Selection of Cutting Fluids: The Basic Guide

In mechanical cutting, when using CNC machine tools to cut, according to the cutting method, there are turning, milling, drilling and boring.

Divided by the amount of material removed, there are rough machining, semi-finishing and finishing.

According to the cutting tool materials used, there are high-speed steel, hard alloy, ceramics, diamond, and cubic boron nitride.

Divided by machine tools, there are mainly machine tools that explicitly require the use of oil-based cutting fluids, machine tools that do not require them, single-piece small batch production types of machine tools, and batch or automatic production lines (flexible manufacturing systems).

For different cutting methods, the cutting characteristics of the same metal are different, and the changes in the difficulty of processing are also significantly different.

Cutting Fluids

For example, using the same Q235 carbon steel material, it is difficult to machine threads to cut other surfaces in guaranteeing the surface roughness quality index.

Finishing is difficult to rough.

Difficult cutting processes also require higher cutting fluids.

The correct use of cutting fluid can effectively guarantee the processing quality, extend the tool life and improve the processing efficiency.

Rough Machining

Rough Machining

When the CNC machine tool needs to separate roughing and finishing, or the work piece is not completed on one machine, you can choose cutting fluid according to the characteristics of roughing and finishing.

During rough machining, both the amount of back cutting depth and the feed are large, resulting in large cutting resistance and a large amount of cutting heat.

The heat transferred to the work piece and the tool also increases accordingly, which makes the work piece’s thermal deformation and tool wear worse.

You should choose water-based cutting fluid that is mainly cooling and has a certain lubricating cleaning and rust prevention effect to continuously flow with large discharge.

In milling or machining with irregular shapes, uneven margins and intermittent machining, the cutting speed is lower than continuous uniform machining.

The effect of cutting heat is smaller than the impact of shock and vibration on the tool and the work piece, and the lubrication and cooling effects of the cutting fluid must be balanced.

When the machine tool conditions permit, you can use the inner hole car, boring knife and grooving knife with internal liquid supply holes during hole processing and cutting, or use pressure to supply liquid as well as the use of spray liquid supply for rough machining of difficult-to-machine materials, all can achieve better results.

Generally, rough machining work pieces have machining allowances.

At the same time, when processing difficult-to-machine materials and non-ferrous metal materials, the accuracy requirements for surface roughness are not high.

Therefore, when processing difficult-to-machine materials and non-ferrous metal materials, rough processing does not require high chemical components in the cutting fluid, and water-based extreme-pressure emulsions can be used.

When rough-casting cast iron and brittle non-ferrous metals, the common characteristic of these materials when cutting is that the chips are chipped.

Related reading: Ferrous vs Non-ferrous Metals

The fine chips are in a flowing state under the impact of the cutting fluid, and most of them will deposit when the cutting fluid circulates through the cutting fluid tank.

One part flows with the cutting fluid, and it collects in the small part of the cutting fluid conveying pipe, which will block the cooling nozzle, and make the chips adhere to the moving parts of the machine tool (such as the guide rail pair).

At the same time, the cutting fluid chemically reacts with certain components in the cast iron, causing the cutting fluid to deteriorate and causing the performance of the cutting fluid to decline.

Because of the difficulties associated with cutting fluids, cutting fluids are generally not used.

In order to reduce the impact of dust and cutting heat, when conditions are available, you can consider using a dust extraction device to suck away dust, fine chips and some heat.

If cutting fluid is used, it is easy to use water-based cutting fluid, and it is necessary to do a good job of filtering and purifying the cutting fluid to prevent deterioration of the cutting fluid and decrease in concentration.

During roughing machining, the concentration of cutting fluid is lower than that of finishing.

Finishing Machining

Finishing Machining

According to the cutting speed, fining can be divided into high-speed finishing and low-speed finishing.

During high-speed finishing, although the use of oil-based cutting fluid can ensure the surface quality of the work piece and prolong the service life of the tool, but the viscosity and flash point of the mineral oil contained in it are low, and often produce oil fume, oil mist and oil vapor, and decompose toxic components, which affects the production environment and damage the physical and mental health of the operator.

Therefore, when finishing high-speed cutting, use an emulsion or micro-emulsion containing extreme pressure additives in water-based cutting fluid as much as possible, and its concentration should be higher than that of rough machining.

In the case of low-speed finishing, the cutting temperature is low, and the above problems are unlikely to occur.

In order to ensure the machining accuracy of the work piece, oil-based cutting fluid should be used.

Compared to roughing, the cutting speed during finishing is high.

The cutting heat generated by chip deformation is lower than the cutting heat generated by friction on the flank of the tool, and the cutting force is small.

Even at high-speed cutting, the thermal deformation of the work piece by cutting heat is not as obvious as that of rough machining.

The reason cutting tools use cutting fluids is mainly to reduce the wear of the tool flank.

When the tool is finishing, the relative cutting speed is high, and the temperature at the moment when the chip is removed from the work piece is very high.

Even if cutting fluid is used, if the cutting fluid has poor permeability or insufficient pressure, the cutting fluid does not sufficiently penetrate into the cutting area, which will make the tool extremely prone to flank wear.

Even if the tool can still be used, the surface roughness quality of the work piece can not meet the requirements, and the tool still needs to be replaced.

Therefore, during finishing, be sure to use cutting fluid with significant lubrication and good permeability to extend the service life of the tool and ensure the machining accuracy and surface roughness quality requirements of the work piece.

When finishing ferrous materials at low speed, active extreme pressure cutting oil can be used. When finishing non-ferrous metals at low speed, non-reactive extreme-pressure cutting oil should be used.

For high-speed finishing of non-ferrous metals, an emulsion of inactive extreme-pressure additives can be used.

Hole Machining

Hole Machining

In CNC machining, hole machining mainly refers to drilling, reaming, turning inner holes on a lathe, and boring on a boring machine.

Due to the difficulty of chip removal and heat dissipation, the rigidity of the tool (rod) is poor, and the cutting speed of hole machining is lower than that of outer contour machining, and the difficulty is higher than that of outer contour machining.

When drilling, a general twist drill is used, which belongs to rough machining.

Chip removal is difficult during drilling, and it is difficult to radiate cutting heat, which often causes the blade to anneal and affects the service life of the drill as well as the processing efficiency.

Selecting a cutting fluid with good performance can significantly extend the life of the drill and significantly increase productivity.

Generally, extreme pressure emulsion or extreme pressure synthetic cutting fluid is used.

Extreme pressure synthetic cutting fluid has low surface tension, good permeability, and can cool the drill bit in time, which is very effective in extending tool life and improving processing efficiency.

For hard-to-cut materials such as stainless steel and heat-resistant alloys, low-viscosity extreme-pressure cutting oils can be used.

When drilling, whether it is ordinary drilling or deep hole drilling, poor heat dissipation conditions will generate a lot of cutting heat.

The direction of chip discharge is opposite to the feed direction of the drill. Cutting fluid needs to penetrate the bit of the drill to lubricate, cool and assist chip removal.

The cutting fluid should first have good permeability, and the fluid supply method, flow rate and pressure must also meet the requirements.

In terms of improving drill life indicators, oil-based cutting fluids are generally better than water-based cutting fluids, and extreme-pressure micro-emulsions in water-based cutting fluids are the best.

The low-viscosity active sulfurized oil in oil-based cutting fluids has the best overall performance of all cutting fluids.

Because reaming is a medium-to-low-speed chip cutting for finishing, when reaming a hole, two quality indicators of dimensional accuracy and surface roughness accuracy of the reaming hole as well as the service life of the reaming tool holding accuracy should be considered.

In terms of controlling the pore diameter index, oil-based cutting fluids all increase the pore diameter, mineral oil makes the pore diameter larger, the extreme pressure cutting oil is smaller, and the active sulfurized and chlorinated oil is the smallest.

Water-based cutting fluids reduce the pore size, sulfur-containing extreme pressure micro-emulsions and micro-emulsions minimize the pore size, the emulsion is centered, and the synthetic cutting fluid is the smallest.

In view of this, in order to control the size of the reaming hole, when using a new reamer, the water-based cutting fluid is used to make the hole difficult to expand.

When the reamer is worn to a certain extent, an oil-based cutting fluid can be used to slightly expand the aperture.

In terms of reducing the surface roughness of the reaming hole, the use of water-based cutting fluid is better than the use of oil-based cutting fluid.

Among oil-based cutting fluids, active sulfurized chlorinated oil has the best effect, followed by chlorine-containing extreme pressure oil, mixed mineral oil, and pure mineral oil is the worst.

Among water-based cutting fluids, the effects of emulsions, micro-emulsions, and extreme pressure micro-emulsions containing sulfur are basically the same, and synthetic cutting fluids are the worst.

In terms of controlling the service life of the reamer, the inactive extreme-pressure cutting oil and anti-friction cutting oil in the oil-based cutting fluid have the best performance.

Among water-based cutting fluids, synthetic cutting fluids are the worst.

Except for floating boring tools, turning inner bores and boring are both single-edged cutting.

The heat dissipation conditions are worse than the outer circle.

When using cutting fluids, as with drilling and reaming, the flow and pressure must be appropriately increased.

Thread Machining

Thread Machining

Thread processing belongs to forming processing.

When tapping and setting threads, it belongs to multi-edge low-speed cutting.

The cutting edge is surrounded by the cutting material, the cutting torque is large, and the chip removal is difficult.

The heat cannot be taken away by the chips in time, the chip fragments are crowded and easily vibrate, and the tools are easy to wear.

In particular, cutting conditions are more severe when tapping:

Chip removal space is narrow, chips are not easy to break and flow out, tapping and chip removal generate large cutting force and friction force, and it is easy to cause taps to break.

The cutting fluid is required to have both a low coefficient of friction and a good permeability to reduce the frictional resistance of the tool and extend the service life of the tool.

When cutting ferrous metal materials, generally use oil-based cutting fluids with more sulfur and chlorine additives, low viscosity and good permeability.

When using high-speed steel for low-speed turning of threads, the selection principles of the cutting fluid are the same as that of tapping and sleeve thread.

When using carbide tools to turn thread, the cutting speed is fast, the impact force is large, the cutting temperature is high, the cutting edge area is small, the cutting force is large, the accuracy of thread profile is high, and it is not easy to produce problems affecting the surface quality of thread such as chip accretion and scale thorn.

The service life index of the tool is high, and the cooling fluid, lubrication and penetrating performance of the cutting fluid are required at the same time.

It should be suitable to use water-based cutting fluid containing extreme pressure additives.

Different Tool Materials

Different Tool Materials

Many times, in order to avoid damage caused by uneven heating of the tool, dry cutting is used when cutting with carbide tools.

However, on CNC machine tools, it is best to use cutting fluid in terms of efficiency, comprehensive cost and machining quality.

When machining with carbide tools, water-based cutting fluids are generally used when the cutting speed is relatively high.

Continue to use it at a large flow rate and observe carefully to ensure that the nozzle is always aligned with the cutting position.

When cutting with high-speed steel tools, oil-based cutting fluids based on lubrication are generally used when the cutting speed is low.

The use of ceramic tools, cermet tools, diamond and cubic boron nitride tools are mainly suitable for many machining methods such as high-speed cutting, dry cutting and hard cutting, it is not necessary to use cutting fluid.

Sometimes, in order to avoid excessive cutting temperatures, cutting fluids are also used in most cases when using these tools.

Due to the high cutting speed, it is advisable to use water-based cutting fluids.

Final Thoughts:

In the process of NC cutting, the rational use of cutting fluids plays a very important role in improving machining efficiency, improving machining quality, extending tool service life, and reducing overall costs.

However, the cutting fluid will cause health hazards to the processor during use, and the disposal and discharge of waste fluid will also cause pollution to the environment.

Aiming at the proposal of green manufacturing, the processor should explore the no-use of cutting fluid or less cutting fluid to complete the processing task.

Relevant enterprises and scientific research institutions should also continue to develop cutting fluids with better performance, green environmental protection, good versatility and economy to meet the requirements of cutting fluid users.

Machine tool managers, process technicians and machine tool operators should follow and pay attention in time, and actively promote and use new environmentally friendly cutting fluids as well as new cooling and lubrication technologies, so that more suitable cutting fluids and technologies can be used in production practice.

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