CNC G-Code Simulator

The CNC G-Code Simulator is an interactive tool for writing, visualizing, and animating CNC programs. G-code is the standard programming language that controls Computer Numerical Control machines, instructing them where to move, how fast to cut, and what operations to perform. This simulator lets you write G-code in a syntax-highlighted editor and instantly see the resulting tool path rendered on a virtual workpiece in 2D or 3D isometric view.

The simulator supports all fundamental motion commands: G00 (rapid positioning), G01 (linear interpolation at controlled feed rate), G02 (clockwise circular arc), and G03 (counter-clockwise circular arc). It also recognizes coordinate modes (G90 absolute, G91 incremental), unit selection (G20 inch, G21 metric), and M-codes for spindle and program control. 30 example programs are included — 17 for milling, ranging from basic squares to bolt hole circles, face milling patterns, contour chamfers, 5-pointed stars, circular pockets and multi-pass depth programs, and 13 for turning with Z-axis colour visualization.

The simulator opens in Simulate mode with a split layout: the syntax-highlighted G-code editor on the left and the tool path canvas on the right. The Control Panel is in two rows: the top row sets what you are machining — a Milling / Turning switch and the example program — plus an SI / Imperial switch. The row below is what to do with it: Run, Step, Reset, Clear, then the two reference tools — S&F Calc and Cheat Sheet — and two menus. View holds the display options (isometric 3D, material removal, animated cutter, cut-direction arrows) and remembers them between visits. Material removal replaces the tool path with the metal itself and cuts it away as the program runs — on a mill a strict plan view of the plate in its vice, on a lathe a side elevation of the bar in its chuck — with the blank size, the volume removed and the cutter it was removed with reported on the canvas. Tick Isometric 3D as well and the plate becomes a solid block you can see the depth of — the plan view shows the shape of a pocket and can only imply how deep it is. The two tool-path options grey out while material removal is on, because they would do nothing; Program holds save, open, share, import and the three exports. Below the canvas, a speed slider sets how fast the program is played back (1× to 10×, where 1× is three times real machining speed). The readout row shows current X/Y coordinates, feed rate, spindle RPM, program line, and total path length. A yellow-blinking guide highlights the next action button.

Switching machine keeps your work. Milling and Turning each load their own first example, which used to throw away a program you had typed. If the editor holds anything other than an untouched example, the switch now asks first — and Cancel is the useful answer: it changes the dialect and leaves your code exactly where it was, so you can read the same program as a lathe would.

To run your first program: (1) Select an example from the dropdown (e.g., Square Profile). The editor fills with syntax-highlighted G-code. (2) Click "Run" to play the program through. The tool travels along each move in proportion to the time that move really takes — a 200 mm cut takes a hundred times longer than a 2 mm one, a plunge at F40 visibly crawls where the profile at F200 sweeps past, and rapids flick across at traverse speed. Watching the run is therefore watching the cycle-time estimate being spent, and the Path Length readout climbs with the tool rather than jumping a whole move at a time. Rapid moves are dashed, cutting moves solid and coloured by depth. Press Run again to pause: the machine stops at the last completed block, so Step carries on cleanly from there. (3) Use "Step" to execute one line at a time, watching the current line highlight in the editor and the tool advance on the canvas. (4) After completion, a Toolpath Statistics panel shows cut distance, rapid distance, total moves, estimated cut time, and bounding box. (5) Click "Reset" to return to the start, or "Clear" to empty the editor and write your own code. Hover over the canvas to see real-time X/Y machine coordinates.

The editor features full syntax highlighting: G-codes appear in green, M-codes in orange, coordinates (X/Y/Z/I/J/R/Q) in blue, S/F/T values in purple, and comments in gray. A line indicator shows which line is currently executing, with a yellow highlight bar on the active line. The canvas renders the complete tool path with distinct colours: rapid moves (G00) as dashed gray, linear cuts (G01) in green, clockwise arcs (G02) in blue, counter-clockwise arcs (G03) in red, and drilled holes as circles with crosshairs, coloured by cycle — violet for G81, purple for G83/G73, amber for a G84 tap.

Canned Cycles (G81/G83/G73/G84): Type a drilling canned cycle to visualise hole patterns. G81 drills at each X/Y position; G83 peck-drills in increments (Q value). Once a canned cycle is active, each subsequent line with only X/Y coordinates repeats the drill at that new position. Use G80 to cancel. The Toolpath Statistics panel counts drill holes separately.

Tool Diameter (⌀): Set a tool diameter with the −/+ stepper in the speed row — hold either button to run through the range — to show a translucent cut-width band along every cutting move. The box is in whichever unit system is selected, and the badge beside it says which. This helps visualise material removal width at a glance. Set to 0 to hide. Keyboard shortcuts: F5 = Run/Pause, F8 = Reset, F10 = Step.

3D View: View → Isometric 3D view gives a true isometric projection: the stock is drawn as a slab with thickness, every move is placed at its real Z, so depth passes step down into the material and a plunge is a vertical line. In this view the coordinate tooltip reports the point on the stock's top face under the cursor. Animated Cutter: View → Animated cutter shows a spinning cutter with a feed direction arrow. Use Ctrl+Scroll to zoom, drag to pan, or use the +/−/reset buttons. On a phone a single finger scrolls the page; zoom in first and the same finger then pans the view. The Cheat Sheet button opens a quick-reference overlay with all G-codes, M-codes, canned cycles, and parameters — and it follows the machine switch. In Milling it says G90 is absolute positioning; in Turning it says G90 is the turning cycle, lists U/W instead of G91, and adds the G71/G70/G76/G96 group. One sheet could not be right for both dialects, so there are two.

Click the Explore tab to access a reference library organized into three categories: G-Codes, M-Codes, and Programming Concepts. Each category presents a grid of selectable concept cards. Click any card to view a detailed explanation with the command's syntax, a code example and a worked problem.

G-Code topics cover G00 (rapid), G01 (linear cut), G02/G03 (arcs with I/J center offsets), G90/G91 (absolute/incremental), G20/G21 (units), and G28 (home return). M-Code topics explain M03/M04/M05 (spindle), M08/M09 (coolant), M00 (program stop), and M30 (program end). Programming concepts cover program structure, block format, coordinate systems, cutter compensation, and tool change sequences. This is your quick-reference guide for G-code syntax. (For the Speeds & Feeds calculator — cutting speed, RPM, chip load and feed for a real cutter in a real material — see section 7.)

Practice mode tests your G-code knowledge with exercises. You might be asked to identify the correct G-code for a specific motion, determine the endpoint of an arc command, or write the coordinates to produce a given tool path. Enter your answer and check it. Solutions explain the reasoning behind each answer.

Quiz mode presents five questions per session covering G-code commands, M-codes, arc programming, coordinate systems, and program structure. Questions range from basic identification (what does G02 do?) to practical application (write the command to cut a clockwise semicircle). Review your score and retake to master CNC programming fundamentals.

• Start with the Square Profile example to understand basic G00 and G01 commands, then progress to Circle, Oblong Slot, and Complex Contour for arc programming. Try advanced examples like Bolt Hole Circle (G02 arcs in a pattern) and Circular Pocket (concentric cuts).
• Use Step mode to execute one line at a time, watching how each command moves the tool. This is the best way to learn arc commands (G02/G03). The editor highlights the active line.
• For arc commands, remember: I and J are the offsets from the current position to the arc center, not the absolute center coordinates. Open the Cheat Sheet for a quick-reference table.
• Always start programs with a safety line: G90 G21 G17 (absolute, metric, XY plane), then M03 S3000 (spindle on), and end with M05 M30 (spindle off, program end).
• View → Isometric 3D view makes spatial relationships in the tool path readable; View → Animated cutter shows spindle rotation and feed direction during the run.
• If a program looks right but the part comes out untouched, turn on View → Material removal. A contour with no Z word never leaves the top face — the tool path looks perfect and no metal moves, and the panel will say so rather than leaving you guessing.
• Hover over the canvas to see real-time X/Y machine coordinates. Use Ctrl+Scroll to zoom in on details, and drag to pan around.
• After running a program, check the Toolpath Statistics panel for cut distance, rapid distance, estimated machining time, and bounding box dimensions.
• Try writing your own programs after studying the examples. Start simple (a triangle or rectangle) and add arcs as you gain confidence. Use the syntax highlighting colours to verify your code structure.

Keyboard Shortcuts: F5 = Run/Pause, F8 = Reset, F10 = Step one line. These work whenever the editor or canvas is in focus.

Exports live under Program: G-code downloads the editor content as a .nc file, toolpath CSV is a spreadsheet-ready file including arc centre/radius data and drill hole depths, and image saves the canvas as a watermarked PNG.

Right-click Menu: Right-click on the canvas for quick access to Save PNG, Export CSV, Export G-Code, or Reset. Fullscreen Mode: Click the yellow fullscreen button (bottom-right of canvas) to enter an immersive workspace. Press Escape or click the close button to exit.

Z-Depth Colour Visualization: When programs use negative Z values (actual cutting depth), tool path colours shift from green (Z=0, surface) to blue (deepest Z). Each pass is a different shade, making multi-pass depth cuts immediately visible. Try the "Depth Passes (Z Color)" example. The Current Z readout shows the active cutting depth.

Cut Direction Arrows: View → Cut-direction arrows overlays small directional arrows at the midpoint of every cutting move. This clearly shows the direction of travel along each segment and arc — essential for understanding climb vs conventional milling and verifying circular arc direction (G02/G03).

Toolpath Scrubber: As soon as a program is loaded — before you have run anything — a Scrub slider sits below the canvas. Drag it to scrub back and forth through the tool path at any step. The canvas updates in real time showing the tool path up to that point, and the readouts reflect the machine state at that exact moment. This is ideal for inspecting exactly where a problem occurs in a complex program.

Saved Programs: Program → Save as… stores the current editor content under a name of your choosing, and Program → Saved programs… lists everything you have kept, with the line count, tool diameter and date, and a Load or Delete button on each. Programs persist between sessions in your browser. (Earlier builds also had three numbered quick slots below the editor; they were a second, weaker save system, so they have been folded into this list — anything you had in them now appears as “Quick slot 1/2/3”.)

SI / Imperial: Both systems sit side by side on the toolbar with the active one lit, so the control is also the indicator — which is why it stays out of the menus. Switching converts everything you read: the X/Y/Z, feed, spindle and path-length cards, the toolpath statistics and bounding box, the canvas rulers and their axis captions, the coordinate tooltip, the metal removed in the solid view, and the tool-diameter box (type 0.25 in and the tool is 6.35 mm). Your choice is remembered between visits and shared with the rest of the site. It is a display setting and never rewrites your program — the program's own units are whatever G20 or G21 declares, so a metric program can be read in inches without touching a line of code.

Speeds & Feeds Calculator (the S&F Calc button): a calculator overlay with 9 material/tool-type presets. Select a material, enter tool diameter, flute count and your Machine Max RPM, and it returns spindle speed, feed rate, chip load and the surface speed you are actually getting. Two things it does that a formula sheet does not. Chip load follows the cutter. A ø1 mm slot drill cannot take the feed per tooth of a ø12 — the tabulated f_z is quoted at ø6 and scaled as (D/6)^0.75 from there, which is why a small cutter gets a small chip rather than a broken shank. The spindle is finite. 200 m/min on a ø1 cutter wants 63,662 RPM; if that is past your machine, a note says so, tells you the cutting speed you will really get, and matches the feed to the RPM you can actually reach — keeping the chip the right thickness, which is the part that saves the tool. It does not pretend to recover the cutting speed. Click "Apply to Editor" and it edits the program: the spindle word is rewritten in place to the calculated RPM — not a second M03 added above your own, which the original one would simply override — and the feed is written onto every cutting move, converted to the units your program declares with G20 or G21. A line under the button says exactly what it will change before you press it, and Apply also sets the Tool ⌀ box to the cutter you just sized — the drawn cut width and the metal removed must be the cutter the numbers were calculated for. Plunge moves are left at the feed you wrote: a straight-down move wants a fraction of the side feed, and overwriting it with the milling feed is how drills get broken. In Turning mode only the spindle is set, because a lathe feeds per revolution and this is a milling calculator.

Warnings: The linter checks for: arc radius mismatch (I/J error), G01 with F=0 (stall warning), cutting moves before M03/M04 spindle start, missing X/Y/Z coordinates, coordinates written before any motion command, arcs programmed outside the G17 XY plane, and any G- or M-code the simulator does not execute (G41/G42 cutter compensation, G43 tool-length offsets, G54–G59 work offsets, G04 dwell and so on). Each distinct code is reported once, not once per block. Warnings appear in a red bar with clickable line-jump buttons (Ln) to scroll the editor directly to the offending line.

Editor Line Numbers: A line number gutter on the left side of the editor shows line numbers that stay in sync as you scroll. This makes it easy to cross-reference warnings, error messages, and the program line readout.

Toolpath Statistics: After running a program, a statistics panel shows cut distance, rapid distance, moves, estimated machining time, bounding box, and drill hole count (when canned cycles are used). The bounding box measures the part, not the machine — it is taken over cutting moves only, so a program that opens with a rapid at the origin does not report a box bigger than the shape it cuts. The statistics also clear the moment you edit the program, rather than describing the code you used to have.

Sound Feedback: Audio cues confirm button presses, program completion (ascending chime), and correct/incorrect answers in Practice and Quiz modes.

G-codes are modal. A motion command stays in effect until another one replaces it, so a block that carries only coordinates continues the last motion. This program cuts all four sides of a square, because G01 is still active on lines 2, 3 and 4:

G01 X50 Y0 F200
X50 Y50
X0 Y50
X0 Y0

The same applies to G00, G02 and G03, and to the drilling cycles — which is why a bare X30 Y10 line repeats a G81 hole at a new position. G80 cancels the mode; after it, a block of bare coordinates is flagged rather than obeyed.

A block may carry several codes. The safety line G17 G21 G90 G40 G49 G80 sets five modes and cancels any cycle in one block, and all of them take effect. Likewise G00 G90 X10 Y10 is a rapid in absolute mode — the order of the words in the block does not matter.

F and S are modal too. A line containing nothing but F300 changes the feed rate for every move that follows, and a bare S2500 changes the spindle speed without stopping the spindle.

A leading / is an optional block skip. On a real control it is a switch on the panel: with Block Delete on those blocks are jumped, with it off they run. There is no such switch here, so they run — and the warning bar tells you that once, so a program written to be skipped does not quietly do the opposite of what its author meant.

M02 and M30 end the program. Nothing written after them is executed, exactly as on a real control. M00 and M01 are operator stops: the simulator notes them in the warning bar and continues, because there is no Cycle Start button to press.

Z travel is real travel. A plunge or a retract is counted in the Path Length readout, in Cut Distance, and in the estimated cut time — a G01 Z-10 F100 is ten millimetres of cutting at F100, not a free move. Each move is timed at the feed rate that was active for that move, not at the program's final feed rate.

Playback is timed, not stepped. Each move is played at length ÷ feed — the same arithmetic behind the Est. Cut Time readout — with rapid traverse taken as 5 m/min. This is why a program can look busy and still be slow: the time is in the deep plunges and the fine finishing feeds, not in the long fast passes. Step is unchanged and still executes exactly one block.

G28 returns through home properly. It retracts Z to zero before crossing the table, so the tool is not dragged through the part, and it will pass through an intermediate point if you give it one (G28 X10 Y20).

What is not simulated. Cutter compensation (G41/G42) is recognised but the path is drawn on the tool centreline; tool-length offsets (G43/G44/G49) and work offsets (G54–G59) are recognised but treated as zero; arcs are interpolated in the XY plane only. Each of these tells you so in the warning bar rather than changing your path silently.

The Milling / Turning switch at the left of the Control Panel changes the whole simulator between the two dialects. In Turning mode the example list, the readouts, the drawing and the interpreter all change together — and the isometric option disappears from the View menu, because a lathe is drawn in section rather than in isometric.

What changes in the readouts. Current X reports a diameter (marked ⌀), the second card becomes Current Z, and the feed is shown in mm/rev. The vertical ruler on the canvas is labelled in diameters too, so what you read off the picture is what you would type into the program.

What is drawn. The bar is shown in section with a three-jaw chuck gripping its left end and the spindle centreline chain-dashed through the middle. The toolpath is drawn above the axis and mirrored faintly below it, because a turned profile is a solid of revolution. The turning tool is drawn with the insert nose exactly on the programmed point — the nose is the coordinate.

Cycles that run: G90 straight and taper turning, G94 facing, G71 roughing with G70 finishing (give both the same P and Q block numbers), and G76 threading. G96/G97 constant surface speed is solved rather than merely recognised — the spindle really does climb as the diameter falls — and G50 is the clamp that stops it running away at the centre. G98/G99 switch between feed per minute and feed per revolution, and the cycle time follows.

Thirteen worked turning programs ship with it, ordered as a course rather than as a feature list. They start with First Turn, which uses nothing but G00 and G01 — face the end, turn one diameter — then a stepped shaft, chamfers and radii (with the trap that a 45° chamfer needs the diameter to change by twice the Z distance, because X is a diameter). The middle of the list is the machine's own vocabulary: the G90/G94 cycles, taper and radius work with U and W, centre drilling and boring, the two feed modes, constant surface speed, and the same job written in inches under G20. It finishes with G71+G70 roughing, G76 threading, grooving and parting off, and a capstone that runs the whole sequence a shop would — face, rough, finish, groove, thread, part off.

Inside diameters. A drill runs in the tailstock, on the centreline, so it has no X word at all — it is a feed move in Z at X0, and its diameter comes from the Tool ⌀ box rather than from the program. A boring bar then goes down that hole and opens it out. The simulator tells the two apart the way the shop floor does: by where the tool is when it enters the metal. That is also why a parting cut, which runs from the outside right down to X0 through solid metal, is still read as an outside cut and not as a bore.

Material removal. View → Material removal switches between the tool path and the workpiece itself, being cut down as the program runs — a bar of aluminium in the chuck on a lathe, a plate of it clamped in the vice on a mill. It is not a picture. Turning produces a solid of revolution, so the material state is exactly two numbers per station along the axis — the remaining radius, and the bore inside it, because a turned part has an inside as well as an outside. Milling on three axes cannot undercut, so its state is exactly one remaining height per station in plan — a height field. Seen from straight above, a pocket wall is not a face you can see but a slope, shaded from its own surface normal; seen isometrically it is a real wall, and the block is drawn as a field of vertical columns, far to near, each clipped against the one in front of it. Z is drawn at the same scale as X and Y — a 4 mm pocket in a 70 mm plate is four seventieths of the plate deep on screen, with nothing exaggerated to make it easier to see. Every cutting move lowers the surface wherever the tool swept. Rapids remove nothing, and a contour with no Z word never leaves the top face, so it removes nothing either — the panel says so rather than leaving you with a blank plate. The panel reports the blank size and how much metal has come off, by volume. Chips fly only while metal is actually being removed, the turned finish is banded at the feed per revolution, and those bands travel as the bar turns. The chuck is drawn in the same side view as the work, with its jaws closed on the bar, and both turn together — though at a speed you can follow rather than the real one: 1,200 rpm is twenty revolutions a second and would only strobe. The rate still rises and falls with the programmed S.

The traps it will catch for you: treating X as a radius, treating I as a diameter, expecting G91 to work, and expecting G90 to mean absolute positioning. The warning bar reports a mistyped arc the same way it does on the mill.