High-speed synchronous tapping processing application
Lead
Because the cutting tool generates high temperature during the machining process and reduces the service life of the tool, the actual cutting speed is low. Various tool materials need to combine high cutting performance and high life. High-speed steel and hard alloy are most common.
High-speed steel has very good strength and toughness, but its high temperature resistance is general. Tungsten-based cemented carbides are generally preferred over high-speed steel because of their higher hardness and their ability to maintain their hardness at high cutting temperatures.
In particular, carbide cutting tools have a cutting speed that is at least 4 times higher than that of high speed steel tools and has a longer tool life. However, compared to high speed steel, cemented carbides have lower fracture toughness, which limits their application in some processes, especially tapping.
Unlike most of the tools used for turning, milling and drilling, the inherent machining of the tap determines its cutting edge and cross section. Cutting edges tend to collapse or rupture, causing tool failure, even in the processing of relatively easy-to-machine materials like steel.
In low carbon steel processing, long continuous chips can block the flutes of the tap, limiting the carbide taps to only those materials that are easier to machine than steel, such as aluminum and cast iron.
Steel and other ferrous materials are the most commonly used materials for threaded joints, which means that carbide tools will have more potential advantages than high speed steel if they solve the problem of chipping and cracking.
Rigid tapping
The accuracy of the internal thread determines the accuracy of the thread itself and the accuracy of the thread fit.
When machining threaded holes, usually the tap is driven by the drill press, or the tap is placed on a machine with a floating tapping chuck to rotate the tap and its feed is approximately equal to the theoretical value of the internal thread pitch.
In the previous flexible tapping equipment, the feed rate is only an approximation. The thread pitch after machining is determined by the pitch of the tap, but there is a slight error between the feed of the machine tool and the pitch of the tap. The wire clamp is adjusted to be coordinated. The axial direction of the floating tapping chuck has a certain amount of expansion and contraction. As long as the accumulated error between the feed of the machine tool and the pitch of the tap does not exceed this amount of expansion and contraction, the machining can be performed normally without causing disorder (or “ Rotten tooth").
In addition, the tapping chuck allows the tap to have a certain amount of runout in the radial direction during tapping, thereby reducing the accuracy of the threading process. These conditions result in extremely low stiffness and uneven tapping load.
It is well known that the successful use of cemented carbide tools typically requires high machine rigidity and uniform feed. The material development of CNC tools can be improved from high-speed steel to higher hardness materials such as cemented carbide, but the brittleness is also avoided. The development of science and technology has not enabled us to economically obtain tool materials that can combine high hardness and high toughness. Therefore, we must consider how to maintain the rigidity of the tool and how to feed control to avoid high brittleness of the tool material during processing. Normally damaged.
For most machining methods, these are not the main problems with the use of carbide tool materials, but for tapping, this is a problem that must be considered.
Today's CNC machine control technology has evolved to maintain spindle rotation and feed synchronization, eliminating the need for floating tapping chucks. In the past CNC machine control, when the machine reached a stable speed, the synchronization of the two can be achieved, but in the start and stop stages, it can not be synchronized - the chaos often occurs at this time.
In addition, when clamping tools of other rotating shanks, such as carbide drills and end mills with precision shanks, the technology has been developed to such a chuck: it can be heated and expanded first, then cold-shrinked, Thereby it can be tightly fitted with the shank of the tool to deliver sufficient torque. There is also a collet that uses hydraulic pressure to clamp the handle of the tool, and it can transmit a large amount of torque.
Another benefit of using heat rise and hydraulic chucks is that they have very little radial runout when gripping the tool compared to the tapping head: for example, the concentricity of the chuck when rotating can be 3 μm or more. Small, these methods can also be used to hold a cylindrical handle with higher clamping force and rigidity.
The powerful TGHP precision collet with high clamping force is effective in tapping processing, although it does not have the same precision as the hot and hydraulic collet.
The creation of such use conditions allows the carbide tap to be machined with less radial runout and higher stiffness, resulting in the possibility of machining the thread at a cutting speed that far exceeds the speed of the high speed steel tap.
However, since the current taps are used with flexible tapping heads, the size of the expression of the runout does not need to be limited to tight tolerances. For example, a high-speed steel cone with a thread diameter of 0.5 inches (12.7 mm), the industry standard for the eccentricity of the shank can reach 20 μ (0.0008 inch).
In addition, we do not need to strictly control the thread diameter and the coaxiality of the cutting cone and the tap handle.
Solid carbide tap enlargement
New high performance carbide tip design
In order to give full play to the advantages of cemented carbide, a new tap fully exploits the advantages of rigid tapping machines and high-precision tool chucks.
Like precision drills and end mills, the tap of the tap is also completely cylindrical, but unlike current high-speed steel taps, the diameter of the drill stem is universal.
For example, the new uniform threaded UNF1/4-20 carbide tip has a shank diameter of 0.25 inches (6.35 mm) and a high precision carbide 0.201 inch (5.1 mm) commonly used to process UNF 1/4-20. The diameter of the shank of the screw hole drill is the same.
In order to fully utilize the thermal fit, hydraulic or precision collet, the diameter deviation of the shank is maintained at h6 of the German Industry Standard 7160.
For example, a 0.5 inch (12.7 mm) shank has a diameter tolerance of -0.0110 mm (-0.000040 in.) and a roundness of 3 [mu]m (0.00012 inch.).
Square heads are not necessary because they have sufficient clamping force to meet tapping requirements when the shank diameter is within specified tolerances.
Further, the threaded portion of the new tap and the conicality of the cutting cone to the shank are within 10 μm. The use of high-precision collets creates a completely rigid process system and reduces the runout of the tap, meeting the two conditions for the successful use of carbide taps: rigid and uniform loads.
Together with good rigidity and neutrality, a newly developed carbide grain with excellent properties, tap geometry and PVD coating greatly increase the tapping speed and service life!
In the case of Kennametal, two materials of carbide can be used for tapping. The KC7542 is specially designed for the processing of new taps for steel and cast iron. It is coated with a newly developed nano TiAlN coating on a high-strength cemented carbide substrate. This new tap guarantees the strength of the cutting edge and Anti-wear ability. KC7512 is used to process aluminum and other non-ferrous metals. The material consists of a wear-resistant cemented carbide substrate and two layers of coating. TiN is the coating on the substrate and CrC/C (chromium carbide) is the coating of the surface. Floor. When processing non-ferrous metals, the outermost coating has a small coefficient of friction, which prevents scales and built-up edges.
Performance of cemented carbide taps in rigid tapping
Advances in machine tool, tapping chuck and tool design techniques, rationally designed carbide taps are not only used for "short chip" materials such as aluminum and cast iron, but are now also being used for the first time in "long chip" materials, including carbon steel. , alloy steel and tool steel.
Ductile iron, malleable cast iron and grey cast iron in "short chip" materials. These carbide taps can successfully process the above-mentioned metallic materials within the specified speed range. The cutting speed can be up to 4 times that of high-performance high-speed steel coated taps, which in essence improves production efficiency.
It is worth noting that not all CNC machines can achieve rigid tapping when machining blind holes. Because the tap and the spindle must be decelerated when they reach the bottom of the hole, a pitch error may occur during the reversal process, and thrust is generated on the tap and the tapping size is increased. During the process of deceleration, the tap is still in the workpiece, the tap is reversed and re-accelerated, and the speed of the tap must be reduced to 40% of the recommended processing speed listed in the table above.
When the spindle of the tap rotates, not all CNC machines can be considered synchronous or rigid, so that the feed is equal to the pitch, but a large number of taps with compensating taps have been used in rigid tapping. These collets allow for slight axial movement to compensate for errors.
The chuck of the "sub-rigid" tap allows a certain amount of axial movement but has a high rotational stiffness and can achieve better results with cemented carbide taps. Because they were put on the market two years ago, these taps have been extensively tested. Manufacturers have transformed their tapping process to take advantage of new carbide tips. The cutting technology website believes that there will always be some manufacturing errors due to the pitch of the tap, and the axial feed of the machine tool will not be exactly the same. The error between the two will make the side of the tap tooth shape more than the other side. Large loads that accelerate the wear of the tap. Therefore, the use of a small amount of compensation (for example, ± 0.2 mm) can effectively solve this problem, which is more significant for a relatively hard brittle carbide tap.
For example, an automated component supplier KC7542 tap processing A536 ductile iron can increase the tapping speed from 110 feet per minute (33m/min) to 400 feet per minute (122m/min), reducing the tapping cycle by 65. %. The life of the tap has also increased to 40,000 holes (four times that of powder metallurgy high-speed wire cones), and the overall cost of tapping has dropped by 66% when considering machine tool and tool costs.
Another manufacturer found that the new KC7512 carbide tip can process speeds of up to 310 feet per minute (94.5 m/min) when machining silicon-aluminum alloy brake components, and can increase the life of the tap by more than three times to 300,000. hole. In this case, the manufacturer's tapping costs were reduced by 49%.
in conclusion
With the development of machine tools, control technology, tap chucks and carbide taps, the range of taps has exceeded the range of traditionally machined materials, including aluminum and cast iron, as well as carbon steel and alloy steel for the first time. The new carbide tip has a cylindrical shank and high precision tolerances that can be used for shrink fit, hydraulic or ER, or higher precision TGHP clamps (with high clamping force) for rigidity or Synchronous tapping on CNC machines.
When this processing system is combined with advanced tapping design, these carbide taps can be reliably used in steel processing, and the speed can exceed 5-6 times that of so-called high speed steel, and 5 times at the same time. Tool life. This progress will greatly increase the productivity of tapping.
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