Knowing the geometry of the tap and selecting the right tap for different workpiece materials helps eliminate the fear of tapping.
Many mechanics regard tapping as a fearful route. The reason is as follows: Since the machine tool is automatically controlled by the machining program, the operator can only stand by and watch the tapping process. If the tap is going to hit the bottom of the hole, it is really bad: the programmed machining cycle must be completed without the feed pause when the tapping process is completed; if the feed rate selected during programming is not suitable, then the luck is too bad: the feed rate It cannot be adjusted. When the tap enters the screw hole, what else can you do besides being scared?
This is one of the reasons why tapping processing can cause concern for CNC machine operators. This concern has forced the operator to take many precautions to ensure that the tap can successfully complete the work of cutting the internal bore threads.
Other factors may also cause concerns about tapping. In general, the feed rate of the tap is much higher than most other cutting tools. Each revolution of the tap requires a pitch to advance. For example, a 5/16-18 American tap has a tapping feed rate of 1/18=0.055ipr (1.4mm/r), while a diameter of 0.257′′ (6.5mm) drill has a drilling feedrate. Only 0.005 ipr (0.13 mm / r).
In order to feel better control of the machining, it is necessary to tap at a slower speed. In order to effectively reduce the feed rate of the tap in the screw hole, reducing the spindle speed is the only feasible method. When a 5/16-18 tap is tapped at a spindle speed of 900r/min, the feed rate is 50ipm (1270mm/min), but if the spindle speed is reduced to 720r/min, the feed rate is only 40ipm ( 1016mm/min).
Understand the geometry of the tap
However, although tapping can be tricky, it is not incomprehensible. By understanding the geometry of the tap and what taps are best suited for a particular machining task, problems with tapping can be simplified and reduced.
For example, reducing the chip load can prevent the tap from wearing too quickly. The chip load is defined as the load on any cutting edge of the tool and can usually be controlled by changing the feed rate. As mentioned earlier, it is not possible to change the feed rate during tapping, but the chip load can be changed by selecting the tap.
One solution might be to use a tap with more chip flutes. Each time a tap is added to the tap, a cutting face is added. As the cutting surface increases, the cutting load of each tooth decreases. For example, a 4-slot tap has a chip load of only 1/2 of the 2-slot tap load. However, this may mislead the standard recommendation for metal cutting, which always uses the largest number of sipe. For the tap, this advice may not be correct.
Ken Miskinis, a hole processing specialist at Kenner, points out that more sipe means less space for cutting chips during tapping. Providing more sipe on the circumference of the same diameter means that the sipe is smaller in width and depth. The smaller the chip space, the greater the risk of squeezing the chips, which may cause the tap to break.
Therefore, when increasing the number of chip flutes may not be an ideal option, choosing a different cutting cone length may be a viable solution.
Dr. Peter Haenle, president of Guhring, points out that in general, longer cutting cone lengths mean longer tool life. In the tapping process, since the cutting load is distributed over a long cutting edge, the chip load it bears is also small.
There are three common tap cutting cone lengths: the initial cone has a cutting cone length of 7-10 threads; the middle cone has a cutting cone length of 3-5 threads; the bottom cone has a cutting cone length of 1-2 threads. To provide more options, the tap manufacturer has added a number of specifications, including a 2-3-threaded cutting cone length (sometimes referred to as a half-bottom cone).
Increasing the length of the cutting cone allows the chip load to be distributed over a longer cutting surface. In fact, there are more teeth that cut the thread at the same time, similar to cutting with a single-point thread cutter.
Miskinis explained that the length of the cutting cone has a large effect on the life of the tap because they affect the chip load. Comparing the length of the 4-thread thread or the shorter cutting cone shows that the life of the tap is doubled for each half-thread of the cutting cone length.
Obviously, it is desirable to increase the length of the cutting cone of the tap. The shorter the length of the cutting cone (such as the bottom cone), the faster the tap wears. Therefore, if possible, avoid using shorter cutting cone lengths. Unfortunately, mechanics are not always free to choose.
Haenle says taps with shorter cutting cones are often used to minimize the difference between hole depth and thread length. Many times, the design requirements of parts force people to use taps with shorter cutting cone lengths.
Another way to more effectively tap the wire is to control the chip thickness. For example, when tapping, the chips may become excessively thin. When the taper is tapped, the strip-shaped chips may be generated and a nest-like chip group may be formed, thereby preventing the lubricant from reaching the tap cutting portion and preventing the chips from being smoothly discharged. As in other types of cutting, increasing the chip load may contribute to chip breaking.
The break of the tap is another problem that the mechanics are afraid of. During tapping, a sudden reversal does not cause the tap to break, and the chip clogging the chip pocket will cause the tap to break. In some cases, this means that the chips are pressed tightly in the chip flutes, causing the newly generated chips to go out of the way, causing the tap to break under heavy pressure.
Solve the problem of chip flute blockage
Even if the accumulation of chips in the chip flutes does not cause the tap to break, the clogging of the chip flutes makes it difficult for the lubricant to reach the tool/workpiece contact interface, and the friction between the chips and the tap generates excessive heat. Chip flow is a key component of successful tapping. The direction of chip flow depends on whether the tapped hole is a through hole or a blind hole. When a blind hole is machined with a spiral groove tap, the helix angle guides the chip to flow upward and exit the hole.
The spiral groove of the tap can be divided into a slow spiral groove (spiral angle of 15°-30°) or a fast spiral groove (helix angle of 40°-60°). The geometry of the faster spiral groove allows for more free cutting, while the slower helical groove has a higher cutting edge strength. Typically, fast spiral groove taps are used to machine workpiece materials that are not too stiff or produce banded chips, while slow spiral groove taps are used to machine workpiece materials that can form short chips or high hardness.
When the through hole is machined, the screw tap pushes the chip forward out of the hole. In fact, the tip itself is a left spiral groove that is only ground at the tip of the cone, which creates a downward flow of chips. In other respects, the screw taps look similar to straight taps.
Since the sipe of the screw tap does not actually need to be used for chip removal, but for introducing lubricant, a shallow groove depth can be used. Therefore, the screw tap has a larger core diameter and a higher strength. This also means that the screw tap can benefit by increasing the number of sipe without the problem of chip clogging.
Reasonable choice of taps
Since tapping is a more complex process and there are so many types of taps to choose from, it makes a reasonable task to choose a tap. The main reason for the wide variety of taps is the wide variety of workpiece materials. Tap manufacturers use taper and shovel back designs to customize taps for different workpiece materials.
The cutting face of the tap is the part of the sipe that is used to cut (or shear) the workpiece between the large and small diameters of the thread. The rake angle is the angle formed by the cutting surface and the line connecting the cutting surface from the center of the tap to the large diameter of the thread.
If the apex angle of the tap cutting edge is in front of the rest of the cutting face, the rake angle is positive. Although the positive taper is not as strong as the negative rake, it has excellent shear properties.
The cutting face angle of the negative rake tap is located behind the rest of the cutting face. Although this geometry is stronger than the positive horn tap, it also requires more torque during tapping and produces more cutting heat.
The shape of the cutting face is also one of the factors that determine the cutting performance of the tap. The cutting surface can be either a flat surface or a curved surface. Andrew Strauchen, design and marketing manager at OSG Tap & Die Inc., says straight-cut faces can increase tap strength, while curved cutting edges improve shear performance. For high performance taps, the choice of cutting angle (fore angle) depends on the material of the workpiece to be machined: a larger front angle can be used when the material is softer; a smaller front angle is used when the material is harder.
The tap back is the removal of a portion of the metal material from behind the cutting edge. The greater the amount of backhoes, the greater the clearance between the tap and the workpiece. There are three main types of tap backs: no shoveling back, full width shingle back and partial shoveling back.
The non-shovel back (also known as the concentric shovel back) refers to the blade back of the tap (the thread portion remaining after the tap prepares the chip flute) is concentric with the thread to be machined, that is, there is actually no shoveling back, so when tapping, The surface of the tap will rub against the surface of the thread being machined.
Hand taps usually do not require shoveling because they are used for manual tapping, and the cutting speed is low, resulting in friction and heat that do not have a significant impact on tool life. Since the blade back is concentric with the thread, the thread on the tap helps to guide the tool into the machined thread on the workpiece during tapping.
The full width shovel back (also known as the eccentric shovel back) means that the back of the tap blade is ground to an arc that gradually descends from the cutting edge and is not concentric with the body. This shovel back provides optimum clearance between the tap and the thread being machined. Since the tool does not rub the workpiece material, friction and heat generation can be minimized.
Part of the shovel back is a hybrid of the other two types of shovel backs, with the blade back height gradually decreasing from a certain width from the cutting edge, a small portion of the blade back remains concentric with the leading edge, while the remaining shovel back is at Eccentric state. This shoveling method provides a balance between the friction reduction of the full width back and the guiding tool of the concentric back.
The concentric shovel back and part of the shovel back tap will rub the workpiece material as it enters and exits the tapped hole (some of the shovel taps are lighter), causing friction and heat, which reduces tool life. Therefore, most high-grade taps use a full width shovel back. Haenle explained that the greater the amount of backhoes, the smaller the friction between the tap and the workpiece. As a result, the larger backing reduces tool wear and extends tool life. However, the smaller shovel back facilitates better guiding of the tool in the axial direction because it reduces the radial cutting tendency of the tap.
High-grade taps are not necessarily suitable for all machine tools. When tapping the thread, the high-grade tap has a poor guiding effect on itself. Therefore, a high-grade tap with a full-width shovel back requires a higher precision of the machine's feed mechanism. Haenle says that on the new CNC machine tools, you can use the larger taps at the back of the shovel. When using old-fashioned machines or drills with poor rigidity and standard tapping chucks, the smaller back angle of the shovel helps to better guide tap tapping.
It is clear to the tap manufacturer that a balance must be maintained between the strength of the tap cutting edge and its shear performance. For high performance taps, Strauchen says, the size of the rake angle depends on the hardness of the material being processed. A larger rake angle can be used when processing softer materials; a smaller rake angle should be used when the workpiece material is harder. Determining the front and back corners of the tap requires a comprehensive trade-off. Increasing the rake angle and the back angle creates a sharp cutting edge and a free cutting surface, but reduces the tap strength. Conversely, reducing the rake angle and the back angle can increase the strength of the cutting edge, but it will reduce the shear performance of the tap and produce greater friction.
In order to develop a suitable tool geometry for a particular process, the design of the tap can be quite complex. OSG designed the Hypro VXL tap for vertical tapping and the Hypro HXL tap for horizontal tapping. Strauchen explained that the VXL tap uses a large rake angle to increase the helix angle to create longer chips, which is essential for smooth chip removal of deep hole vertical tapping. With the unique flute type, it is possible to form tightly wrapped chips that are easily removed from the tool when the tap and collet exit the tap.
Horizontal tapping that can take advantage of gravity advantages requires tap geometry that is different from vertical tapping. He said that we reduced the rake angle and the helix angle of the tap to form frangible short chips, which improved the chip removal performance of the horizontal tapping. This tap design not only eliminates nesting chip clusters, but also extends tool life.
Compared with other cutting operations, the feed rate of tapping processing is higher, and the consequences of processing failure are more serious, which is a high-risk processing. One way to reduce this risk is to use simulation software to simulate the process before tapping to identify and correct potential problems. For example, CGTech's Vericut 6.2 simulation software adds simulation and analysis capabilities for tapping. The tapped hole is intuitively distinguished from other drilled, boring and reaming holes. The Vericut software checks the feed rate and tapping direction used correctly and determines if the tapped hole is too small.
Mechanics can also use Vericut's X-Caliper inspection to measure thickness, volume, depth, clearance, distance, angle, aperture, fillet radius, and arc height. Vericut can directly measure blind hole depth, stepped hole size, and the top and bottom radius of the cone. In addition, the X-Caliper measures the distance between the tool and the workpiece and shows the thread characteristics of the tapped hole after tapping (eg, the number of threads per inch).
Many tap problems can be solved before tapping can be started. Learn more about tap design and help eliminate the mystery and fear of tapping.
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