Leveling CNC waste board surfaces means making the top parallel to the plane your spindle travels in, not level to gravity. Three steps, in order, get you there: seat the machine on an untwisted base, tram the router square to its travel, then let the machine surface the board at 1 mm (0.04”) or less per pass. Shims and a bubble level can’t do it on their own.
The bit never consults a bubble. Z-zero is a distance from the gantry, and every pocket depth assumes the stock sits at that same distance everywhere on the table. Sienci Labs puts “flattening” in quotes in its wasteboard surfacing guide for this reason: the goal is “a surface that’s parallel to the CNC.” A machine on a sloped bench cuts fine. A machine on a twisted bench does not, and shimming the MDF won’t fix that.
What does “level” mean for a CNC waste board?
A level waste board is one whose top face stays the same distance below the spindle wherever the gantry moves, so a single Z-zero holds across the whole bed. The reference is the machine’s own rails, and the only tool that can transfer that reference to the board is the machine itself, running a cutter across it.
Three separate faults can each look like “out of level,” and each one needs a different fix:
- The base is twisted. The rails are not coplanar, so the gantry rocks as it travels and the motion “plane” is a shallow warp. Fix: shim the frame, not the board.
- The spindle is out of tram. The cutter is not perpendicular to travel, so every pass leaves a stepped ridge. Fix: adjust the router mount.
- The board is not parallel to travel. The MDF is cupped, thicker in one corner, or sitting on an uneven support. Fix: surface it.
Work through them in that order. If you surface a board on a twisted machine, the MDF takes on the twist. The board then reads true to the machine, but any flat sheet you lay on it will rock or get clamped into a curve.
Step 1: Seat the frame on a flat, untwisted base
Sienci’s table mounting guide says the LongMill “is designed to be mounted to a flat surface which is provided by the user” and recommends 3/4” MDF as both mounting surface and waste board. Every hobby gantry machine has the same dependency: the frame isn’t stiff enough to ignore what it is bolted to.
The tabletop doesn’t need to be perfect, because surfacing absorbs small errors in the board. The four points where the feet land do need to be coplanar. In a Onefinity forum thread on dead-level tables, experienced owners say to check those four corners and, if one is low, “shim one of the machine corners up a bit” rather than chasing the whole top. The same thread suggests a quick check: lay a 6-foot level across the top and slide note paper under it to find gaps.
Two checks before moving on:
- Twist. Stretch two strings (fishing line works) diagonally between the four corners of the frame. They should just touch where they cross. Swap which string is on top and look again. If there’s a gap one way and a hard press the other, the base is in wind. Shim the low corner until both crossings just touch.
- Square. Sienci’s quick test: tape a pencil to the router mount, jog a large L or box, and check it with a carpenter’s square or a 3-4-5 triangle. Squareness doesn’t change Z, but it’s easier to fix before the board is surfaced and screwed down.
If you bolt LongMill feet down, Sienci warns that countersunk wood screws will crack them. Use the included #8 particle board screws.
Step 2: Tram the spindle to the machine’s travel
Tramming squares the cutter’s axis to X-Y travel. Do it before the full surfacing job, because a tilted cutter leaves ridges however flat the board is. Sienci’s tramming procedure for the LongMill and AltMill holds a dial indicator in the collet, rests the tip on the table, and swings it 180° from side to side at a constant radius. Equal readings on both sides mean that axis is square.
The catch is that the indicator reads against the board, and the board isn’t parallel to travel yet. Sienci’s answer is to surface first. Carbide 3D community regulars describe the full sequence as surface, tram, surface, because at the start the board “may not be in the plane that the spindle moves in.” To save MDF, surface a 6” x 6” patch in the middle of the table, tram against that patch, then surface the whole board once.
Sienci breaks the LongMill correction into two motions:
- Tilt (side to side): loosen the eight small screws on the gantry plate behind the router, rock the mount until the indicator reads the same on both sides, retighten.
- Nod (front to back): loosen the four screws holding the mount to the gantry plate and add shim stock or feeler gauges. Shim on top of the screws behind the mount to tilt the router forward, under them to tilt it back.
Inventables’ X-Carve calibration guide does the same job with a large square. Check the Z-axis against the table and adjust the four button-head screws behind the X-carriage. Then stand the square on the waste board against the wide Makerslide: “There should be no gap.”
How much effort the tram deserves depends on cutter width, because the error grows in proportion to distance. A 0.004” difference across a 6” sweep (3” arm) is about 0.0007” across a 1” surfacing cutter and about 0.0017” across a 2-1/2” insert head. The wider the cutter, the tighter the tram has to be before the ridges disappear.
Step 3: Surface the board to the machine’s plane
Surfacing is the step that actually levels the board. It cuts a shallow pocket over the whole reachable table, and the top ends up wherever the bottom of the cutter traveled. Bit choice, toolpath boundaries and Z-zero handling are covered in our guide to surfacing a CNC spoilboard. Two published starting points:
| Setup | Cutter | RPM | Feed | Depth per pass | Stepover |
|---|---|---|---|---|---|
| LongMill MK2, Makita trim router | 22 mm surfacing bit | about 18,000 (dial 3 to 4) | 2000 mm/min (80 ipm) | 1 mm or less | about 40% |
| Shapeoko, Carbide 3D McFly | 1” (25.4 mm), 1/4” shank | 16,000 | 60 in/min | 0.02” | not published |
The LongMill row comes from Sienci’s surfacing guide and the Shapeoko row from Carbide 3D’s McFly product page. Sienci warns that “cutting any deeper with a large bit can result in an uneven surface,” so a heavy first pass saves no time. Inventables’ X-Carve flattening job cuts about 0.01” deep, which makes a handy benchmark: if a 0.01” skim cleans the entire board, the board was already within 0.01” of the travel plane.
What decides whether the pass works:
- Scribble first. Hatch the whole board in pencil. Graphite left after a pass marks a low spot. Step down another increment and repeat until it’s gone.
- Hold one Z-zero. Zero on the board top once and program every pass from it. Re-zeroing between passes defeats the point.
- Sink every screw head below the deepest planned pass. Carbide inserts chip on steel.
- Oversize where the cutter has clearance. Sienci suggests oversizing the cut so the edges don’t leave a lip. Before running it, check the toolpath extents against your travel limits and fixtures.
- Keep stepover under 50%. How stepover and cut direction interact decides whether the finish comes out scalloped or smooth.
No surfacing cutter? A 1/4” end mill works, only slower. Set its feed with the chip load calculator.
How do you verify the waste board is level?
A level board passes three checks: no pencil left after the final pass, no ridge between passes that catches a fingernail, and dial-indicator readings at the four corners and center within a few thousandths of each other. If it passes all three, the board is parallel to travel.
Take the indicator readings with the spindle stopped. Zero at the center of the surfaced area, raise, jog to each corner, lower to the same Z and read. Don’t re-zero between points.
If a check fails, the pattern points to the step you need to redo:
| Symptom | Likely cause | Fix |
|---|---|---|
| Stepped ridges running parallel to the raster | Tram error across the raster direction | Adjust nod (X raster) or tilt (Y raster), resurface |
| Surface looks clean, but flat stock rocks or pocket depth varies | Base twist copied into the board | Shim the low corner, resurface |
| Carves deeper on one side | Carriage rotated (on X-Carve, wide Makerslide tilted forward, per Inventables) | Re-square the carriage |
| Board was true, now isn’t | MDF moved with humidity | Light skim pass |
The ridges follow from simple geometry. Run a raster left to right with the cutter tilted front to back, and one edge of the cutter sits lower than the other. Each pass leaves a sloped strip, and the next pass starts a step lower, so the floor comes out as a sawtooth. Tilt along the direction of travel only leaves a shallow dish inside each pass. Ridges therefore mostly show tilt across the raster. To read the other axis, cut a second test patch rastered at 90°.
If you trammed against an unsurfaced board, check the tram again after the full surfacing job. Otherwise the tram may still be compensating for a tilt the cutter has since removed.
FAQ
how much should i take off when surfacing a cnc waste board
Take off the least that cleans up the whole board, usually between 0.01” and 1 mm. Inventables’ X-Carve flattening job skims about 0.01”, Carbide 3D lists 0.02” per pass for its McFly cutter, and Sienci recommends 1 mm or less per pass because deeper cuts with a large bit leave an uneven surface.
can you level a cnc spoilboard without a surfacing bit
Yes, any flat-bottomed end mill will level a spoilboard, and a smaller one just needs more passes. Sienci notes a waste board can be flattened with a bit as small as 1/4”. At the same 40% stepover, a 1/4” tool needs about four times as many passes as a 1” cutter, so budget the time.
what thickness mdf should i use for a cnc waste board
Use 3/4” MDF, which Sienci recommends for the LongMill as both the mounting surface and the waste board and describes as “quite stiff.” That thickness leaves room for many skim passes before screw heads or T-track get near the cutter. Replace the sheet once mounting hardware limits how deep a surfacing pass can go.
how often should you resurface a cnc waste board
Resurface whenever cut depth starts varying across the table, not on a calendar. Inventables notes MDF can warp with environmental changes and use, and a Onefinity forum regular warns you might resurface with the seasons. If a 0.01” skim cleans it up, nothing structural moved. If it takes several passes, recheck the base for twist.
