Int J Adv Manuf Technol (2010) 47:21 – 27 27 DOI 10.1007/s00170-009-2055-2
SPECIAL ISSUE - ORIGINAL ARTICLE
New tool-workpiece setting up technology for micro-milling K. Popov S. Dimov A. Ivano Ivanov v D. T. Pham E. Gandarias &
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Received: 9 June 2008 /Accepted: 8 April 2009 /Published online: 10 May 2009 # Springer-Verlag London Limited 2009
Abstract A major issue in micromilling is the setting up with the necessary accuracy the work coordinate system on the machine. The spindle thermal stability during the setting up procedure procedure make this a ver very y dif difficu ficult lt task task.. Thu Thus, s, it is esse es senti ntial al to de deve velo lop p ne new w to tool ol se sett tting ing up te tech chno nolo logy gy to increase the process productivity, reduce machine setting up time ti mess an and d er erro rors rs,, an and d at th thee sa same me ti time me im impr prov ovee th thee precision and quality of machined components. The paper presents a cost-effective and reliable method for setting up the work coo coordin rdinate ate sys system tem for micr micro-m o-milli illing ng ope operat ration ionss employi emp loying ng an onon-line line too tooll – workpiece workpiece voltage monitoring system. The viability of the proposed method was verified in a series of experiments conducted on an ultra-precision micromilli micro milling ng mach machine ine cent centre. re. The expe experime rimental ntal tria trials ls involved the machining of test parts in brass with cutters 100 and 200 µm in diameter. The results of these experiments were analysed and compared with the capabilities of current cur rently ly ava availab ilable le meth methods ods and tech technolo nologies gies on micr micro o milling machine tools for condition monitoring and setting up the working coordinate systems. Finally, conclusions are madee abo mad about ut the effe effecti ctiven veness ess of the pro propos posed ed new too toollworkpiece setting up technology for micro milling.
measuring ring system systemss . Keywords Micromilling . On-line measu Tool – workpiec workpiecee coo coordin rdinate ate sys system tem set setting ting up
K. Popov (*) : S. Dimov : A. Ivanov : D. T. Pham Manufacturing Engineering Centre, Cardiff University, Cardiff CF24 3AA, UK e-mail:
[email protected] E. Gandarias Mondragon Goi Eskola Politeknikoa, Mondragon Unibertsitatea, Loramendi 4, 20500 Arrasate, Spain
1 Introduction
Recent developments developments in machinin machining g technol technologies ogies and machine chi ne too tooll des design igns, s, in par particu ticular lar micr micromi omillin lling g refl reflects ects the constantly increasing requirements towards the accuracy of thee pr th prod oduc uced ed co comp mpon onen ents ts.. At th thee sa same me tim time, e, th ther eree is a growing gro wing tre trend nd for pro produc ductt mini miniatur aturisat isation ion that lead leadss to continu con tinuous ous red reducti uction on of com compon ponent ent feat feature ure size sizes, s, and corresp cor respond onding ingly ly the dia diamete meterr of the cut cutters ters emp employe loyed d in their machining. In particular, the applications that ” push” the micromilling technology to its limits are the manufacture of micro parts for watches, keyhole surgery, housings for micro-engines micro-e ngines,, toolin tooling g inserts for micro injection mould moulding ing and hot embossing, and housings and packaging solutions for micr micro-o o-optic ptical al and micr micro o flui fluidic dicss dev devices ices.. A com common mon challenge challe nge across all these application application areas is the machin machining ing of micro features with dimensions smaller than 100 µm. Many researchers have contributed to the creation of the currently available process knowledge about conventional milling. mill ing. Unfo Unfortun rtunatel ately y, the size eff effects ects are dom domina inant nt in micromi mic romillin lling, g, and therefore therefore it is not pos possibl siblee to ben benefit efit directly from this rich knowledge repository. To advance this technology it is necessary to study systematically the factors that affect the process reliability when it is employed for machining components incorporating micro features. Micro machining using conventional technologies, such as mill milling ing,, pre presen sentt uni unique que cha challen llenges ges in man manufac ufacturi turing. ng. Cutting Cut ting forces and too tooll pre pressu ssures res whe when n the mac machin hining ing is performed with micro tools, cutter diameters smaller than 200 µm, brings a whole new realm of problems. The tool pressure results from the filling of tool channels with a workpiece material, e.g. burrs and chips, especially when drilling operations are perform performed. ed. Und Under er such condition conditions, s, any an y si sign gnifi ifica cant nt va varia riatio tions ns of cu cutti tting ng fo forc rces es an and d th thei eir r directions may lead to the tool breakage. The spindle must be dynamicall dynamically y stable in order to minimise its thermal
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Int J Adv Manuf Technol (2010) 47:21 – 27
enlargement and tool vibrations. In particular, any vibrations or run-outs at the tool tip may have adverse effects on the surface finish and accuracy of the machined micro features. To limit these negative effects it is necessary to control as tightly as possible the whole set of machining variables, associated with all components of Machine tool-Fixturecutting Tool-Workpiece System (MFTWS) and the operating environment, and thus minimising their overall effect on the machining quality. In regards to the dimensions of parts and their feature, the relative accuracy in micro machining has completely new perspectives. While the feature sizes achievable in micro-milling are down to less than 100 µm, their relative accuracy or machining tolerances are in times worse than those routinely attainable in precision manufacturing. In particular, the relative tolerances achievable in ultra-precision machining are in the range of 10-5 to 10-6 of the features ’ nominal dimensions while in micro manufacturing 10-1 to 10-2 can be challenging [1, 2]. Hence, it is necessary to re-think the meaning of precision in micro machining. There are several key areas of concern in regard to MFTWS when machining parts at this scale: 1. Changes in the surrounding environment that affect the resulting process predictability and repeatability; 2. Vibrations (Internal and External); 3. The MFTWS management; 4. The use of cutting fluids and their dynamics. Therefore, machine resolution, control, construction and auxiliary tools all become much more important for the successful production of micro parts. There are solutions for minimising uncertainties in MFTWS when performing micromilling. Examples of such solutions are Tool Condition Monitoring Systems (TCMS) that improve the effectiveness of micromilling operations. Similar approaches together with some existing technical solutions can be adopted for minimising uncertainties associated with other sub-systems of MFTWS. This paper describes a cost-effective solution for setting up MWCS that adopts a new technique for detecting the contact between the cutting tool and the workpiece within MFTWS. The feasibility of this approach is verified experimentally and conclusions are drawn about the advantages and disadvantages of this solution.
2 Factors affecting tool-workpiece setting up accuracy
The main problems related to the setting-up of micro milling operations and the attainment of sub-micron tolerances are: &
The run-out of the cutter-holder assembly. This leads to changes of the cutting forces during the machin-
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ing, which have a detrimental effect on the tool life and the achievable accuracy, as well as the surface finish. The temperature variations during the machining due to thermodynamic stability of MFTWS. There are two types of temperature variations, one associated with the temperature build-up during one machining operation, and those caused by performing machining cycles with different cutters. The thermal expansion of the spindles is a major factor affecting the resulting part accuracy [3]. The spindle must be thermodynamically stable in order to minimize its thermal “growth”. Also, any cutting should be preceded by a “heating-up” stage in order the spindle temperature to stabilise that can takes up to 10 min as shown in Fig. 1. To address this problem new micromilling machines are equipped with hybrid Automatic Tool Length Measurement (ATLM) systems which combine the capabilities of contact and non-contact measuring methods in order to verify the tool tip position in the sub-micron range regardless of spindle thermal expansion [4, 5]. In addition, the machining the cutting Limitations of laser tool measurement systems. In particular, these systems have shown some limitations when measuring cutting tools with diameters below 50 µm. This is associated with the beam spot size used to carry out these measurements. Additionally, when conducting measurements between cutting operations the burrs attached to tools introduce a further uncertainty that can lead to erroneous setting-up values. For example, burrs of up to 50 µm long can be observed on micro cutters as shown in Fig. 2. Thus, for precise machining of micro parts, the tools should be cleaned regularly. In particular, to perform such a cleaning a
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50 ] C [ 40 e r u t a 30 r e p m e 20 T
d=6 n=30000 d=6 n=5000 d=0,5 n=37000 d=0,15 n=39000 d=2 n=24000
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Fig. 1 Temperature change during the spindle, heating-up stage. Note: spindle temperature readings at a given tool diameter in millimetres (d) and revolutions per minute (n)
Int J Adv Manuf Technol (2010) 47:21 – 27
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Fig. 2 Burrs attached to a micro cutter
specially designed ultrasonic bath, depicted in Fig. 3, has be utilised in this research because the commercially available are too big. The bath was installed on the machine table as close as possible to the tool measuring device, and then connected to the machine control system to command its activation/deactivation through a specially developed cleaning cycle. The cleaning procedure lasts around 10 seconds, and should be applied prior to any probing cycle in order to obtain accurate measurements. All these issues place new requirements on the applied techniques for setting up Machine tool Working Coordinate Systems (MWCS) as a means to increase machining productivity, improve the precision and the quality of the parts, and ultimately reduce the production costs. Especially, this is the case when machining complex 3D microstructures with relatively high material removal rates utilising cutting tools with diameters less than 200 µm [4 – 7]. A major problem is the big difference between tools’ Zlevels when setting up them using a touch probe at a
Fig. 3 A specially designed ultrasonic bath - a general view
spindle temperature of around 25-26°C and those measured at a speed above 20,000 rpm when the temperatures reaches 36-38°C, leading to 20 to 30 µm thermal enlargements in Z-direction. Unfortunately, this negative effect cannot be avoided even by operating micromilling machines in a temperature controlled environment. Furthermore, the existing methods for setting up the origins of MWCS for micromilling have an unacceptably high error along the Z axis and are effective only when the machined surface is “relative” to other surfaces milled with the same cutter within one operation. Therefore, more efficient technical solutions are required for setting up MWCS within the machine coordinate system, and thus to reduce the uncertainty when performing micromilling operations. One possible way to achieve this is to adopt solutions already developed for TCMS [6].
3 Tool-workpiece setting up method
In this research, a method for setting up MWCS is proposed that employs an existing technical solution for reducing uncertainty in MFTWS. In particular, the Tool-Workpiece Voltage Monitoring System (TWVMS) developed for detecting a tool breakage during micromilling and drilling described in [8] is adopted for setting up MWCS. Figure 4 depicts the general principles of this system. By creating a close electrical circuit between the spindle and the workpiece, abrupt voltage variations during the cutting process can be detected with a specially designed sensor. For further processing, the measurements taken are converted into a digital signal and then sent to the CNC controller. In the proposed method, the same technique is employed to detect the contact between the cutting tool and the workpiece, and then the machine CNC readings at the time of the contact are used to set up the machine origins along the X, Y, and Z axes. By applying this technique it is possible to avoid any additional errors from tool run-outs (Ero) or spindle thermal enlargement (Ete) which are major sources of uncertainty in MFTWS.
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Int J Adv Manuf Technol (2010) 47:21 – 27 +
tool. If the tool is changed Steps 1 to 4 have to be repeated in order to assure the required repeatability and overall accuracy.
Z _
Spindle Tool holder SENSOR
Tool
CNC controller
Workpiece Y X
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Fig. 4 The general principle for detecting the tool-workpiece contact
The setting-up of MWCS for each cutting tool includes the following steps: 1. Selection of appropriate locations on the workpiece to carry out the setting up operation in all three axes with cutting tools. It is preferable to select them on surfaces that will be machined later on. The positions of these surfaces along the X, Y, and Z axes, X set , Yset and Zset respectively, are measured with a touch probe and recorded for further use. This is because the “standard” procedure for setting up machines is performed with touch probes. 2. Loading in the spindle the cutting tool for which MWCS should be set up. Next, the tool should be left running at its normal cutting speed until the spindle reaches its working temperature. For example, for a 100 µm diameter cutter running at 40 000 rpm, 10 to 11 min are required to stabilise the spindle temperature at approximately 36°C (see Fig. 1). Only then the fine setting up steps can start. It is worth stressing that all steps for setting up MWCS should be performed in a temperature controlled environment. 3. The cutting tool is measured using a laser system utilising standard CNC cycles for calculating the effective tool radius (R t) and length (Lt ). In this way, it is possible to minimise uncertainties introduced by the tool-holder run-out and the spindle thermal enlargement. 4. Fine setting up of the origins in X, Y, and Z axes is carried out employing the set up shown in Fig. 4. In sequence, the cutting tool at fast speed approaches the positions Xset , Yset and Zset recorded in Step 2, and at a safe distance before reaching them switches to a low (measuring) feed rate for detecting accurately the contacts with the workpiece. This distance is usually in the range of 10 to 20 mm in order to avoid any collisions. In this way, the origins along the X, Y, and Z axes are set up and thus MWCS is fully defined for carrying out machining using this particular cutting
A source of uncertainty in the proposed approach for setting up MWCS is the delay with which the system is triggered when the tool contacts the workpiece. For most CNC controllers this is within one millisecond. It is possible to reduce this time by employing special input connectors, though the cutting tools will always “overshoot ” before the latched position is reached. This so called over-travel is a source of uncertainty, Eot , in MFTWS. To minimise it, the feed rate selected for performing the setting up operation should be sufficiently low to assure that Eot
