Welcome to Learn the Lingo! The idea behind this series of articles is to bring you up to speed on the terms you will need to know to understand what someone is talking about in a specific skill. Often, when just getting into a new skill, the lingo that practitioners use can be confusing and have meanings you didn’t realize. For example, a “hook” means completely different things if you’re sewing or if you’re tying a trout fishing fly!

On the slate for today’s vocabulary expansion is CNC machining. Specifically, we’re talking about the process of cutting away bits of metal to make shapes. This can be done with hand based tools called mills or lathes. Though you’ll hear about mills and lathes in woodworking, the term machining tells us that we’re going to be talking about metal.

For many years, this was done with “manual” machines, where a person turns a dial that moves the tools. For this article, we’re talking about Computer Numerical Controll, or CNC machines, which use a computer to move the tools around.

I spoke with Chris Fox from Tormach, to see if he could help us out. Tormach was founded in 2002 in an effort to provide capable CNC machine tools in a price range more comfortable for hobbyists and people starting out. Their machines can mill anything from plastic to titanium and still fit in your garage.

People definitely get confused about terminology when machining, which can be quite a big deal when you’re dealing with high-precision, spinning metal cutters. This can be blamed on the fact that like machining itself, where there are generally bunch of different ways to do the same process, several different machining terms can often identify the same thing or slightly nuanced versions of the same thing. The key to learning machining is to learn by doing, but it helps to have a mentor or guru (YouTube is a great place for this) to keep your lingo on the right path.

Terminology for your first day in machining

Feeds and Speeds – A calculation to determine the best way to cut through a piece of metal. These need to be adjusted according to the hardness of material, the surface finish you are looking for, and what type of cut you are making.

Workholding – An apparatus or structure designed to hold your work, or the material you are cutting with your CNC machine. On a mill, this is most often attached to the table, whereas on a lathe, this is most often the element that is spinning.

Toolholding – The structure or apparatus designed to hold your cutting bit or tools. On a mill, this is often the element that spins, whereas on a lathe, this is the element that is secured to your table.

Fixturing – A term given to the apparatus or structure that is holding your work which is often custom-made to hold your specific part.

CAD – Computer aided design. CAD programs allow you to digitally create a part that can be milled, 3D printed, or rendered.

CAM – Computer aided machining, or sometimes known as computer aided manufacturing. CAM is a computer program that takes a CAD file (IGES, STEP, etc.) and allows you to create tool paths for the cutting process on a CNC machine. CAM tells your CNC machine where to go, how fast to move and spin.

Post Processor – While most CNC controllers run on standard forms of G-code, every controller has a different dialect of control code. A post processor acts as the translator to get a CAM file into the proper code that can be read by a specific machine.

G-code – This is the coding language that a CNC controller reads to tell the machine what movements to make and the speeds at which to move, among many other functions.

Chatter – A noise that can occur during cutting which can be caused by an array of different factors, including (but not limited to) dull tools, incorrect speeds and feeds, and too much tool stick-out. Too much chatter mostly affects the finish on a part, but it can also be signs of inefficient cutting, looming tool breakage, or improper workholding.

Depth of Cut – Closely related to speeds and feeds, depth of cut is how much material is being removed with each pass of the tool. This is another adjustment that can be made to improve surface finish and/or the life of your tools.

Thou – A machinist term for a unit of measurement equaling 0.001″ or a thousandth of an inch.

Tenth – A machinist term for a unit of measurement equaling 0.0001″ or one ten thousandth of an inch, NOT to be mistaken with 0.1″ or a tenth of an inch.

Conversational – An interface for machining that is done at the controller; a method of CNC machining without CAD or CAM, for very simple operations.

Work Coordinate System – Also known as WCS, that tells the CNC machine where a part is relative to the machine’s home.

Concentricity – The relation between two diameters to each other.

Runout – The concentricity of a tool in relation to the spindle that is spinning it. More runout means that a tool will be less accurate in the cutting process – a tool with high runout has a harder time staying true to the programmed tool paths.

(Tool) Offsets – Tells the CNC machine where the tip of the tool is relative to a known surface (like the nose of the spindle).

Facing – machining your stock down to a set height, usually using a larger tool than you use for your other operations.

Here are some further resources for learning the terms used in machining

• Advanced Machining’s term glossary
• CNC Cookbook
• Micromatics Glossary
• All Words Glossary of Machinist’s terms

If you feel that there are more terms that someone should know on their first day, share them in the comments!