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Requirements for discharge conditions of graphite electrode

6 加工速度的調整:
放電加工時,工具和工件同時受到不同程度的電蝕,單位時間內工件的電蝕去除量稱之為加工速度,即生產率.
    在電火花加工過程中,影響加工速度的因素很多,主要有加工極性、電參數、工件材料以及工作液等.提高加工速度常在在粗加工中優先考慮,通常是放電能量越大,加工速度越快,所以:
  ①.依靠較大的電流才能確保石墨電極的高速加工;
  ②.要保證足夠的火花位才能使用較大的加工電流;
  ③.增加脈寬雖然也能提高加工速度,但在石墨電極放電加工中,過大的脈寬會使加工不穩定反而影響加工速度,且容易發生積炭的現象;
  ④.用較短的放電休止可以提高加工速度,但要注意加工穩定性;
  ⑤.用較大極間電壓或高壓電流對提高加工速度有利;
  ⑥.不同的電極材料、不同的工件材和不同的火花機臺要選用不同的放電參數。
7 表面粗糙度的調整 :
表面粗糙度由脈沖寬度、峰值電流、電極材料和工件材料共同決定的.電火花加工的表面和機械加工的表面不同,它是由無方向性的無數小坑和硬凸邊所組成,電火花加工表面粗糙度通常用微觀輪廓平面度的平均算術偏差Ra表示。
表面粗糙度受石墨顆粒直徑大小的影響, 顆粒越大其得到的表面粗糙度就越粗.通常選用細粒徑的石墨來提高表面光潔度;
脈寬越大,單位時間內放電時間越長,放電蝕痕越深,其加工表面也越粗.石墨電極以較小的脈寬來完成放電過程,則形成較淺的蝕痕而利于拋光.銅電極為了損耗小用較長的脈寬來完成放電,其形成的蝕痕較深、表面硬度也大,從而導致拋光困難;
峰值電流越大,加工表面粗糙度, 其沒有脈寬影響大,所以石墨電極的“大電流、小脈寬”原則也是降低加工表面粗糙度的特點.
主軸搖動加工方式也是降低表面粗糙度和獲得均勻紋面的有效手段。
8 電極損耗的調整:
放電加工時,單位時間內電極的蝕除量稱之為損耗速度。在實際生產中,通常采用相對損耗作為衡量工具電極耐損耗的指標.
在電火花加工中存在4種損耗:整體損耗、角損耗、端面損耗和側面損耗。由于角部損耗決定最后加工的精度,所以它的損耗率最重要,特別是電極精修時應重視角損耗。
石墨電極放電加工時,電流越大,電極損耗越小,
正常情況下,脈寬越大,電極損耗越小.但在石墨電極放電加工中過大的脈寬會導致石墨電極出現毛刺的負損現象,所以石墨電極的最大脈寬一般不超過420μs,且隨著放電面積的越小,其脈寬值也越;
合理配合脈寬,也可減少電極損耗,當IP=10A~120A、ON=100μs~300μs時,是石墨電極放電加工無損耗條件。

 

1, the requirements for pulse current (IP):
The characteristics of pulse current are: the higher the value, the higher the discharge machining speed, the larger the discharge gap, the more rough surface roughness, and the smaller the electrode loss.
1). Pulse current discharge affected area, the effect of current density.
The principle of selection of graphite electrode pulse current with the average current standard
Graphite electrode large current density, typically 10 ~ 12A/cm2;
Graphite electrode, the current density is usually set to 6 ~ 8A/cm2.
2). Pulse current by reducing electrode measured in inches (spark position) size effect
If a large area with a small spark or a small area with a spark of graphite electrodes are not suitable for normal discharge machining.
The current shall be determined by the electrode to determine the size of the area, this is the most reasonable selection method.
The average current graphite electrode to 10A ~ 120A, the electrode loss minimum. Also increases with the increase of current electrode loss.
2, the pulse width (TIME ON, pulse width) requirements:
Width: of numerical value is greater, the longer the discharge time, faster processing speed, electrode loss is small, the discharge gap is big, the surface roughness of the more coarse. Machining stability is poor.
The pulse width range of graphite electrode is 0 ~ 1000 us.
Pulse width is larger, processing speed with the increase of pulse width, unstable process, increase the processing time, processing speed slow down and make the workpiece surface ablation; its value is generally not more than 420. When the pulse width in the 100 ~ 300US graphite electrode with minimal loss.
To selection of pulse width according to the magnitude of the electric current and the discharge processing requirements to determine, if discharge area is larger or used as rough machining, in order to improve the processing speed, pulse width takes bigger; small area or finishing, considering the surface roughness, is pulse width from smaller.
Different workpiece materials, different machining polarity, influence of pulse width on the processing effect is also different.
Graphite materials of different manufacturers, different grades, different batches, the influence of pulse width is also different.
Width, graphite particles smaller, the electrode loss is smaller.
3, the pulse interval (TIME OFF discharge end) requirements:
Pulse interval is let automatic discharge Zhe, eliminate ionization, so that the processing of liquid medium to remove debris, and prepare for the next discharge.
Characteristics of pulse interval: only affects the discharge machining speed and stability, and to the influence of other smaller. When its value is, the better the stability of the processing, processing speed is relatively slow, but stable discharge is unstable than to fast;.
The pulse interval to increase electrode loss instead of increasing pulse interval in the range was better than that of the pulse of the broad, between 0 ~ 2500 us. Pulse interval is 100US attains the minimum value.
Graphite electrode for electrical discharge machining in often take pulse interval (off) = pulse width (on), and depending on the processing stability of the adjusted to a pulse width of 1 / 3 to 2 / 3.
When the pulse interval is appropriate, with the increase of the pulse interval, interelectrode dielectric deionization full and is conducive to the formation of the cover layer (in the EDM process erosion except the decomposition products and medium containing carbon particles attached on the surface of the electrode), and reduce the electrode loss, but when the pulse interval greater than 100US, the electrode and the workpiece surface cooling time is too long, a pulse need more energy for the channel forming discharge, and is not conducive to the cover layer formation, electrode loss but increased. If the pulse interval is too small, the elimination of the electrode and the workpiece is not sufficient, and it may produce the phenomenon of the surface of the electrode and the workpiece surface.
Inactivity time usually only affects the discharge rate, and the electrode wear and surface roughness is not obvious.
4 of gap voltage (SV) requirements:
The characteristics of gap voltage, the greater the value, the better the processing stability, the higher the discharge gap size, the effect on the electrode wear and surface roughness:
Different spark machine, set the gap voltage level is also different, generally divided into:
40 ~ 90 ~ 60V files, 120V files, file 150 ~ 190V, 200 ~ 250V file.
Graphite electrode of the current processing to use a larger gap voltage, up to 200 ~ 250V and fine processing with smaller gap voltage (40 ~ 60V).
Do not use file the gap voltage, the working current is also different.
5 requirements for machining polarity:
In electric discharge machining, the polarity of the pulse source determines the direction of the electric current, and the polarity of the electrode directly influences the effect of the electric power.
In positive polarity machining, graphite electrode as cathode, with flowing current, temperature rise, heat emitted by the electron is increased. Therefore, the workpiece is erosion. At the same time, the processing speed quickly.
Machining of die steel and graphite electrode in the negative polarity machining with positive polarity machining: electrode loss, slow processing speed, surface quality good. When the pulse duration is longer, such as pulse width is 200 US and cathode machining electrode loss reached almost zero, and sometimes reverse loss, but the surface roughness increased, rough machining of the graphite electrode, with the pulse duration is longer, if considered emphatically electrode loss should be the positive polarity machining (i.e. graphite electrode design for cathode, the workpiece as anode), electrode wear is small; if first consider the processing speed of the workpiece, the positive polarity machining (i.e. workpiece design for anode, graphite electrode as cathode), processing speed quickly.
Copper electrode in precision machining and micro machining, with shorter pulse width should be negative polarity machining, to get a better surface quality, but electrode loss is bigger:
Workpiece materials of different graphite electrode polarity selection different. By positive polarity machining, general tool steel, stainless steel, copper alloy, aluminum alloy, tungsten steel, and titanium alloy the negative polarity machining
6 adjustment of processing speed:
While the tool and the workpiece at the same time, the tool and the workpiece are also different degrees of electrical erosion. The electrical erosion of the workpiece in the unit time is called the processing speed.
In electrical discharge machining (EDM), there are many factors influencing the processing speed, mainly processing polarity, electrical parameters, workpiece material and working fluid. Improves the machining speed often in rough machining is preferred, usually discharge energy is larger, the faster processing speed, so:
. rely on large current to ensure the high-speed machining of the graphite electrode;
II. To ensure sufficient to use a spark processing large current;
3. Increasing the pulse width, although they can improve the processing speed, but in graphite electrode for electrical discharge machining, high pulse width will enable processing unstable but affect the processing speed, and prone to carbon deposition phenomenon.
The with shorter resting discharge. The processing speed can be improved, but should pay attention to the processing stability;
5. With large voltage or high voltage current to improve the processing speed;
. the different electrode materials, the workpiece material with different spark discharge parameters to choose different machine.
7 adjustment of surface roughness:
Surface roughness by pulse width, peak current, electrode material and workpiece material co decision. Electrical discharge machining surface and mechanical processing different from the surface, it is composed of non directional numerous small pits and hard collar, electrical discharge machining surface roughness is usually used micro profile of flatness arithmetic mean deviation RA said.
The surface roughness is influenced by the diameter of graphite particles. The larger the surface roughness of the particles, the more rough the surface roughness is obtained;
Duration is, time units within the longer the discharge time, discharge corrosion mark more deep, the machined surface is also more thick. Graphite electrode with narrow pulse width to complete discharge process, the formation of shallow corrosion mark and good polishing. Copper electrode to small loss with long pulse width to complete discharge, corrosion marks the formation of deep, surface hardness is high, resulting in difficulties polishing;
The higher the peak current, the roughness of the machined surface, the greater the influence of the pulse width, the "big current and small pulse width" principle of the graphite electrode is also the characteristic of reducing the roughness of the machined surface.
The main axis of the shaking process is also the effective means of reducing the surface roughness and obtaining uniform grain surface.
8 electrode loss adjustment:
In the discharge machining, the amount of the electrode in the unit time is called the loss rate. In practical production, relative loss is usually used as an indicator of the loss of the tool electrode.
There are 4 kinds of losses in the electrical discharge machining: the loss of the whole, the loss of the angle, the loss of the surface and the loss of the side. Because the final machining accuracy depends on the angle of loss, so its loss rate is the most important, especially electrode refinement should be perspective of loss.
When the graphite electrode is discharged, the current is larger and the electrode loss is smaller,
Under normal circumstances, the duration is, electrode loss is small. But in the graphite electrode for electrical discharge machining in a large pulse width can cause in the formation of burrs on the graphite electrode negative loss phenomenon, so the maximum pulse width of graphite electrode is generally not more than 420 Mu s and with the discharge area is small, the pulse width value is smaller.
Reasonable combined pulse width can also reduce the electrode loss, when the 120A ~ ON=100, s ~ 300 s ~ IP=10A, the graphite electrode discharge machining without loss condition.

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