Building a CNC-Flattened Epoxy River Table

A river table — two live-edge slabs joined by a poured band of tinted epoxy resin — is one of the most rewarding crossover projects between woodworking and CNC work, and it's also one of the few wood projects where a desktop CNC router genuinely outperforms hand tools. Flattening a large, uneven slab-and-epoxy assembly by hand with a router sled is slow and unforgiving of mistakes; a CNC router with a surfacing bit and a large-area flattening toolpath does the same job faster, flatter, and repeatably. This project covers the full build on a machine like the Wolfpawn 4040 Pro: slab prep, the epoxy pour itself, and CNC surfacing top and bottom to a dead-flat, consistent thickness.

Slab Selection and Prep

Choose slabs that are properly dried — kiln-dried or air-dried to a stable moisture content (roughly 8-12% depending on climate) is essential; wet or partially dried slabs will continue moving after the epoxy cures and can crack the resin pour along the wood-epoxy boundary.
Seal live edges and any checks/cracks with a thin coat of epoxy or a dedicated wood sealer before the main pour — unsealed end grain and cracks will wick resin and produce bubbles and voids that are hard to fix after the fact.
Plan the river width and shape before cutting or arranging slabs — a consistent 2-4" river gives predictable pour volume and a strong bond area; very narrow or irregular gaps are harder to pour cleanly and more prone to trapped air.

Building the Pour Mold

Build a mold frame from melamine-faced MDF or plywood, sized to the slab footprint plus a small margin, with the base sealed and caulked at every seam — epoxy will find and exploit any gap.
Apply mold release or line the mold with packing tape so the cured piece releases cleanly.
Position and clamp the slabs in the mold at the planned river gap, using spacers or blocks to hold position, and dry-fit before mixing any resin.
Dam the ends of the river channel where needed so poured resin doesn't escape past the slab ends.

Mixing and Pouring Epoxy

StepNotes Choose deep-pour vs. casting resinStandard casting epoxy is limited to shallow pours (often under 1/2" per pour) before excessive exotherm cracks or clouds the cure; deep-pour formulations are designed for thick single pours and are the right choice for most river table gaps Mix ratio and volumeFollow the manufacturer's ratio exactly by weight, not volume, using a gram scale — an off-ratio mix can stay tacky indefinitely or cure unevenly TintAdd pigment or dye to a small test batch first to confirm color before tinting the full pour — resin dyes are concentrated and easy to overshoot Pour in stages for thick riversEven with deep-pour resin, very thick sections (over ~1.5-2") often go better as two pours a day apart to manage heat buildup and reduce cracking risk Torch or heat-gun the surfaceLightly pass a torch or heat gun over the poured surface to pop surface bubbles — don't overdo it, excess heat can yellow or cloud some resins

CNC Surfacing

Once the epoxy is fully cured (check the manufacturer's full-cure time, not just the tack-free time — surfacing too early risks gumming the bit and dragging soft resin) the slab is ready for the CNC. This is the step a router sled struggles with and a CNC router does cleanly:

Mount the slab securely to the CNC bed — screws through waste areas outside the final part outline, or a vacuum table if the slab and bed size allow, since any movement during a surfacing pass ruins the flatness the whole operation is trying to achieve.
Probe or shim to find the slab's actual high points before committing to a Z-height for the first pass — set the initial cut depth to just clear the highest point on the surface, since a large flattening pass is one of the few operations where taking too aggressive a first cut can stall the spindle or overload the machine.
Use a wide surfacing bit (a large-diameter flycutter or insert-style surfacing bit) with a generous stepover for the flattening toolpath — a small single-flute end mill will technically work but takes drastically longer and leaves more visible tool marks.
Run the flattening toolpath at a conservative feed rate for epoxy sections specifically — resin generates more heat under the cutter than wood and can gum or melt slightly if fed too aggressively, which both dulls the bit faster and leaves a poor surface finish over the resin.
Flip and repeat on the underside once the top is flat, referencing off the now-flat top surface to dial in a consistent final thickness across the whole piece.
Finish sand progressively after CNC surfacing — start around 120-150 grit to remove the surfacing toolpath's scallop marks, and work up through progressively finer grits (typically to 400-600+) before finishing, since epoxy in particular needs a fine sanding progression to reach a clear, glassy final look rather than a hazy one.

Finishing

Wood sections take a standard wood finish — oil, hardwax oil, or a wipe-on polyurethane all work well and are covered in this site's wood finishing guide. The epoxy sections generally don't need a separate finish coat if sanded through a fine enough progression and polished, though a final buffing compound pass brings up more clarity and shine on the resin specifically. Round over or ease all edges, including the transition between wood and resin, since a sharp epoxy edge chips more easily than the wood does.

Safety

Epoxy resin and hardener are sensitizers — repeated skin contact can trigger an allergic reaction that gets worse with continued exposure, so wear nitrile gloves for all mixing and pouring, and work in a ventilated space since uncured resin gives off fumes during the pour and initial cure. Sanding cured epoxy produces a fine dust that should be treated the same as any fine particulate — a properly rated dust mask or respirator and, ideally, dust collection at the sander. On the CNC side, follow the same workholding and E-stop practices covered in this site's CNC router safety guide — a large, heavy slab under a spinning surfacing bit is exactly the kind of workpiece where a workholding failure has serious consequences.

A river table is a genuinely large undertaking — slab sourcing and drying alone can take weeks if starting from green wood — but the CNC surfacing step is where a shop with a router like the Wolfpawn 4040 Pro has a real advantage over a purely hand-tool build: a flat, consistent-thickness result without the risk of a hand-guided sled wandering partway through a large pass.

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