Coolants For Milling Titanium
Coolants are an essential part of machining titanium. If you choose the right coolant, it can extend tool life, speed up machining times and reduce waste. Anyone searching for the right coolant
must remember the three main functions of any good coolant. These are; lubrication, cooling and chip removal. If you can find a coolant that can manage all three of these tasks it can drastically increase productivity.
Types of Coolant
The 5 types of coolants generally available in machining are liquids, pastes or gels, aerosols, CO2, air or other gasses. All of these can be effective but each has its flaws so the type of coolant you use will be based on the type of machining you use.
Liquid coolants fall into three categories, mineral, semi-synthetic and synthetic.
Mineral lubricants are petroleum based. These offer a great deal of lubrication but lack the cooling power of their synthetic and semi-synthetic cousins. To get proper chip removal using this type of coolant, you need high flow or high pressure spray depending on the application.
Semi-synthetic lubricants combine oil with water. Usually, small amounts of oil are suspended in large amounts of water using an emulsifying agent like detergent. This is often used in CNC machining because the water is excellent at dealing with heat while the oil provides lubrication and rust resistance. Using high pressure or high flow rates also mean this type of coolant is excellent for chip removal.
Synthetic lubricants are usually water based man-made coolants that have a variety of characteristics. Like semi-synthetic, these coolants provide cooling and lubrication. They can also improve chip removal depending on the application of the coolant.
Paste or Gel Coolant
Paste or gel coolants are suitable for some types of metal cutting, drilling or tapping. They are not recommended for industrial use though and they are not able to improve chip removal.
Traditional aerosol coolants were wildly ineffective. They were bad for workers, impossible to direct and generally put coolant everywhere except where it was needed. Now, using MQL (minimum quantity lubricant), aerosol coolant can be directed to the cutting surface precisely running directly through the tooling. The only issue with aerosol coolants is heat dispersal. MQL provides plenty of lubrication, which helps reduce heat, but because very little fluid used, heat can stay in the tooling.
C02 coolant works using phase changes. Liquid CO2 is allowed to expand, which lowers its temperature and turns the liquid to tiny solid crystals. These crystals are forced into the cutting zone through the spindle or a sprayer. This forces chips out and provides a super cooling stream that reduces heat. Unfortunately, the CO2 provides no lubrication, which can affect cutting.
Air or Other Gas Coolants
Traditionally air, either ambient or compressed, was used as a coolant. A forced stream of air removes chips and provides some degree of cooling. In modern milling, the gas of choice is nitrogen. The liquid nitrogen is sprayed onto the tooling or used with MQL to refrigerate the tool to such a degree that it can afford to absorb the excess heat created during machining. This does not provide any lubrication but it can help increase tool life significantly.
Coolant Delivery Methods
Along with the types of coolant, there are methods for delivering coolant to the tooling. These have evolved significantly in recent years to become far more effective. Coolant can now be applied quickly and precisely to suit even the highest speed and feed rates.
Flooding coolant was and still is a commonly used technique for many machining applications. This method applies coolant using low pressure, high volume nozzles. The benefit is plenty of cooling and chip removal. The drawback is needing to use a large amount of coolant to do the job.
Spraying coolant is different from flooding because spray is often under high pressure. This means it is applied to the precise area needed. The benefits of spray coolant are lower coolant usage and better chip removal. The drawback is coolant is still wasted as it bounces off the tooling and work surface.
Misting has traditionally been an inaccurate spray using aerosol coolants directed via nozzles. It has recently evolved to be used in a through-tool coolant system. This can now be used with MQL systems and is very efficient. Its only drawback is it does not provide the same heat dispersal as flooding or other high flow cooling systems.
By using liquid, aerosol or gas coolants delivered through the tooling, this system wastes almost no coolant. Depending on the coolant type, it can provide lubrication as well as dealing with heat.
Once you have chosen your coolant delivery system and coolant type, it is time to look at your coolant recovery system. High flow systems like flooding and spraying usually employ a sump to recover and reuse the coolant. Gas systems simply let the nitrogen or CO2 disperse into the atmosphere. Through-tool and misting coolant delivery systems do not always even need a coolant recovery system as they use such small amounts of coolant.
Remember, the right type and application of coolant is crucial for good machining. Without it, tool life and overall material finish will suffer significantly as will productivity.