Thwart router-bit tear-out
Tear-out -- when a router bit rips chunks of wood from your workpiece instead of cleanly shearing it -- happens more often in porous-grain wood species, such as red oak, ash, and hickory. But even tight-grained woods (maple, cherry, and walnut) can fall victim. A few simple precautions reduce or eliminate tear-out with any type of wood:
- Keep your router bits clean and sharp. A bit needs attention if it's consistently burning or tearing out wood.
- Choose the appropriate router speed. (See the chart at right.) Remember that small diameter bits (under 1") perform best at the top speeds, while large-diameter bits, such as panel-raisers, need the slowest speeds.
- Select wood with straight grain that runs the length of your project parts, especially for door frames. Wavy or figured grain or grain that runs diagonally across a workpiece's width proves more prone to tear-out.
- Feed tear-out-prone stock at a rate slower than you normally would -- even if it burns the wood slightly. Then make a light final pass removing about 1/32" at a normal feed rate.
- Resist the temptation to take deep cuts. Instead, rout in 1/8"-deep increments.
Like a zero-clearance tablesaw throat insert, a zero-clearance auxiliary fence for your router table supports wood fibers next to those being cut away. And it's easy to make one. We recommend 1/2"-thick medium-density fiberboard (MDF) for auxiliary fences because it's inexpensive, flat, smooth, and easy to work with.
Begin by cutting out a blank about as long as your router table's fence. The blank height need only be taller than your bit by an inch or so, unless you're routing a large workpiece on edge and it needs the added support of a taller auxiliary fence.
With your bit installed in the router and set to the height of your final pass -- you'll need to make test cuts in scrap before adding the auxiliary fence -- hold the MDF blank against your fence and centered on the bit. For bits with a bearing, mark the width and height of the bearing (and its mounting screw) on your MDF blank. Saw out a relief slot for the bearing. (Remember, you only need zero-clearance protection where the cutters meet the workpiece on the infeed side.)
Now, clamp the MDF blank to your fence. Position the fence behind the bit as close as possible without touching the bit, and secure one end. Turn the router on and slowly pivot the fence into the bit, as shown top right. Turn off the router and align the fence to the bit, as shown middle right.
Feather boards, or similar accessories, help to eliminate tear-out by holding stock tightly against the fence and table, as shown bottom right. Without these, accidental lifting or jerking could cause the bit to tear away chunks from the profile.