Tech Talk
What is the "Touch Dressing Technique"?
Conventional dressing techniques when applied to CBN wheels are far from ideal. After dressing, the grinding force is very high and removal rates are reduced unless an appropriate conditioning technique is employed to open up the wheel. The grinding power after conventional dressing takes a long time to decrease from an in initial high value to an acceptable steady lower value. The total power decrease is about one third of the initial power. The high initial grinding power is attributed mainly to the flattening of the CBN wheel surface with conventional dressing. Because of the high hardness of CBN, the force required for the dresser to cut through the CBN grains is high. If the dressing depth is large, the large dressing force may pull out grains, leaving bond material at the wheel surface. The subsequent grinding is conducted with bond as well as grains. This is equivalent to grinding with a blunt wheel, which increases rubbing adn reduces cutting. With a conventional dressing operation, effective grinding only takes place after bond material at the wheel surface is worn away so that only grains are used for grinding.
A large dressing depth of cut closes up the wheel surface. If the grain pullout is considered, the wheel surface will be even poorer. If the dressing depth is reduced as in the case of touch dressing, the dresser may cut through the grains without pulling them out, leaving sharp grains on a more open wheel surface. Therefore a lower initial grinding power is expected.Touch dressing is a technique of dressing a vitrified CBN grinding wheel with minimal dressing depth (usually less than 5 um) so that the correct profile of the wheel is restored without loss of cutting ability.A decrease of the dressing depth increases the usable wheel life. The consumption of the wheel using touch dressing is less than one third of that with normal dressing conditions. The improvements clearly demonstrate the potential of touch dressing to reduce the cost of grinding.The main challenge of applying touch dressing is how to determine the initial contact between the dressing tool and the wheel and how to achieve a small constant dressing depth of cut of less than 5 um. A CNC machine has the ability to position a machine axis to a high accuracy and to achieve an increment of 1 um. However, although modern CNC systems can position the dressing tool to a high accuracy, there is still a requirement to detect the position of the grinding wheel surface. The position of the grinding wheel surface relative to the machine axis positions changes due to grinding wheel wear, thermal expansion or contraction of the grinding wheel. Diamond wear also changes the position of the diamond dressing tool. The effect of this variability is that the wheel position stored in the CNC is often incorrect by the time it is necessary to redress the grinding wheel. To overcome this problem the machine user often specifies sufficient dressing infeed to guarantee that the dressing tool will the grinding wheel. An alternative strategy is to estimate the mean grinding wheel wer rate and use this within the CNC to set the required dressing increment. The latter strategy relies on the grinding wheel wear rate being constant. For workpieces with varying stock allowance this strategy will fail.A better strategy is to detect the initial contact between the grinding wheel and the dressing tool and then apply the required dressing increment. Acoustic emission sensors are used for this purpose. Acoustic emission is employed because the high frequency signals of dressing contact can be discriminated from background noise. The acoustic emission from the contact of dresser and grains contains some very high frequency harmonic elements. The AE signal is characterized using a band-pass filter, a rectifier and a low-pass filter. Inital trials on an external grinding machine showed that the detection of dressing depths of cut of 1 um are easily achieved.However, detection of contact between the dresser and the wheel for a high frequency internal grinding machine was found to be difficult, because of the much higher frequency of the background noise. The high frequency components are attributed to harmonics of the high speed of the grinding wheel spindle and the motor driven rotary dressing cup. The signals to be monitored for touch dressing should be in a higher frequency range than the background noise to give a satisfactory signal-to-noise ratio. High frequency AE signal energy is heavily reduced during its transger to the sensor. For an AE sensor mounted on the body of the rotary dressing tool the AE signal is required to pass from the dressing tool to the sensor via the dressing tool shaft, the support bearings and the dresser body.To reduce the transmission distance and the number of elements in the transmission path, a coolant coupling method was used. This method allowed the transfer of a much stronger contact AE signal to the sensor. The amplitude of the high frequency components of the AE signal remain at a higher level and can be distinguished from the background noise. Based on the ability to detect a 1 um dressing depth using an AE sensor, the trueness of the grinding wheel shape can also be monitored. This may provide a new method to monitor the grinding wheel shape.
