G-Code Simulation and Verification Before You Cut: CAMotics, NC Viewer, and Backplotting

Our CAM software guide covers generating toolpaths in Fusion 360, VCarve, and Carbide Create, and our workholding guide covers keeping stock in place once you hit cycle start. There's a step in between that a lot of hobbyists skip entirely: actually looking at the G-code before it touches material. A CAM program that looks correct in the toolpath preview can still generate a rapid move through a clamp, a Z value that's off by the thickness of your spoilboard, or a tool change that never got assigned a real tool number. On a desktop router like the Wolfpawn 4040 Pro, that mistake costs you a bit, a spoilboard, or a trip to the ER — not a machine crash alarm and an automatic stop like you'd get on industrial equipment. Simulating and backplotting the file first is the cheapest insurance in the whole CNC workflow, and it costs nothing but a few minutes.

What Simulation Actually Catches

ProblemHow Simulation Catches It Wrong work offset / originToolpath preview shows the cut happening off the stock entirely, or mirrored/rotated from what you expected Z depths referencing the wrong surface3D solid simulation shows the tool plunging into the spoilboard or stopping short of full depth Rapid moves crossing clamps or fixturesBackplotting the rapid (G0) moves in a top-down view reveals travel paths that cross your workholding Missing or wrong tool numbersTool change simulation flags an undefined tool or a diameter mismatch against your CAM setup Feed rate typos (e.g. a stray G0 where you meant G1)Time estimate is wildly off from what the CAM software predicted, or the sim shows a rapid-speed "cut" move Post-processor errorsLine-by-line G-code view shows malformed blocks, duplicate line numbers, or missing M-codes the post-processor should have inserted

CAMotics: Free, Solid-Body Simulation

CAMotics is open-source and does full material-removal simulation — it renders your stock as a solid, cuts it exactly the way the machine would, and lets you inspect the finished part for gouges, missed material, and depth errors before you ever load the file onto the controller.

Set up your stock — enter the actual dimensions of your workpiece, not just the CAM software's default bounding box, so the simulation reflects your real setup.
Load the G-code — CAMotics reads standard RS-274/NGC G-code directly; most GRBL-flavored output from Fusion 360, VCarve, and Carbide Create works without modification.
Run and scrub the timeline — step through the cut a move at a time rather than just watching the final result, especially for the first few operations where a bad rapid does the most damage.
Check the "resultant" view — CAMotics highlights areas where the tool cut deeper than intended (red/over-cut) versus areas that were missed, which is the single most useful thing it does that a simple line-preview tool doesn't.

NC Viewer: Fast Web-Based Backplotting

NC Viewer is a free, no-install, browser-based G-code viewer — you paste or drag in a file and get an instant 3D toolpath trace. It doesn't do material removal simulation the way CAMotics does, so it won't catch a gouge, but it's faster to reach for and genuinely useful for two things: sanity-checking the overall shape and travel before you export from CAM, and eyeballing a G-code file someone else sent you (a downloaded sign file, a shared nesting layout) without opening your CAM software at all. It supports GRBL, Marlin, and most common dialects out of the box.

Reading What You're Looking At

Line Color / Move TypeWhat It Means Rapid moves (G0)Travel at full speed, tool should be retracted above the material — any rapid line that dips into the stock outline is a red flag Feed moves (G1/G2/G3)Actual cutting moves at your programmed feed rate — this is where you check depth and shape against the design Tool change markersConfirms the sequence and numbers match your physical tool list — mismatched numbers are one of the most common causes of a ruined part on a manual tool-change machine Bounding box / travel limitsCompare against your machine's actual work area — a toolpath that exceeds the Wolfpawn 4040 Pro's travel will either soft-limit fault or, if soft limits aren't configured, crash into the frame

A Practical Pre-Cut Checklist

Simulate in CAMotics (or at minimum backplot in NC Viewer) after every CAM export, not just for new designs — a re-exported file after a small edit can still pick up a stale operation or wrong stock setting.
Confirm the stock dimensions in the simulator match your actual material, including thickness — a common source of "it cut through the spoilboard" is a stock model that's thinner than the real board.
Step through the first three operations manually rather than letting the animation run — this is where origin and depth errors show up first.
Cross-check tool numbers and diameters in the simulation against your physical tool list and the numbers loaded in your controller's configuration.
For jobs with an X/Y home you're not 100% sure of, run the job once with the spindle/router powered off and the bit retracted well above the stock — a physical "air cut" catches origin mistakes that even a good simulation can miss if the stock model itself was wrong.

Limitations to Keep in Mind

Simulation is not a substitute for good work zero setup or a final visual check before hitting cycle start. It can't catch a physically loose clamp, a bit that isn't actually seated to the length you told the CAM software, or a stock board that's warped. It also won't catch errors introduced after simulation — always simulate the exact file you're about to run, not an earlier version, and re-simulate after any manual G-code edits. Treat it as one layer in a stack of checks that also includes physically walking through the toolpath by eye and confirming your work offset on the machine itself, not as a green light that replaces those checks.

Adding a five-minute simulation pass to your workflow is one of the highest-value habits a desktop CNC owner can build. It catches the mistakes that are cheap to fix on a screen and expensive to fix in aluminum, hardwood, or a bent bit — and unlike most quality-of-life upgrades in this hobby, it costs nothing but the time to open the file.

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