Guide · 8 min read
CNC glossary: the words on a desktop CNC spec sheet
26 terms from backlash to Z clearance, each with a sourced definition and how many machines in the record it applies to.
CNC spec sheets use a lot of words that nobody explains. These are the 26 terms that come up most on the 35 maker pages in this site's record, each with a short definition, a quote from a public source, and a link to the guide that covers it properly. Where the record can say how many machines a term applies to, the count is given; those counts are rebuilt from the data every time a machine is added.
Jump to: Backlash · Ballscrew · CAD · CAM · Closed loop · CNC · Collet · Enclosed · End mill · Footprint · G-code · Gantry · GRBL · grblHAL · Leadscrew · Linear rail (profile rail) · NEMA 17, NEMA 23 · Offline controller · Open loop · Router (trim router) · Sender · Spindle · Spoilboard · Stepper motor, step, microstep · Work area (travel) · Z travel, Z clearance
- Backlash
Wikipedia defines it as "a clearance or lost motion in a mechanism caused by gaps between the parts". When an axis reverses, the slack has to be taken up before the axis moves. Wikipedia adds that it "can be minimized by choosing ball screws or leadscrews with preloaded nuts".
- Ballscrew
A screw whose nut rides on recirculating steel balls instead of sliding on the thread. Wikipedia: "A typical ball screw may be 90 percent efficient, versus 20 to 25 percent efficiency of an Acme lead screw of equal size." Codes like "1610" give diameter and pitch: Carbide 3D describes its 1610 screw as "16mm diameter, 10mm pitch".
In the record: 14 of the 35 pages say ballscrews on every axis.
- CAD
Design software. In Carbide 3D's words, CAD "is the CNC software that lets you draw your designs."
- CAM
The software that turns a design into the path the bit follows. Carbide 3D says CAM is "responsible for taking your designs and creating a toolpath for the machine to follow." The last step, which Autodesk calls post-processing, "converts CAM toolpaths into G-code that machines can read."
- Closed loop
A stepper motor with a sensor that checks its position. Sienci Labs describes the driver sending steps "while simultaneously monitoring the position of the motor using a sensor". See also open loop.
In the record: 12 of the 35 pages say their motors are closed loop.
- CNC
Computer numerical control: a machine tool moved by a computer. Wikipedia says "the first NC machines were built in the 1940s and 1950s" and were "rapidly augmented with analog and digital computers, creating the modern CNC machine tools".
- Collet
The sleeve that grips the bit. Wikipedia calls it "a segmented sleeve, band or collar", and notes "a given collet holds any diameter ranging from its nominal size to its 1-mm-smaller collapsed size". ER20 is one size in the common ER family; Sienci Labs' AltMill spindle kits use it.
- Enclosed
On this site, a machine counts as enclosed when its maker describes an enclosure as standard. A page that doesn't say so is counted as not enclosed.
In the record: 4 of the 35 pages say the machine is enclosed: Makera Z1, Makera Carvera Air, Carbide 3D Nomad 3 and Makera Carvera.
- End mill
The usual CNC router bit. Wikipedia: "While a drill bit can only cut in the axial direction, most milling bits can cut in the radial direction." That sideways cutting is what lets a router cut a shape.
- Footprint
The space the whole machine takes up, motors and all, as opposed to the area the bit can reach. On small machines it can be much larger than the work area.
In the record: 24 of the 35 pages say how much space the whole machine takes up.
More: Work area, travel and footprint: the three sizes on a CNC spec sheet
- G-code
The plain-text list of moves a CNC machine follows. Wikipedia describes it as the language "that tells the motors where to move, how fast to move, and what path to follow". G20 and G21 set inches or millimetres; GRBL starts in G21 (millimetres) unless the file says otherwise.
- Gantry
The bridge that spans the work surface and carries the X axis. The Z axis hangs off it and holds the router or spindle.
- GRBL
Free, open-source controller firmware. Its project page calls it "a no-compromise, high performance, low cost alternative to parallel-port-based motion control for CNC milling", and says it "accepts standards-compliant g-code".
In the record: 16 of the 35 pages say their controller runs GRBL (not grblHAL).
- grblHAL
A descendant of GRBL for faster chips. Its README says it is "based on the Arduino version of grbl" and "mainly aimed at ARM processors (or other 32-bit MCUs)".
In the record: 2 of the 35 pages say grblHAL: Sienci Labs LongMill MK3 30x30 and Sienci Labs AltMill 4x8.
- Leadscrew
A screw whose nut slides directly on the thread. Wikipedia says leadscrews have "a high degree of friction on the threads" and "most are self-locking (cannot be back-driven)". Sienci Labs describes the LongMill's as "T12".
In the record: 8 of the 35 pages say leadscrews on every axis.
- Linear rail (profile rail)
A rail with a bearing block that runs along it. Spec sheets write them as sizes or codes: Sienci Labs lists "15mm profiled linear guide rails and bearing blocks on all axes"; Carbide 3D writes "HG15". Other guides in the record are shafts, steel tubes and V-wheels in an aluminium extrusion.
- NEMA 17, NEMA 23
Motor frame sizes, not power ratings. Wikipedia: NEMA 17 means "a 1.7 by 1.7 inches (43 mm × 43 mm) faceplate". Torque in Nm is the figure that describes strength, and only some makers give it.
- Offline controller
A handheld unit that runs a job from a memory card, so a computer doesn't have to stay connected during the cut. SainSmart: "Wireless and offline controls offer convenient alternatives to a computer connection." It runs a file; it doesn't make one.
In the record: 11 of the 35 pages say offline control, a touch screen or a pendant.
- Open loop
A stepper motor with no position sensor. Wikipedia says such systems "track position simply by counting the number of steps that each motor has been instructed to take", and that under some conditions "steps may be missed".
In the record: 2 of the 35 pages say their motors are open loop: Onefinity Apprentice and SainSmart PROVerXL 6050 Plus. 21 don't say either way.
- Router (trim router)
The handheld woodworking tool, mounted in the machine to spin the bit. Wikipedia describes the trim router as "a smaller, lighter version of the router". Several larger machines are sold without one: Sienci Labs' LongMill page says "Router and other accessories sold separately."
In the record: 12 of the 35 pages say the price doesn't include a router or spindle. 5 don't say.
- Sender
The program on your computer that streams G-code to the machine during a job. gSender's README says it "accepts standard, grbl-compliant g-code from all the common CAM programs"; Universal G-Code Sender describes itself as "a Java based, cross platform G-Code sender".
- Spindle
A motor made to live in the machine, with its speed set electronically. Where makers give a speed range it varies widely: Carbide 3D gives the Nomad 3's as "9000 - 24000 RPM".
In the record: 18 of the 35 pages say something to cut with is included.
- Spoilboard
The board on top of the frame that the work sits on.
- Stepper motor, step, microstep
The motor most desktop routers use. It turns in fixed steps; Wikipedia gives a typical one as "1.8 degrees per full step or 200 full steps per revolution". Microstepping divides each step: "as the microsteps become smaller, motor operation becomes smoother". Carbide 3D publishes a "Step length" of .012 mm on the Shapeoko 5 Pro's X and Y.
- Work area (travel)
How far the bit can move in X and Y, and so the largest area it can cut in one go. SainSmart: "Working travel describes the cutting range; pass height describes clearance for stock and fixtures". Makers also say "cutting area" or "active working area".
In the record: All 35 pages in the record give one; it's what the sizes chart draws.
More: Work area, travel and footprint: the three sizes on a CNC spec sheet
- Z travel, Z clearance
Z is up and down. Travel is how far the bit can move vertically; clearance is how tall a piece fits under the gantry. Sienci Labs gives both for the LongMill MK3 30x30: "Z-axis clearance: 4.6″" and 5.1″ of maximum travel. Most pages give one Z number without saying which.
More: Work area, travel and footprint: the three sizes on a CNC spec sheet
Nothing here is advice on how to run a machine or which one to buy. The definitions say what the words mean on a spec sheet; the makers' pages are the place to check what a particular machine has.
Sources
Quotes were checked against these pages on 1 and 2 November 2026, when the guides they first appeared in were written. Quotes are copied from these pages.
- Wikipedia: Backlash (engineering)
- Wikipedia: Ball screw
- Wikipedia: Leadscrew
- Wikipedia: Stepper motor
- Wikipedia: Numerical control
- Wikipedia: G-code
- Wikipedia: End mill
- Wikipedia: Collet
- Wikipedia: Router (woodworking)
- Carbide 3D: free CNC software (definitions)
- Autodesk: free CAM software
- GRBL README (github.com/gnea/grbl)
- GRBL documentation: commands (github.com/gnea/grbl, doc/markdown/commands.md)
- grblHAL core README (github.com/grblHAL/core)
- gSender README (github.com/Sienci-Labs/gsender)
- Universal G-Code Sender README (github.com/winder/Universal-G-Code-Sender)
- Sienci Labs Closed-Loop Stepper Motor Kit
- Sienci Labs LongMill MK3 30x30 product page
- Sienci Labs AltMill MK2 4x4 product page
- SainSmart Genmitsu desktop CNC buyer's guide
- Carbide 3D Shapeoko 5.1 Pro product page
- Carbide 3D Shapeoko 5 Pro spec page
- Carbide 3D Nomad 3 product page
- Each machine's maker page (counts), linked from its page in the record