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Wire Cut EDM
What is the OFFSET?
The OFFSET is the radius of copper wire plus discharge gap, the wire radius is a fixed value, and the discharge gap will change due to different energy; the smaller energy get a smaller gap, and the larger energy get a large gap.
Introduction of cutting wire
Foreword
Cutting wire plays an important role in Wire EDM because it would largely influence machining results such as surface roughness and dimension accuracy.
This chapter is going to discuss different properties of cutting wire commonly, which is used by “Slow Cutting” Wire EDM.
History of Cutting Wire
At the initial phase of Wire EDM development, cutting wire was all made of copper and brass. In 1979, people started using zinc-coated wire for better machining efficiency, and coming along with technology improvement of Wire EDM in 1990, cutting wire was diversified by a variety of ingredients compositions and production methods for different performances.
Wire Properties
01.Electrical
Electrical power passes to cutting wire through wire itself, therefore, electrical properties such as over-current withstanding (max.:700A; ave.:45A) and conductivity of cutting wire can greatly affect the quality of power output.
02.Mechanism
UTS (Ultimate Tensile Strength) of cutting wire depending on what material and production method it is made of. UTS indicate material’s capacity of resisting against breakage when undergoing radial force. UTS unit is PSI (British) and N/mm2 or kgf/mm2 (Metric), which represent the weight undertaken by per unit of cross-section area.
03.Physical Feature
Melting Point：During erosion, cutting wire undergoes continuous vaporization and melting phenomenon, so basically the wear rate of cutting wire would be lower if melting point is higher.
Vapor pressure：During eroding process, when work-piece’s melting point reached by the heat from cutting wire, vapor will be generated and create vapor pressure to help take away debris. Therefore, it is considered work-piece vaporized is more helpful with debris removal than melted.
Wire Materials
Wire diameter for use is usually considered based on followings.
All of the material of cutting wire listed above can be applied to CHMER Wire EDM. It is recommended to consult CHMER technicians for choosing suitable material of wire and instructions of AWT system tune up for best performance.
Remark: The content of data sheet above is only for reference; actual information such as composition, cost and production methods should be according to manufacturers.
Wire Diameter
Wire diameter for use is usually considered based on followings.
01. Work-piece thickness
Generally speaking, thicker work-piece requires higher current for machining, so wire diameters are supposed to be increased with work-piece thickness to undergo additional current volume. Below is recommendation for choosing wire (Brass) diameter based on different work-piece thickness.
※Note: Max work-piece thickness allowed could be different among each machine model; please assure Z axis travel before performing a job.
02. Corner (R) Size
Actually, cutting wire is circular object so it will leave a “round angle” after passing by a corner, whose value is associated with wire diameter. The round angle can be roughly estimated as (wire diameter ÷ 2) + sparking gap; it has to be smaller than every R angle in NC program to avoid over-cut.
Introduction of dielectric
Why need Insulation
In order to concentrate the energy (current) on cutting wire, it requires insulated substance between cutting wire and work-piece during eroding process; therefore, how good the insulation is would result in different machining accuracy and stability. The insulation, in fact, is from the electrical resistance of dielectric, and it is commonly referred as water conductivity.
Machining Abnormality
What is Taper Cutting
“What is Taper Cutting?” means the status that upper/lower guide are not vertically aligned; instead, they are located diagonally to each other. And in the meantime, cutting wire would be stretched tightly and difficult to get cooled down by flush water from upper/lower head so as to cause wire break more easily. The wider taper angle is, the more frequently wire break will happen.
S-CODE Adjustment for Taper Cutting
- Reduce ON time for 1~2
- Reduce AN time for 1~2
- Reduce SN time for 1~2
- Increase OFF time for 2~10
- Increase AFF time for 2~10
- Increase SFF time for 2~10
- Increase SV for 10~20
- Reduce WT for 2~3
Note: Tune up range should be widened when taper angle increased.
Points for Attention
01.Choose suitable nozzle
Normal nozzle allows only 15˚; when taper angle >15˚, it is suggested use special “Wide Range Nozzle,” which has flush hole of larger diameter for additional taper angle. Notice: In addition, to avoid cutting wire interferes with Diamond guide or Nozzle in actual taper cutting, please dry run the program and pay attention to table movement, especially when passing 90° right angle since taper angle tends to be enlarged at corners.
02.UV movement of taper cutting can be calculated by formula
(Z height +DA+DB) × tan(taper angle)=U or V movement
e.g.: Taper angle=18° DA=6 DB=6 Z height=50mm
U or V movement=(50+6+6) ×tan18=62×0.3249=20.14
03.Sample deformation
(1) Inverted Triangle
Cause: Z axis (upper head) gets too close to work-piece.
Solution: Increase OFF and AFF value.
(2) Shrink in the middle
Solution：Reduce SV and increase FR .
(3) Bulge in the middle
Solution：Increase SV and reduce FR.
(4) Triangle and a line scar
Cause: Z axis (lower head) gets too close to work-piece.
Solution: Increase SV and reduce FR.
Eliminating the Cavity Caused by Approach
Cavity is usually generated to damage the contour line of a die and punch after approach.
About Work-piece
To better ensure the accuracy of finished items, it is essnetial to choose a suitable location for start-hole to avoid deformation of work-piece.
The Start-Hole Dimension
Generally speaking, start hole can be any dimension as long as larger than wire diameter. For example, if 0.2mm is used, ≧0.3mm start hole dimension is suggested.
Location of start-point
Most of the time, it is better to start cutting from inside of workpiece, not from the outside. Below shows that a start-hole is located inside as the start-point of a job. Theoretically, once the machining begins, wire must go forward for a 0.2~0.3mm distance off start point before getting stable; therefore, the skill, “approach,” is commonly used by reseving 2~5mm distance before reaching contour line to help obtain stable machining and reduce work-piece deformation as well.
Heat Treatment
Most of the time, work-piece used in wire-cut process have underwent heat-treatment, so normally there is no residual force existing inside and the hardness can reach HRC58~62, which can decrease the chances of deformation and breakage. It is necessary to assure work-piece is well heat-treated before running a cut.
Another way to increase the accuracy is before performing a cut, lay the work-piece next to the Wire –Cut machine or submerged in the dielectric to equalize the temperature of Wire-Cut machine and work-piece.
NOTICE: If the work-piece has magnetism please subdue its magnetism first, preventing the work-piece influences any components of WEDM cause any faulty during processing.
Consumable Parts
The selection of working fluid
01. Flare point of at least 100˚C- the higher the less it would cause fire
02. No poison, no odor – for minimum hazard to human beings
03.No corrosion – for minimum hazard to human beings
04.No smoke – for minimum hazard to human beings
05.Low viscosity (＜2.8mm²/S) – for removal works
06.Stability, low volatility – for long – term use
Type of dielectric oil
| Brand | Viscosity at 20°C | Flare point | |
|---|---|---|---|
| CST | E° | °C | |
| Mobil oil VELOCITE 4 | 9 | 1.75 | 118 |
| Castrol HONILO 409 | 6.4 | 1.52 | 135 |
| Chevron discharging fluid 71 | 5.7 | 1.46 | 116 |
| BP insulation fluid 250 | 6 | 1.48 | 120 |
| Esso LECTOR 40 | 6.8 | 1.55 | 132 |
| Esso UNIVOLT 64 | 2.0 | 2.9 | 156 |
| Socal Fina LYRAN D 50 | 1 | 2.05 | 132 |
| Fuchs RATAK FE | 5.6 | 1.46 | 115 |
| Gulf Mineral Seal Oil | 5.8 | 1.48 |
132 |
| Mobil oil VELOCITE 6 | 19.1 | 2.8 | 158 |
| Esso MENTOR 20/SOMENTOR 43 | 7.4 | 1.6 | 124 |
| Kerosene(for comparison, no used allowed) | 2 | --- | 78 |
Instruction for flushing
“Flushing” means a normal circulation of the dielectric oil between the electrode and work-piece, a factor very important for electric discharge machining. Only an adequate flushing can bring the most effective working.
Injecting Flushing
- It is a flushing for the dielectric oil to inject from the work-piece through an injection table or by passing the electrode.
- In the first situation, the work-piece is previously drilled a hole and installed on the injecting table to connect with flushing system.
- In the second situation, a hole is drilled on the electrode for the dielectric oil to be input directly through the electrode holder.
- The components which were processed by injecting flushing are a little bit conical. The effect results from the reason that the particles were extruded to the two sides of the electrode to cause an electric discharge on the edges. This flushing is usually used for the extrusion mould with conical, gap which of course has to be manufactured in a counter-direction machining to maintain the exact cone.
- The flushing situation will be very well while doing the deeper through-hole with the graphite. Apply the high pressure dielectric oil into electrode of the newest technical; therefore more enough dielectric oil can be easily flushing within the gap due to the porosity of graphite.
Sucking Flushing
- This is a flushing that the dielectric oil is sucked in by passing the work-piece or electrode through the button hole.
- Compared with injecting flushing, sucking flushing will result in an exact cylindrical hole instead of conical effect for the electric discharge of particles distributing on the two sides of the electrode .Too strong vacuum sucking should be avoided. For some example in application, injection had better be substituted for sucking to have a higher pressure than that of sucking.
- This will perform a more rapid machining for the sucking flushing to pass through electrode than work-piece.
Edge Flushing
When it is not available to drill one or more holes on the electrode or workpiece, this flushing method shall be applied.
It needs to adjust the injecting outlet cautiously in edge flushing that the complete working surface of the electrode may obtain an even flushing. Generally this flushing shall match the electrode interval vibration to perform a better effect.
In machining on flat surface, the flushing direction must be corresponding with the molding angle. For the machining of commemoration badge mold, a specific injecting outlet shall be used for its un-flat surface.
If the flushing direction is not in parallel with the edge of the electrode it will cause an interrupted flow with a small volume of dielectric oil to go through the gap to result in a poor flushing.
On the other hand, if the flushing is in a proper direction, then most of the dielectric oil will enter the gaps.
For edge flushing, it must prevent the dielectric oil from flowing in from the two sides of the polarity; otherwise, the particles will not be excluded.
In machining in rectangle slot, the dielectric oil shall flow from the longer side of the electrode to lead the oil reach the hole bottoms.
Dielectric “Extruding” Flushing
It is a flushing taking advantage of the interval spasm of the electrode. When the electrode ascends, the gap will widen to have more dielectric oil to flow in to mix with the polluted oil. The particles will be flowing excluded when the electrode descends.
Pulsed Injecting Flushing
During the machining with metal electrode, it can be checked that an abnormal electrode wear happens in the injecting area and the wear will increase when the flushing pressure comes higher.
To prevent such a situation, pulsed injection is hereby developed. That is, the dielectric oil will perform flushing in the gap under the pressure only when the electrode ascends but not during machining.
Flushing when Machining “Open-sided” Cavities or Holes
To avoid dielectric escaping through the open side, a piece of aluminum can be fixed to the side and machined at the same time as the workpiece. As a result, the flushing pressure is improved and machining can take place under good flushing conditions.
The types of electrode material
Electrode materials can be divided into the following three categories:
(a) Metal
Electrode copper
Copper tungsten
Aluminum alloy
Brass
Tungsten (mostly in the form of line)
Steel
(b) Non-metallic
Graphite
(c) Complex of metal and non-metallic
Copper graphite
The installation of electrode pipe
Install the electrode pipe carefully to avoid bending it, then put the seal rubber on the electrode pipe, and making the electrode pipe out of the seal rubber at 2mm~3mm. Insert the electrode pipe to the drill chuck from the top side, and tighten the drill chuck with seal rubber. Rotate the drill chuck to tight up the electrode pipe (do not over tight), and make sure if the electrode pipe at the central of drill chuck.
Precautions for using electrode pipe
- It is required for seal rubber and guide using the same diameter as electrode pipe, the other is common used.
- Please careful to avoid drop or bump the electrode pipe, sometimes will caused bending on it, especially with the extremely fine.
- Please take a detailed inspection at electrode pipe while installed it, sometimes will affecting the flush which caused by the flattened brass or burrs at the end of it.
- Please check if there is bending or warping by rolling the electrode pipe on a flat table.
How to use dielectric fluid
- The dielectric is the distilled water or the same treatment of water of wire cut EDM.
- The discharge speed will get better if the temperature of dielectric is under 10˚C.
- The discharge speed will get faster and less of electrode consumption if added such as K1CS, KICN to the dielectric.
The appropriate distance between the electrode guide and work piece
The distance between electrode guide and work piece is 2mm~3mm.
The classification and diameter range of electrode pipe
The electrode is electrode pipe (copper, brass and bronze), the diameter range is Ø0.2～Ø3.0mm.
The problem of large consumption of electrode due to unstable machining
Check whether the electrode is pendulum or bending by rotate it, it is not only unstable with machining but also speed up the electrode consumption when the electrode is pendulum or bending seriously, and also affecting the machining precision; sometimes, it cannot machining for more deep when reaches to a depth if the pendulum is excessive.
The machining technology
- Enlarge the “Gap voltage” with 1 step and reduce the “Servo speed” with 1 step, also adjust the “Current selector (I)” to 1 (the smallest) before starting machining when complete the setting of machining condition.
- Enlarge the “Current selector (I)” with 1 step every two seconds when reaches to the workpiece after starting the machining; and adjust the “Servo speed” and “Gap voltage” till same as the setting of technology table when the “Current selector (I)” achieve to the setting from technology table follow by the preceding steps. (The “Gap voltage” could be an appropriate adjusting maintained at 25V ~ 30V by the instruction on V meter.)
- Reduce the “Current selector (I)” with 2~3 steps, lower the “Pulse-ON duration” with 2~3 steps and enlarge the “Gap voltage” with 1 step in the meantime, also lower the “Servo speed” with 1 step till the electrode pipe through out of the hole completely when machining with hole drill (appear the spark at the bottom of workpiece).
Select the appropriate electrode depending on the workpiece
- Work Piece: Steel (SKD-11) To Brass Tube
- Work Piece: Tungsten Carbide (KG5, W88%, CO12%) To Copper Tube
- Work Piece: Brass To Copper Tube
- Work Piece: Aluminum To Brass Tube
- Work Piece: Graphite To Brass Tube
Tool application
The high speed milling machine use a variety of tools and materials, and the materials are very wide, ranging from hard metals and marble to soft PVC, plexiglass, two-color board, etc. Therefore, different workpiece materials correspond to suitable tools. It is very important to machining a good product with a good tool. A good tool is not only required for sharpness, but also for its model, size, material and parameter, which affect the quality and efficiency of machining.
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