Guide to improving the efficiency of high-speed machining with micro tools on CNC machines

This guide describes the benefits of using ultra-high speeds when machining non-ferrous metals and plastics with micro tools. Topics covered include the definition of micro tools for high-speed machining, the challenges of machining with micro tools, the technology available, and the maximised advances and speeds that result from using the new technologies.

Micro tools involve cutters and drills with a diameter of 0.250” or less. They are required for very intricate or detailed machining and work best with spindles designed for high speed.

High-speed machining does not have an established definition or absolute parameters, but a feasible definition is spindle machining at speeds of 25,000 RPM or higher.

The challenges of improving the efficiency of high-speed machining

With a trend towards miniaturisation in manufacturing, workpiece sizes are decreasing and part versions are increasing. As a result, the use of micro tools is becoming more common. However, the efficient and cost-effective use of these small tools requires both the foresight to employ equipment designed specifically for them and the willingness to deviate from standard traditional machining practices and acquire new technologies. This is mainly due to the fact that the spindles of conventional CNC equipment cannot achieve the required accuracies at the required working speeds.

The trend in precision machining productivity is to work at higher RPM speeds for increasingly smaller diameter tools. As an example, a conventional CNC machining centre working with tools less than ½» in diameter at 10,000 RPM or less will result in unfavourable feed rates and costly tool breakages.

Tool breakage is often attributed to operator error, incorrect machining parameters, or, worse still, simply the nature of small parts. The reality is that these high tool breakage costs are due to the heavy spindle force of a conventional machine and its inability to reach the high RPMs necessary to effectively evacuate chips from the cutting channel.

Technology available to optimise the efficiency of high-speed machining

The best approach to efficient machining with small tools is a three-step process. The three interrelated elements are:

1. High-speed machining technology: The smaller the tools, the higher the spindle speed required to efficiently machine parts to the required quality and prevent tool breakage. High-performance spindles operating at speeds of 40,000 RPM and above are ideal for milling, drilling, thread milling and engraving using micro tools.
   High-speed machining technology uses high RPM rates, with a smaller pitch but significantly higher feed rates. Suppose you move your hand through the flame of a lit candle. If you move it too slowly, there is enough time for the flame to cause damage. But if you sweep your hand quickly through the flame, there is not enough time for the fire to damage your skin. The principle applies to high-speed machining with microtools. Move quickly, and there is not enough time for the heat to return to the workpiece and cause problems.
   During the machining process, the tool continuously removes a chip from the workpiece. Approximately 40% of the heat generated is developed by friction on each side of the tool and 20% by deformation (bending) of the chip. Therefore, around 60% of the heat is inside the chip. High-speed machining attempts to evacuate most of the heat with the chip, providing a cleaner cut. The best machining quality is based on cooler tools, lower machining forces and, therefore, less vibration.
   The high spindle speed reduces the chip load to less than 0.005”. Such a low load significantly reduces the forces between the tool and the material. High-speed/low-force machining performance with less heat reduces tool deflection and allows for the machining of thinner-walled workpieces. All this results in cooler machining, superior surface and edge quality, better accuracy and, as a by-product (of low force), easier clamping, as modular vacuum tables can be used for quick setup and job changeover.
2. Optimised design of microtools: Reducing the geometry of larger diameter tools to a smaller format results in unacceptable feed rates and unsatisfactory finishes. Working requirements change when the tool diameter is reduced and the spindle speed is increased. Conventional tools are not suitable for applications where micro tools are required. This is mainly due to the high RPM speeds required when working with smaller diameters.
   Increased RPM rates require properly balanced tools with significantly more chip space to ensure proper chip removal and prevent burning. Efficient machining with small tools requires that they be specifically optimised for high-speed work in machining applications. The right micro-tool geometry, combined with high-speed spindles and the ideal coolant, can completely eliminate deburring and degreasing as secondary operations.
3. Low-viscosity coolant: While high-speed machining inherently reduces heat, the task of cooling a rapidly moving micro-tool often requires a good coolant. Machining technicians who routinely work with small tools understand that the coolant used with conventional CNC equipment is not optimal, and this is a perfect example of where it is necessary to think «outside the box» when undertaking applications that require high-speed machining.
   A small tool with intricate geometry that rotates at extremely high RPMs requires a cooling system and a lubricant with a viscosity lower than water. Lower viscosity is needed because the coolant must reach the cutting edge of the tool despite the high spindle speeds involved. Emulsion-based coolants have a higher viscosity than water and are therefore not effective as a lubricant for high-speed machining with micro tools.
   Some micro-volume coolant spray systems can use ethanol, a form of alcohol that occurs naturally in the fermentation process of sugar and has a lower viscosity than water. Ethanol's low evaporation point makes it an extremely efficient cooling and lubricating agent for high-speed machining operations. Furthermore, while conventional flood coolant is petroleum-based and must be disposed of properly, ethanol simply evaporates. This eliminates the costs associated with disposal. In addition, ethanol as a coolant leaves no residue on machined parts, thus eliminating the costly secondary operation of degreasing parts.

Machine dynamics to increase machining efficiency

Using small microtools is not as easy as finding an adapter to hold a tiny tool in a 40-taper spindle on a conventional CNC. That spindle was designed for large tools such as a 3-inch fly cutter intended to «gobble up» deep cuts in dense areas. As such, it has so much torque and force that it only breaks small tools, which is inefficient and very costly over time.. The only option available to an operator in this situation is to reduce the RPM and feed rates to a minimum, and this is also inefficient because it results in unacceptable cycle times.

A vivid, and perhaps comical, analogy is the pick-up truck with an engine versus the sports car. The reality is that it is unthinkable to compare the two vehicles or even consider them competing with each other. Why? Because the pick-up truck was designed with the power and strength to transport or tow enormous loads, while the sports car was designed for speed and manoeuvrability.

Essentially, conventional CNC machine manufacturers who promote the ability to run microtools are like a car manufacturer who puts a spoiler and racing stripes on a racing vehicle claiming that it now has the same qualities as a Porsche. Well, just as you cannot put a spoiler and racing stripes on a pickup truck and expect it to perform like a sports car, you cannot adapt a high-speed spindle to a clunky conventional machine and expect it to efficiently achieve high-speed machining with microtools.

When designing a machine, you can go in one of two directions. You can build your machine with a large 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 designed specifically for micro tools.

Certainly, both types of machines can be multipurpose and perform a variety of functions, such as milling, engraving, drilling, and threading. But that is where the multifunctionality ends. Ultimately, if efficiency and quality are important to you and you need to produce both large and small parts, you will end up with both types of machines operating side by side on the same factory floor. While this may seem like a duplication in terms of equipment expenses, the costs are quickly amortised through ROI, as the efficiency and versatility will produce better parts, faster, at a lower cost.

Considering high-speed machining centres exclusively, the best way to approach micro-tool applications to improve machining efficiency 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.

When applied together, this triple process can deliver impressive manufacturing speeds and improved product quality. But the benefits do not stop there. In addition, this process can completely eliminate secondary operations such as deburring and degreasing.

It all comes down to using the right tools for the right job. Conventional CNC machines with low-speed, high-torque spindles cannot meet the criteria for efficient machining with small tools. Only a machine built from the ground up, with the sole purpose of high-speed machining with micro-tools, will deliver the efficiency and quality needed to manufacture the smallest and most intricate parts.

High-speed machining with micro-tools offers lower force, less tool breakage, no increase in temperature, better surface finish, elimination of deburring and degreasing operations, and less tool vibration. Spindle speeds between 25,000 and 60,000 RPM result in efficiency with small tools, better part quality, and improved cycle times.

Since you are interested in improving the efficiency of high-speed machining with microtools on CNC machines, we invite you to access the video guide on the scope of new clamping solutions for lathes, milling machines and grinding machines in the automotive industry, as well as to learn about the New clamping technologies for lathes using high-precision chucks, which can provide machining workshops that manufacture all types of parts with faster changeovers, a greater clamping range, high rigidity in heavy chip removal, and reduced maintenance.