FEATURE
Basic Machining Tools – Terminology
Machine shops and machining schools use many specialized tools. Each tool has a particular purpose, and the best choice depends on the material and operation. Understanding these basic tools is essential before selecting one for a job.
End Mill
An end mill is one of the tools most frequently used on milling machines. It can side-mill the ends of a part to produce clean, parallel surfaces, cut pockets, and create square or round features on a CNC milling center.
Different end mills are suited to tasks ranging from roughing solid steel to making finish passes on aluminum. Important selection variables include diameter, flute length, tool material or coating, roughing or finishing geometry, and number of flutes.
Drill
Drills create holes, whether an operation requires one hole or several hundred. Although drills are less versatile than end mills, selecting the correct drill remains important.
High-speed-steel or coated drills with a 118-degree point are common because they work with many basic materials. Specialized applications—such as hard stainless steel, copper, deep holes, or close-tolerance work—may require a different drill.
Tap
A tap creates internal threads in a previously drilled hole. Tool selection depends on whether the hole is blind or through and whether the required thread follows a metric or U.S. standard. Common milling options include cutting taps, roll-form taps, spiral-point or spiral-flute taps, and thread mills.
Cutting Tap
A cutting tap removes material and produces chips while forming the thread. The drilled hole must meet the specified tap-drill diameter for the required thread. Cutting taps are commonly used in through-holes, where chips can move ahead of the tool and exit the hole.
Roll-Form Tap
A roll-form tap displaces material to form threads rather than cutting it, so it does not produce chips. It is useful for suitable materials and blind holes because chips cannot accumulate at the bottom of the hole.
Reamer
A reamer is used after drilling or boring to bring a hole to a close dimensional tolerance and improve its finish. A typical process leaves approximately 0.010–0.015 inch of material in the hole for the reamer to remove, though the proper allowance depends on the hole size, material, and tool.
A reamer does not correct a crooked or substantially out-of-round hole; it tends to follow the existing path. When position, straightness, or roundness must be corrected, the hole may need to be drilled and then bored before being reamed to its final size.