Effectively protecting machine tool spindles--which means primarily the spindle bearings that support the shaft--from contamination by coolant, moisture and chips has challenged some of the best and brightest engineering minds.
Add ECD to the alphabet soup of metalworking abbreviations becauseelectrochemical dressing may indeed be coming to a superabrasive grinding machine near you. Swiss grinding machine builder Agathon, working with the Swiss coolant maker Blaser and a consortium of Swiss manufacturers as well as the Swiss academic community, is commercializing this innovative dressing process. Designed for superabrasive grinding, it is available on Agathon's new 350 COMBI machine. Its trade name is EcoDress.
Efficient dressing of metal bonded diamond and CBN grinding wheels is generally problematic since the metal bonding is very resistant to wear and because the grain materials are numbers one and two on the hardness scale. One problem with conventional dressing of superabrasive metal bonded grinding wheels, using silicon carbide or corundum dressing wheels, is the low protrusion of the grain and the rather high cycle times to accomplish the dress.
In addition, the kinematics of the conventional dressing process build up on the surface of the grinding wheel in taking the form of grooves in the rotating direction of the wheel. Unfortunately, this leads to a sharpness direction of the grinding wheel in its rotating direction. The average protrusion of grain is limited for conventional dressing techniques of 15 to 25 percent of the grain diameter. It's like an iceberg with 15 to 25 percent floating above the water line while more than three-quarters of it is submerged.
When the top of the grain fractures and leaves the cutting area, the wheel bonding is now rubbing the workpiece, thus increasing heat and horsepower requirements. This low protrusion of grain in ratio to its diameter also leads to rapid wheel loading because there is very little clearance for swarf to clear the cutting zone. This condition too will increase heat and reduce the wheel's free cutting.
It takes frequent or continuous dressing to keep heat and cutting forces in an acceptable range. Of course, frequent dressing can waste expensive supera-brasive wheels and slow down the grinding cycle.
The ECD process is the solution to dress metal bonded superabrasive grinding wheels. It offers the ability to expose new cutting grains continuously (in-process) without physical contact between the dressing tool and the wheel. It works by means of electrochemical metal dissolution.
The process developers use a bronze-bound diamond or CBN grinding wheel. Vitrified or resinoid bonding will not work because the wheel must be capable of carrying an electric current. In the EcoDress process, the grinding wheel is the anode. Electrical connection to the wheel is integrated into the grinding wheelhead. The cathode is located on the periphery of the wheel using an adjustable electrode holder.
Coolant acts as an electrolyte in the gap between the electrode and the grinding wheel. In this process the coolant used is a synthetic that is water soluble in a mixture of 5 percent. When the "juice" is turned on, electrolysis dissolves the bronze bonding away from and between the inert diamond or CBN grains.
The speed of the dissolution, meaning the speed of the grain exposure, is controlled by the amount of current that is sent through the wheel. The amount of the current flow is controlled by the actual wheel sharpness, which is detected by in-process measuring of the forces of the grinding process.
The advantage of the ECD process in grinding is that the average depth of the grain increases to 100 percent of the grain diameter. This provides a significantly improved coolant flow and swarf dispersal around the individual cutting grains. Free cutting action resulting from the extra large protrusion of the grain allows faster grinding infeed speeds without additional heat developments and grinding forces.
The grinding ratio is controllable by the current in a wide range. Because of that, the end-user will be able to use the same grinding wheel for different kinds of materials by changing the dissolution rate only. In addition, by using the dressing process continuously, it will significantly reduce grinding cycles and wheel wear.blog comments powered by Disqus