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MACHINE TOOLS Growth of machine tool through dynamic efficiency and precision

Aug 1, 2019

Dynamic efficiency focuses on all processes with high cutting forces and high metal removal rates. As for dynamic precision, it makes precise machining operations quicker. The following article discusses how dynamic efficiency and dynamic precision make it possible for machinists to fully apply the potential of their machine tools.

Dynamic efficiency allows greater removal rates and thus, raises output without compelling the user to turn to other tools. As for dynamic precision, it boosts production rates, cuts down dynamic deviations of a machine tool, and positively increases machining operations. Eventually, the two lead to high improvement in accuracy.

Dynamic efficiency: More chips in less time

With dynamic efficiency, Heidenhain exploits the power of machine and tools, in order to make heavy machining even more efficient. At the same time, the mechanical load is limited in order to reduce wear on machines and keep tools in use as long as possible. Dynamic efficiency supports all processes with high cutting forces and high metal removal rates. It combines performance-enhancing controller functions with time-saving machining strategies. For example, Active Chatter Control (ACC) suppresses the inclination of a machine to chatter, whereas Adaptive Feed Control (AFC) always ensures the best possible machining feed rate. The “trochoidal milling” machining strategy serves to reduce wear on the tool while roughing slots and pockets, and can very easily be used as a cycle. The effort is worth it. Metal removal rates 20% to 25% greater are possible, which significantly increases the cost efficiency.

ACC — Active reduction of chatter vibrations

High cutting forces come into play during roughing and particularly during machining of hard-to-cut materials. This can result in chatter vibrations. ACC is a powerful controller function that reduces the tendency toward tool chatter. Chatter vibrations leave blemishes on the workpiece surface. At the same time, the tool is subject to heavy and irregular wear. In unfavorable situations, the tool can even break. This chatter also places a heavy mechanical load on the machine tool.

ACC protects the machine tool against the effects of chatter vibrations, and at the same time, increases its performance. The ACC algorithm actively counters the disturbing vibrations. This permits greater infeeds, leading to higher metal removal rate. For certain machining tasks, the increase is easily more than 20%.

AFC — Profiting from the best feed rate possible

AFC shortens the machining time by automatically increasing the feed rate in machining zones with less material removal. This depends primarily on the spindle power and further process data. AFC, therefore, always ensures the best possible feed rate when there are fluctuations in the cutting depths or material hardness. This increases the efficiency.

The application is simple: Before machining, one should specify the maximum and minimum limit values for spindle power in a table. The values are determined by having the TNC record the maximum spindle power consumed during a teach-in cut. The adaptive feed control then continuously compares the spindle power with the feed rate, and attempts to maintain the maximum spindle power during the entire machining period.

AFC also offers another advantage: As a tool becomes blunt, the spindle power increases and the control reduces the feed rate. AFC can initiate an automatic tool change if the maximum spindle power is reached. This reduces the mechanical load on the machine and effectively protects the spindle from overloading.

Trochoidal milling — using the tool’s potential

The control supports the trochoidal milling machining strategy with an easily programmable cycle. This significantly accelerates roughing of any contour slots. The cycle superimposes a circular tool movement over a linear feed movement. For this one needs an end mill that can remove material over its entire cutting edge. “Scraping out” the material in this manner lets the machine work with large cutting depths and high cutting speeds. Circular plunging into the material places less radial force on the tool. This reduces the mechanical load on the machine and prevents vibration.

Combining trochoidal milling with AFC

A significant gain in efficiency can be expected if trochoidal milling is combined with adaptive feed rate control. Since the tool moves on a circular arc, a part of this movement is in the air. In this situation, AFC moves the tool at a much higher feed rate.

Software combination for efficient heavy machining

A high metal removal rate in the least amount of time possible is the measure for efficient roughing operations. The functions for heavy machining place great importance on ensuring that the machine’s dynamic behaviour is not impaired, while at the same time maintaining high accuracy — regardless of whether the functions are used separately or in combination.

The role of dynamic precision

With dynamic precision, Heidenhain exploits the accuracy potential of the machine tool. Dynamic precision makes it possible to compensate for dynamic deviations of the machine tool, and ensures that workpieces with increased contour accuracy and even better surfaces are produced, while also increasing the machining velocity.

Dynamic Precision comprises:

Cross Talk Compensation (CTC): CTC compensates for position deviations that result from the compliance between two axes. With it, the jerk can increase by a factor of 2 and machining times can be reduced by up to 15%.

Active Vibration Damping (AVD): AVD actively dampens vibrations. It suppresses dominant low-frequency vibration (machine setup vibrations or elasticity in the power train). To attain comparable surfaces without AVD it would be necessary to reduce the jerk values by up to a factor of 3.

Position Adaptive Control (PAC): PAC regulates the feed rate depending on the position. PAC changes machine parameters depending on the axis positions. This achieves improved contour accuracy within the entire range of traverse of the feed axes.

The machining of a workpiece is often a conflict of interest; if the workpiece is to have exact contours, the milling operations can’t be too fast. But if higher feed rates are required, the contour accuracy and surface definition usually suffer. So what’s to be done? Modern manufacturing enterprises are always faced with the challenge of simultaneously achieving higher accuracies and shorter machining times. Increased production rates and cost pressure force parts manufacturers to reduce their door-to-door times. Stringent demands on the accuracy & the surface definition should be met without time-consuming touch-up work.

It would appear that this conflict cannot be resolved. But this is exactly where dynamic precision comes into play. Dynamic precision makes exact machining operations even faster, and increases production rates. Machine operators don’t waste time or money on unnecessary scrap. Dynamic precision for TNC controls is a package of optional functions that ideally complement each other. These controller functions improve the dynamic accuracy of machine tools. Milling operations on a machine with dynamic precision can be performed faster and more accurately.

Dynamic deviations are the cause

Dynamic deviations are transient position/angular deviations or vibrations at the tool center point (TCP). They increase when the speed of NC program execution increases. The drive control usually cannot compensate the dynamic deviations completely. This leads to a following error between the nominal position and the actual position of the feed axes. The following error is a measure of the quality of the feedback control, i.e. how well the control traces a nominal contour. The dynamic deviations change over the lifetime of a machine, since the frictional forces in the guideways change due to wear, for example. Dynamic deviations usually also increase in machines with table kinematics when heavy workpieces are clamped on.

How do dynamic deviations arise?

Dynamic deviations are the direct result of machining operations. Machining forces, i.e. high moving forces and torques, briefly deform parts of the machine. The tool is continuously accelerated & decelerated again. Due to the inertia of the masses, the nominal and actual positions of the tool then no longer match. But even the drive train itself is not completely rigid. Vibrations can arise due to a certain elasticity of the components.

In order to accomplish a change in direction while machining complex path contours, the axes must be braked and accelerated. The more rapidly this occurs, the greater is the jerk. The jerk is the measure for the duration of changing acceleration. The greater the jerk, the more the machine begins to vibrate. This leads to dynamic deviations, and especially on slightly curved surfaces to visible shading. Until now this could only be prevented by slower feed rates. But now the time has come for dynamic precision.

What does dynamic precision do?

Dynamic precision reduces the dynamic deviations of a machine tool. Particularly at high feed rates and rapid accelerations can dynamic precision show its strength, by compensating for the deviations that arise. This enables machinists to fully utilise the potential of their machine tools. Test operations have shown that the accuracy can be improved, even if the jerk is increased by a factor of 2. At the same time, it was possible to reduce the milling time by up to 15%.

How does dynamic precision work?

The Heidenhain controller functions compensate for deviations, dampen vibrations, and regulate machine parameters in dependency of the current position, inertia and velocity. This is done without modification of the machine’s mechanics. Dynamic precision maintains the accuracy, subject to the current motion and load.

Conclusion

Dynamic precision significantly speeds machining operations, while at the same time, improving accuracy. This means that machinists much less frequently need to turn the feed-rate potentiometer to the left in order to reduce the feed rate. High precision is possible together with fast machining, no matter how heavy the workpiece is.

Courtesy: Heidenhain India

Image Gallery

  • The comparison shows that the blemishes are avoided on the surface machined with ACC

  • Abhilash Tripathi

    Managing Director

    Heidenhain India

    tripathi@heidenhain.in

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