4. Number of electrodes
In case of the most common machining which process only one point on the workpiece of iron-based material, "2 electrodes, one for rough machining and one for finish machining, are made" in 90% opportunity, and the troublesome judgment is unnecessary. However, when work materials are different, or when machining points increase, it is necessary to increase electrode number. How to think about the decision of electrode number is explained below.
1) Number of rough electrodes
The number of rough electrodes is determined primarily by the amount of electrode wear. Electrode wear ratio differs depending on the work material.
In the case of the workpiece material is aluminum-based, you can reduce the number of electrodes because electrode wear ratio is about 1 / 3 of iron by replacing it to volume wear ratio (which is usually expressed weight wear). For the copper alloy-based materials, weight wear ratio is 3-10%, and it is 15-20% for cemented carbide. More number of electrodes are required in these cases.
The following is calculation for "Guidelines for the electrode number". The coefficients in this formula don't have special basis, and are determined based on experience. Please use them as easy formulas for determining the number of electrodes in a short time.
Guidance value for the rough electrode number = 0.02 [coefficient] × Machining depth [mm] × Electrode wear ratio [%] × Number of machining points
Note) machining depth [mm]
The actual cutting depth. Please use the deepest machining depth for complex shapes and for machining after pre-machining.
Note) electrode wear ratio [%]
The electrode wear ratio is read from the machining data such as instruction manuals attached to machines. However, the data attached to machines are data for shallow machining depth. When machining depth is deepened, the machining stability worsens, and the electrode wear increases to around 1.5-10 times generally. Because it is difficult to predict an accurate electrode wear ratio, please use the following data for the standard value.
<Data of the electrode wear by the combination of typical work - electrode materials>
Work material: Iron-based - Electrode material: Copper or Graphite ⇒ Electrode wear ratio [%] = 1
Work material: Aluminum-based - Electrode material: Copper or Graphite ⇒ Electrode wear ratio [%]= 0.3
Work material: Copper Alloy - Electrode material: Copper or Graphite ⇒ Electrode wear ratio [%]= 3
Work material: Cemented carbide - Electrode material: Copper tungsten ⇒ Electrode wear ratio [%]= 20
<Examples of expected number of rough electrodes>
Machining example 1)
- Work material: Iron-based
- Electrode material: Copper
- Machining depth: 20mm
- Number of machining point: 1
Guidance value of rough electrode = 0.02 × 20 × 1 × 1 = 0.4
⇒ Number of rough electrodes: 1
Machining example 2)
- Work material: Iron-based
- Electrode material: Copper
- Machining depth: 20mm
- Number of machining points: 4
Guidance value of rough electrode = 0.02 × 20 × 1 × 4 = 1.6
⇒ Number of rough electrodes: 2
Machining example 3)
- Work material: Cemented carbide
- Electrode material: Copper tungsten
- Machining depth: 5mm
- Number of machining points: 2
Guidance value of rough electrode = 0.02 × 5 × 20 × 2 = 4
⇒ Number of rough electrodes: 4
2) Number of finish electrodes
The number of finish electrodes is related to required machining accuracy, finish surface roughness and electrode wear ratio, but it is determined according to machining contents as follows because a judgment method is troublesome.
Guidance value for finish electrodes = Number of machining points ÷ Number of possible machining points
| Machining contents | Number of possible machining points |
|---|---|
| Accurate edge part | 0.5 |
| High quality cavity | 0.7 |
| General cavity | 2 |
| Lib groove | 2 - 3 |
| General machining part | 3 - 8 |
Note) Number of possible machining points
The number that is possible to be processed with one electrode
Note) Machining contents
・Accurate edge part
Machining which needs sharp edges at the machining corner such as
core pin of the connector pin mold
・High quality cavity
Machining of cavity part (product part) when you want to avoid polishing
work for maintenance of the shape quality as much as possible
<Examples of expected number of finish electrodes>
Machining example 1)
- Machining contents: Accurate edge part
- Number of machining point: 1
Guidance value of finish electrode = 1÷0.5＝2
⇒ Number of finish electrodes: 2
Machining example 2)
- Machining content: General cavity
- Number of machining points: 4
Guidance value of finish electrode = 4÷2＝2
⇒ Number of finish electrodes: 2
3) Number of common use electrode for rough and finish
When there is much machining number, and also when electrode reduction can be made same for rough and finish electrodes at the time of design, you can reduce the number of electrode by not distinguishing rough and finish electrode and using the used finish electrode as the electrode for the next rough machining.
To determine the number of the common use electrodes for rough and finish electrodes, it is determined as described below after calculating the expected number by each aforementioned procedures of "Number of rough electrodes" and "Number of finish electrodes".
(1)When the expected number of finish electrodes is larger than that of rough electrodes
Guidance value for the number of common use electrodes for rough and finish = Number of finish electrodes +1
(2)When the expected number of rough electrodes is larger than that of finish electrodes
Guidance value for the number of common use electrodes for rough and finish = Number of rough electrodes +1
<Examples of expected number of common use electrodes for rough and finish>
Machining example 1)
- Work material: Iron-based
- Electrode material: Copper
- Machining depth: 10mm
- Number of machining points: 20
Guidance value of rough electrode = 0.02×10×1×20＝4
⇒ Number of rough electrodes: 4
- Machining content: General cavity
- Number of machining points: 20
Guidance value of finish electrode = 20÷2＝10
⇒ Number of finish electrodes: 10
Thinking about common use electrodes for rough and finish, the number of finish electrodes is larger than that of rough electrodes. Therefore, the formula to be used is
Guidance value for the number of common use electrodes for rough and finish = Number of finish electrodes +1
Guidance value for the number of common use electrodes for rough and finish = 10 + 1 = 11
⇒ Number of common use electrodes for rough and finish: 11
Machining example 2)
- Work material: Iron-based
- Electrode material: Copper
- Machining depth: 30mm
- Number of machining points: 20
Guidance value of rough electrode = 0.02×30×1×20 = 12
⇒ Number of rough electrodes: 12
- Machining content: General cavity
- Number of machining points: 20
Guidance value of finish electrode = 20÷2＝10
⇒ Number of finish electrodes: 10
Thinking about common use electrodes for rough and finish, the number of rough electrodes is larger than that of finish electrodes. Therefore, the formula to be used is
Guidance value for the number of common use electrodes for rough and finish = Number of rough electrodes +1
Guidance value for the number of common use electrodes for rough and finish = 12 + 1 = 13
⇒ Number of common use electrodes for rough and finish: 13