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With a variable geometry end mill, this tremor can be reduced. Although this type of tool is often referred to as a "variable helix angle tool," it is a more general concept. ?
Variable spiral end mills have two basic designs. One design feature is that unequal (or different) helix angles are the same along each cutting edge length, while the other design is along the length of the cutting edge. Its helix angle is widely adjustable. For example, the helix angle of a rake face of a tool is 30°, the angle of a certain point in the middle is 37.5°, and the helix angle of a flank face can reach 45°. The helix angle can also be changed from 37° to 30°. "In the cutting process, the tool will be affected by the damping, because the end mill is cutting along the same helix at different angles," explained Paul St. Louis, president of Benchmark Cemented Carbide. ?
Bob Srail, the workshop director of Melin Tool, added that as long as the tools and tool grinders produced by tool makers can be machined, the helix angle of the cutters can be varied within a wide range. "Most machine tools are capable of grinding any helix angle." ?
Both end mills have different spiral pitches (or different pitches) and therefore can produce a different phase (out-of-phase) cutting motion to prevent resonance. For example, the pitch of a 4-slot end mill (or variable end mill) is 89° between the first and second grooves, 91° between the 2 and 3 grooves, and so on. Add up to equal 360°.
“Usually when a conventional end mill is used, continuous harmonic vibrations are generated. This is based on the assumption that the milling cutters have the same helix angle and even distribution of the cutting edges.†pointed out by Jeff Burton, executive vice president of SGS Tools. "Anything that can interfere with harmonics can reduce the tremor that results." SGS Tool Co. claims that by minimizing harmonic vibrations, Z-Carb's solid carbide end mills (with the same profile but helix angle) Variable) Maximize material removal.
According to Dennis Noland, technical director of Niagara Tool Co., the patent-pending rotary cutter developed by Ny Foundations can improve the situation. This design has improved the concept of “Variable End Mill†through the special selection of sound patterns generated by the tool. Not only changed the index between the flutes and the spiral, but also changed the rake face and the undercut. Therefore, the cutting edge of each slot is different from the others, and the lead of each slot is also different. “In general, for a set of end mills with a given geometry, the change in helix angle cannot exceed the range of 5° when you look from all directions, otherwise stability problems will arise.†The result is that each cutting edge of a tool produces a clear sound pattern when it comes into contact with the surface of the workpiece and moves along the surface of the workpiece. "Because the cutting edge produces different sound patterns, the potential danger of resonance may be overlooked." Although Niagara's variable helix angle end mills have the same geometry along each cutting edge, they are In the case of the cutting edge, it is still possible to produce a complete change of the rake angle, undercut, spiral, and even from the rake face to the flank. When machining workpieces with this kind of milling cutter, the tool will constantly change its sound pattern. "For this, we have not yet completed the study," he said. "This involves a large number of very complicated test procedures." In processing high temperature, high strength When alloyed, Niagara's Stabilizer HT end mills have different pitches, rakes, undercuts, and helixes to effectively reduce chatter during machining.
VRK's patent-pending Silent Pitch 3-, 4-, and 5-slot end mills have offset grooves and variable helix angles, and each end mill has only two identical helices. The variable geometry allows the end mill to "almost chatterlessly cut and complete the finishing process at nearly the speed of roughing."
3-D simulation software
If it is not the emergence of 3-D simulation software for grinding machines, it is impossible to change the helix angle of the end mill. When sharpening this type of end mill, hundreds of tools and a large number of grinding wheels can be damaged without the aid of a simulator. This analog device, which enables many design changes to be completed in a few minutes, allows more work to be done in the office without spending time on the production floor. In the past, it took 15 minutes to grind a 1/2′′ diameter end mill, but now it takes only 2 to 3 minutes to pass the simulation device.
However, simulation software and grinding machines that can be used to grind variable helix angle cutters can cause many end users of such tools not to grind existing old cutters. It is also not a good idea to send the tool back to the manufacturer for emphasis. In response, Jeff Davis (Deputy Technical Vice President of Harvey Tools) said: "Our company is not really a tool dressing manufacturer. Our tool price is low enough and it is very competitive, so compared with the old knife Buying a new tool may be a better option."?
Contrary to this, Mr. Steve Abrams, Kennametal's marketing manager for end mills and carbide rotary tools, believes that a large number of end-users are expected to be able to perform heavy-duty after end mill wear. The end users of Ken Miller's Vari Mill cutters can perform heavy cutting and re-coating of cutters at their own company or at a local tool grinding service center, which can also achieve the effect of a new knife. He explained that the Vari Mill end mills do not have variable or different helix angles, but instead have offset flutes and a constant rake angle along the length of the flute to prevent the resonance from occurring. Make a 4 slot end mill work like two independent 2 slot end mills. ?
Roughing time
With the disappearance of tremors, variable helix angle end mills can achieve very high material removal rates. Noland of Niagara Company thinks, “In some cases, by eliminating the possibility of resonance, it is entirely possible to increase the depth of cut by 2 times, 3 times, or even 4 times. For example, 2mm can be machined on the surface of 4340 steel with a length of 600mm or more. Deep slot."?
SGS's Burton said: Although this type of end mill is also suitable for finishing, roughing is generally considered the most significant and effective machining process because roughing can increase the material removal rate. In many cases, you can increase the depth of cut, the width of the cut, and the amount of feed. When you are not affected by unwanted chatter, you can increase the above parameters and force the cutter more aggressively. At this time, since the variable end mill avoids the jump along the workpiece surface, it can make the finishing quality the same as or better than the standard end mill with a small feed finishing. “You can often see unfinished workpieces with satisfactory surface quality.†Burton commented on the effects of variable end mills. “Even if the final milling is still required, the use of a variable end mill can increase the amount of roughing operations, thereby increasing machining efficiency.â€
Conversely, a variable end mill is usually not the best choice if an end user only needs a small feed cut. Because it does not have the advantage of being able to cut under high cutting loads during processing. Benchmark Carbide's St. Louis said: "For users who only perform small feed finishing, I recommend that they not use the variable end mill and use the cheapest tool they can find; if only the variable The end mill is used to machine the hard skin of the billet, so it will not be able to play its advantage of reducing chatter during machining."
Mechanical problem
Because variable helix end mills cost 20% to 25% more than standard endmills, they are not suitable for a wide range of machine tools. According to Melin Tool's Mr. Srail, "The use of milling cutters depends on the type of equipment. It is economically uneconomical to use a variable helix angle end mill on older Bridgepots."
In order to use the variable helix angle end mill more effectively, the ideal machine tool must have at least one spindle with a 40 degree taper. Using a spindle with a 30 degree taper will produce a lot of deviations, but it can also be used, but it is necessary to reduce the feed during use. the amount. ?
Mr. Noland said: "A machine with a power of 15-20 hp, a spindle taper of 40°, a maximum speed of 10,000 rpm, and a 1/2" (or smaller) variable helix angle end mill will have a lot of productivity. improve. Even on old Bridgepot machines, the use of variable helix angle end mills can double their efficiency. "?
Mr. Noland emphasized that regardless of the machine tool used, it is crucial to maintain tool cooling at all times when heavy cutting (such as milling 2 mm deep slots) is performed. Therefore, it is necessary to supply a large amount of coolant at a high speed. One important step is to allow the coolant to enter the cutting zone, keeping the cutting zone temperature low enough to avoid material sticking. Once material bonding occurs, the tool will quickly fail and break. Internal cooling tools are an option, but they add a lot of cost. "This internal cooling structure has not yet been adopted in our company's variable end mill design, but this is definitely the direction we have been paying attention to."
Due to the geometry of the variable helix end mill, coolant is not required for cutting. Its surface is usually coated with TiAlN or AlTiN coating. Both coating tools are suitable for dry cutting and quasi-dry cutting.
To maximize productivity, you need to optimize every factor in the machining system, as well as the tool itself. After the tool geometry is determined, the coating provides greater advantages than other factors. The perfect combination of tool coating and geometry will maximize output for the user. If you use a high-quality carbide substrate again, the end mill will machine deeper.
When playing music, harmonic resonance of the instrument forms a pleasant and harmonious harmonic, which is often intoxicating. However, in the metal cutting process, the vibration and tremor generated by the resonance can cause the quality of the surface of the part to be reduced, repaired, and may even make the workpiece. scrapped. ?