Thread Rolling, Types and Appications

tagsThread Rolling Machine For Bolt

The thread can be made by

By using some type of

Or equipped with suitable threading rollers for automatic screw machines or turret lathes. If one

When using the machine, (automatically or manually) place unthreaded screws, bolts or other "blanks" in the

It has a thread-like ridge that sinks into the blank and forms a thread of the desired shape and pitch by replacing the metal. In the case of a large number of bolts, screws, studs, screws, etc., the thread rolling process is used. Compared with any other method, this method can produce threads within the rolling process range faster. Due to the cold working effect of the die, the strength of the rolled thread is 10% to 20% higher than that of the cut or ground thread, and the increase in fatigue strength may be much higher. Other advantages of the rolling process are that no raw material is wasted when forming the thread, and the surface of the rolled thread is harder than the surface of the cut thread, thereby improving the wear resistance.

A type of machine widely used

With a pair of flat or right-angle molds. When using the machine, one die head is stationary, and the other die head is reciprocating. The thread-forming ridges on these molds are inclined at an angle equal to the helix angle of the thread. When manufacturing molds for precision thread rolling, threads can be formed by milling and grinding after heat treatment, or they can be formed by grinding "from the solid" after heat treatment. Use vitrified wheels.

In the thread rolling machine, a wire is formed in a channel of the workpiece, inserted into one end of the mold manually or automatically, and then rolled between the mold faces until it pops out at the other end. The relationship between the position of the mold and the thread being rolled is such that the top of the thread-shaped ridge of one mold directly faces the bottom of the thread groove in the mold at the point of contact with the thread. Others died at the point of contact. Some form of mechanism ensures that the blank is started and aligned with the mold at the correct time.

For this type of machine, the blank is tightened while rolling between two or three cylindrical molds (depending on the type of machine), and then the permeability is adjusted according to the hardness or wall thickness of the material to be pressed into the blank. Perform threading operations on pipes or hollow parts. The mold has ground or ground and ground threads, and the pitch diameter is a multiple of the pitch diameter of the thread to be rolled. Since the diameter of the mold is much larger than that of the workpiece, multiple threads are required to obtain the same lead angle as the workpiece. The thread can be formed in one turn or less, or it may take several turns (for example in rolled hard materials) to obtain the same progressive permeability as a flat or straight die (if extended to a length of about 15 inches) or 20 feet (4.6 or 6 m). The important feature of these machines is the precise adjustment or matching of the threaded rollers so that they are correctly aligned with each other.

: On a two-roller machine, the workpiece rotates between two horizontal electric threading rollers and is supported by a hardened support rod on the lower side. A roller is conveyed inward by hydraulic pressure to an automatically adjusted depth.

: On this machine, the blank to be threaded is kept in the "floating position" while rolling between three cylindrical molds. These molds are moved inward at a predetermined penetration rate through the toggle arm until the desired Pitch diameter. The movement of the die is driven by a cam that is driven by a selected variable gear to provide the required compression, holding and release cycles.

The productivity of thread rolling depends on the type of machine, the size of the machine and the workpiece, and whether the parts to be threaded are inserted manually or automatically. Reciprocating flat die machine suitable for ordinary steel, threading 30 or 40 parts per minute, diameter ranging from 5/8 to 1 1/8 inches (15.875-28.575 mm), and threading 150 to 175 mm per minute. Screw sizes range from No. 10 (.190 inches) to No. 6 (.138 inches). For the usual heat-treated alloy steel with a hardness of 26 to 32 Rockwell C, the output may be 30 or 40 minutes or less per minute. For cylindrical mold type machines mainly used for precision machining and hard metal design, the usual production speed is 10 to 30 parts per minute, the number depends on the hardness of the material and the allowable mold penetration speed per revolution. These production rates are for general reference only. The diameter of rolled threads usually varies from the smallest machine screw size to a maximum of 1 or 1.5 inches (25.4 or 38.1 mm), depending on the type and size of the machine.

Both flat and cylindrical molds are used in aviation and other factories for precision machining. Under the condition that the precise die and blank diameter are kept close to the limit, it is feasible to produce the rolled thread that meets the American standard level 3 and level 4. The blank sizing can be carried out by centerless grinding or a mold combined with a die operation. The blank should be round. Generally, the diameter tolerance should not exceed ½ to 2⁄3 of the pitch diameter tolerance. The blank diameter should be within the correct size (close to the pitch circle diameter, but should be determined by actual tests), and be reduced to the minimum allowable value, even if the large diameter may reduce the tolerance to ensure that the correct pitch circle diameter is slightly different . Precision thread rolling has become an important method for threaded connection of alloy steel studs and other threaded parts, especially in aviation operations that require precision and high fatigue resistance. Micrometer screws are also outstanding examples of precision thread rolling. Although it is common practice to finish rolling taps by grinding when 3 and 4 levels are required, the process has also been applied to tap manufacturing.

The types of steel range from soft low-carbon types of ordinary screws and bolts to nickel, nickel-chromium and molybdenum steels such as aircraft studs and bolts, or are used for any work that requires excellent strength and fatigue resistance. Typical SAE alloy steel numbers are 2330, 3135, 3140, 4027, 4042, 4640, and 6160. After heat treatment, these steels usually have a hardness of 26 to 32 Rockwell C and a tensile strength of 130,000 to 150,000 psi (896). –1034 MPa). Although harder materials may be rolled, grinding is more feasible when the hardness exceeds 40 Rockwell C. Thread rolling is not only suitable for various steels, but also for non-ferrous metals, especially when threads are difficult to cut due to "tearing".

The diameter of the screw blank or cylindrical part on which the thread is to be rolled should be smaller than the outer diameter of the screw, which should just compensate for the metal that is displaced and lifted above the original surface due to the rolling process. The increase in diameter is approximately equal to the depth of a thread. Although there are rules and formulas for determining the diameter of the blank, it may be necessary to change the calculated size slightly to ensure a good thread is formed. The diameter of the blank should be verified by experiments, especially when rolling precise threads. Due to the higher hardness or toughness of metals, some blanks have higher resistance to displacement than others. The following figures may be useful for determining the scale of the experiment. The blank diameter of the screw that varies from 1/4 to ½ is 0.002 to 0.0025 inches (50.8–63.5 μm) larger than the pitch diameter, and the blank diameter of the screw from ½ to 1 inch (12.7–25.4 mm) or more is larger than the pitch diameter. The pitch circle diameter is 0.0025 to 0.003 inches (63.5-76.2μm) larger. The blanks slightly smaller than the pitch diameter are used for bolts, screws, etc., and they should fit relatively freely. The diameter of the blank for this type of work may be 0.002 to 0.003 inches (50.08–76.2 μm) smaller than the pitch diameter of thread sizes varying from ¼ to ½ inch (6.35–12.7 μm), and vary from 0.003 to 0.005 inches (76.2–7). small. For sizes over ½ inch, it is 127μm smaller than the pitch circle diameter. If the thread is less than 1/4 inch, the blank is usually 0.001 to 0.0015 inches (25.4–38.1 μm) smaller than the pitch diameter of the normal working grade.

When the thread is behind the shoulder, the thread is sometimes rolled in an automatic screw machine and a turret lathe so that it cannot be cut with a die. In this case, the advantage of winding the thread is to avoid the second operation. Round rollers are used to roll threads in screw machines. The rolls can be processed by any method that produces satisfactory threads in the tangential or radial direction. In the former case, the roller gradually comes into contact with the periphery of the workpiece and completes the processing as it passes through the surface to be processed. When keeping the roll in a radial position, it is only necessary to press it to one side until a complete thread is formed. The method of applying the roll may depend on the relationship between the tapping operation and other machining operations. Because it is difficult to roll threads in steel, thread rolling in automatic screw machines is usually only suitable for brass and other relatively soft metals. However, threaded rollers made of chromium-nickel steel with a carbon content of 0.15% to 0.20% are quite effective when applied to steel. 3% nickel steel with about 0.12% carbon has also been shown to penetrate brass well.

The diameter of the threading roller used in the screw machine may be approximately the same as the diameter of the thread, but for sizes less than 3⁄4 inches (19.05 mm), the diameter of the roller is several times the thread diameter minus to obtain a better rolling effect. When the diameter of the thread and the roller are actually the same, a single thread roller is used to form a single thread on the screw. If the diameter of the coil is set to twice the diameter of the screw, in order to avoid using a smaller coil, the coil must have double threads. If the diameter of the threaded roller is three times that of the thread, a triple thread is used, and so on. When the roll diameter is a certain multiple of the workpiece, these multiple threads are necessary in order to obtain the corresponding helix angle on the roll and the workpiece.

The pitch circle diameter of a threading roll with a single thread is slightly smaller than the pitch circle diameter of the thread to be rolled, and in the case of a multi-threaded roll, the pitch circle diameter is not an exact multiple of the thread pitch. But it has also decreased. The reduction recommended by a screwdriver manufacturer is given by the formula shown at the end of this paragraph. The description of the terms used in the formula is as follows: D = the pitch diameter of the threading roll, d = the pitch diameter of the thread, N = the number of single threads or "starting points" on the roll (this number refers to the required roll diameter), T = single thread depth:

: Use the above formula to find the pitch diameter of the double-threaded roll used to roll ½ inch U.S. standard threads. The pitch diameter d = 0.4500 inches, and the thread depth T = 0.0499 inches.

One or more threads on the roll should be left-handed in order to roll right-hand threads, and vice versa. The roller should be wide enough to overlap the part to be threaded, provided that there is a gap at the end, and if possible, a gap should be formed. When rolling, the threads on the rolling line should be sharp, and the American (national) standard form of thread should be rolled so that less pressure is required to replace the metal when rolling the thread. The bottom of the wire groove on the coil can also be kept sharp or flat. If the bottom is sharp, assuming that the roll is an American roll, the roll can only sink far enough into the blank to form a coil with a flat top. Generally, the number of threads on the roll (whether double, three, four, etc.) is selected so that the diameter of the threaded roll is between 1¼ and 2¼ inches (31.75–57.15 mm). When making a threaded roller, the end is inclined at a 45-degree angle to prevent the thread from breaking at the end of the roller. Precautions should be taken when hardening, because if the sharp edges are burned, the rollers will be useless. Wire rolls are usually ground after hardening. The method is to fix the wire rolls on the spindle of the lathe and use emery and oil on a piece of hardwood. In order to obtain good results, the threaded roller should be tightly fixed in the support. If the installation of the reel is loosened, the threads will be damaged.

The shape of the workpiece and the operating characteristics required to produce the workpiece largely determine the method of applying the threaded roller. Some points to consider are as follows:

When the diameter to be rolled is much smaller than the diameter of its front shoulder, a cross slide knurling tool holder should be used. If the part to be threaded is not behind the shoulder, a bracket fixed according to the swing principle should be used. When the workpiece is longer (the length is greater than two and a half of its diameter), a swinging roller stand should be used with supports. If the workpiece can be cut after rolling the thread, the cross slide rolling bracket should be used. The method of applying the support to the workpiece also determines to some extent the method of applying the line roll. When there is no other tool working with the needle at the same time and there is no chip, the needle can be fixed more firmly by rolling the needle from below instead of over the workpiece. When the roller passes the workpiece, it may increase lateral slip. If the part to be threaded is tapered, it is best to fix the roller in a laterally sliding roller holder to present it on the workpiece.

Wire mills will accept specifications with a diameter tolerance of plus or minus 0.002 inches (50.8 mm). It is especially important to maintain this tolerance on stocks of long screws with small diameters. On shorter length screws, the material will flow, and if the wire size is too large, there will be little trouble, but for screws with a length greater than ten times its diameter, the material will be limited and will "burn" if tolerances It occurs if it is greater than the specified tolerance. If the size of the wire is slightly smaller, the thread after rolling will have a jagged appearance due to the incomplete formation of the wave crest. On screws with sizes below 10-24, tolerances of plus or minus 0.001 inches should be observed to ensure good results.

When the threaded roller is made of high carbon steel and used on brass, surface speeds of up to 200 feet per minute can be used. However, better results can be obtained by using a lower speed than this. When the reel is fixed in a bracket connected to the transverse slide rail and presented tangentially or radially to the workpiece, a higher speed can be used compared to fixing it in a swing tool. This is due to insufficient rigidity of the swing type cage. When using the "cross sliding roller bracket", the above table provides the feed to be used when using the cross sliding roller bracket; the lower half of the table provides the feed for thread rolling with a swing tool. When the root diameter of the blank is not less than five times the two-fold depth of the thread, these feeds are suitable for rolling threads without support. When the root diameter is less than this value, a support should be used. When the width of the roll is greater than one-half of the smallest diameter of the workpiece to be rolled, the support should be used regardless of the pitch of the thread. When the minimum diameter of the workpiece to be rolled is much smaller than the root diameter of the thread, the minimum diameter should be used as the determining factor for the feed to be used.

This article provides an overview of thread rolling, including the machine used, productivity, material, pitch diameter, and machine speed and feed rate. Learn more about

Excerpted from the 30th edition of the Mechanical Manual, which has been published and

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