WO2017064583A1

WO2017064583A1 - Spark assisted chemical engraving machine, a work piece machined by the machine, and a process related thereof

Info

Publication number: WO2017064583A1
Application number: PCT/IB2016/055662
Authority: WO
Inventors: Rolf WÜTHRICH; Lucas Abia HOF; Jana D. ABOU ZIKI; Giuseppe Cusanelli; Patrick Thibaut
Original assignee: Posalux Sa
Priority date: 2015-10-13
Filing date: 2016-09-22
Publication date: 2017-04-20

Abstract

The present invention relates to a spark assisted chemical engraving machine for machining a glass surface of a work piece, the machine comprising: - A support for fastening the work piece during machining, said support being dipped into an electrolytic; - A machining head operated by a feed motor for translating said machining head along a feed axis (Z1) extending along said machining head; - A tool electrode protruding from the machining head toward the support to dip at least partially into the electrolytic; - Guiding means for guiding the translation of the machining head with respect to the support along said feed axis (Z1); - A counter electrode dipped into the electrolytic; - A sensor for measuring the axial machining force exerted on the tool electrode during machining; - A control module arranged for progressively adapting the translation speed of the tool electrode to said machining force.

Description

Spark assisted chemical engraving machine, a work piece machined by the machine, and a process related thereof

Field of the invention

[0001 ] The present invention concerns a spark assisted chemical engraving machine, a work piece machined by the machine and a process related thereof. Description of related art

[0002] Technics available for machining or engraving materials are commonly classified into different categories depending on the

technologies involved.

[0003] Chemical machining involves the use of chemical reagent such as photoresist activated by specific radiation to remove material and is frequently used for instance to design the surface of a wafer. One of the major hurdles of chemically etching relates to the uses of hazardous or difficult to handle chemical reagents.

[0004] In mechanical machining, chemical reagent are replaced by tools designed for contacting the surface to be machined to ensure the removal of material.

[0005] Other technologies are listed among thermal machining which is based on the local supply of energy to remove part of the material.

Electricity can be used for thermal machining conductive and non- conductive materials.

[0006] Conductive material can be machined by electrical discharge machining (EDM) wherein the material, for instance a metal, serves as a conductive medium. The process is limited to electrically conductive material. Electrical arcs between the tool and the work piece allow the removal of material by melting and evaporation. [0007] For non-conductive material such as glass or ceramics, one of the machining process is called electrochemical discharge machining (ECDM) or spark assisted chemical engraving (SACE) and is often described as a hybrid process which associates chemicals and electric sparks to machine non- conductive surface of work pieces. Basically, the current and heat produces reactive species nearby the tool electrode opposite the surface and promote the chemical etching of the material and cause the removal of part of it.

[0008] SACE was developed by Kurafuji and Suda in the late 1950s as an innovative process to machine glass surface of a work piece. Ever since, the original process was improved by focusing on five major aspects :

- The composition of the electrolytic

- Optimal flushing of machining zone

- Tool electrode feed mechanism - Correct tool geometry

- Avoiding tool electrode / glass breakage.

[0009] Regarding the electrolytic, the key is the availability of hydroxyl (OH) radicals in high concentration. The viscosity/density of the electrolytic also influence the local flow of the electrolytic and therefore flushing of machining zone usually improved SACE.

[0010] Optimal flushing is required in order to bring fresh electrolytic and to remove machined material from to the machining zone. Further it allows to avoid excessive heating and the formation of heat affected zones and micro-cracks in the work piece. Another important aspect is to design the machining cell in such a way to have a constant flushing around the tool electrode in order to guarantee a constant level of the electrolytic. The melting and evaporation temperature of the electrolytic also influence the temperature of the machining zone. [001 1 ] Tool feeding mechanisms describe in literature can be broadly divided into two: gravity-feed and constant velocity-feed.

[0012] The document WUTRICH, R., (Machining of non-conducting materials using electrodischarge phenomenon, International Journal of Machine Tool & Manufacture, 2005, pages 1095 to 1 108) provides an overview of electrochemical discharge machining methods for machining non-conductive material, for instance glass, via gravity feed or constant velocity feed.

[0013] Another document WUTRICH, R., (In Situ Measurement and Micromachining of Glass, International symposium on micromechatronics and human science, 1999, pages 185 to 191) describes a tool holder for an electrochemical discharge device that allows to scan a substrate surface before machining it.

[0014] In the gravity feed mechanism, a constant force is applied on the tool which machines to ensure a constant close contact between the tool and the work piece. This method is well recognized as a simple to

implement method for machining work pieces. The major drawback of this method is that by maintaining a close contact between the tool and the work piece during machining, the risk of damaging the tool and/or work piece is very high. Additionally, when it comes to quality machining, this method often provides unsatisfying results because the hole formed is frequently deformed by the constant force applied on the tool during machining.

[0015] In constant velocity-feed mechanism, the tool is moved at a constant speed, for example in order to drill a hole.

[0016] However, in this latter mechanism it is difficult to choose an appropriate feed rate because during the machining, the machining rate of the tool varies over time: typically, the machining is efficient at the beginning of the process but tends to decrease over time. The reasons for such a decrease remain an opened question but it is known that the variation of the composition of the electrolytic nearby the tool is one of said reasons. Moreover, the flushing of the electrolytic is less efficient when the tool is engaged in a deep hole. As a result, the machining becomes more difficult over time. Hence, for constant velocity mechanism, the feed rate is maintained very slow in view of the expected decrease likely to occur over time. The aim is to avoid tool/work piece damage.

[0017] Therefore, spark assisted engraving is usually considered as a slow machining process, to the point that it is not commonly used in industry. [0018] Therefore, there is a need for a faster spark assisted engraving machine.

[0019] There is also a need for a SACE machine capable of producing high quality machining in an efficient manner.

Brief summary of the invention [0020] One of the aim of the invention is to provide a SACE machine free from the limitations of the known machines.

[0021 ] In particular, one aim is to provide an SACE machine capable of producing high quality machining in an efficient manner.

[0022] According to the invention, these aims are achieved by a spark assisted chemical engraving machine for machining a glass surface of a work piece, the machine comprising:

A support for fastening the work piece during machining, said support being dipped into an electrolytic;

A machining head operated by a feed motor for translating said machining head along a feed axis (Z) extending along said machining head; A tool electrode protruding from the machining head toward the support to dip at least partially into the electrolytic;

Guiding means for guiding the translation of the machining head with respect to the support along said feed axis (Z);

A counter electrode dipped into the electrolytic so that an electric current flows between said counter electrode and the tool electrode within the electrolytic;

A sensor for measuring the axial machining force exerted on the tool electrode during machining;

The machine further comprising a control module arranged for

progressively adapting the translation speed of the tool electrode depending on said machining force.

[0023] The invention also relates to a work piece machined by the machine according to the present invention.

[0024] The present invention also relies on a process for machining a glass surface of a work piece by using a machine according to the present invention.

[0025] According to the invention, these aims are achieved by a process for machining a glass surface of a work piece with spark assisted chemical engraving machine, the process comprising the successive step of:

i) Holding the work piece into an electrolytic, the machine comprising a tool electrode placed close to the glass surface of the work piece so that a gap (d1) exists between said glass surface and said tool electrode, the machine further comprising a counter electrode dipped into said electrolytic; ii) Simultaneously a. Moving the electrode tool with respect to the glass surface with a feed rate (f1) when an electric current is flowing between the tool electrode and the counter electrode within the electrolytic; and b. Detecting an axial machining force (B) exerted on the tool electrode during the step ii)a); iii) Progressively adapting the feed rate (f1) to said axial machining force (B). [0026] During machining, the tool is displaced at a controlled feed rate, i.e., the speed of displacement of the tool with respect to the work piece along a feed axis (Z) extending along said machining head. Additionally, the tool can also rotate around an axis (C) parallel to said axis (Z).

Optionally, in a milling process, the tool can be moved parallel to the surface of the work piece, at a horizontal feed rate.

[0027] During machining, the machining force, i.e., the reaction force that is exerted by the machined part on the tool, varies over time, depending notably on the composition and/or availability of the electrolytic nearby the tool. [0028] In the case of drilling, the machining force is parallel to the longitudinal axis of the tool. The machining force increases when the tool touches the piece or penetrates into more viscous layers near the machined portion of the piece. The machining force thus depends on the distance between the tool tip and the part. [0029] In the present invention, the machining force is monitored constantly during the machining of the work piece by a control module of the machine. Advantageously, the control module is connected with the sensor to monitor or compute the machining force. [0030] The control module is arranged for progressively adapting the translation speed of the tool electrode depending on said machining force.

[0031 ] The control module permits to adapt the translation speed of the tool depending on the machining force. For instance, if the machining force is higher than a predetermined threshold, the control module will progressively adapt the translation speed of the tool accordingly, by decreasing the translation speed. On the contrary, if the machining is essentially close to zero, the control module will progressively adapt the translation speed of the tool accordingly, by increasing the translation speed.

[0032] The control module offers a real-time monitoring of the machining force to adapt the position of the tool depending on the machining force.

[0033] The control module allow optimizing the parameters of the tool to ensure high quality machining. For instance, the parameters can be the translation speed, the rotation speed or the electric current.

[0034] In an embodiment, the control module is arranged for adapting the vertical position of the tool electrode to said machining force. To that end, the control module is adapted for arranging the translation speed of the tool. When the control module detects a high machining force the control module is arranged to reduce the feed rate in order to reduce the risk of collision between the tool and the glass surface, and to maintain a gap. In one embodiment, the control module is arranged to retract the tool, i.e., to move it in an upward direction, when the machining force exceeds a predetermined threshold. In one embodiment, the control module is arranged to augment the gap between the glass surface and the tool.

[0035] On the contrary, when the control module detects a low machining force, the control module is arranged to increase the feed rate of the tool, so as to reduce the gap between the tool and the glass surface. [0036] In an embodiment, the control module adapts the electric current flowing between said counter electrode and the tool electrode within the electrolytic. Thus, when the control module detect a high machining force when the electric current flowing in the electrolytic is V1 , the control module is arranged to increase the current to an electric current V2, V2 being superior to V1. Alternatively, the control module is also arranged to decrease the current to an electric current V3, V3 being inferior to V1 and/or V2. In the present invention, said electric current is defined as the machining voltage. [0037] In an embodiment, the control module adapts the rotation speed of the tool. Thus, when the control module detect a high machining force when the tool has a rotation speed of r1 , the control module is arranged to increase the rotation speed up to a rotation speed r2. On the contrary, when the control module detect a low machining force when the tool has a rotation speed of r1 , the control module is arranged to decrease the rotation speed down to a rotation speed r3, said r3 being inferior to said r1 and/or r2.

[0038] In an embodiment, the control module is arranged to adapt the horizontal feed rate of the tool (for example the milling speed), preferably by controlling the horizontal speed of the tool.

[0039] In an embodiment, the control module measures or computes the derived function of the machining force. Advantageously, the derived function of the machining force is used to adapt the translation speed of the tool accordingly. [0040] In an embodiment, the machine, in particular the control module, is arranged for controlling the flushing the electrolytic nearby the tool electrode and the glass surface. Thus, when the machining force exceeds a predetermined threshold, the control module is arranged to adapt the flushing to said machining force. [0041 ] In the present invention, the machine head is operated by at least a motor so that said machine head is movable with respect to the support.

[0042] In one embodiment, at least one motor is a brush motor. [0043] In an embodiment, at least one of said motors comprises a rotor and a stator, the rotor being fixed relative to the machine head. In particular, the rotor is mounted at one end of the machine head, while the tool is mounted at the opposite end.

[0044] In an embodiment, the rotor comprises at least a magnet whereas the stator comprises at least an electric coil. In the present invention, the control module controls the motor so that the control unit is capable of modifying the position of the rotor in function of the force machining.

[0045] In an embodiment, the feed motor is a voice coil motor. Voice coils motor are known to be reactive motor. Thus, the tool can be operated quickly depending on the force machining variation.

[0046] In one embodiment, the machine further comprises a rotation motor arranged to rotate the electrode tool around an axis (C) substantially parallel to said feed axis (Z1). The control module is arranged to adapt the rotation speed of said rotation motor to said machining force.

[0047] In one embodiment, the feed motor comprises a first translation motor arranged to translate the electrode tool along said feed axis (Z1). The control module is arranged to adapt the translation speed of said translation motor to said machining force. The first translation motor allows the displacement of the tool electrode with a first stroke (A) between about 100 micrometer to about 1000 micrometer, preferably between 400 micrometer and 600 micrometer. [0048] In one embodiment, the machine further comprises a second translation motor arranged to translate the electrode tool along a secondary axis (Z2) substantially parallel to said feed axis (Z1).The control module is arranged to adapt the translation speed of said translation motor to said machining force. The second translation motor allow the

displacement of the tool electrode with a second stroke (B) between about 1 centimeter to about 10 centimeter, preferably between 4 centimeter to 6 centimeter.

[0049] In one embodiment, the feed rate is determined by the

displacement of the tool along the feed axis (Z1) and along the secondary axis (Z2).

[0050] In an embodiment, the first stoke (A) is inferior to the second stroke (B). Therefore, the first translation motor allows a more accurate displacement of the tool electrode than the second translation motor. In an embodiment, the first translation motor is a voice coil motor.

[0051 ] In one embodiment, the feed motor comprises the first

translation motor and the second translation motor.

[0052] In one embodiment, the control module is arranged to control the first translation motor along the axis (Z1) and the second translation motor along the axis (Z2). Advantageously, the control module is arranged for retracting said tool electrode when the machining force exceeds a predefined threshold by actuating the first translation motor and/or secondly the second translation motor, preferentially by actuating only the first translation motor. [0053] In one embodiment, the machine head comprises a headstock, said headstock comprising two portions, the tool portion comprising the tool at one end and motor portion comprising part of the motor at the opposite end, said headstock being incompressible; in particular, a bearing, preferably a ball bearing, is positioned between said tool portion and said motor portion. Thus, the tool portion can rotate while the motor portion stands still. Advantageously, the motor portion comprise the sensor.

Rotation of the sensor during machining may disturb the measuring of the machining force.

[0054] In the present invention, guiding means are designed for guiding the translation of the machining head with respect to the support along an axis. Advantageously, said guiding means allow an essentially friction less displacement to minimize the contribution of said friction forces to the measurement of the forces exerted on the tool, in particular the machining force. In one embodiment, the guiding means comprises at least a bearing, in particular a plain bearing, more in particular ball bearing.

[0055] In one embodiment, the machine further comprises magnetic bearing. Magnetic bearing allow essentially frictionless displacement.

[0056] In one embodiment, the machine further comprises a titling element. The tilting element cooperates with the tool electrode to distance the distal end of the tool electrode from the feed axis (Z1) with a

predetermined angle, for instance between 1 degree and about 60 degree, preferably between 1 degree and 30 degree. In one embodiment, the control module is arranged to control the tilting element. Thus, during machining, in particular depending on the machining force, the tilting element displaces the distal end of the tool to increase the diameter of the hole.

[0057] In the present invention, a sensor is designed for measuring the translation of the tool during machining. In one embodiment, the sensor is connected to the control module. [0058] In one embodiment, the control module takes into account the measurements provided by the sensor to compute the machining force.

[0059] In an embodiment, the sensor is an optical sensor. The sensor could also be a capacitive, inductive, or magneto resistive sensor. [0060] In one embodiment, the current applied to the motor is used for determining the machining force.

[0061 ] The arrangement results in a machining head with a controlled stiffness. The stiffness k is defined as the resistance offered by the machining head to deformation d when a machining force F is applied:

F

k = i where F is the machining force applied to the tool and d is the deformation of the machining head, i.e., its variation of length between the distal extremity of the tool and a fixed point of the machine that is caused by F. [0062] The control module is programmed so as to adapt the stiffness k of the machining head during machining.

[0063] In particular, the machine head according to the present invention has a low stiffness. The low stiffness of the machine head is provided by a reactive motor that reacts rapidly to actuate the tool electrode, for instance a voice coil motor. The low stiffness allows high quality machining by minimizing the constraints on the tool and on the glass during machining.

[0064] In one embodiment, the machine head has a stiffness comprised between about 0.01 kN/m and about 50 kN/m, preferably between about 0.1 kN/m and about 25 kN/m, in particular between about 0.3 kN/m and about 5 kN/m.

[0065] Surprisingly the machine according to the present invention provides high quality machining when the stiffness of the machine head is below 5 kN/m. [0066] In particular, the machine according to the present invention provides excellent quality machining when the stiffness of the machine head is below 0.5 kN/m. [0067] The stiffness may be adaptable.

[0068] The stiffness may be adapted by programming the control module, or by entering parameters into the control module.

[0069] In one example, the stiffness is adapted to the tool. A thin, easily breakable tool may require a lower stiffness than a comparatively more solid tool.

[0070] A low stiffness, and ability to prevent breakage of the tool, requires a sensor, a control module and a motor able to react rapidly to a machining force. In one embodiment, this system permits an acceleration of at least one millimetre per square milliseconds.

[0071 ] In the present invention, low machining voltage ranges from about 20 Volt to about 29 Volt. On the contrary, high machining voltage ranges from about 30 Volt to about 40 Volt.

[0072] Surprisingly, the machine according to the present invention provides high quality machining when the voltage is about 30 Volt.

[0073] In the present invention, low machining force ranges from about 0 N to about 0,1 N. On the contrary, high machining forces ranges from about 0,1 N to about 1 N.

[0074] In the present invention, low feed rate ranges from about 1 micrometer per second to about 10 micrometer per second. On the other hand, high feed rate range from about 100 micrometer per second to about 500 micrometer per second.

[0075] In the present invention, the instrument and the process related thereof allow to drill hole with minimum diameter between about 100 micrometer to about 150 micrometer. [0076] In another embodiment, the tool rotated along the axis C during machining. The rotation speed ranges between about 100 rpm and about 1000 rpm, preferably between about 400 rpm to about 600 rpm. In particular, the control module is capable of adapting the rotation of the tool to the detected machining force.

[0077] In an embodiment, at least a portion of the tool is covered with an insulating layer designed for isolating said portion from the electrolytic.

[0078] The tool can have several geometries. In an embodiment, the tool is cylindrical. In another embodiment, the tool is cylindrical and terminated by a drill bit, in particular a print circuit board (PCB) drill bit. For example, the tool is made at least partially with tungsten carbide.

Advantageously, the tool is essentially wear free. In one embodiment, the tool has a bevelled shape. In another embodiment, the tool further comprises a flute. [0079] In the present invention, the terms "tool" and "tool electrode" are synonyms and interchangeable.

[0080] The embodiments described herein can be used alone or in combination in a machine according to the present invention.

[0081 ] The embodiments described herein can be used alone or in combination in a machine according to the present invention, said machine being used in a process for machining a glass surface of a work piece according to the present invention.

Brief Description of the Drawings

[0082] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: Fig. 1 shows a view of a machine according to the present invention.

Fig. 2 shows a cross section view of a machine according to the present invention. Detailed Description of possible embodiments of the Invention

[0083] The machine 1 according to the present invention comprises a headstock 2 positioned in between a rotation element 3 and a translation element 4. The headstock 2 comprises a first translation motor 5 whereas the translation element 4 comprises a second translation motor 6. [0084] The translation element 4 is affixed to a support 7. The

translation element 4 allows to position the headstock 2 with respect to a work piece to be machined, especially prior to the machining. The second translation motor 6 allows the displacement of the head stoke 2 along a secondary axis (Z2). Once a work piece to be machined is placed opposite the headstock 2, a user actuates the second translation motor 6 to bring closer the head stoke 2 and the work piece. In the present example, the second translation motor 6 is a steeper motor with a stroke of 10

centimeter.

[0085] The rotation element 3 comprises a rotation motor 8. In the present example, the rotation motor 8 is a DC motor. The rotation motor 8 is coupled to the headstock 2 via a belt 9 cooperating with a coupling element 10 so that the rotation motor 8 allow the rotation of the

headstock 2 around an axis (C) parallel to the secondary axis (Z2).

[0086] The head stoke 2 has a rectilinear profile extending along a feed axis (Z1), with a tool electrode 1 1 at one end and the first translation motor 5 at the opposite end. The coupling element 10 divides the head stoke 2 in two portions, the tool portion 12 and the motor portion 13. The tool portion 12 corresponds to the lower part of the head stoke 2 whereas the motor portion 13 corresponds to the upper part of the head stoke 2. [0087] The tool portion 12 comprises successively the tool 1 1 , a tool holder 14, a tilting element 15 and tool guide 16. The holder 14 allows maintaining the tool 1 1 . The tool 1 1 is made of tungsten carbide which is known to be wear resistant. Additionally, the tool 1 1 has a beveled shape to enhance the removal of machined material out from the hole during machining. The tilting element 12 is designed for orientating the tool 1 1 with an angle a1 with respect to the feed axis (Z1). In the present example the angle a1 ranges from about 0 degree to about 30 degree. The tool 1 1 , the tool holder 14, the tilting element 15 are integral with the tool guide 16, said tool guide 16 being fitted into the coupling element 10. The tool portion 12 of the head stock 2 further comprises a first air bearing 17 opposite the tool guide 16. The first air bearing mainly aims at guiding a friction less translation and rotation of the tool portion 12 respectively along the feed rate axis (Z1 ) and the axis (C). [0088] The motor portion 13 of the head stock 2 comprises successively a ball bearing 18, a motor guide 19 opposite a second air bearing 20 and a voice coil 21. The ball bearing 18 allows that when the rotation motor 8 rotates the headstock 2, the tool portion 12 rotates accordingly whereas the motor portion 13 stands still. The second air bearing 20 permits to guide the friction less translation of the motor portion 12 along the feed rate axis (Z1). In the present example, the translation along the feed axis (Z1) is actuated by the voice coil 21. The stroke of the voice coil ranges from about 0 to about 1 mm. The stroke of the voice coil 21 is inferior to the stroke of the second translation motor. In most cases, the second

translation motor 6 is used prior to machining to bring the tool 1 1 in close proximity with the surface of the work piece to be machined; whereas during machining, the first translation motor 5, the voice coil in the present example, allows to modify the feed rate of the tool 1 1 with respect to the work piece. [0089] The machine 1 further comprises a counter electrode (not shown in figures) dipped into the electrolytic so that an electric current flows between said counter electrode and the tool electrode 1 1 within the electrolytic. [0090] The motor portion 13 further comprises an optical sensor 22 for measuring the displacement of the head stoke 2 along the feed axis (Z1). The sensor 22 is further connected to a control module (not represented on the figure). [0091 ] In the present example, the control module is an electronic device connected to a computer. The control unit is further connected to the rotation motor 8, the first translation motor 5 and the second

translation motor 6. During machining, the control unit is designed for real time monitoring of the machining force to optimize the machining parameters for high quality machining of the work piece.

[0092] For example, if the machining force is above or below a threshold limit, the control unit can modify the machining parameter to enhance the machining. For instance, the control unit can

- increase/decrease the feed rate of the tool 1 1 by actuating the voice coil 21 and/or the second translation motor 6 and/or

- increase/decrease the voltage between the tool 1 1 and the counter electrode and/or

- increase/decrease the rotation speed of the rotation motor 8 and/or

- increase/decrease the flushing of electrolytic nearby the tool 1 1 and/or - retract the tool 1 1 from the hole.

Reference numbers used in the figures

Machine

Headstock

Rotation element

Translation element

First Translation motor

Second translation motor

Support

Rotation motor

Belt

Coupling element

Tool electrode

Tool portion

Motor portion

Tool holder

Tilting element

Tool guide

First air bearing

Ball bearing

Motor guide

Second air bearing

Voice coil

Sensor

Claims

1. A spark assisted chemical engraving machine (1) for

machining a glass surface of a work piece, the machine (1) comprising: a support for fastening the work piece during machining, said support being dipped into an electrolytic; a machining head (2) operated by a feed motor for translating said machining head along a feed axis (Z1) extending along said machining head; a tool electrode (1 1) protruding from the machining head (2) toward the support to dip at least partially into the electrolytic; guiding means for guiding the translation of the machining head with respect to the support along said feed axis (Z1); a counter electrode dipped into the electrolytic so that an electric current flows between said counter electrode and the tool electrode (1 1) within the electrolytic; a sensor (22) for measuring an axial machining force exerted on the tool electrode (1 1) during machining; the machine (1) further comprising a control module arranged for progressively adapting the translation speed of the tool electrode (1 1) depending on said machining force.

2. Machine (1) according to claim 1, wherein said control module is further arranged for retracting said tool electrode (1 1) when the machining force exceeds a predefined threshold.

3. Machine (1) according to claims 1 or 2, wherein the control module is arranged to adapt the electric current flowing between said counter electrode and the tool electrode (1 1) within the electrolytic to said machining force.

4. Machine (1) according to any one of claims 1 to 3, wherein the machine (1) further comprises a rotation motor (8) arranged to rotate the electrode (1 1) tool around an axis (C) substantially parallel to said feed axis (Z1), the control module being arranged to adapt the rotation speed of said rotation motor (8) to said machining force.

5. Machine (1) according to any one of claims 1 to 4, wherein the machine (1) further comprises a second translation motor (6) arranged to translate the electrode tool (1 1) along a secondary axis (Z2) substantially parallel to said feed axis (Z1), the control module being arranged to adapt the translation speed of said second translation motor (6) to said machining force.

6. Machine (1) according to any one of claims 1 to 5, wherein the machine (1) comprises a headstock (2) protruding from the machine

(1), said headstock (2) comprising the electrode (1 1) at one end and at least part of the motor at the opposite end, said headstock being incompressible.

7. Machine (1) according to any one of claims 1 to 6, wherein the machine (1) further comprises a tilting element (15).

8. Machine (1) according to any one of claims 1 to 7, wherein the machine head (1 1) has an adaptable stiffness.

9. Machine (1) according to any one of claims 1 to 8, wherein machine head (1 1) has a stiffness comprised between about 0.01 kN/m and about 50 kN/m, preferably between about 0.1 kN/m and about 25 kN/m, in particular between about 0.3 kN/m and about 5 kN/m.

10. A process for machining a glass surface of a work piece with spark assisted chemical engraving machine (1), the process comprising the successive step of:

i) Holding the work piece into an electrolytic, the machine (1) comprising a tool electrode (1 1) placed close to the glass surface of the work piece so that a gap (d1) exists between said glass surface and said tool electrode (1 1), the machine (1) further comprising a counter electrode dipped into said electrolytic; ii) Simultaneously a. Moving the electrode tool (1 1) with respect to the glass surface with a feed rate (f1) when an electric current is flowing between the tool electrode (1 1) and the counter electrode within the electrolytic; and b. Detecting an axial machining force (B) exerted on the tool electrode (1 1) during the step ii)a); iii) Progressively adapting the feed rate (f1) to said axial machining force (B).

1 1. The process according to claim 10, wherein step iii) further comprises rotating the tool electrode (1 1) with respect to the glass surface, and wherein the rotation speed is adapted to said machining force.

12. The process according to claims 10 or 1 1 , wherein step iii) comprises augmenting said gap between the glass surface and the tool electrode (1 1).

13. The process according to any one of claims 10 to 12, wherein step iii) comprises retracting said tool electrode (1 1).

14. The process according to any one of claims 10 to 13, wherein step iii) further comprises adapting said electric current flowing within the electrolytic between the counter electrode and the tool electrode (1 1) to said machining force.

15. Work piece comprising a glass surface machined by a machine

(1 1) according to any one of claims 1 to 9.