Air is the devil in grinding

In a rudimentary experiment, I poured three distinct types of grinding oils into three electric mug warmers, allowing them to sit for three months. The oils were: a mineral-based oil – a highly refined derivative of oil extracted from the earth; a semi-synthetic oil – a similar substance but with artificial additives; and a purely synthetic oil – essentially human made. After three months, the cup containing the mineral-based oil was 2/3 empty, while the other mugs remained full.

The selection of oil should never be regarded as an afterthought. In this context, the adage price vs. cost holds significant relevance. Fully synthetic oils, while typically commanding a higher price, undoubtedly deliver improved grinding performance, including extended longevity. Known for exceptional thermal stability, they sustain their effectiveness even under elevated operating temperatures. This stability helps maintain consistent grinding quality for extended periods, while still providing sufficient lubricity.

Metal cutting is typically based on using a positive rake angle to remove material, while abrasive machining (grinding) relies on friction to remove material. This friction generates heat, and it’s well-established that grinding requires substantially more energy, necessitating higher coolant pressure and greater coolant volume compared to conventional machining processes.

An entire field of study is dedicated to coolant nozzles and optimal methods for delivering coolant to the grinding point. The relatively high peripheral speeds in grinding create an air barrier around the outer edge of the wheel, diverting fluid away from the grinding area. The primary objective of the coolant nozzle is to breach the air barrier, reaching the precise heat zone where the friction of grinding occurs. To successfully breach the air boundary layer surrounding the grinding wheel, coolant velocity must be sufficiently high. If air becomes entrapped within the coolant supply, it diminishes the coolant’s effectiveness in breaking through this air barrier.

I advise against the plastic modular nozzle systems which inherently introduce turbulence into the coolant delivery. Their internal structure relies on a ball-and-socket assembly to provide flexibility to form a coolant line, but each socket introduces additional turbulence. Consequently, by the time the coolant reaches the grinding point, it becomes saturated with excessive air, making the delivery lack coherence.

Some grinding machine builders offer their own solution of metal coolant hoses and nozzles. While still flexible, they direct the oil at high velocity with minimal air presence to the focal point of friction.

MyGrinding Inc.