6. Amount of reduction
With an electrical discharge machine which raises sparks to process, the machined shape becomes larger depending on the strength of the spark than the electrode. Therefore, the dimensions of the electrode must be reduced for the objective shape beforehand. The amount to reduce the electrode is called "electrode reduction amount". Here is the explanation how to decide "electrode reduction amount".
1) Considerations by size of "electrode reduction amount"
(1) With a large "electrode reduction amount", rough machining speed is fast.
The following data show the relationship between the "electrode reduction amount" and expected machining time. (Electrode material is copper, depth of machining is 10mm, and the size of the electrode is 10, 50, and 100 mm). You can see that expected machining time changes greatly due to "electrode reduction amount".
| Size of the electrode (mm) | |||
|---|---|---|---|
| Electrode reduction amount | □ 10 | □ 50 | □ 100 |
| 0.4 | 1 hour | 3 hours | 20 hours |
| 0.3 | 1 hour | 4 hours | 25 hours |
| 0.25 | 1 hour | 7 hours | 40 hours |
| 0.2 | 1 hour | 10 hours | 60 hours |
| 0.15 | 2 hours | 20 hours | 120 hours |
| 0.1 | 3 hours | 60 hours | － |
| 0.07 | 15 hours | 120 hours | － |
Table 1) expected machining time by the electrode reduction amount of using a copper electrode of 10mm in depth
Note that when the electrode size is □ 10mm, there is no difference in the expected machining time for "electrode reduction amount" 0.4, 0.3, and 0.25mm ". EDM is not in effect for increasing machining speed even if using stronger machining condition for a small machining area.
(2) A large "electrode reduction amount" tends to make a large shape error.
In a large "electrode reduction amount", you have to use orbit movement of a large radius to make objective shape dimensions, and it increases risks for producing a shape error necessarily. Therefore, you must avoid raising "electrode reduction amount" needlessly with the finish electrode in particular.
(3) A large "electrode reduction amount" tends to make a large electrode wear of a corner.
A large "electrode reduction amount" makes larger wear of the corners during EDM, and you have to avoid it in machining that requires a sharp corner shape.
2) How to decide "electrode reduction amount"
"Electrode reduction amount" has to be decided by amount of machining or required shape accuracy, etc., but when thinking about efficiency of production of electrodes, it is not good to adapt them for various kinds of "electrode reduction amount". It causes problems that complicate tool management to make electrodes, programming of cutting for machining electrodes, and judgment method that is required to electrode designers, and so on. Therefore, it is necessary to keep kinds of "electrode reduction amount" within three kinds for rough electrode, within two kinds for middle finish electrode, and within three kinds for finish electrode at most.
| Types of Mold | 1 | 2 | 3 | |
|---|---|---|---|---|
| General home electronics molds Automotive molds General Die Casting molds |
Rough electrode | 0.8 | 0.5 | 0.3 |
| Middle finish electrode | 0.5 | |||
| Finish electrode | 0.3 | 0.25 | 0.2 | |
| Small plastic mold Small die-casting mold |
Rough electrode | 0.3 | 0.2 | |
| Middle finish electrode | 0.15 | 0.1 | ||
| Finish electrode | 0.15 | 0.1 | 0.07 | |
| Precise plastic mold | Rough electrode | 0.2 | 0.15 | |
| Middle finish electrode | 0.07 | |||
| Finish electrode | 0.1 | 0.07 | 0.05 |