In machining and every other type of contact technique, heat generation is inevitable. In fact, majority of the energy generated in a CNC cutting and milling process is transformed. Heat generation, if not handled properly, can be a tool destroyer. In some cases, heat accumulation in a tool can be easily detected. Other times, heat generation may not be immediately obvious, leading to scrapped parts, tool crack, breaking and deformation. As any cutting tool will generate heat, it is therefore important to understand the causes and possible ways to counter and dissipate the heat generated during machining.
Heat Generation During Machining
As discussed above, more than half of the energy generated during machine is represented as heat. Heat is produced all through the process at the primary shear zone – This is where the major part of generated energy is converted to heat. Heat is generated where metal deformation is taking place to create chips. This occurs at the cutting edge. It is produced as a result of friction between the chip, cutting too and workpiece. Heat is also generated at the point of separation where the metal part is removed (subtractive manufacturing process) to form the final workpiece.
Heat accumulation has an adverse effect on both surface of the workpiece and the tooling. And while heat itself does not have much of this detrimental effect, it is the constant increase in temperature at the same point that allows for tool damage to occur when machining.
Understanding Cutting Temperatures
In order to calculate the heat generated during machining, one must understand that cutting temperature isn’t constant across the tool, chip and workpiece. Heat generation may be estimated directly through calculation or indirectly by the use of visual cues such as thermo-sensitive paints and temper color of a chip.
How to Reduce Heat Generation During Machining
High-efficiency Milling
High-efficiency milling (HEM) is machining strategy that employs the use of a larger axial depth of cut (ADOC) combined with a lesser radial depth of cut (RDOC) to deliver optimal performance and results. It is widely used in roughing for removing large amounts of material from a workpiece
Using that HEM machining strategy results in a higher material removal rate with increased feeds and speeds. What this means is that your tool is not engaged for prolonged periods of time at only a small point at its edge; the full potential of the tool is explored, and the heat generation is uniformly dispersed across the cutting edge of the tool.
Control Speed and Feed rates
This is perhaps the easiest way to reduce heat generation during machining operations. Many researches have documented a direct relationship between cutting speed and feed rate with the temperature generated at the tool. In other to reduce heat generation, the best practice would be to lower the cutting speed and feed rate during operations. Consequently, the cutting time of the tool will become longer, and the overall production rate can be expected to decrease. However, if you’re not in too much of a haste on your project, reduced speed and feed rates will reduce heat generation and prolong the life span of your tool.
Application of cutting fluids
Cutting fluids, widely referred to as coolants, are one of the most effective means of reducing heat generation in machining. This is because coolants can be used to mitigate tool breakage in a variety of ways. For instance, coolants are responsible for flushing away chips from aluminum workpieces and preventing built-up edges. In materials such as titanium, where heat transfer isn’t very good, they serve their primary function of heat moderation and dissipation around the tool lowering the chances of overheating. Coolants may also deliver some type of lubricative function, allowing generated chips to easily exit the toolpath when machining.
As coolants are considerably expensive, it is imperative to know how to deliver the coolant. Coolant fluids may be applied in a number of ways, including as flood, minimum quantity lubricant, high-pressure or mist. Using the wrong delivery method with the wrong tool can result in tool damage. Coolants may be air, water or oil based. They may also be synthetic or semisynthetic. It is therefore imperative to know what is best for your tooling and operations before you proceed.
Consider positive rake angles
Tool geometry and cutting angle play an important role in heat generation and power requirements during machining. In other to reduce heat generation and tool damage during machining, cutting with a positive rake angle is highly recommended whenever possible.
Generally, the rake angle of any tool is the angle of the tool’s rake surface with reference to the plane. A positive rake angle in machining involves cutting at an angle when the sum of the wedge angle of the cutting tool and its clearance angle, measured on an orthogonal plane, fall below 90 degrees. The benefits of a positive rake angle is reduced power requirements, lesser tendencies of built-up edge and less temperature and heat generation during cutting. During machining, cutters with positive rake angles can pull the tool into the work for better results; the positive axial rake lifts up the chips in the toolpath and disperses them from the completed workpiece, into the inner side of the cutter body. Increased flank angles lower the rubbing and abrasion that occurs between the tool and workpiece, reducing heat generation and extending tool life.
Use climb milling
Climb milling is a milling technique that engages cutter rotation with the feed. This method, when used in place of conventional milling where the cutter rotates against the feed, can help to reduce heat generation and favors heath transfer.
During climb milling, the chipping begins at the widest with and decreases as the operation goes on. This prevents heat generation and accumulation in the tool or workpiece, ensuring that the heat is well transferred to the chip. This will not only keep the workpiece safer but reduce tool rubbing and tendency of the job to require a re-cut.
Why Heat Dissipation is Important
In every machine shop, tools are some of the most priced possessions. Heat accumulation can be a damaging factor, increasing machining costs and reducing productivity. In order to ensure that heat is evenly dissipated during machining and transferred to the chip as much as possible, consider climb milling strategies, high efficiency milling and working with positive rake angles when possible.
The importance of coolant fluids cannot be overstated. Ensure that you get the right coolant for your tooling needs and apply with the appropriate method as mistakes during application can cause certain tools to break or lower their effectiveness.
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