Surfacing and Spoilboard Levelling Bits: Which One, and How to Use It

Every CNC router needs a surfacing bit at some point: to level a new spoilboard, to re-true it after a few hundred jobs, to flatten a slab or a glued-up panel, or to skim a batch of parts to thickness. The questions I get asked most are which bit, what size, and why the result has ridges. Here is the short version.

What a surfacing bit actually is

A surfacing bit (also called a spoilboard cutter, bottom cleaning bit, fly cutter or slab flattening bit) is a wide, flat-bottomed cutter with a short cutting length. The job is to remove a thin layer across a large area quickly, so the diameter is large and the depth per pass is small. There are two families:

• Brazed carbide tipped (TCT), like the ARDEN surfacing bit and the small bottom cleaner. Cheapest to buy, sharpenable, good for occasional use.
• Replaceable insert, like the insert surface planer, the spoilboard leveller and the Australian made CARBiTOOL insert planer. When the edge dulls you rotate or swap the inserts and keep cutting. Better for anyone surfacing regularly or working abrasive material like MDF and epoxy.

Picking the diameter

Bigger is faster, but only if your machine can drive it. A rough guide:

• Makita RT0700 / 65mm spindles up to 1.5kW: 22mm to 40mm on a 6mm or 6.35mm shank. The small bottom cleaner or the 26mm spoilboard leveller.
• 2.2kW spindle, 12.7mm collet: 50mm to 80mm. The 52mm spoilboard leveller or a 55mm to 80mm insert planer is the sweet spot for most hobby and small commercial routers.
• Hand router in a sled: 50mm to 80mm, keep the RPM down (the TCT bits are rated to 12,000rpm) and take light passes.

Check the shank: a 12.7mm (1/2") shank bit needs a 12.7mm collet. If you only have a 6.35mm collet, stay with the smaller cutters rather than using a reducer on a wide cutter.

Feeds and speeds that work

Surfacing is a light cut over a big area, so the numbers are different from profiling:

• Depth per pass: 0.5mm to 1mm for a spoilboard clean-up, up to 2mm on timber with a stiff machine. Deeper is not faster if it stalls the spindle or chatters.
• Stepover: 40% to 60% of the cutter diameter. Going wider leaves ridges because the cutter is never perfectly perpendicular to the bed.
• RPM: as low as your spindle allows with good torque, typically 12,000 to 16,000 on a VFD spindle. Large diameters have a high tip speed; running them at 24,000rpm burns the edge and the timber.
• Feed: 2,000 to 4,000mm/min on MDF and softwood, slower on hardwood and epoxy. If the finish looks burnished rather than cut, feed faster or drop the RPM.

The full method for working out your own numbers is in the Feeds & Speeds Guide.

Why you are getting ridges

If you can see or feel lines after surfacing, it is almost never the bit. Nine times out of ten the spindle is not square (trammed) to the bed. A spindle leaning by a fraction of a degree means one side of the cutter sits lower than the other, so each pass leaves a shallow step. Fixes, in order:

1. Tram the spindle. Put a dial indicator in the collet, sweep a circle on the bed and shim the spindle mount until the readings match. Even a piece of paper under one side of the mount makes a difference.
2. Reduce the stepover to 40%. It hides small tram errors.
3. Check the inserts are seated and all at the same height. One high insert does all the cutting and leaves a track.
4. Surface in one direction (climb or conventional, not alternating) for the final pass on visible timber.

Spoilboard tips

• Surface a new MDF spoilboard before the first job, then re-surface it whenever the cut-through marks get deep enough to affect hold-down or vacuum. A 0.5mm skim is plenty.
• Reset Z to the new surface after every skim. It sounds obvious, and it is still the most common cause of a cutter ploughing into the bed.
• If you use a vacuum table, seal the edges of the MDF with a thinned PVA coat after surfacing so the vacuum pulls through the face rather than out the sides.

Which bit should I buy?

• Occasional spoilboard skims on a small machine: 22mm or 30mm bottom cleaner.
• Regular use on a 2.2kW machine: 52mm spoilboard leveller or the insert surface planer.
• Slabs, end grain boards and epoxy: CARBiTOOL insert planer with a spare set of inserts.
• Aluminium plate on a router or mill: the insert aluminium surfacing bit, which has the right rake for metal.

Still not sure? Call me on 0448 431 833 with your spindle size and what you are flattening and I will point you at the right one.