Tapping is a relatively difficult machining process because the tap is almost buried in the workpiece for cutting, and its cutting load per tooth is larger than that of other tools, and the tap has a large contact area with the workpiece along the thread, and it must accommodate and discharge chips when cutting the thread, so it can be said that the tap works under very harsh conditions. In order to tap smoothly, various problems that may arise should be considered in advance. Such as the performance of the workpiece material, what kind of tool and machine tool to choose, what kind of cutting speed to choose, feed rate, etc.

Tapping on special workpiece materials

The machinability of the workpiece material is key to the ease or difficulty of tapping. Hansen-Whiney company designersVernon Hill said:“Today's tap manufacturers are mainly focused on developing taps” for special material machining. . For high-strength workpiece materials, the tap's lead angle and hook are usually smaller to increase the strength of the cutting edge. Taps with a larger hook are used in situations with higher cutting torque. However, if the hook is too large, the cutting edge may chip and embed in the thread. Generally, materials that produce long chips require a larger lead angle and hook to coil and break the chips. On the other hand, too sharp an angle can make the cutting edge too weak.1)see Figure(the degree of hook on the frontHOOK)—

Another significant angle of the tap affected by the machinability of the workpiece material is the back angle. Processing harder workpiece materials requires a larger back angle to reduce friction and allow the coolant to reach the cutting edge, but too large a back angle will also reduce the self-centering ability of the tap when cutting into the workpiece. When cutting soft materials, a too large back angle can cause the thread hole to expand. For cutting workpiece materials with high hardness and strength, the tap should choose an eccentric back angle starting from the cutting edge; while for easily machinable materials, choose a tap with a composite eccentric back angle(Con-eccentric), which is characterized by the absence of a back angle edge below the cutting edge and then grinding out the back angle
(see Figure2).

Spiral flute tap is mainly used for tapping threads in blind holes. When processing workpieces made of materials with high hardness and strength, the spiral flute tap with a smaller spiral angle can improve its structural strength. For example Hanson- Whitney company's ULTI-XT spiral flute tap with a spiral angle of15°, is mainly used to process400 series of strong and tough materials Stainless steel. And the company'sULT－XT300 spiral flute tap, with a spiral angle of41°, is mainly used to process300 series stainless steel.

For tough-to-machine materials, use a tap with a shorter chip flute length to reduce torque during cutting. For materials with elastic memory(With elastic memory)such as titanium, the tool should have a larger cone, gradually reducing the friction caused by the material“bounce”from the front of the tap to the handle.

When optimizing tapping conditions, tapping manufacturers also consider factors other than the tool.Emuge company's technical department manager Alan Shepherd said that high-performance tapping can significantly increase cutting speeds, from the previous 35 - 50sfm on cast iron materials to 250sfm. However, such cutting speeds can only be achieved with specific tapping tools and machine tools with special performance. He said:"Maximum cutting speeds are sometimes limited by machine tool performance. For smaller tapping tools, the spindle speed may already exceed the maximum spindle speed to achieve the ideal speed [ rpm= (sfm x 3.8)/ tapping diameter ]. On the other hand, high-speed cutting with larger tapping tools can generate large torques, which may be greater than the horsepower provided by the machine tool.”

Alan Shophedsaid:"Applications of tools with 700psi internal cooling capability can achieve250sfm cutting speeds on the machine tool without internal cooling facilities, the cutting speed is limited to150sfm. Unlike most metal cutting tools, a tap is unique in that it has a large contact area with the wall of the workpiece hole, so cooling is essential. If a high-speed steel tap becomes too hot, it can break or seize."Shepherdsaid:"The geometry of the high-performance tap fromEmuge company is characterized by a larger back angle and a counterclockwise cone."

The geometry of the above mentioned tap, together with a special coating surface(such asTiN,TiCN,CrNorTiAlN), can significantly increase the life of the tap. These heat-resistant, smooth coatings reduce cutting forces and allow tapping at higher cutting speeds. In fact, the development of high-performance taps has greatly contributed to the increase in spindle speed and power of machine tools.

The use of hard-faced gold twist chisels is increasing.

Just as hard metal cutting tools gradually replace high-speed steel cutting tools in lathe turning, hard metal alloy tap also begins to be more used in tapping operations, compared with high-speed steel, hard metal alloy has high hardness and large brittleness, and there are problems with chip removal when tapping with hard metal alloy tap. Although so, hard metal alloy tap has a good effect on processing cast iron and aluminum alloy materials, and the failure form of the tap is mainly mechanical wear.

Alien BenjaminVice President of the companyAmy Haywoodsaid that the use of tungsten alloy wire cutters is preferred to obtain a long tool life, as a large number of cast iron and aluminum alloy parts are processed in the automotive industry. Tungsten alloy wire cutters have a longer life than high-speed steel wire cutters when machining workpieces of these materials. In the automotive industry, a significant reduction in tool change time is an important factor, and the long life of tungsten alloy wire cutters will minimize tool change time. The effect of using a small spiral angle tungsten alloy wire cutter with a surface coating on an aluminum alloy workpiece with a silicon content of8%-12%is very good. He also said that the use of tungsten alloy wire cutters made of submicron-grain hard substances can increase the toughness of the tool without reducing its hardness, which is very good for cutting normalized steel, plastics and difficult-to-machine nickel-based alloys. The company'sPRXseries of wire cutters can continuously tap more than200 threads on nickel-chromium-iron alloyInconel 718 under certain conditions, while it was necessary to grind before.

Regal Cutting ToolCompany International Sales ManagerGene Delettsaid that the company's fastest growing product in the market is the hard alloy taps. He said:"The new fine particle hard alloy taps is also suitable for some general machining, the cutting speed can be increased5-6times, and the tool life can be increased100times. Of course, the price is many times more expensive than the high-speed steel taps, but the benefits that can be used are much greater than the expenditure, especially suitable for the mass production of the automotive industry."But it should be noted that when using hard alloy taps, it is necessary to use a rigid tapping method(that is, without a floating tapping chuck and a turn around protection device, the lead of the workpiece is controlled by theCNCsystem to control the speed of the tap, the feed, and rigidly ensure——the translator).

There are also tap designs developed for certain processed materials that have also achieved good machining results when used on other materials. For example, a tap originally designed for tapping on nickel alloys(a tap made of a high-quality steel, with an appropriate rear angle, and a very short thread length)was also used to processHRC38hardness4340steel, and the results were also very good.

Mskinissaid, despite the continuous development of tap, they learned that many users are not interested in increasing cutting speed, what they care about are the problems that often occur during tapping, such as tap breakage, hole expansion, chip removal, etc.

Choice of tap and cutting data

First, five questions should be clarified before tapping:(1)What is the material of the workpiece to be machined?(2)How strong is the workpiece material?(3)Is the thread hole to be machined a through hole or a blind hole?(4)How deep is the thread hole(or what is the allowed thickness?)(5)What is the category and size of the thread hole to be machined.

Choose the proper tap according to the above content. There are many ways to find the tap needed for machining various workpiece materials, such as referring to samples and selecting charts.Protoyphas developed a computer cutting service(CCS)programCD-ROM, which contains a tapping database to help users select the proper tap, tapping cutting parameters, and finally complete tapping on special materials economically and efficiently. The content related to changing the tapping process, such as tapping milling and rolling forming, is also included in theCCSprogram. It can give users a clear concept: what method to process a tapping hole and how much it costs. It can be said that this program is a new starting point for economical and efficient tapping.