External Threads

In theory, external threads can be produced using a variety of thread-cutting tools. In practice, however, thread rolling is the preferred method, especially when using hard-anodized cutting dies with pronounced spiral points.

When working with non-hardened or non-heat-treatable alloys, challenges such as lower strength and poor resistance to mechanical stress can arise. Threaded aluminum engagement lengths should be longer than those used in steel, and the pre-machined diameter of the thread pin should be reduced by 0.2 to 0.3 times the thread pitch relative to the nominal thread diameter. This ensures proper thread depth and minimizes excessive material displacement.

Using emulsion or cutting oil is essential, especially for longer threads, to reduce friction and extend tool life.

Thread forming is the most effective method for producing high-quality external threads. It performs well even with non-hardened or non-hardenable alloys and offers multiple advantages: reduced material waste, shorter production time, and stronger threads due to work hardening. These benefits result in improved fatigue resistance, making thread forming ideal for applications involving cyclic loads.

Nevertheless, if milling is used, the quality of threads is generally lower. Milled threads are prone to the notch effect—a phenomenon where sharp corners or tool marks create stress concentrations that can lead to cracking and premature failure. In fatigue-critical applications, rolled threads with rounded root radii are preferable, as they minimize stress risers.

Guidelines for thread rolling and forming are machine-specific and should always be followed closely for optimal results.

Internal Threads

Internal threads can be produced using conventional methods, provided the right tools and materials are used. For power-assisted tapping in aluminum, special taps for light alloys are available. These feature:

• Large, precision-ground flutes (straight or spiral) with exposed teeth
• Spiral point taps for through holes (spirals run opposite the cutting direction to push chips forward)
• Spiral flute taps for blind holes (spirals follow the cutting direction to extract chips)
• Cylindrical lateral surfaces with radial relief to prevent chip entrapment during tool retraction

Two main tap types are used for hand threading: a pre-cutter and a finish cutter. Core hole diameters for these are standardized.

Chipless (thread forming) methods are generally preferred for internal threads due to their ability to deliver consistent tolerances and smooth, score-free flanks. The cold working involved improves abrasion and pullout resistance. Because the forming process displaces material, special cutters must be used for thin-walled parts to prevent deformation.

Thread formers resemble taps but lack flutes. They are available for all standard thread sizes and do not require special machinery, as long as the machine can perform thread cutting. Ensure that the chuck supports flexible length compensation and has an adjustable safety clutch for overload protection.

Thread forming may generate torque levels up to ten times higher than thread cutting, depending on tool geometry and lubrication. While standard cooling lubricants are partially suitable, high-fat greases or emulsions are recommended for softer materials.

For best results, use forming speeds of approximately 20–30 m/min. Speed variations have minimal impact on quality. The core hole should be slightly larger than in the cutting process. T o avoid bead-like bulges at the thread outlet and ensure smooth engagement of the thread former, provide beveled or tapered edges at the hole’s entry and exit

Internal Tap Core Drilling Values

*When milling is used to produce internal threads, the quality is generally lower than with chipless methods. Milled threads also carry a higher risk of the notch effect. If milling is necessary, use speeds between 20–60 m/min with generous cooling lubrication to reduce risk and improve finish.

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