Carbide End Mill Sets

Archive for the ‘Common Questions’ Category

When using a silicon carbide grinding wheel, it has to either be absolutely dry, or have enough coolant to flood both the wheel and tool.

Dry grinding is the method most commonly used because when an operator uses wet grinding, they tend to us an insufficient amount of coolant in order to have a better visual of the grinding operation. Unfortunately, this increases the likely hood of checking or cracking.

However, wet grinding does have several advantages over dry grinding. When sufficient amounts of coolant are used on the wheel: The wheel can be used approximately one grade harder than in dry grinding, which increases the life of the wheel. The use of sufficient amounts of coolant prevents thermal stresses and cracks. There is also less of a tendency for the wheel to load. A dust exhaust system is not required with wet grinding.

When used correctly, both wet grinding and dry grinding provide satisfactory results.

A list of the three main coatings used today:

Titanium nitride (TiN) TiN-coated carbide end mills must be run at speeds and feed rates close to that of uncoated carbide end mills. However, the TiN coating provides a better wear and lubricity to the end mill.

Titanium carbon nitride (TiCN) TiCN coatings are perfect for slow feeds and speeds, however it is often the choice coating for high-speed steel end mills. For carbide tools coated with TiCN, you can run at speeds approximately 80 % faster than uncoated solid carbide end mills. However, TiCN is prone to failure under extreme heat, which is why it is often used in slower feed and speed applications.

Titanium aluminum nitride (TiAlN) or Aluminum titanium nitride (AlTiN) TiAlN- or AlTiN-coated end mills are becoming increasingly popular. The aluminum in both coatings helps to create a gaseous aluminum oxide layer at the cutting edge that can reach temperatures greater than 1800-degrees. This layer of aluminum oxide gas protects the carbide in the tool from the damaging effects of heat. This is why these two coatings have become popular for high-speed and hard milling, especially in dry cutting. Aluminum titanium nitride contains greater amounts of aluminum than titanium aluminum nitride.

There are other coatings available on the market today, however, they are variations of the main three. When machining nonferrous materials, aluminum, brass, and plastics non-coated carbide end mills that have polished flutes are recommended in order to prevent edge buildup. These materials require finely keen edges and end mills that are coated don’t allow as sharp an edge.

These are only a few of the factors to consider using carbide end mills. Other factors include the specific geometries of the end mill, rake angles, gash lands, relief angles, as well as programs, and the machine. Understanding the process as well as the material that will be machined is only the first step in deciding which end mill should be selected for the application.

When manufactures of machine tools simply scale down a tool’s geometry, it produces unacceptable feedrates as well as unacceptable finishes. Smaller tools with smaller diameter require increased spindle speeds. Higher rpm rates require tools, which are properly balanced and have increased chip room for proper chip evacuation in order to prevent chip burn-up. Therefore, HSM applications require small tools that are optimized for this purpose.

Use of Low-Viscosity Coolant:

Just by the nature of the process of high-speed machining with micro tooling heat. In fact, the process creates enough heat that can require the use of coolants, even though HSM technology contains advantages, which reduce heat. There are, however certain applications that still require the use of an efficient coolant system.

Coolants perform two task, first the reduce heat and second they serve as a lubricant, which enabling the tool to move smoothly over the surface of the workpiece. Micro tooling requires a lubricating agent that has a viscosity lower than water in order for the coolant to make it to the cutting edge of the tool at the higher spindle speeds. Emulsion-based coolants are ineffective as a lubricant for HSM with micro tooling because they have a higher viscosity than water.

Ethanol can be used in micro-volume coolant spray systems and is suitable for nonferrous metals as well as some plastics. However, oil-based coolants are required on steel-based materials because of the risk of sparks that can be produced by the carbide tooling on steel surfaces can produce a safety hazard when the sparks come in contact with the ethanol.

Although ethanol is flammable, its low evaporation point makes it an efficient coolant for HSM operations. Other benefits of ethanol include that fact that it doesn?t leave any residue on the machined parts, and eliminates the process of degreasing.

Improving HSM Technology:

Machining involves math, a lot of math. We use math to calculate the areas and dimensions of plane figures, we use math for measurements and inspections, to check pitch, to calculate clearance angles, and speed and feed rates, in fact there probably isn?t a step of machining that does not relay on a mathematical formula. Therefore, it is perfectly logical and mathematically sound when we state that a smaller tooling requires a higher spindle speed in order to machine the parts efficiently. This is due to the fact that the higher spindle speed is required to efficiently evacuate chips to prevent chip burn-up.

HSM technology does use higher rpm rates, however it also uses considerably increased feedrates as well. When the spindles spin fast, there’s not enough time for heat to feed back into the workpiece and cause concerns. However, the tool continuously cuts chips from the workpiece, which produces heat and friction on both sides of the tool and chip. Approximately 60% of the heat is inside the chip. HSM process is based on evacuating the chips, thus removing the heat along with the chip to provide a cleaner cut. However, if the tooling cannot or has insufficient chip evacuations, then the heat is not removed, creating chip-burn and eventually the tooling cracks, and breaks. A better principle would be machining process that is based on cooler tooling, meaning lower machining forces to reduce vibration. Higher spindle speeds reduce the amount of chip load to < 0.005″, which in turns reduces the amount of forces between the tool and the workpiece. A higher speed lower force machine produces less heat, which decreases tool deflection resulting in a better surface quality, as well as better accuracy.

Dynamics of Machining:

Improved Micro tooling designs, low-viscosity coolant, and improved HSM technology can drastically improve the dynamics of machines, which perform HSM with micro tooling. Traditional machines are heavier meaning that they do not have the speed or feed rates required for micro tooling. Therefore, you cannot expect the same quality, precision, and tolerances from retrofitting a high-speed spindle onto a traditional machine as you would from a high-speed machine with micro tooling that is designed precisely.

Machines can either be large and powerful capable of using tools with large diameter spindle sized, or they can be lightweight, designed specifically for micro tooling. When you attempt to merge the two, it is like trying to breed a Great Dane and a Tea Cup Chihuahua. Therefore, to achieve proficient high-speed machining there has to be improved micro tooling designs, the use of low-viscosity coolants, and improved HSM technology. HSM with micro tooling provides less force, reduced heat, which means less tool breakage, and a better surface finish. In addition, there is the elimination of deburring and the degreasing process.