Runout, Tearout and Chatter

Three words for three different problems

These terms show up in reviews, manuals and woodworking forums. They are related because they all describe a rough or inaccurate cut, but they are different problems with different causes.

• Runout is a wobble in a spinning part. The blade, bit or shaft does not rotate in a perfectly true circle.
• Tearout is ripped or splintered material at the cut, where fibers lift out instead of being sliced cleanly.
• Chatter is vibration between the tool and the work, usually leaving a pattern of ripples, marks or a loud rattling noise.

One can lead to another. A wobbling blade can cause rough cuts, and vibration can encourage tearout. Sorting out which symptom you have is the first step.

Runout

What it is

Imagine a spinning saw blade viewed from the side. If it is perfectly true, it stays in one plane. With runout, the rim moves slightly side to side as it turns, or a bit or cutter spins a little off-center. Runout is usually small, often a tiny fraction of an inch, but small amounts matter at high speed.

What you might notice

• A cut that is wider than the blade’s normal width, or has a rough, uneven wall.
• Vibration, noise, or a blade that heats unevenly.
• Burn marks or visible lines on the cut.
• Router bits that leave a ring or an uneven profile.
• Drill bits that wander or drill an oversized hole.

Usual causes

• A bent or damaged blade or bit, for example after being dropped or pinched.
• Debris or pitch between the blade and the flange, or in the collet.
• A worn or damaged arbor, flange or bearing.
• A collet that is dirty, worn or not tightened correctly. A bit not fully inserted can also lead to wobble.
• Bent shank, common on an old or abused bit.

General fixes

Unplug the tool or remove the battery first. Clean the mating surfaces on the arbor, flange and collet. Reinstall the blade or bit following the manual, with the shank seated to the recommended depth. Replace anything that is visibly bent. If the problem remains with a good blade or bit, the tool itself may need service, and a periodic check as part of tool calibration is a good habit. Measuring runout accurately usually needs a dial indicator, so most users judge by cut quality and by checking the blade for visible wobble.

Tearout

What it is

Tearout is damage to the surface where fibers break away instead of being cut. It is most common on wood and wood products, where it shows as splinters, ragged edges or chips along the cut line. It often happens on the exit side of the cut, where there is nothing to support the fibers.

Usual causes

• A dull blade or bit. Dull edges push and tear instead of slicing.
• The wrong blade for the job. Different saw blade types suit different cuts. A blade with few, large teeth is built for fast, rough cuts and often leaves more tearout than a fine-toothed blade made for finish work.
• Cutting against the wood grain. Fibers that rise toward the cut are more likely to lift. Understanding the direction of wood grain helps you plan the cut.
• A fast feed rate. Pushing the work too quickly leaves less time for clean cutting.
• No backing support. Without a support at the exit, the fibers break out.
• Veneered or coated sheet goods. Thin surface layers are especially prone to chipping.

General fixes

• Use a sharp, clean blade with a tooth style suited to the material.
• Slow down the feed and let the tool do the cutting.
• Support the exit side, for example by clamping a scrap board behind the cut, or using a zero-clearance insert on a table saw.
• Score the cut line with a knife or make a shallow first pass.
• Apply masking tape along the line on delicate surfaces.
• With plywood, remember that different saws tend to chip different faces, depending on whether the teeth exit through the top or the bottom. The manual or a test cut on scrap shows which face to protect.

Chatter

What it is

Chatter is a repeated vibration between the cutter and the workpiece, leaving evenly spaced marks, ripples or a rough finish. It can sound like a buzzing or rattling. In metal drilling and machining, chatter can be loud and can chip cutting edges.

Usual causes

• Loose or flexible workholding, so the work vibrates.
• Too much tool stick-out or long overhang, for example a long bit extended from a collet.
• A dull or poorly matched cutter.
• Speed or feed that is not right for the material. Both excessive and insufficient feed can cause rubbing and vibration.
• Worn bearings, or runout, which can drive vibration.
• An unsupported or thin workpiece that acts like a drumhead.

General fixes

• Clamp the work firmly and support it close to the cut.
• Use the shortest, stiffest setup you can.
• Replace or sharpen dull cutters.
• Change speed or feed in small steps. When drilling metal, slower speeds with firm, steady pressure are often used.
• Check the tool for loose parts, damaged bearings or runout.

Telling them apart

Sometimes several overlap. Work through the likely causes one at a time, and change one variable between tests so you know what helped. Always wear eye protection and secure the work.