Much research has been done on the issue of wire breakage during the machining process. But before we explore the types of wire that can be used to decrease wire breakage in certain materials, we must first look at what causes wire breakage in general.
There are several key factors involved when determining what caused the wire breakage. The first and sometimes the obvious is the current density: the amount of current on any given surface area. When that exceeds the physical limitations of electrical current flowing on the surface of the wire, it creates enough heat to melt the wire.
For example, a 2″-thick piece of tool steel as a contact area along the wire of 2″—this is the surface area that will conduct the cutting current to the workpiece. The lesser thickness of a .5″ workpiece would have much less contact area. This should not be confused with peak current, which is the available amperage or power on the wire. Peak current can be compared to pressure and current density can be compared to volume; both can be controlled by our choice of settings.
One of the easiest ways to control the current density is to decrease the frequency of the spark by increasing the time between sparks. This is accomplished by adjusting the off-time of the spark through the control panel, either manually setting the off-time, or setting a range or safety setting on the control in which the power supply will automatically detect the current density.
Another cause of wire breakage can be the flushing or water cooling of the wire. This is the first thing we usually reach to adjust and the easiest to see immediate results. Bad flushing can come from poor head placement, usually too far away from the workpiece. If moving the head closer is not an option because of clamps or other obstructions, then increasing the volume of water could resolve the situation.
Sometimes you will notice air in the cutting area due to cavitation from high water pressure; this is most common when starting a cut from an outside edge of your part. Usually, the condition will only last until the wire has cut far enough into the part to create a wire slot into which the water can flow, and cool and clean the cutting area.
The point at which power is applied to the wire is a point of wire break that is sometimes overlooked. You will notice that the wire will break at a point near the top surface of the part. This is caused by worn or misadjusted power contacts. By replacing or adjusting the contact plate, the breakage will stop immediately.
Poor maintenance is not as common of a cause but can be just as important to consider and one of the more difficult to pinpoint. This will usually be in the wire transport system and can be dirty wire rollers, worn guides, small bits of broken EDM wire, misadjusted wire tension or bad water conductivity. A check of the wire path from roller to scrap bin is best done by a careful inspection of every component in order, and may take more time to do during a job than if an inspection was done just prior to starting the cut. Check the hours on the DI resin, and if there is any question about the life left, replace the bottle or resin charge. It will not hurt to make a resin change when in doubt. Beyond these common causes of wire breakage, power supply board failure or grossly selected cutting settings would be less likely, but just as important to have on the checklist as you look for the cause and cure. More to come in the next blog about how different wire types will aid in decreasing wire breakage and increasing speed.