Do you press the big red button when you first see smoke, or when the fire alarm next door goes off? It is an unavoidable fact that if you cut or shear metal with a cutting tool or rub two metal surfaces together at speed it will generate heat. But how hot is too hot, and what can you do about it?
Signs of overheating
How do you know that a CNC machining tool is overheating? The reality is that if you wait for sparks or smoke you have probably already caused unnecessary damage to the cutter or the workpiece. Even a short period of exposure to excessive heat is enough to soften the tool material and damage the cutting edge.
You can often see (or hear) the signs of overheating long before tool damage occurs. Coolant evaporation, tool blackening or discoloration, and even unusual changes in pitch can all be signs that things are heating up. If any of these signs present themselves and you are not expecting it, it’s time to stop and rethink your machining strategy.
Coolant & chip clearance
Liquid coolant doesn’t only cool down the CNC machining process. It also lubricates and moves waste material away from the cutting tool. This ensures that the tool is not recutting old material and working harder than it has to. The waste material, or chips, are also hot! Bending, cutting, and shearing these small pieces of metal off of the workpiece has the effect of heating them up. You don’t want your tool wrapped in a blanket of hot pieces of metal. So evacuating the chips quickly also removes heat from the process.
Often a liquid coolant isn’t needed and forced air will do. This won’t cool the tool as effectively as a liquid, but it will do a better job of chip clearance.
Whether you are using a liquid coolant or forced air, these are the top considerations
- More coolant or indeed any coolant is not always essential when cutting a part.
- Coolant is only needed when cutting materials with low thermal conductivity, such as titanium and some steels.
- Liquid coolant can be essential when machining deep cavities as forced air may not be strong enough to vacate all the chips.
- Direct coolant jets at the tool, not the part to be effective.
- Consistent application of coolant throughout the cut is important to avoid hotspots and sudden thermal shocks. Don’t suddenly turn the coolant on when the tool is getting hot! Wait for the tool to cool down first!
Machined chips have an essential role in removing heat from a cutting tool. A cutting strategy that creates chips that are too small or too few, will reduce the amount of heat removed from the machining process. Choosing the correct tool in terms of geometry, size, flute count, and length of cut will help to reduce heat build-up. For example, a 2-flute end mill is great for removing material from soft, thermally conductive materials like aluminum and copper. But it has less mass, or thermal capacity, to handle the heat generated by harder, less conductive materials.
Other important factors are tool wear and the coating. A worn or damaged tool will be less effective at cutting, and generate more heat when doing so than a newer tool. Tool coatings are often optimized for use with specific materials. Make sure that you know which materials are suitable for machining with your tool. Coatings designed for reducing friction and heat build-up when milling steel may bond with other materials like aluminum at typical cutting temperatures. So choose carefully!
Feeds & speeds
It’s logical to think that the faster the tool moves relative to the part, the more heat is generated, and therefore choosing lower speeds should produce less heat. This is true of the spindle speed, particularly if you find that the CNC machine is creating very fine chips. Even small reductions in RPM can reduce heat generation and increase tool life. At the expense of course of a lower material removal rate and higher machining time.
Reducing the feed rate alone can have the opposite effect as it will reduce the chip size and chip load – both key factors in taking heat away from a cutting operation. Sub-optimal chip clearance also means that the tool is spending some of its time rubbing against the workpiece rather than cutting. This causes unnecessary additional heating of the tool and can result in the work hardening of the material.
CNC turning tends to produce steady, predictable results in terms of chip size, material removal rates, and heat build-up in the tool. Except for very simple tool paths, material removal rates when milling are not as consistent. Conventional milling tool paths will use the very end of a tool, at a constant feed rate. The effect of which is to have a high radial depth of cut, low axial depth of cut, and a variable tool engagement angle. This causes uneven tool wear and a lot of thermal stress on the active part of the tool.
Dynamic milling is one machining strategy used to reduce this, through radial chip thinning (reducing the radial depth of cut) and maintaining a constant engagement angle. High-Efficiency Milling is another. It uses more of the axial cutting edge to sustain the work done and resultant heat over a larger surface area. Both of these strategies reduce the amount of time that each flute is engaged with the material, thus reducing the amount of heat transferred to the tool. But beware! It’s likely that you will need to increase the feed rate for both of these strategies to maintain chip thickness and chip load considering the lighter radial engagement.
Still stuck? Get in touch. CNC Solutions has extensive experience machining parts from a wide range of materials and could machine the part for you. There may also be a suitable CNC training course in our fully equipped CNC Training Center.