Machining metals can create a lot of heat from the friction between the cutting tool and material. It’s necessary to keep that heat under control to save the tool and workpiece from damage. Cutting oils are special oils or lubricants that cool and lubricate during precision CNC machining. They also reduce friction from cutting tools, extend tool life, and wash away metal chips.

There are a few different types of oils for a CNC machine. Each has its advantages for different materials and applications. Knowing which type is suited for the operation helps get better tool life and surface finish. 

We’ll discuss some common types of lubricants for a CNC machine, which can keep your tools sharp and protect your investment.

What is Cutting Oils or Lubricants?

Cutting oils, also called cutting fluids or lubricants, are special liquids that are used when cutting metals. These oils help you cut different materials, like steel or aluminum, on machines like lathes, mills, and CNC machines. They cool the cutting tool and the workpiece during operation and also lubricate to reduce friction and wear. 

There are a few different types of oils for a CNC machine. Some are made from petroleum while others use plant or animal oils. Water-based cutting fluids that mix oil and water are also common. No matter the type, all of them have additives that give them useful properties. The ingredients are combined carefully to work well together.

A high-quality cutting oil for CNC must meet certain standards. Some common properties are:

• Conducting heat well, so heat transfers away from the cutting tool and workpiece.
• Absorbing heat without getting too hot or boiling.
• The oil should have the correct thickness to flow easily and prevent chips from getting stuck.
• The oil does not corrode or chemically react with the metal or tools.
• Protecting against rust and oxidation on cut parts.
• Does not have any harmful effect on people or the environment.
• Does not leave any bad smells or marks.
• It stays stable and does not break down fast with use over time.

With the right type of oil, you can produce high-quality parts through CNC turning, milling. 

Common Types of Cutting Oil or Lubrication for a CNC Machine

There are two major types of lubricants for a CNC machine: 

1. Oil-based fluids: Soluble and straight oils
2. Chemical-based fluids: Synthetic and semi-synthetic oils

Some oils are better for certain metals and cutting speeds. We will discuss each type, their performance, and pros and cons. 

a. Soluble Oils (60-90% Petroleum Oil)

Soluble oils, also called emulsions or emulsifiable oils, are a popular type of cutting fluid used for CNC machines. They are 60-90% petroleum or mineral oil and the rest is water and additives.

To use it, the concentrated form is mixed with water in a particular proportion (1 to 20%) of oil to water. Special chemicals called emulsifiers help blend the oil into the water to form a milky mixture. This allows the oils to stay mixed together instead of separating.

The oil helps lubricate and protects the metal parts from wear during machining. The water helps draw heat away from the cutting area. Together, they can handle light to medium cutting operations on many types of metals. Some have additional ingredients that let them work for heavier duty machining too.

Because it uses both oil and water, soluble oil cools parts effectively while leaving a lubricating film. This protects machine components and slides from corrosion and wear. The emulsions also resist washing away other lubricants, like greases.

While soluble oils work well overall, they do have some downsides compared to other cutting fluids. Incorrect proportion of oil to water can cause rust and residues can form when mixed with hard water. Bacterial growth is not uncommon too. It also needs more maintenance to keep its good properties. Still, most CNC machine shops use soluble oil for its versatility. 

Pros

• Provides lubrication from oil and cooling from water
• Prevents wear on machine parts
• Suitable for a variety of ferrous and non-ferrous metals
• Handles light to medium cutting operations and some heavy-duty uses
• Resists washing away of other lubricants

Cons

• More prone to rust and bacterial growth without additives
• Needs frequent maintenance to prevent degrading
• Harder to clean off parts than other fluids
• Evaporates more easily than straight oils

b. Straight Oils (100% Petroleum Oil)

Straight oils work well for basic machining of metals, like aluminum and steel. They don’t contain any water, so they are just mineral or plant-based oils with some additives mixed in. They are available in different viscosities. Thinner oils work better than thicker ones for some applications.

For easy jobs like lightly shaping metals, straight oils usually don’t need many extras added to them. But for tougher tasks, they have to be mixed with ingredients, like fatty oils or special soaps, to work better. These additives help the oil stick to the metal pieces and cutting tool more. 

Another good thing about straight oils is that they feel slippery and cushion the metal from the tool. This helps make the surface of the metal very smooth. Though they cost a lot, using them can make tools last much longer for difficult types of machining. 

Straight oil mixtures are still the best choice for things like grinding metal in a crusher or drilling deep holes. They also do a pretty good job of preventing rust. Another advantage is that bacteria don’t grow in them like they can in ones mixed with water.

On the downside, the oil does not do a good job in reducing heat. It also creates smoke that isn’t healthy to breathe. So, you must use it in machines running at low temperatures and speeds. It can also be hard to clean off parts, requiring cleanup liquids. 

Pros

• Provide excellent lubrication
• Long tool life when used for appropriate applications
• Good rust protection properties
• Relatively easy to maintain
• Resist contamination from bacteria

Cons

• Poor heat dissipation
• Increased fire risk at high temperatures
• Can create unhealthy mists or smoke during use
• Difficult to clean off parts, may require solvents
• Limited to low-speed, low-temperature cutting applications

c. Synthetic Oils (0% Petroleum Oil)

Synthetic oil is one of the chemical-based lubricants for a CNC machine. Unlike traditional petroleum or mineral-based cutting oils, it contains no petroleum at all. It’s made from chemical ingredients and rust inhibitors that are dissolved in water.

Like other water-soluble fluids, synthetics come as a concentrate that needs to be mixed with water. This creates a fluid that has high cooling ability and lubrication. It also prevents corrosion better than oil-based fluids. These properties make synthetics work well for operations that generate a lot of heat, like surface grinding. 

In recent years, new “heavy-duty” synthetics have been developed that can handle most machining tasks. They have variations, including simple, complex, and emulsifiable types. Simple synthetics are good for light grinding. Complex synthetics work for moderate to heavy jobs and allow higher speeds and feeds. Both simple and complex synthetics stay clear after mixing.

Emulsifiable synthetics have additives that give them lubricating power like oil-based fluids. This allows them to both cool and lubricate during demanding cuts, like grinding tough alloys. They look cloudy or translucent mixed instead of clear.

Synthetics have benefits like long lifespan, non-toxicity, corrosion protection, minimum misting, and easy maintenance. While foaming can still occur, they are a good option for many CNC operations.

Pros

• No petroleum content, more environmentally friendly
• High cooling ability and corrosion protection
• Provide good lubrication for difficult materials and operations
• Creates less foam and mist than oil-based fluids
• Resistant to bacterial growth, long fluid life
• Easy to separate from chips for simplified cleaning

Cons

• Higher initial cost compared to oil-based fluids
• Some types can still foam or mist under high agitation
• Improper maintenance can lead to residues and contamination

d. Semi-Synthetic Oils (2-30% Petroleum Oil)

Semi-synthetic oils are made by blending a small amount of petroleum oil, usually between 2-30%, with synthetic compounds and additives. The petroleum oil provides lubrication while the synthetic parts help the oil mix with water.

Regular oils are just petroleum and don’t mix with water well. Semi-synthetics can be diluted with water, like water-based coolants. But they have some oil in them for better lubrication during metal cutting.

The oil stays mixed in the fluid after adding emulsifiers. Emulsifiers are ingredients that allow the oil and water to mix together without separating. They help keep the oil droplets very small, usually less than 1/100 of a millimeter across. This makes the fluid look transparent or slightly cloudy instead of two separate layers.

Semi-synthetic oils provide better lubrication than water-based fluids because of the petroleum oil. This allows machinists to cut metal at higher speeds and force without damaging tools. 

Compared to regular oil-based fluids, semi-synthetics are easier to maintain. Bacteria grow less in them since there is less oil for bugs to feed on. They also produce less smoke and airborne mist during machining. 

While semi-synthetics work well in many applications, hard water can sometimes cause issues. Minerals in hard water may drop out of solution and form deposits. Foaming is also more likely than with oil-based fluids. But it’s a balanced lubricant overall for CNC machines.

Pros

• Provides good lubrication 
• Can be diluted with water
• Emulsifiers allow oil and water to mix without separating
• Allows cooling and lubrication at higher speeds and feeds than regular oil
• Easier to maintain than straight oil-based fluids
• Produces less smoke and airborne mist during machining

Cons

• Hard water can cause instability and deposits
• Have a higher risk of foaming compared to oil-based fluids
• Less lubrication than straight oil-based fluids

Applications of Cutting Oils or Lubricants

Cutting fluids play an important role in CNC machining. They help tools last longer and smoothen the machined parts. There are different types of oils for a CNC machine for applications like:

• Drilling
• Milling
• Grinding
• Turning 

Petroleum oils work well for drilling because they help drill bits move smoothly through metal without wearing out fast. Water mixes are better for grinding and turning since these processes make a lot of heat from metal scraping away. 

How the fluids get applied depends on the machine too. In CNC milling, the tool does the moving while the part stays put. So, flooding works best here. The whole work area gets covered in fluid, so chips wash away and the tool slides freely. 

Drilling mostly needs chip removal more than lubrication. The drill doesn’t rub much since it goes in and out. A spray works to flush chips out of holes being made. 

Turning spins parts very quickly, generating heat. A fluid jet cools hot spots that form on the outer edge being shaped. It also washes away swarf as it releases from the cut.

Choosing fluids and application methods make a difference in precision, finish quality and tool life. Taking advantage of each type of liquid’s strengths leads to smoother machining runs. 

Applications of Different Cutting Oils in CNC Machining:

Oil Type	Suitable Machining Process	Common Applications
Soluble	Light to heavy-duty cutting and grinding of metals.	Hardware parts, automotive, construction, aerospace, bikes.
Straight	High-speed machining, hard metal machining, thread cutting, tapping, screw machining, gear cutting.	Automotive, heavy machinery, sheetmetal fabrication.
Synthetic	High-speed and hard material CNC machining.	Aerospace machining where precision and surface finish are critical.
Semi-synthetic	High-speed machining, hard metal machining, thread cutting, tapping, screw machining, gear cutting.	Automotive, heavy machinery, tool and die manufacturing, precision engineering.

Conclusion

Choosing the right cutting fluid is important for getting quality parts and long tool life from a CNC machine. With some knowledge of the different oil types and their applications, you can pick the best lubricant to machine all kinds of metals smoothly and efficiently. Using the suitable cutting fluid can help maximize a company’s investment in its specialized CNC equipment.

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