This gives the retrofit spindle access to the machines entire work zone. The disadvantage is that the machining centre may not be able to perform slow, deep cutting.
Fixed centreline spindle:
An independently powered spindle can be mounted on to the original spindle. It can be removed to let the original spindle take slower, heavier cuts. But, frequent tool changes are difficult to incorporate.
Spindle speed increaser:
A tool holder, with a speed-increasing gearbox built into it, can increase spindle speed at the cost of some lost torque and lost Z-axis travel.
Air powered spindle:
If HSM is limited to very light milling and drilling with small tools in softer materials, then an air-powered spindle may be sufficient.
Secondary spindle:
An independent high-speed spindle can be attached alongside the main spindle. The original spindle can be used for slower, heavier cuts. Disadvantage is that the secondary spindle is not centered along the X-axis.
Cutting dry
Coolants for HSM operations is a controversial issue. Dry, mist, and flood cooling are all used. The problem is that, at present, there is no way to get the coolant to the actual cutting surface, even with very high pressure, through-the-tool delivery systems. So the coolant in all cases has only a peripheral influence on tool and workpiece temperature.
Certain experts feel that the use of coolants in HSM is not practical as it does carry heat away, but also causes thermal damage to coatings. In contrast, air blow is more effective and it increases tool life by 10 to 20 times over the use of coolant. Hence, experts recommend compressed air, or an oil mist in an air stream, to move the chips, and not fluids that can cause thermal cracking of the tool coating. Sometimes, mist coolant is used when a very fine finish is required. It is used for its lubricating properties, and not for the heat indulgence quality.
Tracing the trends
Even with numerous issues challenging the technology, HSM is gaining popularity across its application spectrum. In fact, the trends in HSM can be best traced by examining the main application fields of high-speed machining and the technological advancements arising out of the requirements in these fields. The major application fields can be classified as aerospace, automotive, die/mould and general manufacturing.
Today, the machining centre market constitutes at least 60 per cent of the total market for milling and drilling machines. Special purpose and transfer machines for large volume manufacturing, and take up the remainder by milling and drilling machines for small batch and one-off manufacturing. Within this market, high-speed machining centres used in small, medium and large batch production are gaining grounds.
Action in the automotive sector
High-speed machining centres for large batch manufacturing are mainly required in the automotive sector. These could be more strictly called high speed positioning machines, whose main aim is to be able to move from point A to point B in the shortest possible time. These machines typically are horizontal machining centres with linear motor drives that offer rapid traverse rates of 100 to 120 m/min and accelerations of 2 to 3 g. Linear motors, with their high feed rates and accelerations, offer the benefits of higher productivity, which outweighs the drawbacks of higher energy consumption, higher heat generation and a higher cost of up to 30 per cent compared to their ball screw counterparts. Spindles on machines for automotive applications need to be versatile to handle a variety of tools and have sufficient torque in the low ranges for drilling and tapping operations. Spindles here are generally in the 16,000 to 24,000 rpm range. New synchronous spindles, that generate high torque at low speeds, offer good potential for the future. For high speed positioning applications, hexapod and similar parallel kinematic or non-cartesian machines continue to be developed. Accuracy and repeatability, issues with controlling thermal effects, programming and control challenges, and a large physical size for a small working envelope have kept these machines at an experimental stage. Trade shows, nevertheless, keep exhibiting interesting new designs.
In pace with the aerospace industry
The aerospace sector makes ample use of high speed machining centres for its one-off small and medium batch manufacturing. Aerospace machines are a class in themselves, as the application is high speed profiling of aluminium and aluminium alloy aircraft structural components. The machines feature large travels where the benefits of linear motors are clear. High horsepower spindles, with speeds of up to 40,000 rpm, are designed for machining aluminium. Optimizing metal removal rates by running at or near the resonant frequency of the machine and minimising chatter is the main objective here.
Serving the die & mould sector
The requirements of the die/mould and general manufacturing sector is a tad different for high speed machining centres. Complex 3D shapes need to be machined with speed, accuracy and high surface finishes. In addition, a wide range of materials, ranging from hardened steels, steels and aluminium to copper and graphite need to be machined. With the available tooling technology, practical feed rates in hardened steels are up to 8 m/min and up to 12 m/min in graphites, beyond which there is material breakout.
For such applications, linear motors are beneficial in larger travel machines, but considering their cost and drawbacks, they are not practical or economical for small to medium travel machines. These machines are equipped with ballscrews, and today, they can achieve feedrates of 80 m/min and up to 2.5 g vectorial acceleration in three axes. With diverse materials manufactured, there is a wide range of tool sizes, and a variety of spindles with speeds ranging from 12,000 to 60,000 rpm. Depending on the application, a careful selection of the spindle is critical for success.
Linking it up
High speed machining is growing more common as manufacturers seek to machine hard parts to near-net shape. Technological advancement, newer ideas, innovative approach driven by application requirements is driving HSM to higher grounds. It is interesting to note that this innovative technology makes extraordinary demands on each link in the HSM chain: machine tools, cutting tools, tool holders, and programming. For instance, dedicated HSM machine tools need both structural rigidity an control flexibility to exploit higher speeds and feed Even with rigid, accurate tool holders, rigid machines and set ups are essential to prevent runout with the higher machining forces of HSM. Simultaneously, HSM programmers strive to keep tools in the cut until the job is done in order to maintain consistent machining loads throughout the part. The ease with which programmers can optimise tool paths impacts productivity.
HSM, with all its promises, is scaling dizzy technological heights as it improves productivity, ensures better surface finish and better part quality, while it keeps expanding its application scope. This innovative technology is still young, and has a long way to go... ahead! |
