HEAT GENERATION IN CUTTING TOOLS

Anyone who is even casually involved with a machining process knows that cutting tools generate large amounts of heat. Heat is generated in three ways; by the deformation of the metal in the shear zone ahead of the cutting edge, at the point of separation when the metal is physically pulled apart and by the friction of the chip as it rubs along the surface of the tool as it is pushed out of the way. In fact, much of the horsepower drawn at the spindle motor winds up as heat which is concentrated in a very small area at the cutting edge of the tool, and in the chip itself.

As described elsewhere in this manual, heat that is allowed to accumulate can be very detrimental to tooling, as well as to the surface of the work piece. The nice blue color in a chip means that the metal has seen very high temperatures. Most steels need to get to a temperature of at least 800° F to acquire an obvious blue oxide surface and the temperature at the tip of the cutting tool can often be over twice as hot. At these temperatures water is completely vaporized before it can reach the cutting zone.

HEAT vs. TEMPERATURE

There is no way to eliminate the heat generated at the cutting edge. The use of cutting fluids has an effect on the heat generated by friction; however, the majority of the heat is produced by the deformation of the metal itself as it is removed. The idea is not to allow the heat to accumulate in the tool to cause the temperature to rise. A brief review of heat transfer may be in order at this point. Heat is the measure of the amount of motion of the atoms in a material. All substances at a temperature higher than absolute zero contain heat. Absolute zero is the temperature at which all molecular motion ceases.

Heat is measured in British thermal units (Btu) in the English system, or calories in the metric system. A Btu is the amount of heat required to raise the temperature of a pound of water one degree on the Fahrenheit scale. Similarly, a calorie is the heat required to raise the temperature of a gram of water by 1° C. Temperature increases in other materials can be predicted by knowing the ratio of the temperature gain for a given heat input expressed as a ratio to that of water. By knowing definition of the units, it becomes obvious that, all other things being equal, an increase in the heat content of a substance causes an increase in temperature.

HEAT and MACHINING

So how does all of this relate to high pressure cooling? The fact is that it is not the heat generated by a machining process that does the damage, it’s the increase in temperature. Heat that’s allowed to accumulate where the tool meets the work will raise the temperature to a point where tool damage occurs. Heat remaining in the material will cause the shear plane to elongate, resulting in a thick chip that won’t break or damaging the structure of the work piece surface.

Heat can be removed by simply pouring the coolant over the tool as it cuts. This is referred to as ‘flood’ cooling, and has been the standard method for years. The coolant picks up heat as it washes over the area. A problem is that, even with the best operators, the coolant line is rarely aimed at the critical point. Even with the most careful coolant application, however, at the high performance levels available with modern machine tools, so much heat is generated that the coolant is heated to beyond its boiling point. A blanket of vapor forms over the very area we’re trying to cool, insulating it from the coolant. The only way heat can be drawn out of the area is by radiating it through the vapor blanket, and by conduction back through the tool. Either way, only a fraction of the heat-carrying capacity of the coolant is being used.

The high pressure cooling, pioneered by ChipBLASTER, allows the coolant to be introduced in such a way as to remove the heat at a high enough rate and pressure to eliminate the vapor barrier. This allows a direct heat transfer from the mass of the insert to the mass of the coolant. The temperature of the tool, in some cases, is only slightly above the temperature of the coolant.

ChipBLASTER tooling uses highly specialized jets which are precisely aimed to insert high pressure, high velocity coolant into exactly the spot where the heat is generated.