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Requirements for discharge conditions of graphite electrode

6 加工速度的调整:
放电加工时,工具和工件同时受到不同程度的电蚀,单位时间内工件的电蚀去除量称之为加工速度,即生产率.
    在电火花加工过程中,影响加工速度的因素很多,主要有加工极性、电参数、工件材料以及工作液等.提高加工速度常在在粗加工中优先考虑,通常是放电能量越大,加工速度越快,所以:
  ①.依靠较大的电流才能确保石墨电极的高速加工;
  ②.要保证足够的火花位才能使用较大的加工电流;
  ③.增加脉宽虽然也能提高加工速度,但在石墨电极放电加工中,过大的脉宽会使加工不稳定反而影响加工速度,且容易发生积炭的现象;
  ④.用较短的放电休止可以提高加工速度,但要注意加工稳定性;
  ⑤.用较大极间电压或高压电流对提高加工速度有利;
  ⑥.不同的电极材料、不同的工件材和不同的火花机台要选用不同的放电参数。
7 表面粗糙度的调整 :
表面粗糙度由脉冲宽度、峰值电流、电极材料和工件材料共同决定的.电火花加工的表面和机械加工的表面不同,它是由无方向性的无数小坑和硬凸边所组成,电火花加工表面粗糙度通常用微观轮廓平面度的平均算术偏差Ra表示。
表面粗糙度受石墨颗粒直径大小的影响, 颗粒越大其得到的表面粗糙度就越粗.通常选用细粒径的石墨来提高表面光洁度;
脉宽越大,单位时间内放电时间越长,放电蚀痕越深,其加工表面也越粗.石墨电极以较小的脉宽来完成放电过程,则形成较浅的蚀痕而利于抛光.铜电极为了损耗小用较长的脉宽来完成放电,其形成的蚀痕较深、表面硬度也大,从而导致抛光困难;
峰值电流越大,加工表面粗糙度, 其没有脉宽影响大,所以石墨电极的“大电流、小脉宽”原则也是降低加工表面粗糙度的特点.
主轴摇动加工方式也是降低表面粗糙度和获得均匀纹面的有效手段。
8 电极损耗的调整:
放电加工时,单位时间内电极的蚀除量称之为损耗速度。在实际生产中,通常采用相对损耗作为衡量工具电极耐损耗的指标.
在电火花加工中存在4种损耗:整体损耗、角损耗、端面损耗和侧面损耗。由于角部损耗决定最后加工的精度,所以它的损耗率最重要,特别是电极精修时应重视角损耗。
石墨电极放电加工时,电流越大,电极损耗越小,
正常情况下,脉宽越大,电极损耗越小.但在石墨电极放电加工中过大的脉宽会导致石墨电极出现毛刺的负损现象,所以石墨电极的最大脉宽一般不超过420μs,且随着放电面积的越小,其脉宽值也越小;
合理配合脉宽,也可减少电极损耗,当IP=10A~120A、ON=100μs~300μs时,是石墨电极放电加工无损耗条件。

 

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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