How to Edit G-Code Files

You can edit a G-code file in any plain-text or code editor, but safe edits are limited to commands you understand; for changes to size, shape, infill, supports, or most print settings, return to the model and re-slice it. G-code is the machine’s finished toolpath, not an editable design file, so a small change can affect every command that follows. The safest workflow is to preserve the original file, make one targeted edit, inspect the result, and preview it before running it.

What Editing G-Code Can and Cannot Change

G-code is a line-by-line instruction file for a 3D printer, CNC machine, laser, or other motion-controlled device. It can contain movement commands, temperatures, speeds, tool changes, pauses, fan controls, and comments.

Editing it directly works best when you need a narrow, predictable change, such as:

Adding a known machine-specific command at the start, end, or a marked layer.
Changing a value you can identify and verify for your printer or CNC controller.
Removing or revising a comment.
Adding a pause or material-change routine supplied by your slicer or machine documentation.
Correcting a post-processing mistake when you understand the command sequence.

Direct editing is usually the wrong tool for changes that require the slicer to recalculate paths:

Scaling, rotating, cropping, or reshaping a model.
Changing infill percentage or pattern.
Adding, removing, or redesigning supports.
Changing layer height, wall count, or top and bottom layers.
Reworking retraction, seam placement, cooling, or variable speed behavior throughout a print.

Those changes affect thousands of coordinated moves. Start with the original CAD, STL, 3MF project, or slicer project file, change the setting there, and export a fresh G-code file.

A G-code file can be valid text and still be unsafe machine code. The file’s active modes (units, positioning, feed rate, extrusion behavior, tool selection, and temperature state) matter as much as the line you add.

How to Edit a G-Code File Safely

Choose the least invasive way to make the change.

Goal	Best Method	Why
Change a model’s dimensions or geometry	Edit the CAD, STL, or project file, then re-slice	G-code is already a generated toolpath
Change infill, supports, layers, or general print quality	Adjust slicer settings, then re-slice	The slicer must recalculate internal paths
Add a pause, message, wipe, or other known routine	Use the slicer’s custom G-code feature when available	The slicer places the command in the right stage of the job
Make a small, documented machine-specific change	Edit a copy of the G-code in a text editor	A focused edit can preserve the existing toolpath
Modify a CNC `.tap` program	Change the CAM source and repost when possible	Controller dialects and toolpath state vary widely

Step 1: Identify the File’s Source and Target Machine

Check which program created the file: a slicer, CAM package, machine utility, or printer cloud service.
Confirm the exact machine and controller that will run it.
Confirm whether the file is intended for a 3D printer, CNC router, mill, laser, or another device.
Find the original project or source model before changing anything.

A G-code command is not automatically portable between machines. Firmware and controllers can use different command sets, startup routines, tool-change behavior, and comment syntax. A command that is harmless on one printer may be ignored, interpreted differently, or cause a dangerous move on another.

Step 2: Create a Working Copy and Open It as Plain Text

Copy the original file into a separate working folder.
Rename the copy so it is obvious that it is edited.
Open the copy in a code editor or plain-text editor, not a slicer’s viewer mode.
Keep the original extension, such as .gcode, .gc, .nc, or .tap.

I use a code editor with a terminal here because search and a diff make a single-line alteration much easier to verify than a basic text editor.

Do not save the file as a rich-text document. A word processor can add formatting or change characters in ways that make a machine file unreadable.

Step 3: Find the Correct Insertion Point

Read the first section of the file before editing it.
Look for comments that mark startup code, layer changes, tool changes, or shutdown code.
Search for a distinctive existing comment, layer marker, or command you know belongs near the change.
Read several lines before and after that point to understand what the machine is doing.

Many slicers produce verbose comments that make it easier to locate a layer or section. A comment marking the start of a layer is much safer than estimating a location by line number.

Placement matters. A pause added before homing behaves differently from a pause inserted after the nozzle has reached print temperature. A movement command inserted during a print can be interpreted relative to the printer’s current position or absolute coordinate system.

Step 4: Read the Active Machine State Before Changing Motion

Check whether the file establishes units and coordinate mode near the top.
Check whether movements use absolute or relative positioning.
For 3D printing, identify whether extrusion is handled as an absolute or relative value.
Look for recent feed-rate, tool, spindle, fan, and temperature commands before the line you plan to change.

For example, this common line format represents a linear motion command:

G1 X10 Y20 F300 ; Move to X10, Y20 at 300 mm/min

G1 requests a controlled linear move. X and Y identify coordinates, while F sets the feed rate. Whether that move is safe depends on the machine’s current mode, position, limits, and tool state. Treat examples like this as code to interpret, not as a universal snippet to paste into a job.

Step 5: Make One Narrow, Documented Edit

Change only the command or block needed for your goal.
Preserve surrounding lines, spacing, and comments unless you know they are unnecessary.
Do not globally replace values such as feed rates, extrusion values, or coordinates.
Add a clear comment beside your change if the file’s dialect supports comments.

If you need to add a pause, temperature adjustment, material-change routine, or tool action, use code supplied for your exact firmware or controller. Do not copy a command from a different printer family, slicer profile, or CNC control just because the command name looks familiar.

Step 6: Compare the Edited Copy With the Original

Use your editor’s compare or diff feature.
Confirm that only the intended lines changed.
Check that the beginning and ending routines are still present.
Make sure no lines were accidentally duplicated, truncated, or moved.
Confirm that the file did not gain an extra extension such as .txt.

This check catches the most common editing mistakes: a misplaced insertion, an accidental replacement of every matching value, or a save operation that changed the file type.

Step 7: Preview and Run the File Cautiously

Load the edited file into the intended slicer viewer, CAM simulator, controller preview, or machine software when available.
Confirm that the toolpath still looks complete and that the edit appears in the expected section.
Perform the machine’s normal preflight checks before starting.
Stay with the machine when it begins the edited job and stop it if motion, heating, or tool behavior is unexpected.

A preview can catch an obvious path error, but cannot prove that every machine-specific command is safe. The original project and a fresh slice remain the better choice whenever the requested change affects generated geometry.

Edit G-Code for Common 3D Printing Changes

Add a Pause or Material Change

Use the slicer’s custom G-code or layer-change feature when it offers one. The slicer can place the command at an actual layer boundary and retain the rest of the generated toolpath.

If you must edit manually, locate a clearly labeled layer transition, add only a routine documented for your printer firmware, then inspect the preview.

Change Infill, Supports, or Print Size

Do not try to change these by editing the generated file line by line.

Infill is a set of internal toolpaths generated from the model, wall settings, layer settings, and overlap rules. Supports are similarly generated from the model’s overhangs and the slicer’s support settings. Rescaling a completed G-code file also changes coordinates, extrusion amounts, speeds, layer positions, and machine limits.

Open the original model or slicer project, make the change, review the new preview, and export a new G-code file. If you only have the G-code file, recreating the model or obtaining the original STL is generally more reliable than trying to reverse-engineer the toolpath.

Change a Temperature or Speed

A direct change can work when you know exactly which command controls the intended stage of the print. The risk is that slicers may set the same value more than once: during startup, after a tool change, at a layer change, or in a filament-specific block.

Search for the relevant command, determine every place it is set, and change only the occurrence that corresponds to the stage you intend to alter. If the goal is a broad material or quality adjustment, change the filament or process profile in the slicer and re-slice instead.

Handle TAP and GCODE.3MF Files

TAP files may need controller-specific handling, while GCODE.3MF archives require editing the G-code payload and preserving the package structure.

Edit a G-Code TAP File

A .tap file is often a CNC program, but the extension alone does not guarantee a universal format. It may contain G-code, controller-specific commands, macros, or code generated for a particular post-processor.

Open a copy as plain text, then identify the program’s commands and macros before editing. For changes to cutting geometry, toolpaths, stock setup, tool selection, or feeds and speeds, edit the CAM project and post-process a new .tap file. Direct edits are best reserved for a machine-specific correction you can verify in a simulator or controller preview.

Edit a GCODE.3MF File

A .gcode.3mf file is not necessarily a normal text G-code file with a different name. It can be a packaged archive containing metadata, thumbnails, configuration, and one or more G-code payloads.

For some Bambu Lab exported plate archives, the G-code payload can be stored in the Metadata folder of the package, as shown in this Bambu Lab community workflow. Extracting and changing that payload requires preserving the archive’s internal structure when repacking it.

Make a copy first, extract it with an archive tool, edit only the G-code payload, repack the archive correctly, then load it into the associated software to verify that it still opens and previews.

Recover, Crop, or Resume a Failed Print

Cropping a print file to resume partway through is a recovery project, not a routine edit.

A partial 3D print is not at the same starting state as a fresh job. The printer must know the correct position, layer height, coordinate mode, extrusion state, temperatures, and clearance above the existing part. Removing earlier lines from a file without rebuilding that state can drive the nozzle into the print or bed.

Plan recovery before cancelling rather than assuming the existing G-code can be cropped to resume the print.

When the model and project are available, correct the support issue, re-slice the full object, and preview the result before starting a new job. Use a documented power-loss or pause recovery feature according to its instructions. Keep the original job file until the replacement print has completed successfully.

Verify the Edited File and Prevent Future Problems

Before transferring an edited file to a machine, confirm all of the following:

The original file remains unchanged.
A diff shows only the intended edits.
The startup and shutdown blocks are intact.
The edit is placed at the intended layer, tool change, or program stage.
The file opens in the expected viewer, simulator, or controller software.
The machine profile, material, tool, and work setup match the generated file.
You have the original model and slicer or CAM project saved separately.

For future work, save three things together: the original model, the slicer or CAM project, and the exported machine file. The project is what lets you make normal design and print-setting changes safely; the G-code is the final output intended to run a specific job.

Frequently Asked Questions

Can I edit infill in a G-code file?

Technically, you can alter individual infill movement lines, but you should not use direct editing to change infill percentage or pattern. Infill is generated from the model and slicer settings, so changing it correctly requires re-slicing the original model or project. Open the project, adjust the infill setting, inspect the preview, and export a new .gcode file.

Can I edit .gcode.3mf files?

Yes, but only if you treat .gcode.3mf as a package rather than ordinary text. Some exported plate files contain a G-code payload inside an archive, often alongside metadata and preview files. Extract a copy, edit the contained G-code file, preserve the internal folder structure when repacking, and verify that the printer software can still open the package.

How Do I Edit a G-Code TAP File?

Edit a .tap file in a plain-text or code editor only after identifying the CNC controller and post-processor that created it. For geometry, tools, stock setup, or cutting-strategy changes, modify the CAM project and generate a new .tap file instead. Direct changes should be limited to code you understand and can verify in the intended controller or simulator.

How Do I Save Something as G-Code?

You save something as G-code by opening the model in a slicer for 3D printing or a CAM program for CNC work, configuring the machine profile and job settings, then exporting or post-processing the toolpath. The resulting file is specific to that machine configuration. Keep the project file too, because it is the editable version of the job.

Can I Edit an STL File From G-Code?

No, G-code is not a reliable editable substitute for an STL file. It contains machine movements and material or tool instructions rather than the original mesh, dimensions, and design history. Find the original STL, CAD model, or slicer project, edit that source, and generate fresh G-code for the revised job.

How can you add G-code at a specific layer in Bambu Studio?

According to a Bambu Lab community post, you can insert G-code at the beginning of a layer in a sliced Bambu Studio job by right-clicking the layer slider. The post also says that generated G-code includes plentiful comments, which can help you locate the matching layer when working with the file.