Resin contamination changes lubricant properties differently than dust. Wood resin that reaches the bearing interior mixes with grease, increasing viscosity and tackiness. The thickened lubricant-resin mixture creates more drag on bearing motion. The sticky mixture also tends to bind surfaces together rather than allowing smooth rotation.

Moisture from humid environments or wet wood causes lubricant breakdown and promotes corrosion. Water doesn't mix with grease-based lubricants, instead forming separate droplets that displace lubricant from bearing surfaces. The water droplets promote rust formation on steel components, creating rough surfaces that increase friction.

Lubricant eventually depletes through combination of thermal degradation, contamination, and physical loss. The bearing transitions from having adequate lubrication to having minimal or no lubrication. At this point, metal-to-metal contact occurs continuously during operation. Friction spikes dramatically and bearing wear accelerates rapidly.

Some router bit bearings are permanently lubricated sealed units designed to operate their entire lifespan without maintenance. When these bearings wear out, the entire bearing assembly requires replacement. Other bearings are serviceable - they can be cleaned and relubricated. But servicing router bit bearings is difficult because of their small size and the challenge of disassembling them without damage.

Diagnosis Through Observation

Distinguishing bearing friction problems from cutting problems requires careful observation of symptoms and patterns. Several indicators point toward bearing friction as the primary heating source.

Burning that appears immediately at cut start suggests bearing problems. If the workpiece shows scorch marks from the instant the bit contacts wood, before significant cutting has occurred, bearing friction likely dominates. Pure cutting heat takes time to accumulate - if burning happens instantly, a different heat source is present.

Burning concentrated at specific locations where bearing pressure is highest indicates bearing friction contribution. If corners or curves show more burning than straight sections, bearing load variation might be the cause rather than cutting mechanics.

Bearing temperature after routing provides direct evidence. Immediately after a cut, carefully touch the bearing (wait a moment for chips to clear). If the bearing is too hot to touch comfortably - above 140-150 degrees - bearing friction is significant. Normal bearing operation produces modest warmth but not painfully hot temperatures.

Comparing top-bearing and bottom-bearing bits helps isolate bearing effects. If both bit types burn identically, cutting is probably the dominant heat source. If bottom-bearing bits burn worse than top-bearing bits of similar design, bearing proximity to cutting edges suggests bearing heat contribution.

Sequential part burning progression indicates thermal accumulation. If the first few parts route cleanly but burning appears on later parts without any technique changes, heat is building up in the system. Bearing friction combined with cutting heat causes this progressive heating because the bearing never cools between parts.

Template surface examination shows bearing friction evidence. Darkened or melted streaks on the template surface indicate extreme bearing heat. Indentations or grooves worn into template material show bearing dragging rather than rolling. Clean templates without surface damage suggest bearing friction is minimal regardless of burning on workpieces.

Bearing spin test provides the most direct assessment. Remove the bit from the router and manually spin the bearing. The resistance level and smoothness indicate bearing condition. This test isolates bearing friction from all other variables.

FAQ

Why does template routing burn more than freehand routing?

Template routing adds bearing friction heat to cutting heat. When bearings develop resistance from contamination or wear, they generate their own heat source independent of cutting. This bearing heat preheats the bit body before cutting begins, reducing the thermal margin before burning occurs.

Can bearing problems cause burning with a sharp bit?

Yes, bearing friction generates heat regardless of cutting edge condition. A sharp bit with excellent cutting characteristics can still burn if bearing friction preheats the system enough. The combined heat from bearing friction and cutting exceeds wood's charring threshold even though cutting alone would be acceptable.

How does bearing position affect burning?

Bottom-bearing bits burn more readily from bearing friction because the bearing sits immediately adjacent to cutting edges. Heat conduction path is very short. Top-bearing bits provide more thermal separation between bearing and cutting edges, requiring worse bearing problems before burning occurs.

Why do bearings fail faster in template routing than other routing?

Template routing exposes bearings to sawdust, resin vapor, and impact loads continuously during operation. The bearing contacts abrasive wood surfaces and accumulates contamination that degrades lubrication and damages bearing surfaces. Other routing operations don't subject bearings to these conditions.

Does template material affect burning?

Template material affects bearing friction through surface hardness and smoothness. Hard, smooth materials like phenolic create less friction than soft, porous materials like MDF. Lower friction generates less heat, reducing bearing contribution to burning. Template choice can noticeably affect burning tendency.

Can bearing heat damage the template?

Severely degraded or seized bearings generate enough heat to melt or scorch template materials. Hardboard can scorch and MDF can burn from bearing friction heat alone. Phenolic and metal templates resist damage better but can still develop surface discoloration from extreme bearing heat.

Why does burning get worse across multiple parts?

Bearing and bit temperatures accumulate across sequential cuts without adequate cooling between parts. Each part adds more heat to the system. By the fifth or sixth part, the bearing operates hot from previous cuts. Additional heat from current cutting pushes total temperature above burning threshold.

How hot do bearings get during normal routing?

Functional bearings with adequate lubrication and cleanliness reach 120-140°F during normal routing - warm but not painfully hot. Degraded bearings with friction problems can exceed 200°F. Severely degraded or seized bearings may reach 300-400°F, hot enough to cause immediate burns if touched.