Manufacturers always look for ways to make their manufacturing processes take place at the fastest rate possible. Besides, with a trend towards miniaturisation in manufacturing, workpiece sizes are diminishing and part versions are growing. So, the use of micro-tools is becoming more dominant.
Micro-tooling and high-speed machining defined
Micro-tooling involves mills and drills with a diameter of 0.250” or less. It is required for very intricate or detailed machining and works best with high-speed spindles. High-speed machining has no set definition or absolute parameters, but one workable definition is machining with spindle speeds of 25,000 RPM or more.
Challenges of machining with micro-tooling
Workpiece sizes are decreasing and part versions are increasing. So, the utilisation of micro tools is becoming more and more prevalent. However, efficient and cost-effective use of these small tools requires both the foresight to employ equipment specifically designed for them and a willingness to deviate from standard machining practices. This is primarily due to the fact that the spindles on conventional CNC equipment cannot achieve the higher RPM speeds required for small diameter tools. Even if they can, it puts undue stress on the equipment by constantly red-lining their spindles.
Often this tool breakage is due to the force of a conventional machine’s heavy spindle and its inability to reach the high RPM speeds required to effectively evacuate chips from the cutting channel.
The best approach to efficiently machine with small tooling is a three-fold process. The three inter-related elements are:
High-speed machining technology: The smaller the tools, the higher the spindle speed you will need to efficiently machine quality parts and avoid tool breakage. High-frequency spindles with speed ranges up to 60,000 RPM are ideal for milling, drilling, thread milling and engraving using micro-tools. High-speed machining technology uses high RPM rates, taking a smaller step over, but with significantly increased feed rates.
During the machining process, the tool continually carves a chip out of the workpiece. The generated heat develops approximately 40% from friction on each side of the tool, and 20% from the deformation of the chip. Therefore, about 60% of the heat is inside the chip. High-speed machining tries to evacuate the bulk of the heat with the chip, providing for a cleaner cut. The better machining quality is based on cooler tooling, lower machining forces, and therefore, less vibration.
The high spindle speed reduces the chip load to less than 0.005”. Such a low chip load significantly reduces the forces between the tool and the material. High-speed machining yields less heat and allows machining of thinner walled workpieces. This all results in cooler machining, better accuracy and, as a by-product (of low force), easier work holding.
Optimised micro-tool design: Scaling down the tool geometry of larger diameter tools to a smaller format yields unacceptable feed rates and unsatisfactory finishes. Tooling requirements change when tool diameter is decreased and spindle speed is increased. Conventional tooling using inserts is not appropriate for micro-tooling applications. This is primarily due to the high RPM rates rather than the tool diameter. Increased RPM rates require properly balanced tools with significantly increased chip room to assure proper chip removal and to prevent chip burn up. Efficient machining with small tools requires the tools to be optimised specifically for high-speed machining applications. The proper geometry of micro-tooling, together with high-speed spindles and the ideal coolant, can totally eliminate de-burring and de-greasing as secondary operations.
Low-viscosity coolant: While high-speed machining inherently reduces heat, the task of cooling a rapidly moving micro-tool often requires coolant. Those dedicated solely to high-speed machining with small tools understand that coolant used with conventional CNC equipment is not optimal. A small tool with intricate geometry turning at an extremely high RPM calls for a cooling and lubricating agent with a lower viscosity than water. Lower viscosity is needed because the coolant needs to make it to the cutting edge of the tool despite the high spindle speeds involved. Emulsion-based coolants have a higher viscosity than water, and thus, are ineffective as a lubricant for high-speed machining with micro-tooling.
Using small micro-tools just isn’t as easy as finding an adapter to hold a tiny tool in a 40 Taper spindle on a conventional CNC machine. Because that spindle was designed for large tools like a 3 inch fly cutter intended to “hog” out deep cuts in dense substrates. As such, it has so much torque and force that it just breaks small tools which is both inefficient and very costly over the long haul. The only option an operator has in this situation is to slow the RPM and feed rates down to a crawl.
When designing a machine, you can go in one of two directions. You can build your machine with a big motor and heavy mass to provide the force and torque to drive large tools. Or you can build a lighter machine with a high-speed, low-force spindle specifically designed for micro-tooling. Certainly both types of machines can be multi - purpose and perform a variety of functions — like milling, drilling, taping and routing. But that’s where multi - function ends. In the end, if efficiency and quality are important to you and you need to produce both large and small parts, you’ll end up with both types of machines working side by side on the same shop floor.
In consideration of high-speed machining centres exclusively, the best means of tackling micro-tooling applications is to employ equipment that exhibits the key attributes detailed above (high-speed machining technology, optimised micro-tool design and low-viscosity coolant) all working together synergistically. If applied together, this three-fold process can provide you with breath-taking manufacturing speeds and improved product quality. In addition, this process can totally eliminate secondary operations like de-burring and de-greasing.
Here are two examples of high-speed machining, as done on Datron machines. A ¼” single flute cutter in 6061 aluminium, going 1/8” deep. The machining runs at 45,000 RPM and is cooled by Ethanol. The feed rate is 250”/min.
Secondly, using a 1/8” double flute high-speed cutter (HSC+) with low helical angle to machine through a 1/8” 6061 aluminum sheet. The machining runs at 50,000 RPM and is cooled by Ethanol. The feed rate is 200”/min.
There are certain rules of thumb for high-speed machining. First of all, avoid red-lining your spindle, as this increases wear and tear on it and significantly reduces its lifetime. Machine with maximum half the tooling diameter in Z. Machine with a smaller step-over but with higher feed rates. And finally, move fast and evacuate the heat with the chip.
It all comes down to the right tools for the right job. Conventional machines with low-speed, high-force spindles can’t meet the criteria for efficiently machining with small tools. Only a machine built from the ground up, for the sole purpose of high-speed machining with micro-tooling, will deliver the efficiency and quality needed to manufacture most intricate parts.
High-speed machining with micro-tooling offers lower force, no thermal growth, and elimination of de-burring and de-greasing operations. Spindle speeds between 25,000 and 60,000 RPM result in efficiency with small tools and improved cycle times. Datron’s line of machines offer the features and advantages mentioned above and can help manufacturers to achieve quality in small part production with micro-tools.
Courtesy: DATRON Dynamics