Routing Bit Selection: 5 Rules for Clean Depaneling

A routing bit is the carbide cutter that separates boards from a panel along a programmed path. It removes material rather than fracturing it, so the edge it leaves is machined and the stress it puts into the assembly is low. The trade is speed and tool cost, and both depend on choosing the bit geometry and running it inside the window the material allows.

This guide covers five rules for routing bit selection and use, the wear measures that decide when a bit should be changed, and the burr limits that show the process has drifted. It applies to CNC depaneling routers on FR-4, high-frequency laminates and metal-backed boards.

What the Routing Bit Has to Cut

A depaneling cut passes through solder mask, copper, glass fibre and sometimes metal, and each of those materials behaves differently. Copper smears and work-hardens under a dull edge, glass fibre abrades the cutting edge, and a filled laminate can chip if the cut is too aggressive. The bit has to survive all of them in one pass.

The cut is also shallow relative to its length. A typical depaneling path is a few millimetres deep in a board a millimetre or two thick, with the bit entering from one side and cutting a slot wider than the board thickness. Chip clearance in that narrow slot is the limiting factor for feed rate.

<img src="https://www.gopcba.com/wp-content/uploads/2021/05/section-3.jpg" alt="Routed PCB edge inspected for depaneling burrs after a bit change” />

Rule 1: Match Routing Bit Geometry to Material

Two flute end mills with a diamond-cut or fish-tail geometry are the standard choice for FR-4, because they clear chips well and leave a clean wall. Bits with more flutes produce a finer finish at a lower feed rate, which suits a visible edge on a consumer product rather than a hidden rail.

Coating matters more than flute count where abrasive filler is present. A diamond-coated bit can outlast an uncoated one by a large multiple on a ceramic-filled laminate, which changes the cost per panel even though the purchase price is several times higher. The comparison should be made in cut length, not in bit price.

Rule 2: Set Spindle Speed and Feed Rate Together

Spindle speed and feed rate define the chip load, which is the thickness of material each cutting edge removes per revolution. Too small a chip load polishes the edge and generates heat, and too large a chip load chips the laminate and bends the bit. The window between them narrows as the routing bit diameter falls, which is why small cutters need a different program rather than a scaled one.

A useful starting point for a 2 mm bit in FR-4 is a spindle speed near 30,000 rpm with a feed rate that gives a chip load around 0.02 mm per tooth. The value should be adjusted from the sound of the cut and the appearance of the chips, which should be powder rather than dust or slivers.

Rule 3: Control Cut Depth and Pass Count

Depth per pass should be kept below about half the bit diameter. A deeper cut loads the flutes and pushes the routing bit sideways, and the resulting wall is tapered and rough. Two or three passes at a controlled depth give a straighter edge than one heavy pass, even though the path is longer.

The final pass should be a light one, because it sets the finish. Shops that run a single full-depth pass often compensate with a slower feed rate, which increases the heat in the cut and shortens bit life. Our notes on depaneling methods compare this behaviour with other cutting processes.

Rule 4: Manage Routing Bit Wear by Cut Length

Bit wear shows first as a change in edge appearance and later as a rise in the cutting force, which the machine reports as a current or a following error. The reliable measure is cut length, recorded in metres of path, with a change point set from the point at which the first bad edge appeared.

Recording bit wear by time in service is unreliable, because a bit running a short path in a thin board wears far more slowly than the same bit cutting a thick panel. Where several products share a router, the log should be in metres and the change point should be per material family rather than plant-wide.

Rule 5: Hold the Panel and Support the Cut

The panel has to be held flat and supported close to the cut. A board that lifts or vibrates at the cutting line produces a rough wall, a broken routing bit, or both. Vacuum holding plus a support pin pattern that follows the path is the usual arrangement on a depaneling router.

Support distance matters. Where the nearest support is far from the cut, the board deflects under the cutting force and the bit rubs rather than cuts, which glazes the edge and dulls the tool. A support within about 15 mm of the path is a workable target for a board of ordinary thickness.

Depaneling Burrs and Edge Finish

A routed edge is judged by burr height, wall straightness and the amount of loose fibre. A routing bit that has begun to dull leaves a brighter, smeared wall before the burr height changes, so the wall appearance is the earlier signal. Burrs come from copper smearing and from a dull bit, and they are worse on the exit side of the cut. The acceptance limit should be stated in the drawing notes and checked on the first article.

Where a burr limit cannot be met with a single pass, the answer is usually a change of bit geometry or a slower final pass rather than a manual deburr, because hand deburring is inconsistent and can lift solder mask. Our notes on edge quality describe the measurement.

Records, Spares and Change Points

Every routing bit should carry an identity so that the edge quality can be traced back to a specific tool and change point. A simple log with the bit number, the material, the cut length and the observed edge condition is enough, provided it is filled in at each change rather than at the end of the week.

Spares should be held at the change point rather than at zero stock, because a dull bit that is kept in service costs more in rework than the replacement costs to buy. The hole quality that follows a good cutting practice is covered in our guide to drill quality, and the general requirements for separable assemblies are published by IPC.

FAQ

How long should a routing bit last? Between about 20 and 200 metres of cut depending on material and coating. The change point should be set from the shop’s own edge quality data rather than from a supplier figure.

Can a dull routing bit be resharpened? Only on a machine that restores the original geometry. A hand-sharpened bit loses its diameter and its rake, so the chip load changes and the edge quality falls even though the tool cuts again.

Does coolant help on a depaneling router? It controls dust and heat on thick boards but adds a cleaning step. Most shops run dry with extraction, and add coolant only where the material loads the flutes.