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CNC Technology

SINGLE-POINT TOOL THREADING

The advantages of making internal threads with a singlepoint tool are that large threads of various forms can be made and that the threads are concentric to the axis of the work. The threads may not be concentric when they are tapped. Difficulties are encountered when making internal threads. The tool is often hidden from view, and tool spring must …

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MAKING INTERNAL THREADS

Many of the same rules used for external threading apply to internal threading. The tool must be shaped to the exact form of the thread, and the tool must be set on the center of the workpiece. When you are cutting an internal thread with a single-point tool, the inside diameter (hole size) of …

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ADVANCED METHODS OF THREAD MEASUREMENT

The most accurate place to measure a screw thread is on the flank or angular surface of the thread at the pitch diameter. The outside diameter measured at the crest, or the minor diameter measured at the root can vary considerably. Threads may be measured with standard micrometers and specially designated wires (Figure I-331) or …

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BASIC EXTERNAL THREAD MEASUREMENT

The simplest method for checking a thread is to try the mating part for fit. The fit is determined solely by feel with no measurement involved. Although a loose, medium, or close fit can be determined by this method, the threads cannot be depended on for interchangeability with others of the same size and …

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CUTTING THE THREAD

The following is the procedure for cutting right-hand threads: Step 1: Move the tool off the work and turn the crossfeed micrometer dial back to zero. Step 2: Feed it in .002 in. on the compound dial. Step 3: Turn on the lathe and engage the half-nut lever (Figure I-310). Step 4: Take a …

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SETTING UP FOR THREADING

Begin setup by obtaining or grinding a tool for cutting Unified threads of the required thread pitch. The only difference in tools for various pitches is the flat on the end of the tool. For Unified threads this is .125P, as discussed in Unit 10. If the toolholder you are using has no back rake, no grinding …

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HOW THREADS ARE CUT ON A LATHE

Threads are cut on a lathe with a single-point tool by taking a series of cuts in the same helix of the thread. This is sometimes called chasing a thread.A direct ratio exists between the headstock spindle rotation, the lead screw rotation, and the number of threads on the lead screw. This ratio can be …

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METRIC THREAD FORMS

Several metric thread systems such as the SAE standard spark plug threads and the British Standard for spark plugs are in use today. The Système International (SI) thread form (Figure I-297), adopted in 1898, is similar to the American National Standard. The British Standard for ISO (International Organization for Standardization) …

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PITCH DIAMETER, LEAD OR HELIX ANGLE, AND PERCENT OF THREADS

The making of interchangeable external and internal threads depends on the selection of thread fit classes. The clearances and tolerances for thread fits are derived from the pitch diameter. The pitch diameter on a straight thread is the diameter of an imaginary cylinder that passes through the thread profiles at a point where the width of …

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UNIFIED AND AMERICAN NATIONAL FORMS

The American National form (Figure I-290) is now obsolete and has evolved into the Unified form (Figure I-291). The basic geometry of the two systems is similar. Taps and dies are marked with letter symbols to designate the series of the threads they form. For example, the symbol for American Standard taper pipe thread is NPT, …

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