3. PROBLEM DESCRIPTION 3.1 Micro-milling at TU Delft

3.

PROBLEM DESCRIPTION

3.1

Micro-milling at TU Delft

The micro-milling setup (Fig. 3.1) present at Delft University of Technology is a three axis high speed machine.

Fig. 3.1-Micro-milling at TU Delft

This machine has been assembled with components produced by different companies. In particular the axis were constructed by Aerotech, whose engineering specification is listed in Tab. 3.1. In Appendix A the drawings of the whole machine and in particular of the stages are shown; the drawings of the external part of the spindle indeed are in Appendix C together with those of the tool-holder, unfortunately no drawing of the internal part was provided by the constructor.

As visible in the picture above the material of the foundation and the bridge of the machine are granite, a material which is widely used. The density is 2,93kg/m−3, the compressive strength is 240MPa while the flexural strength is 22MPa. The flexural modulus of elasticity is 52,5GPa in the direction parallel to rift and 75,2GPa in the perpendicular direction.

The max travel of the stage is 100mm and the movement depends on the control system. This setup has a software based controller, Automation 3200. The drive system for the setup is shown in Fig. 3.1 near the machine. The linear encoders LT10AS Aerotech with a resolution of 0,005µm are equiped on each stage.

The X and Y axis are ALS25010 Aerotech linear motor. Linear brushless servomotor and mechanical bearings are installed on each stage and the positional accuracy is 0,2µm for the X axis and 0,3µm for the Y axis. The Y stage is mounted on the X stage, the latter is fixed to the granite foundation (Fig. 3.1 and Appendix A).

The Z axis is a ATS20010 Aerotech ground ball-screw stage with a positional accuracy of 0,7µm fixed to the bridge of the machine (Fig. 3.1 and Appendix A).

Tab. 3.1 summarizes the specifications of the three axis mounted on the micro-milling machine present at TU Delft and Fig. 3.2 shows some specifications of the X and Y axis.

Tab. 3.1-Aerotech Engineering Specification of the three axis [53]

NPAQ Model:

Axis Designation: Axis-1 (X) Axis-2 (Y) Axis-3 (Z)

Motor Cable Type: C19801-50 C19801-50 C19801-50

(Port): (AXIS-1) (AXIS-2) (AXIS-3) Feedback Cable Type: C16501-50 C16501-50 C18391-50

(Port): (AXIS-1 J1) (AXIS-2 J2) (AXIS-3 J3) Amplifier/Driver / Bus

Voltage: DP32020E / +/- 80 vdc DP32020E / +/- 80 vdc DP32020E / +/- 80 vdc

ALS25010-ES15813-1 ALS25010-ES15813-2 ATS20010-ES15813

ALS25010-M-10P-LT10AS-NC-XY CMS ALS25010-M-10P-LT10AS-NC-Y CMS ATS20010-M-40P-NM-NC-P2

(Lower stage of XY pair) (Upper stage of XY pair) (Vertical Z-axis) Lead / Gear Ratio: Direct drive Direct drive 4 mm ball screw

Motor: BLM-142-A BLM-142-A BMS100-AH-D25_AS-E1000AS-BK1

Motor I peak / I

cont-peak:: 20.0A / 4.86A 20.0A / 4.86A 8.6 / 2.2 1,000 line/rev, 1vpp

rotary encoder Encoder Multiplier: MXR (x1,000) MXR (x1,000) MXR (x100)

Encoder steps / rev / cycle: 12,192,000 12,192,000 14,000,000

Machine Resolution: .000 005 mm .000 005 mm .000 01 mm

Programming Resolution: .000 01 mm .000 01 mm .000 01 mm

Max. speed encoder counts

/ sec.): 40,000,000 40,000,000 14,000,000 Max. speed Metric: 200 mm/sec 200 mm/sec 140 mm/sec

Max. Feedrate Limiting

Factor: ~ ~

Encoder max frequency rate

(+) Move Direction: Machine cw / + Machine cw / + Machine cw / +

Home Direction: Machine ccw / - Machine ccw / - Machine ccw /

-Home Location: @ Stage (ccw) end of travel @ Stage (ccw) end of travel @ Stage (ccw) end of travel

System Parameter File Name:

Axis Calibration File Name: System Computer: System A3200 Software:

137480-A-1-1.prm 137480-A-1-1.cal (All axes)

ECZ01370 (COMPUTER, DELL,OPTIPLEX GX620,512MB,XP PRO with

NFIRE-PCI-TI-LP pcb) (See sheet-2 for configurations)

A3200 / FULL / NMOTION SMC-6 / NVIEW MMI

NPAQ-B-80B-80B/ULTRA /STAND COOL /GERMANY /BRAKE-3 /STAND /AC

Stage:

Encoder: RGH22B 1vpp analog read-head with 20um tape scale

RGH22B 1vpp analog

Fig. 3.2-Aerotech Engineering Specifications of ALS25010 (X and Y axis) [53]

The three axis are equipped with PID control loop system (Fig. 3.3) – Proportional, Integral and Derivative controller [41]. The controller takes a measured value from the process and compares it with a reference set point value. The difference (or "error" signal) is then used to bring the process back to the set point value (the desired set point of the controller). Unlike simpler controllers, the PID can adjust process outputs based on the history and rate of change of the error signal, which gives more accurate and stable control. It follows the meaning of the terms PID:

• Proportional: to handle the immediate error the error is multiplied by a constant P (for "proportional"). Note that when the error is zero, a proportional controller's output is zero. However the P controller cannot always guarantee that the set point will be reached if the set point is not fixed in time.

• Derivative: to anticipate the future the first derivative (the slope of the error) over time is calculated and multiplied by another constant D.

Fig. 3.3-Example of a PID controller [41]

The spindle (Fig. 3.4) is a prototype made by a German company. It can reach a maximum rotational speed of 120.000rpm, as required by this means of micro-machining and is equipped of magnetic bearings. Those are a non-contact bearings thus friction loss is negligible and no wear is supposed; this kind of bearing offers higher reliability and elimination of the lubrication, that however implies sensitivity to contamination. A magnetic bearing system includes the electromechanical hardware (bearings and sensors), the software program and the control system, that in the present application is a PID control system. The advantages in using an active magnetic bearing (AMB) [38] are resumed in:

• high reliability;

• clean environment (no oil, grease or solid particles of lubricator);

• suitability for high speed applications;

• capability of operating in extreme conditions of temperature, pressure and corrosive fluids.

Anyway some disadvantages characterize the ABMs:

• low load capacity; it requires that the bearings are physically large;

• higher complexity that means elevate price of the bearing system;

• necessity of electrical power to drive the control systems, the sensors and the electromagnets.

At the spindle tip there is clamped the tool-holder, visible in Fig. 3.5. The final cylinder part is a piece of steel that is in the place of the tool.

Fig. 3.4-Spindle Fig. 3.5-Tool-holder at the spindle tip The shank and the collet are integrated with the spindle, as visible in the pictures above.

The endmills used in this research have cutting diameters of Ø0,3mm and Ø0,5mm with two or four cutting edges and are made of tungsten carbide. It will be discussed about the tool in the following Chapters.

In the modern micro-machining technology scenario, the Laboratory for Precision Manufacturing and Assembly (PMA) of TU Deflt intends to develop a reliable industrial micro-milling process for metal moulds applied in injection moulding. In this research project, in approaching at a new setup, three main actions are considered necessary:

• acceptance of the machine: check of the machine’s components delivered by the constructors before to assembly them;

• examination of the control system: it is opportune to verify that the control system is appropriate for application and can satisfy the required performance; if it is not so it will be necessary to improve the control system to let it be suitable for the setup;

• technical knowledge of the machine: given the purpose of the PMA it is fundamental to have a deep technical knowledge of the machine before conducting the research work. The three apsects above are preliminary to the micro-milling process studying , aim of this research, and they will be the foundmentals of the next work.

3.2

Goal of the assignment

The thesis-work presented here belongs to the aforementioned research project of the Delft University of Technology. As a consequence of the aspects considered in the previous paragraph, it has been decided to delve into the knowledge of the micro-milling machine as topic of the present task. In particular the goal of the assignment is to design a procedure to evaluate the performances of the micro-milling setup.

Based on the state of the art known from the literature (see Chapter 2) and applying it to the specific case under consideration, in order to design the evaluation method it is considered opportune to follow several steps (Fig. 3.6):

• error budget: after a general background the first deeper step toward the knowledge of the machine is to make an error budget, meaning as a list of possible sources of error that may influence both the process and the machine itself;

• choice of issues: knowing the possible errors it is easier to understand which are the most important issues to consider and it is possible to choose the performances of which an evaluation method will be designed;

• experimental design: in order to evaluate some performances an experimental design is necessary; that allows to program the way to measure, including the experimental setup, the needed instruments and the possible obstacles that could be found;

• experimental conduction: the application of the experimental design and the data collection;

• results analysis: at the end of the experiments the obtained results must be elaborated, analysed and presented with help of graphs and tables;

• recommendations and improvements: analysis the data it has to be checked whether the results are satisfactory; in the affirmative case in order to improve the setup some recommendations are given, so as there will be a basis for future works, while if the data are not exhaustive it will be necessary to improve the experimental design, modifying either the setup or the instruments used, in order to repeat the experiment obtaining results more satisfactory.

Fig. 3.6-Assignment flow-chart

It follows in the paragraphs below an explanation of the steps that regard the error budget and the choice of the performances to be evaluated.

3.3

Error budget in micro-milling

In order to choose the performances of the micro-milling machine to analyse, it is useful to start drafting an error budget, which is a “way to allocate sources of error among the different components of machine, finding their influence on its accuracy” [26]. This enables sources of error to be identified, so that subsequent efforts could be detected as those problems which proves to have most effect on precision and accuracy. The budget includes all the elements that affect the final accuracy of the micro-workpiece, i.e. the machine, the process, auxiliary equipment and the interactions between them [13].

It is necessary to tell here that the error budget that follows is just an estimation of the possible sources of error for a micro-milling machine. Infact it comes from the literature survey and some preliminary test done on the machine. Therefore at the end of this report a more accurate error budget will be given, which take into account all the results found during the tests.

It is possible to identify as sources of error:

• low stiffness of the machine components: one of the main error in a micro-milling machine depends on the problem of the spindle and tool-collet interface. It can be due to the misalignment of the tool with the tool-holder and to the deflection of the elements involved, spindle, tool-holder and tool. If these elements are not stiff enough, the errors are big and compromise the work-piece accuracy. In a list of components, applied to the TU set up and classified from the most compliance element to the less one, it is possible to identify:

o Tool and tool holder

o Stages

o Bridge

o Basement and connections

It has been seen from the literature, that the machine’s components, except the tool, can amount the 45% of the global compliance [14], so it is necessary to check the stiffness level of the elements. About the Bridge, the results of some preliminary tests bring to check the stiffness of that even if its material is granite and the section is big.

• machine’s inaccuracy: the lack of the requested machine accuracy, both positional and dynamic. In this report, positional accuracy means all the geometric tolerances due to geometric errors, such as linear errors, straightness, squarenes, roll, pitch and yaw, whereas the dynamic accuracy includes all the error due to the motion parameter of the machine, such as spindle and stages motion errors, vibrations of the machine structure, controllers errors.

• thermal expansion errors: the thermally induced errors can degrade the positional accuracy of machine tools significantly resulting in geometrical deviations of manufactured workpieces [23]. This kind of errors can reach the largest contributor to the machine inaccuracy, caused by heat sources that exist within the structure, including bearing, axis and spindle motors and process itself [39]. Due to the fact that the setup

Fig. 3.7- Wear evolution in a ball-end milling tool (Ø 0,2mm) [13]

• process error: it includes the vibrations created by both the process and the environment, the run-out and the wear of tool (Fig. 3.7), tool-holder and spindle. Infact the run-out can increase the cutting force, causing variation in dimension and the need to change the tool, leading to a loss of accuracy. Furthermore the micro-milling machine is not placed in a vibration-controled room, so it is necessary to consider the influence of the vibrations on the machining process.

3.4

Performances to be evaluated

Given the previous error budget, some performances for the machine under exam have been chosen to be evaluated , such as:

• static stiffness of the machine components;

• dynamic accuracy;

• geometric accuracy.

The stiffness of the machine components has been chosen as performance to be evaluated because from former research it has been noticed that in micro-machine the deflection of the machine may be in the same order of the deflection of the tool [14]. Therefore is has been necessary to have an estimation of the overall compliance of the machine tool, in order to have a forecast of the deflection error. Furthermore the machine under exam is a prototype and it has been designed by the TU Delft so there is no done tests on the machine’s components compliance.

During a demonstration done on the machine by the Heidenhain, a German company, some dynamic tests have been done, using a new measurin instrument, the KGM grid encoders (Fig. 3.8) [40]. This instrument setup allows to perform circular interpolation tests to inspect the dynamic performance of the machine tool and the parameter settings of the control loop consisting of CNC control, drives and position feedback systems.

The results of this test have shown an error that was not expected, about the capacity of the machine to follow a contour under specific motion parameters. Even if the data collected from this test were not enough to give a reliable evaluation, the suggestion to analyse the dynamic accuracy of the machine has started from here. Furthermore, given that the TU machine is a high speed machine, it is necessary to have data that involve the motion parameters. This is important since acceleration requirements for the micro-machine tools are one to two orders of magnitude larger at the micro-scale than the macro-scale [20].

Fig. 3.8-KGM grid encoders, from Heidenhain [40]

About the geometric accuracy, the need to evaluate this performance starts from the Heidenhain data too. In fact some elaborations of these data have shown that the geometric accuracy, such as linear error and straightness error of the X and Y axis, did not achieve the expections from the machine tool. Given that the tests were not statistically enough to rely on them, it has been necessary to find a method to check the geometric error for the second time, in order to corroborate the Heidenhain data. Furthermore, given that the geometric accuracy is one of the main performance of a micro-machine tool for obtaining a quality product, it is necessary to deeply know the behaviour of the machine tool from this point of view.

The micro-machining is a new generation technology nowadays evolving in the scientific researching area. That is the reason why few performances evaluation methods have been found in the literature applied directly on the micro-machine tools. Therefore, in this thesis work some existing performance evaluation methods for macro-scale machine tools have been tailored to the micro-scale machine and the performances discussed above have been evaluated by these methods.