Tapping is a machining process for producing internal threads. A tap is a cylindrical or conical thread cutting tool having threads of a desired form on the periphery. Combining rotary motion with axial motion, a tap cuts or forms the internal thread.

Most taps are made of high-speed steel. General-purpose high-speed steels, such as M1, M2, M7 and M10 are most widely used for taps. For tapping difficult to machine metals, (e.g., heat resistant alloys, steels harder than about 35 HRC, etc), taps made of M15 or T15 are justified. However, they cost 2 to 3 times higher than the similar taps made of general purpose type; the highly alloyed types are more difficult to grind and more susceptible to grinding burn. Most users sacrifice tap-life rather than pay for maintenance and repair.

Surface treatment of taps (nitriding, chromium plating or titanium nitride coating) increases tap-life by 500 per cent or more and improves thread finish, but it is an expensive and inconvenient proposition. The taps must be re-treated each time after they are ground.

The principal factors that influence the selection of equipment and procedure for tapping and those which affect the thread quality, productivity and cost are:

• Size and shape of the workpiece
• Thread size and depth
• Tolerance and finish specification.
• Whether blind or through holes are being tapped.
• Speed.
• Use of lead control, and
• Lubricant.

Stainless steels pose maximum problems in tapping. Various suggestions are offered to overcome specific problems.

Lubrication is more important in tapping than in other machining operations, because tap teeth are more susceptible to damage from heat than most other cutting tool surfaces. Chips are more likely to get congested in tapping than in operations in which the cutters are not surrounded by the work material.

Lubricant is used in tapping all metals.

The lubricants most commonly used for tapping are sulphurised or chlorinated oils, soluble oil emulsions or mineral oils (blended with lard oil or animal fat). Sulphurised oil is generally satisfactory for many tapping applications including stainless steels, but it stains some of the metals. Soluble oils are less effective than neat oils for preventing the adherence of tools to workpieces and preventing built up edges. Mineral oils are less effective for cooling and flushing away chips than soluble oil emulsions. Also, they have high viscosity resulting in clinging of chips.

A lubricating paste is a product of dispersion of a thickening agent and a number of solid lubricants with various melting points in a liquid lubricant. Compared to oil, which is generally used in tapping, paste lubricant gives the following advantages:

• It does not drip like oil
• Less frequent application
• Provision of a better mechanical lubrication cushion for extreme conditions, such as stock loading, reversing operations, low speeds and high speeds.

The proprietary ingredients in the primary lubricant dramatically improve the overall tapping performance. During tapping, the paste melts and forms a molecular film, which reduces coefficient of friction. Performance of various lubricants while tapping is presented in Table-1.

In hand-tapping of stainless steels, the use of paste lubricant brings down tapping time by up to 45 per cent, at the same time offering highest ease of tapping. Worker-fatigue is reduced. These factors lead to increased productivity.

Stainless steels are difficult to tap. The common problems encountered in tapping stainless steel are tap breakage, oversize threads and metal build-up on tooth. Various aspects of tapping stainless steel are discussed below.

Selection of taps for stainless steel

High speed steel taps are successfully used for tapping all grades of stainless steel. Recommended hook angle and chamfer relief angle for different grades of stainless steel are shown in Table-2.

Number of flutes

Two-fluted taps are suitable up to 6 mm, for sizes from 6 mm to 12 mm three-fluted taps and above this range four-fluted taps are recommended.

Tap Style

• Hand taps: Hand taps are suitable for through-hole tapping. For effective chip clearance in blind hole tapping, frequent reversing of taps is needed.
• Spiral point tapping: Spiral point of the tap pushes the chips ahead of tap and hence suitable for through holes.
• Spiral fluted taps: Spiral flute provides lifting action for the chips and forces the chips along the flute. Hence, these taps are suitable best for blind-hole tapping.

Modification of tap design

The major problems faced during stainless steel tapping (especially austenitic and chromium ferritic grades) are rough threads and tap breakage. In these grades of steels, because of their 'gumminess', immediately after tapping operation, the metal "flows back" and tends to seize the tap when direction of tap is reversed. In such cases, grinding a hook is recommended.

For improving tapping performance, the following modifications of tap design are recommended:

• Grinding cylindrical grooves along the land.
• Providing relief behind the cutting edge.
• Narrowing the land width by grinding off approximately 2/3 of land from heal side.
• Omitting the cutting edges on alternate threads.

Tapping practice

More power is required for tapping stainless steel than other steels.

Selection of sharp taps, adequate lubricant and chip disposal together enable to prevent work hardening.

Tap drill selection

Tap drill selection should be made to limit the thread depth between 60 to 70 per cent of full thread form. To prevent frequent tap failures, the thread depth should not exceed 75 per cent of full form.

When holes are drilled or reamed for tapping, care should be taken to prevent work hardening of the surface. Whenever possible, drilling and tapping may be done in the same setting, using machine spindle to guide the tap to minimise the tap cutting oversize threads. If material tends to flow back during tapping as a result of cutting while reversing, frequent reversals for tap is needed to reduce the final load on tap and risk of tap breakage.

Lubricant

Right kind of lubricant improves thread finish, carries the chip during tapping, reduces friction and cutting temperature, lowers cutting force and torque, retains sharpness and hardness of cutting edges, and prolongs tap-life. A variety of tapping lubricants in fluid and paste form are available now.

Summary

• Tapping troubles are often caused by too small tap drills.
• For economic and quality threads, thread depth should be limited to 75 per cent of full form.
• To prevent work-hardening of stainless steel during tapping, sharp taps with proper cutting angle should be selected.
• Certain modifications in the tap design and the use of right type of lubricant contribute significantly towards improvements in tapping performance.

S P Shenoy, M.Tech, (Met Engg), M.I.I.M., is CEO, Steel Plant Specialities, Mumbai. Tel: 022-67978060. Fax: 91-22-25552459. Email: info@steelplantspecialities.com

Acknowledgements

The author expresses his sincere thanks to Dr J Krishnan Central Workshop, BARC, Trombay, Mumbai 400 085 for the valuable guidance given to write this paper and also for conducting a series of trials to compare the performance of various tapping lubricants.

Suggested Reading

• Metal handbook, Vol.16 - Machining-Tapping pp. 259 to 265 (9th Edition).
• Drills, Reamers & Taps - Information Handbook by Addison & Co Ltd, Madras pp. 59 to 67.
• Handbook of Machining Data for Cold Finished Steel Balls published by Republic Steels Inc., pp. 142 to 150.
• Product Information Bulletin on ESPON-TA Tapping Lubricant published by Steel Plant Specialities, 211, Raikar Chambers, Govandi (E), Mumbai 400 088. Tel: 022-67978060.