There are many reasons for wire breakage in wire EDM. This article discusses one of the factors and its solution.

Relating to the working piece

The molybdenum wire breaks when the machine processes thin workpieces. Thin workpieces usually refer to the thickness below 3mm. The main reason for the broken molybdenum wire is that the distance between the upper and lower guide wire wheels of the wire frame is fixed, generally about 70mm. When cutting thin workpieces, in the case of high-speed wire feeding, the electrode wire loses the damping effect of the coolant produced when machining thick workpieces, and the silver wire is easy to shake due to the influence of spark discharge. The solution is to adjust the processing voltage to about 50V; adjust the processing current to about 0.3A, adjust the pulse width to less than 10 microns; reduce the jitter of the molybdenum wire. If the wire storage drum is driven by a DC motor, it can be changed Armature voltage reduces the speed. If it is driven by an AC motor, connect a 10-15O ohm, 75W wire wound resistor in series in any two phases of the three-phase to reduce the phase voltage to make the commutation transition time slightly It is long to achieve soft commutation, which can effectively reduce jitter. The method of thickening auxiliary materials between the upper and lower guide wheels is to increase the thickness and enlarge damping. It can also prevent the molybdenum wire from jittering. This method is relatively simple and does not need to adjust the processing electrical parameters.

1. The molybdenum wire breaks when cutting thick workpieces

Thick workpieces generally refer to the workpieces larger than 100mm in thickness. The molybdenum wire breakage occurs during cutting thick workpieces when sparks are generated just after the feed Or during the cutting of the workpiece and when the workpiece is cut. The main reasons for broken molybdenum wires are listed as below.

2. Wire breaks at the initial cutting

The molybdenum wire breaks as soon as sparks are generated during cutting from the outside of the workpiece. This is because during the initial cutting, the molybdenum wire is outside the workpiece, and the upper and lower guide wheels have a large distance. Because the molybdenum wire is not damped and jitters, the gap between the molybdenum wire and the workpiece is in a poor state. Or excessive emulsion makes the insulation resistance reduce. The arc extinguishing performance is not good, the arc discharge is contained in the discharge gap, and the copper wire is burned. In wire cut EDM machining, arc discharge is the main factor that causes corrosion and damage to the negative electrode. Coupled with poor gap, arc discharge is easy to form. As long as the arc is concentrated in a certain section, it will cause wire breakage. Moreover, the greater the short-circuit current, the more severe the arc burns the molybdenum wire, and the greater the possibility of wire breakage.

3. Wire breaks during processing

When the molybdenum wire cuts into the workpiece, the emulsion is difficult to penetrate due to the narrow slit. And the electric corrosions in the slit, the carbon black and metal, do not come out, which makes the processing conditions worse. Electric discharge Machining often happens in the slit twice or three times which causes the slit to widen. Like cutting thin workpieces, the gap is in a poor state, causing the pulse to form an arc discharge. If the arc discharge is concentrated in a certain section, the molybdenum wire will soon be burned.

4. Wire breaks when the cutting is almost finished

The molybdenum wire breaks when the cutting is almost completed and there is still a few millimeters or even tens of microns. In addition to the reasons mentioned above, the reason for this kind of broken wires is the deformation caused by the weight of the workpiece and the internal stress of the workpiece material, which causes the clamping wire to break. The solution is to make simple tooling fixtures, and the materials are subjected to necessary heat treatment before processing.

5. Wire breaks due to the non-conductive materials inside the workpieces

When the material that seems normal to the outside is cut normally, a ‘short circuit’ occurs suddenly, and no matter how it is eliminated, it will not work. In this case, you may consider that possibly most of the impurities are trapped in the forged or smelted materials. These impurities do not have good electrical conductivity, resulting in continuous short circuits during processing, and eventually the steel wire is strangled. The solution is to compile a program that retreats 0.5 to 1 mm every time it enters 0.05 to 0.1 mm, and uses it repeatedly during processing and increases the coolant flow. Normally, it can wash away impurities and resume normal cutting.

The workpieces processed by wire cut EDM machines are mostly after flat grinding. According to the normal process, they should be demagnetized after flat grinding. If the workpiece is not demagnetized, the electro-corrosive particles produced during the wire cutting process are easy to be absorbed in the slit. Especially when the workpiece is thick, the non-demagnetization will easily cause uneven cutting feed, and the increase of surface roughness will cause short circuit and wire breakage.

The molybdenum wire is broken when the wire cutting is automatically centered. This is because there are oil stains, burrs or some non-conductive materials on the wall of the process hole. When the electrode wire moves to the wall of the hole, there is no spark discharge, so that the machine tool cannot automatically change direction. The workpiece supports the molybdenum wire to bend and finally breaks the wire. Therefore, the process holes must be cleaned before processing.