Home > Knowledge > Content
Tapping process of inner thread of stainless steel pipe fitting
Feb 11, 2019

Taps are usually used for tapping internal threads on stainless steel pipe fittings.Due to the high viscosity of stainless steel material, poor chip breaking performance, so in the process of tapping, it is easy to appear chip scraping workpiece thread or tap collapse blade phenomenon, affecting the processing efficiency and thread quality.In order to extend the service life of taps and improve the quality of thread processing, the following points should be noted.


(1) choose better tap material.Adding special alloying elements to ordinary high speed tool steel can significantly improve the wear resistance and toughness of the tap.


(2) coating the thread surface of the tap with titanium nitride coating can significantly improve the wear resistance, heat resistance and lubricity of the tap.


(3) appropriately increase the leading Angle of the tap.However, it should be noted that if the leading Angle of the tap is too large, it is easy to cause the blade breakage of the tap and the multi-edge thread of tapping.


(4) appropriately increase the tap back amount.However, it should be noted that if the amount of shovel back is too large, it is easy to produce the phenomenon that the chip is inserted into the back Angle of the tap, and the tap thread smoothness is not good.


(5) reasonable selection of tool heat treatment method to take into account the hardness and toughness of the tap.


Prejjjn-r of workpiece bottom hole


(1) should be based on the principle of tapping processing capacity as small as possible, at the bottom of the pre machining away L size, especially the NPT thread and the logo f, r thread, its taper is 1:16, whereas most stainless steel factory is according to the small end of the thread diameter cast straight hole as tapping away at the end of L, the second part of the tapping processing capacity is larger, easy to cause the following consequences:


Tapping twist distance is large, tap easy to fracture or collapse edge;


(2) tapping thread finish is poor;


(3) tapping cylindrical thread with plug gauge detection when the diameter is small, the reason is not the thread diameter is not consistent, but because of chip scratch thread, there are burrs.


2 the bottom hole size should preferably be the size of the path being processed with 0.1-0.2mm margin.


(3) before tapping, the hole expanding L processing should adopt custom reamer.In case of American Nf, r thread and British f, r thread, it should be processed into taper bottom hole.At present, many enterprises use waste taps to make reamer by hand.But in the system reamer should pay attention to two problems: 1.(2) the tap edge front corner should be kept straight.


3. Reasonable use of taps


(1) different chip trends should be considered when processing different types of workpieces.When necessary, the new tap also need to repair grinding, grinding principle is to tap when trying to make chip easy chip groove discharge, to prevent scratches have been processed thread.


Examples are as follows:


Case 1.Processing collar (SPU) : the collar tapping processing is best to use vertical tapping machine (or three head tapping machine).The grinding method of the tap is: (1) lengthening the leading part of the tap, so that the tap gradually into the cutting, to


Increase the service life of the tap;(2) the tap on each blade on the blade Angle, but the first grinding should not be too long (about 3 -- 4mm), later can gradually lengthen.The standard to judge the grinding quality of edge inclination Angle is cutting chips during tapping


The horn is discharged from the end face of the coupling without damaging the tapped thread (see figure 1).After tapping for a period of time, if it is found that the quality of tapping thread is not good, the blade Angle can be extended.


Case 2.Processing blind hole, tee and elbow: spiral groove tap can be adopted to discharge the chip from the handle of the tap as far as possible.


(2) the different cores of the tap and the workpiece processed are also the important reasons affecting the service life of the tap, which will cause uneven force of the tap in the tapping process and cause premature fatigue failure or even fracture of the tap.The main factors affecting the different centers include the machine tool spindle, tap joint, tap handle, the processed workpiece, the manufacturing quality of the fixture and the adjustment quality of the fixture and the machine tool working table.The solution is to carefully adjust the machine tool using qualified tap joints and fixtures.In addition, in the process of the tap joint and the machine tool rod do not pull too tight, so that the tap joint a little bit of activity;When the tap is connected with the tap joint, the top wire should not be too tight so that the tap can be self-centering.


(3) the service life of the tap is closely related to the re-grinding quality of the tap.When regrinding the tap, the following questions should be paid attention to:


  1. When processing elbow and tee of British standard FRR, American standard NFF thread, when tapping to thread size, there may be two tap end face collision, the reason is the tap base face is long.In this case, the end face of the tap needs to be ground short, but after the end face is ground short, the performance of the tap may be adversely affected.It can be seen from the relationship between the tap and the workpiece processed before and after the end face grinding of the tap shown in figure 2 and figure 3 that the thread part of the tap after the end face grinding cannot be normally imported into the hole, but is pressed into the hole by the end face, thus affecting the performance of the tap.

In order to solve this problem, some users have to grind the leading part of the tap by themselves. The leading part of the tap before and after grinding is shown in figure 4 and figure 5 respectively.Because of the shape change of the front guide part after grinding, the performance of the tap will still be affected.The way to solve this problem is to maintain the original geometric shape of the front guide part when grinding, it is best to use a tap shovel grinding machine for grinding (or manual grinding on the grinder).The best way to solve the long base surface of the tap is to customize the tap with short base surface according to the specific requirements of the thread to be machined.


(2) tapping the tap after period of time will become dull, at this time must be timely grinding taps, and after grinding tap often is not working or not durable, the reason is that heavy grinding taps in general is on the floor grinder manual grinding (the thickness of the grinding wheel for 5 ~ 6 mm or thicker) commonly, there is no guarantee that the tap after grinding groove type, rake Angle and slot finish.The groove shape before and after grinding and the chip flow direction during tapping are shown in FIG. 6 and FIG. 7 respectively.


Practice has proved that the best tap Angle for stainless steel pipe fitting is 13.The left and right sides.If the front Angle is too large, it is easy to break the blade after tapping the tap.If the front Angle is too small, then tapping thread finish is poor, and tap tapping resistance is large.In addition, grinding tap easy to form a blade Angle, will also change the chip flow direction.


4. Selection of tapping oil


Tapping oil is also very important when tapping stainless steel pipe.Tapping oil must have good cooling, lubrication and rust prevention functions.If the tapping oil lubrication effect is not good, then tapping thread smoothness is poor, and tap tapping resistance is large.If the tapping oil cooling effect is not good, then the tap is easy to stick to the teeth, that is, in the tap thread and the front corner surface easy to bond stainless steel chip.Special tapping oil made of stainless steel has better effect and can be used directly.In addition, can also be used to tap oil and oil configuration, but the ratio must be appropriate.Slurry pump industry machining tool selection domestic enterprises engaged in the production of slurry pump and its accessories processing, are facing the problem of machining tool selection.Slurry pump is the most important characteristics of all fluid channel has the very high abrasion or corrosion resistance, so it is suitable for conveying abrasive or corrosive slurry, its products are widely used in mining, metallurgy, petroleum, coal, transportation, electric power, building materials, municipal engineering and other industries, used in mining, desilting, conveying tail, concentrate, ash, coal, iron powder and so on strong abrasion, high concentration slurry transportation.At present, most of the domestic slag slurry pump wear-resistant parts are made of high chromium cast iron, high chromium cast iron casting hardness can be up to about 65HRC, is one of the best wear-resistant materials at present, the amount of alloy, the production process is complex, since the 1980s in our country began to apply.To the 1990s, the general use of cemented carbide machining tools, machining process complex, high cost and low efficiency, its difficult machining has been troubled by many mechanical processing manufacturers.Ceramic cutting tool is a new material cutting tool in modern metal cutting processing, which is characterized by high hardness, high strength, high red hardness, high wear resistance and excellent chemical stability and low friction coefficient. It can not only process some ordinary blades can not process the super hard material, but also can improve the production efficiency.The use of ceramic tool in many slurry pump manufacturers has proved that it can indeed solve the machining difficulties of high chromium cast iron, not only can effectively reduce the production cost, but also can greatly improve the production efficiency.At present, there are a lot of domestic production of ceramic tool manufacturers, tool material types more, greater performance differences, to solve the high chromium cast iron cutting, the correct choice of ceramic tool is extremely important.If the choice is improper, not only affect the promotion and use of ceramic tools, but also affect the production of users, resulting in users to lose confidence in ceramic tools, has been the use of ordinary tool processing, affecting the development of the company.