Router feed rate
When router feed rate is too slow, the wood is ground up into powder and smoke begins to rise. Moving too slowly trying to smooth a cut, your good intentions backfire by increasing friction. While you are pausing or lingering cautiously in a mortise with the bit spinning, it might be burning.
An inner corner is a likely place for a burn. Forward momentum halts as the cutter enters a corner. Suddenly it is contacting more material around its circumference. Shavings erupt and have to escape somewhere. In concave internal shapes or enclosed spots, a small 2 flute bit does not have much room for conveying swarf. If it cannot keep up, it fills and compacts. Feeding slows to a snail's pace. This obstructed situation commonly causes a flare up.
Single flute bits have less contact area and more chip load to win the scooping race. Their aggressive attack is suited for machining dense materials faster. For rapid production at high volume, a slight compromise in quality may be acceptable in exchange for extended tool life. This option is a way of avoiding burns if the bit must be small and feed rate is slow, or if feeding must stop momentarily with the cutter still running.
Dense, rigid materials are slower to cut than porous ones. Routing MDF takes more time than particle board. Hard maple and oak cut at a slower rate than softwood. If you try to make too deep a pass in the end grain of dense hardwood, the motor will strain. If it almost stalls, don't be tempted to compensate by drastically slowing the feed rate along end grain. Keeping moving with a lighter touch is how to avoid burning. After you reach the end or corner, back away slightly.
The other sort of router speed is rotation. If you have variable speed control, it could range widely between 8,000 to around 35,000 RPM, or Revolutions Per Minute. Your bit size suggests the proper setting. Some big diameter bits come with a speed chart. You might see numbers amounting to a precaution, but not a recommendation to improve the quality of finish. A chart can't evaluate the edge or the material hardness, so it can't substitute for making a test cut in a scrap.
Appropriate speeds are somewhere below maximum RPM, so begin a test cut with the variable speed control setting on slow rotation and gradually increase until the cut is acceptable. Speeds near full throttle are only suitable for a 1 in. bit or smaller.
To keep it simple, a bit twice as wide should rotate half as fast. At the same RPM, a big bit has a faster peripheral surface speed than a small one. If a big bit rotates fast, it burns. To spin big diameter router bits slowly, you'll want a strong motor. A shaper cutter rotates slower and has the power for very wide profiles.
Laminates and solid surface Corian are cut by a carbide bit. The abrasiveness of these materials is an issue. At a slow feed rates the operation can get messy. If it is allowed to warm up, friction burning can melt it. In the wake of a bit spinning too fast, fresh powder sticks to the melting cut. It subsequently hardens crumbly and bumpy. The scorch mark isn't always dark, so the reason isn't immediately obvious. The mystery is solved if the melting goes away when using a new bit at lower RPM.
- A friction burn mark can be made by any place on a bit that isn't a keen edge, such as the guide bearing.
- A flush trim bit has a round section near its end which can't cut, but mars laminate.
- A straight plunge bit cuts on the side, but has no edge on the end. It leaves a burnt chunk standing in the middle until you nudge it out sideways.
- A knife or scraper gets off superficial burns without so much fuss as sandpaper.
- It is easier to prevent burn marks than to painstakingly scrape them off.
Bit burn out
A bit's small size is a versatile attribute, but realistically its life span is limited even in normal use. Solid hardwood and manmade wood composites use up little bits much more quickly than a softwood such as pine. Diminutive bits can do a big project, but waste time. Continuous runtime has limits, so you'll probably need extra ones. If there's a choice of a big bit, do not select a smaller bit that may burn out early with the project incomplete.
Steel bits wore out so fast that carbide router bits all but replaced them. Routing MDF eats up bits. It generates heat and causes tip edges to erode. If you have a fresh spare, do not persevere with a stale, overloaded one that is deflecting and overheating. Burnout is greatly hastened by continuing to spin a tired one constantly without pausing.
If the cutter is not hot enough to be burning your fingers, it's okay. By the time you smell smoke, it has risen a few hundred degrees; that's not so great. If it is blue, that's bad. A bit with a fat stem runs cooler; its hefty mass can help manage the heat.