|Year : 2011 | Volume
| Issue : 1 | Page : 2-6
Spark erosion process: An overview
Liju Jacob Jo
Department of Prosthodontics, KMCT Dental College, Mukkam, Calicut, Kerala, India
|Date of Web Publication||2-Feb-2011|
Liju Jacob Jo
Department of Prosthodontics, KMCT Dental College, Mukkam, Calicut, Kerala- 673 603
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Spark erosion is a metal removal process using electric current under carefully controlled conditions. It is used for precise and accurate fabrication in the field of fixed, removable and implant prostheses. The scope of this article is to discuss the mechanism of action of the process and its significance in implant dentistry along with critical evaluation of its merits and demerits.
Keywords: Spark erosion, electric discharge machining, passive fit
|How to cite this article:|
Jo LJ. Spark erosion process: An overview. J Dent Implant 2011;1:2-6
| Introduction|| |
Modern precision laboratory procedures have a profound edge over traditional laboratory procedures in fabricating more ideal and precise restorations. This article discusses one such procedure i.e. spark erosion process, otherwise known as electric discharge machining (EDM). It is a process by which, a metal is precisely contoured into a desired shape by erosion by using accurately controlled electric discharge through two conductive objects immersed in a liquid medium. Basically there are two types of EDM -wire and probe type, of which the latter can be effectively used in dentistry.
| History|| |
Joseph Priestly  initially discovered this phenomenon while observing the erosive effect of electric current on metal conductors. But it was only in 1943 that an electric discharge machining unit was invented by the Lazerenko brothers  and since then, it has been used in various industries, especially aerospace, medical and micro engineering. The advent of computer-aided EDM in the early seventies, helped it gain significance in manufacturing processes. In 1982, it was introduced into dentistry by Rubeling to fabricate precision attachments.  Later, in the same year, Windeler AS  received a patent for improving the fit of cast restorations using EDM and since 1990 it has been used widely in implant prostheses  .
| Scope in Dentistry|| |
Passive fit is the simultaneous and circumferential contact between components of prostheses.It is imperative to limit stresses induced by prostheses within physiologic limits. This implies that the fit of the prosthesis should be such that, the tooth or bone should be able to adapt or remodel to the stimuli. It is mainly applicable in implant prostheses where there is a lack of resiliency at the bone - implant interface. Only 0-5μm of horizontal movement is possible in implants as compared to 8-28μm at the bone-root interface in natural teeth. A non-passive fit transmits undue tensile, compressive and shear stresses to the implant and bone. This is detrimental to long-term maintenance of osseo-integration and can cause crestal bone loss with possibility of screw loosening, pain, soft tissue irritation, implant fracture and prostheses fracture.
Factors  causing non-passivity include, errors in impressions and impression techniques, elastic deformation of impression materials, stone or investment material expansion, wax distortion, casting defects, soldering inaccuracies, non parallelism of abutments or implants, stresses induced on metal frame works during heat treatment of porcelain and manufacturer's variance in implant components to name a few. Some of these are inevitable and most are not under the control of the clinician. Therefore, passive fit ideally in the range of 10μm is essential for preventing biologic and technical failures of implants and fixed partial dentures. This can be easily achieved by the spark erosion process.
Spark erosion can achieve passive fit of metal sub/super structures and porcelain veneered frameworks which are to be placed on implant abutments and at the metal coping-tooth interface in fixed partial restorations.  Swivel latch attachments,  precision attachments,  friction pins,  titanium copings for Procera All-Titan system  and telescopic crowns  can also be fabricated with this technology. Spark erosion surface treatment for better metal-resin bonding  is another important application being investigated.
| Mechanics of Spark Erosion|| |
In a spark erosion machining unit [Figure 1], an electrode to work piece relationship is maintained in a liquid medium (di-electric fluid). The electrode (anode or positive potential) is usually made of graphite, copper, tungsten or zinc and is shaped into the negative form of the desired shape by CAD/CAM or milling. The work piece (negative potential or cathode) is the metal which is to be shaped into the desired form. A space is maintained between the electrode and work piece through out the machining process which is known as the cutting gap [[Figure 2]a]. The electrode moves towards and away from the work piece assisted by a hydraulic ram connected to it during the process. The di-electric fluid functions as a conductor and coolant during the procedure. This whole unit has a power source that maintains a direct current. The power level selection is dictated by alloy properties used, size of object and amount of erosion required. When the cutting gap is sufficiently small, the fluid ionizes allowing electric discharges to occur. These electric discharges occur at regular intervals and such cycles takes place about 250,000 times a second.  The sequence of events in a single cycle are illustrated here [[Figure 2]b to h].
|Figure 2 :Schematic representation of the mechanics of spark erosion process|
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As the voltage increases, the hydraulic ram brings the electrode nearer to the work piece. The electric field is strongest at the closest point between the electrode and the work piece i.e. electric field is directly proportional to the cutting gap [[Figure 2]b]. With increasing voltage, the di-electric fluid breaks down into ionized particles and an ionization channel is established [[Figure 2]c]. When sufficient ionized particles have accumulated to overcome the insulating effect of di-electric fluid, a current is established. With increasing current, an electric discharge channel is formed [[Figure 2]d]. The subsequent build-up of heat results in a vapor cloud formation [[Figure 2]e] which gradually expands [[Figure 2]f]. Meanwhile, the voltage at the cutting gap is monitored with a reference voltage within the power supply. When voltage at the cutting gap exceeds the reference voltage, the power gets cut off.  This drastically reduces the temperature at the cutting gap, triggering a collapse of the vapor bubble and generates a high-energy spark of temperature ranging from 8000-12,000 o C [[Figure 2]g]. The sudden energy produced causes vaporization of the work piece. The eroded particles [[Figure 2]h] are flushed away by introduction of fresh di-electric fluid. When the voltage at the cutting gap falls below the reference voltage at the power source, the cycle repeats itself as the power source is activated automatically.
| Procedure for Achieving Passive Fit of Implant Super-Structures|| |
Gunter Rubeling introduced the SAE Secotec Spark Erosion technique to implant dentistry in the early nineties. Frameworks which produce passive fit on implants can be fabricated with considerable ease.
This system is compatible with most implant systems. It consists of implant analogue sleeves, implant analogues (depending on the implant system used), copper implant electrodes that are identical to the implants used, plastic cylinders for waxing, insertion screws, torque wrench and short and long screws for the laboratory phase.
A spark erosion prosthesis is basically an implant-supported removable complete denture.  Impressions are made after the second healing stage. Implant body analogues are screwed into implant analogue sleeves and secured to the impressions [Figure 3]. The sleeves are connected with a copper braid to ensure conductivity during the process. The impression is then poured in resin-reinforced extra-hard die stone after blocking out the analogues with soft tissue model material. Conventional procedures are followed till casting and finishing of the sub-structure is completed.
|Figure 3 :Implant body analogues in implant analogue sleeves which have been secured onto the impressions|
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Sheffield tests are carried out before machining to determine passivity. In order to perform this test, the metal sub-structure should be inserted over the supporting implants or abutments.Then the most distal retaining screw should be tightened and the rest of the retaining screws should be kept out. If a gap appears between the remaining supporting implants or abutments and the metal sub-structure, it indicates that the metal framework does not fit passively
The cast with the implant analogues is placed horizontally in the spark erosion unit [Figure 5]. The cradle of the machine which can be moved in all planes is then aligned over the cast. The cast bar (the workpiece), is attached to the cradle of the machine with pattern resin. The cradle moves the bar towards and away from the electrodes during machining. The framework and the cast are connected to the power source by a copper braid [Figure 6]. The hydraulic ram lifts the framework from the model. Copper electrodes, identical to the implant analogues are used to replace them [Figure 7]. The torque setting for the wrench used to screw the copper electrodes should be the same as that used for tightening the implant analogues into the implant analogue sleeves. This avoids discrepancy in height or fit. The di-electric fluid is introduced after the cutting gap is set in the unit [Figure 8]. Then, the cradle moves the sub-structure onto the copper electrode and the erosion process is initiated. This takes place at the area where the sub-structure fits onto the implant abutment. The electrodes erode rapidly [Figure 9] and need to be replaced during the process. Every time an electrode is changed, the power level should be reduced before initiating subsequent machining. The finished sub-structure will exhibit a passive fit [Figure 10].
|Figure 6 :The framework and the cast are connected to the power source by a copper braid|
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|Figure 7 :Copper electrodes, identical to implant analogues are used to replace them|
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|Figure 10 :Sheffield tests after machining showing passive fit of the finished sub-structure|
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The Sheffield test, when executed after machining, shows no gap between the metal sub-structure and abutments, thus pointing to a passive fit at the abutment-framework interface [Figure 10]. A super-structure secured with custom latch receptacles is then fabricated over the metal sub-structure.
Spark erosion can also be used in an identical manner for implant supported fixed complete restorations. The cast super-structure on which porcelain is to be veneered is initially eroded at the implant abutment -super-structure interface as mentioned earlier [Figure 11]. Once spark erosion has resulted in a passively fitting metal super-structure, porcelain is fired to achieve the desired contours. Compressive stresses induced in the framework by firing of porcelain can be relieved by subjecting the finished prosthesis to another round of spark erosion machining, at a low voltage.
|Figure 11 :Cast super-structure for implant supported fixed complete restoration undergoing machining|
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| Advantages of Spark Erosion|| |
- Passive fit of restorations is achieved.
- Complex 3-dimensional structures can be shaped regardless of metal hardness since it is a thermal process.
- An extremely thin work piece can be machined without distortion as no mechanical forces are created.
- There is decreased stress on the work piece due to the cooling action of the di-electric fluid.
- Smooth finish of final restoration is ensured.
- There is decreased oxidation of metals during the procedure (especially useful in titanium to porcelain bonding).
- It is rapid, efficient and accurate (within 0.0254 mm). 
- Frameworks with porcelain can be spark eroded without any stress on the porcelain due to the cooling action of the di-electric fluid.
| Disadvantages of Spark Erosion|| |
- Eroding tends to affect the corrosion resistance of titanium.
- Skilled personnel and specialized lab equipment is mandatory.
- The high cost of the technique limits its usage.
| Summary|| |
An overview of the mechanism of action and applications of the spark erosion process in dentistry is presented. Such advances in dental technology help attain a passive fit of implant prostheses and fixed partial restorations, which is imperative in avoiding failures. The cutting-edge accuracy thus achieved, helps perfect critical adjustments in individual components and the fabrication process, thereby increasing the quality of treatment and hence, patient contentment and clinical success.
| Acknowledgments|| |
- Mr. Gόnter Rόbeling, (Director of SAE DENTAL GMBH- INTERNATIONAL), for providing me with information on the SAE Secotec system.
- Dr. K. Kamalakanth Shenoy (Head of Department) and Dr. Sanath Shetty (Professor), Department of Prosthodontics, Yenepoya Dental College, Mangalore, Karnataka) for their points of critique that has worked well for the betterment of this article.
| References|| |
|1.||Serman G. Practical guide to electro-discharge machining. 2nd ed. Geneva: Ateliers Des Charmilles SA; 1975, chapter 2. |
|2.||Rubeling G. Funkenerosion in der zahntechnik moglichkeiten und grezen. Dent Labor 1982;30:1697-702. |
|3.||Windeler AS. Method of Fabricating a Dental Prosthesis.United States Patent No.4, 363, 627, December 14, 1982. |
|4.||Roekel Ned BV. Electric discharge machining in dentistry. Int J Prosthodont 1992;5:114-21. |
|5.||Misch CE. Natural teeth adjacent to multiple implant sites: Effect on diagnosis and treatment plan. Dental Implant Prosthetics, 3rd ed, St.Louis: Mosby; 2005. |
|6.||Misch CE. Principles of retained prostheses. Contemporary Implant Dentistry, 1st ed, St.Louis: Mosby; 1993. |
|7.||Rόbeling G. New techniques in spark erosion: The solution to an accurately fitting screw-retained implant restoration. Quintessence Int 1999;30:38-48. |
|8.||Janda R, Roulet JF, Latta M, Damerau G. Spark erosion as a metal- resin bonding system. Dent Mat 2007;23:193-7. |
|9.||Lineham AD, Windeler AS. Passive fit of implant-retained prosthetic super-structures improved by electric discharge machining. J Prosthodont 1994;3:88-95. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]