METHOD AND APPARATUS FOR ROLL FORMING INTERRUPTED THREADING TAPS

Abstract: The present invention relates to a method of roll forming taps. By utilizing the plastic deformation of metals and using roll tapping wheels (1, 2) of skip threading makes tap block to become a skip threading screw (2) by roll forming once and afterwards to become skip threading taps after slotting and grooving. To compare the said method of roll forming with the conventional grinding method, the efficiency of the former increases almost 180 times.

Description: Background of the Invention Taps are generally divided into regular taps and skip threading taps. Both of them are standard tools widely used in the machining internal thread of cylinders and cones. The differences between regular taps and skip threading taps are that the thread pitch S of skip thread taps is two times than that of regular taps, i.e. 2S, provided that the pore diameter "dop" and elevation angle β of two threads are same. As thread pitch S of internal thread of the workpiece to be machined is determined by the elevation angle β of taps, the thread pitch S of internal thread machined by skip threading taps and regular taps are same if the elevation angle β of both is the same. Due to the thread pitch of skip threading taps is 2S, which is two times the thread pitch S of regular taps, skip threading taps avoided slogging of filing, mashed threading, thread cracking of taps, grip of the workpiece against taps, etc. because of large tenacity of material when machining internal thread by regular taps, thereby improve the efficiency 2-3 times and increase the service life 2-3 times as well. At present, there are three methods in machining skip threading taps. The first method employs the grinding away method by hand, that is to grind away every other thread and keep one on the finished products of regular tap; the second method employs the relieving machine to back-off every other thread and keep one on the finished product of regular tap and the third method is to grind skip thread tap directly by thread grinder, that is to place the quenched tap block on the thread grinder with auxiliary equipment to grind thread and skip thread at the same time. The quality of skip thread tap processed by the third method is guaranteed and the technology is more advanced than the first and the second methods. However, the working hours are longer, which can not meet the requirements of the development of production. Description of the Invention and Figures To overcome the backward process of skip threading tap of the abovementioned three methods, the present invention provides the method of roll forming of skip threading tap. By utilizing the plastic deformation of metals, and using roll tapping wheel composed of slip threading, makes tap block to become a skip threading screw by roll forming once and afterwards to become skip threading tap after slotting and grooving, thereby greatly increase the efficiency of making skip threading taps and consequently causing the skip threading tap by roll forming features good cutting performances. The method of roll forming of skip threading tap of the present invention is such a way that a pair of skip threading roll tapping wheels are installed on the two shafts of roll tapping machine, after calibration, put the prefabricated tap block between two skip threading roll tapping wheels. While two skip threading tapping wheels move in the same direction toward the tap block, the movable wheel feed and extrude continuously causing the block plastic deform and becoming the skip threading screw. After slotting and grooving, it will become a skip threading tap. Figure 1 is a further description of the method of roll forming of skip threading tap of this invention, 1 and 3 in Figure 1 are skip threading roll tapping wheels, the material is Cr₁₂ and heat treatment is HRC60-63. The outside diamter D of two skip threading roll tapping wheels equal the pore diameter dop of the skip threading tap to be machined multiply number of threads N (N must be even number) arranged in group on the circumference of roll tapping wheel. The thread elevation angle β of skip threading roll tapping wheel and thread pitch 2S equal those of skip threading tap to be machined respectively. The deflecting angles  opposite each other of the thread joint of skip threading roll tappping wheels equal 360° which divided by the number of threads and multiply by 2, i.e.  =. The length of two skip threading rolled tapping wheel is round number multiples of thread pitch 2S of skip threading tap to be machined. 2 in Figure 2 is skip threading tap block, the material is W 18r4v (high speed steel), and quenched to the hardness of HRC₁₂ by sealed quenching. Cut and machined left and right to the size of the tap, the error, generally, will be 0.01mm. As shown in Figure 1, before the skip threading roll tapping wheel put into operation it must adjustment in the following process: 1. Adjust two shaft keys of roll tapping machine in the same direction toward tap block and make the central line of two shaft keys balance each other; 2. Install the roll tapping wheel to the shaft key with the end surfaces of two roll tapping wheels adjusted in the same straight line; 3. The relative distance of two roll tapping wheel equal the outside diameter of the tap block minus H (H is the teeth depth of the tap); and 4. At first, test roll after flushed with cooling fluid during the test run of roll tapping wheel. After the clearness, finish and angle of teeth of the test rolled threads meet the requirements of technology, then the roll tapping wheel can be put into normal operation. Figure 2 shows the skip threading tap after slotting and grooving of the skip threading screw by roll forming of this invention. 2 in Figure 2 is the skip threading screw by cold roll forming, walls 1 is the skip threading tap after slotting and grooving of the skip threading screw 2. The geometric shape, technical precision, finish of thread, neibouring thread pitch and accumulated thread pitch measured by tool microscope of the skip threading tap made by roll forming of this invention reach the technical precision requirement of roll tapping wheel. Table 1 is the comparison of datum measured before and after heat treatment of skip threading tap of cold rolled left 2M18 x 1. Comparison between rolled skip threading tap and ground skip threading tap: 1. The internal metallurgical structure, martemper, carbide and residual austenite of both rolled skip threading tap and ground skip threading tap and ground skip threading tap are basically the same; 2. The structure of rolled skip threading tap is compact and its fibre structure shows continuity to enhance the strength of threads, while the fibre structure of ground skip threading tap is in broken shape; 3. The finish of internal threads of rolled skip threading tap and ground skip threading tap are same. Both reach the finish of ▿6 - ▿7. Besides the above mentioned comparison, especially compared with the roll forming of skip threading tap of this invention to the ground forming, the machining efficiency is greatly enhanced. For instance, in machining two skip threading taps of left 2K18 x 1 and M14 x 1.25, when machined by thread grinder, each tap will require 30 minutes, while by the method of roll forming, each tap will need only 5 seconds, thereby increase the efficience by 180 times and greatly save the production cost.

Claims: 1. A method of roll forming a skip threading tap by roll tapping machine, characterised in that a pair of roll tapping wheels are installed on two shafts of roll tapping machine with a prefabricated tap block between two skip threading roll tapping wheels, rotating the two skip threading tapping wheels in the same direction toward the tap block so that the movable wheels feed and extrude continuously causing plastic deformation of the tap block to form a skip threading screw and there after slotting and grooving the strip threading screw to form a skip threading tap. 2. A method according to Claim 1 characterised in that the outside diameter of skip threading roll tapping wheels is equal the pore diameter "dop" of the skip threading tap to be machined multiplied by the number of threads N (N must be even number) arranged in group on the circumference of roll tapping wheel, in that the thread elevation angle  of skip threading roll tapping wheel and thread pitch 2S equal those of skip threading tap to be machined respectively, in that the deflecting angles  opposite each other of the thread joint of skip threading roll tapping wheel equal 360° which is divided by number of threads mutliplied by 2, i.e.  =, and in that the length of the two skip threading roll tapping wheels is a round number multiple of thread pitch 2S of skip threading tap to be machined.

Inventors: GENG, JIANG YAN
Applicants: SHANGHAI BICYCLE FACTORY
Application year: 1987
Main classification: B21H3
IPC codes: B21H3, B23G5, B23G7