The metal parts used in assemblies and mechanisms, from hydraulic pumps and valves to engines and industrial and agricultural machinery, all go through precision machining. This is the process of cutting away metal from a bar, billet, casting, or forging to produce the precise geometric features needed.

Bolt holes, screw threads, flat mounting or locating surfaces, and precisely finished bores are just some of those features. In some cases, parts need pockets milling, tapers turning, or complex sculpted surfaces producing.

Once machining is complete, parts are usually inspected. This is to verify that they meet the requirements on the drawing, and to find any errors or defects that would make the part unfit for use.

People who haven’t been immersed in precision machining often struggle with the terminology employed. This blog explains the terms used most often.

A

Accuracy: This refers to the closeness of a measurement to the target value. People working in precision machining prefer terms like “process capability,” “tolerance,” and “repeatability.

C

CAD (Computer-Aided Design): Software used to design products and parts, and produce detailed drawings from which to make and inspect those parts.

CAM (Computer-Aided Manufacturing): This software generates movement instructions for CNC machines to follow in order to machine parts as needed.

CNC (Computer Numerical Control): This refers to using a computer to control the movements of a machine tool. The computer executes instructions developed by a CAM program or in response to instructions entered directly at the machine.

Coordinate Measuring Machine (CMM): When machined parts need a complex series of measurements, this is usually done on a CMM. Typically designed as a bridge over a granite table, a CMM uses a touch-trigger probe to sense the location of each edge or feature. This location data is then used to determine dimensions of important features.

M

Milling: A metal removal process that uses a rotating cutter. Milling produces flat surfaces and pockets, although more complex forms can be produced by rotating the cutter axis or the workpiece. Most milling machines have three axes of motion, although machines capable of machining complex contours have four or five.

P

Process capability: A measurement of the variation in a process. A batch of parts is measured, and the mean and standard deviation are calculated. Process capability is a measure of how well the standard deviation fits within the tolerance band.

R

Repeatability: The degree to which a particular measurement is achieved on repeated cycles or parts. When repeatability is high, accuracy can be improved by adjusting the machining process. If repeatability is so low that many parts are outside the tolerance band, the process might be considered out of control.

Resolution: An indication of the precision required, and that a measuring device can achieve. A dimension stated and measured to three decimal places is of higher resolution than one with two decimal places.

S

Surface Roughness: Surface irregularities may be measured in various ways. Ra is a commonly used measurement of surface roughness, although some argue Rz does a better job of indicating roughness. Rz is the average of the five largest peak-to-valley distances over the sample length.

Swiss machining: A form of turning done on a particular type of lathe, (the Swiss screw machine). Swiss machining is preferred for cylindrical parts with a large length-to-diameter aspect ratio.

T

Tolerance: This refers to the variation permitted in a specific dimension or other measured value. It is often written as “+/-”, so for example, a dimension of 2.500” +/- 0.005” means any measurement in the range of 2.495” to 2.505” is acceptable.

Turning: A type of machining performed on a lathe to produce cylindrical shapes. The workpiece is held and rotated in a chuck while the cutting tool moves along the part in an axial direction to reduce (“turn down”) the diameter. The tool can also move across the end face of the workpiece (“facing”) to reduce its length.

Bring Your Precision Machining Needs to Impro

With few exceptions, metal parts all go through one or more precision machining operations. This glossary has explained some of the most widely used terms, but there are many others. If you’re looking for precision machining services and need help understanding the terminology or what’s possible, we’d like to help. Contact us with your needs and questions.