Prepared from parser behavior, machining-file examples and visual checks performed in CNC Viewer. Always confirm the result against the CAM setup and machine documentation before production.
CNC file guide
Understanding CNC machining file formats
A file extension tells you which parser to start with, but it does not guarantee that every machining detail is stored in the file. This guide explains what CNC Viewer can read and which values still deserve a manual check before a program reaches the machine.
What a useful preview needs
A solid simulation needs three kinds of information: the workpiece dimensions, the path followed by the tool and the volume removed by that tool. The path alone is not enough. A line cut by a 3 mm router and the same line cut by a 12 mm router produce very different parts. Depth and machining surface are equally important for blind holes, through-holes and bottom-face operations.
- Stock length, width and thickness
- Tool diameter or cutting width
- Cutting depth and reference surface
- Coordinates, arcs and compensation mode
Supported format families
NC, TAP, NGC and GCODE
These text-based programs usually describe machine motion with G0/G1 linear moves, G2/G3 arcs, tool changes and drilling cycles. They may include stock dimensions and cutter information in comments, but this is not guaranteed. When diameter or stock data is missing, the viewer asks for the values needed to build a useful preview.
MPR
MPR is commonly associated with HOMAG and woodWOP workflows. A file can contain workpiece dimensions, top and bottom drilling, horizontal drilling, contour geometry and routing operations. Some MPR programs call external component macros or reference a machine tool database; those external resources are not embedded in every file.
HOP
HOP files are used in NC-HOPS and HOLZ-HER environments. They can define finished-part dimensions, drilling macros and milling paths with SP, G01 and EP records. Z coordinates may be expressed relative to the top surface or from the bottom of the panel, so the viewer evaluates the safe position and the cutting level together.
PLT
PLT files primarily describe two-dimensional plotter paths. They often lack reliable stock thickness, machining depth and cutter diameter. CNC Viewer requests these values when they cannot be derived from the file, then applies them consistently to the generated model and technical drawing.
NCD and XML machining files
Machine-specific NCD and XML exports can carry structured workpiece and operation data. Support depends on the schema used by the exporting application. Dimensions, hole coordinates, surfaces and channel information are read when the corresponding fields are present and unambiguous.
Why two files with the same extension can behave differently
CNC formats are frequently customized by the CAM system, post processor and machine configuration. One NC file may include a clear stock declaration and tool table, while another contains only motion commands. An MPR file may describe every hole directly, or it may call a separate component macro. A HOP program can use different Z origins depending on how the post processor was configured.
CNC Viewer therefore prefers explicit values from the file, falls back to recognizable operation metadata, and finally asks for user input when a safe interpretation is not possible. The preview is an inspection aid, not a replacement for machine simulation and setup verification.
Checks to make before machining
- Confirm that the displayed stock dimensions match the physical panel.
- Compare each colored toolpath with the expected tool number and diameter.
- Check whether Z zero is the top surface, bottom surface or machine table.
- Inspect through-cuts, edge operations and horizontal holes from several views.
- Use the technical drawing to compare hole positions and overall dimensions.
Worked example: the extension is only the first clue
Imagine two files named panel.nc. The first declares an 18 mm stock, lists T1 as a 6 mm cutter and uses negative Z from the top face. The second contains only coordinates and tool numbers because its post processor expects the machine database to provide the missing values. They share an extension, but only the first can produce a fully determined preview without user input.
For the second file, a responsible viewer asks for thickness and cutter diameter, marks those values as assumptions and keeps the colored source path visible. If changing the assumed diameter changes only the swept width, the interpretation is behaving consistently. If stock size, path position or depth also changes, the parser is mixing independent pieces of data and the result needs further review.