G-code viewer and simulator

What G-code is

G-code is the list of moves a machine is told to make, one to a line. A slicer writes it from a 3D model for a printer; a CAM program writes it for a mill, a router or a laser.

A line such as G1 X50 Y20 E1.2 F1800 reads: move in a straight line to X 50, Y 20, push 1.2 mm of filament through the nozzle on the way, at 1,800 mm a minute. G0 and G1 are straight moves, G2 and G3 are arcs, and commands starting with M switch things on and off: heaters, fans, a spindle. Anything after a semicolon is a comment for people. Slicers use comments to mark where each layer starts, what is being printed and what they estimate the print will take.

The file is the whole print. A printer does what the lines say and nothing else, which is why looking at them before you start is worth a minute.

What to check before you print

• The size. The size shown is that of the part itself, measured from the paths with the width of the plastic included. If a part that should be 60 mm long reads 6 or 1,524, the model was scaled or its unit was misread before slicing.
• The first layer. Drag the layer slider to the bottom. The first layer should be a complete outline, filled, with nothing missing where the part touches the bed.
• The inside. Step up through the layers to see the infill, and whether thin walls and small holes came out as you meant. Choose “This layer only” to look at one layer on its own.
• Overhangs, bridges and supports. Where the slicer marks them, bridges and overhanging walls have colors of their own. Supports show where the slicer put them, and where it did not.
• Speed. Color the paths by speed to see where the printer slows down, which is often where quality matters most.
• Travel. Switch on travel moves to see how the nozzle gets from one place to the next, and how often it crosses open air on the way.
• Filament and time. Both are in the panel beside the view, with the weight for the material you choose.

Stepping through a print

The slider up the side of the view chooses the layer. Everything below it stays in place, so the print is shown as it will stand when that layer is done. The slider under the view moves through the layer on top, one move at a time.

Play runs the print forward from wherever it stands, at 1, 10, 100 or 1,000 times the speed of the real thing. The nozzle moves at the speeds in the file, so slow outer walls and quick infill look as they will on the printer.

Under the view is the line of the file being made. Choose Code to see the lines around it: the list follows the nozzle, a click on a line shows the print as it stands once that line has run, and the arrow keys walk the file a line at a time. A click on any path in the view finds the line that made it.

How the numbers are worked out

Filament is what the file pushes through the nozzle: the E values of every move, with retractions and the plastic pushed back out after them counted once. Weight is that length times the cross-section of the filament times the density of the material:

When the file names its material, its density and the width of its filament, those are used. When it does not, as with files from Cura, you choose the material, and the width is told from how much filament each line takes.

Layers are counted from the marks the slicer leaves at every layer change. A file with no such marks is cut into layers by the heights it prints at.

Print time is the slicer’s own estimate when the file carries one. Otherwise it is the length of every move divided by its speed, a figure the real print can only exceed.

Size is the box around the walls, infill and surfaces of the part. Skirts, brims, supports and prime towers are left out of it, and when they make the print take up more room on the bed, the larger size is given as well.

Files from each slicer

Binary G-code is the packed form that PrusaSlicer writes for the Prusa MK4, MINI, XL and CORE One. It holds the same lines in about a sixth of the space, and opens here like any other file.

Arcs written by Arc Welder or by a slicer’s own arc fitting (G2 and G3) are drawn as arcs, to within a hundredth of a millimeter.

CNC programs

A program for a mill, a router or a laser opens in the same view. Cuts are solid lines and rapid moves are dashed. Cuts are colored by depth, from the deepest to the shallowest, or by feed rate.

• Size and depth. The width and length of everything cut, and the lowest and highest point the tool reaches while cutting.
• Passes. Each stretch of cutting at one depth is a pass. The slider up the side steps through them in the order the machine makes them.
• Time. Cutting moves at their feed rates, plus rapid moves at 5 meters a minute. Tool changes and the spindle coming up to speed are on top of that.
• Units. A program in inches is shown in inches, with millimeters beside the size.

Straight moves, arcs in any of the three planes, drilling cycles (G81 to G83 and their kin), absolute and relative positions and moved origins (G92) are followed. A laser’s moves at zero power are drawn as travel. Variables, subprograms and rotary axes are not followed; when a program uses them the page counts the lines and says so, so you know the picture is incomplete.

What is not shown

• What the firmware adds. A macro such as PRINT_START runs on the printer, and the file does not say what it does. Homing, bed leveling and heating take time that is not in the picture.
• Speeding up and slowing down. The paths are exact; the timing of the playback is that of the speeds asked for, without the slowing at corners.
• Plastic. The beads are drawn with the width and height the slicer gives them. How the real plastic sags, strings or bridges is for the printer to show.

Questions