In plastic CNC machining, the decision of whether to use cutting fluid must be made on a case-by-case basis. In metal machining, cutting fluid serves the dual purpose of cooling and lubrication, but in plastic machining, some plastics are sensitive to liquid contact and may swell due to moisture absorption or suffer surface contamination when exposed to cutting fluid. Meanwhile, the heat generated when cutting plastics is far lower than that generated when cutting metals. Under the right conditions, stable dry cutting can be achieved through proper parameter control and compressed-air assistance. Dry cutting is not, however, simply a matter of turning off the coolant; it involves material selection, tool matching, parameter adjustment, and chip management.
Applicable Conditions for Dry Cutting
The Material Has Sufficient Heat Tolerance
Whether dry cutting can be used depends on how the plastic material behaves at cutting temperatures. The majority of the heat generated during cutting is carried away by the chips, with only a small portion remaining in the workpiece and the tool. If the material does not soften, deform, or degrade within its normal operating temperature range, the material foundation for dry cutting is in place. POM (polyoxymethylene) is a typical engineering plastic well-suited to dry cutting: it has a relatively high heat-deflection temperature and stable cutting performance, and can deliver good surface quality under standard parameters without cutting fluid.
Tool Sharpness Is Well Maintained
Dry cutting places a higher demand on tool sharpness than wet cutting. Without the lubricating effect of cutting fluid, the coefficient of friction between the tool and the material increases, subjecting the cutting edge to greater thermal and mechanical loads. When dry cutting, the tool-change interval should generally be shortened, and tools should be replaced at the first signs of wear. Tools intended for dry cutting should preferably have well-polished flutes to reduce frictional resistance as chips travel through the flute channel.
The Part Geometry Is Conducive to Chip Evacuation
Chips do not flow as freely under dry cutting as they do under wet cutting—there is no fluid flushing action to propel them, so dry chips rely entirely on the tool’s helical flutes and gravity to escape. Dry cutting is therefore better suited to open geometries such as face milling, profile machining, and shallow slotting. Deep cavities, narrow slots, enclosed pockets, and other features where chip evacuation is inherently difficult are not suitable for dry cutting.
Common Plastics Suitable for Dry Cutting
Different plastics vary considerably in their suitability for dry cutting.
POM—One of the most dry-cutting-friendly engineering plastics. Low cutting resistance, good chip form, and a relatively high heat-deflection temperature mean that cutting fluid is unnecessary in most conventional machining operations. Surface quality under dry cutting is usually indistinguishable from that achieved under wet cutting.
Acrylic—Can be dry cut, but requires strict attention to tool sharpness and parameter matching. Compressed-air assistance is highly effective at preventing chip accumulation and the associated thermal damage in the cutting zone. Oil-based cutting fluids are not recommended, as acrylic may develop stress cracking upon contact with certain formulations.
PEEK—Excellent high-temperature resistance gives it good compatibility with dry cutting. However, PEEK generates relatively high cutting forces, and tool wear under dry cutting will be somewhat faster than under wet cutting, so tool life should be monitored. For larger production runs, compressed air or minimum-quantity lubrication can be introduced to extend tool life.
PTFE—Its inherently very low coefficient of friction makes dry cutting the standard practice. Using cutting fluid offers no meaningful benefit.
PC—Can be dry cut but carries higher risk. PC is sensitive to cutting heat; at elevated temperatures, stress whitening can readily appear along edges. If dry cutting is chosen for PC, conservative cutting speeds and feed rates are advised, together with compressed air for forced chip evacuation and cooling.
Situations Where Dry Cutting Is Not Appropriate
Materials Prone to Thermal Deformation or Melting
Thermoplastics such as ABS, polyethylene (PE), and polypropylene (PP) have relatively low softening temperatures. Even a slight temperature rise in the cutting zone can cause chip melting, material sticking to the tool, or a tacky surface. These materials generally require cutting fluid or at least minimum-quantity lubrication to carry away heat and reduce friction.
Deep Cavities or Confined Spaces
In deep-cavity machining, tool overhang is large and the chip-evacuation path is long. Under dry cutting, chips tend to accumulate at the cavity bottom, where they are repeatedly recut and can score the finished surface. At the same time, the cooling effect of compressed air diminishes with depth at the cavity floor, and heat is not easily dissipated. In such cases, compressed air combined with minimum-quantity lubrication is recommended at a minimum.
Glass-Fiber-Reinforced or Filled Plastics
Fillers such as glass fiber and carbon fiber significantly accelerate tool wear. In machining these materials, cutting fluid serves not only to cool but also to flush abrasive particles out of the chip stream, slowing tool deterioration. Dry cutting of glass-fiber-reinforced plastics leads to a drastic reduction in tool life and is not recommended.
Високоточні деталі
For precision plastic parts with tolerances within +/-0.05 mm, the dimensional drift caused by cutting heat during dry machining may fall outside the allowable tolerance band. If dry cutting must be used for precision parts, the workpiece should be allowed to cool fully to room temperature between roughing and finishing, and the finishing allowance should be kept tight.
Parameter Adjustments for Dry Cutting
Dry cutting cannot simply inherit the parameters used for wet cutting. The following adjustments are worth noting.
Reduce spindle speed: Without cutting fluid to remove heat, spindle speed should be lowered to reduce frictional heat generation. A reduction of 15 to 25 percent relative to wet-cutting speeds serves as a practical starting range; the exact value should be determined through trial cuts based on the material and tool diameter.
Limit depth of cut: Both the axial depth of cut and the radial stepover should be reduced to moderate the cutting load per pass and allow more time for heat dissipation from the tool and workpiece.
Favor climb milling: In climb milling, the chip thickness decreases from thick to thin, and the tool experiences lower impact upon entering the material, generating less cutting heat than conventional milling. For thin-walled or easily deformed features, climb milling also reduces the pushing force exerted by the tool on the wall.
Keep tools sharp: Dry cutting is more sensitive to tool condition. If chip form changes from uniform to irregular, surface quality begins to degrade, or the cutting sound becomes dull, the tool should be replaced promptly.
The Role of Compressed Air in Dry Cutting
Dry cutting does not mean operating without any auxiliary measures. Compressed air is the most common aid employed under dry cutting conditions. Its functions include: blowing chips away from the cutting zone to prevent them from being recut and scoring the surface; carrying away a portion of the heat as the air stream passes over the tool and workpiece, providing a basic cooling effect; and forming a thin air film over the tool surface that partially substitutes for the lubricating action of cutting fluid.
Considerations when using compressed air: air pressure is generally set between 0.4 and 0.6 MPa—too low and chip clearing is ineffective, too high and chips may be scattered too rapidly to observe the cutting condition. The nozzle should be aimed at the cutting zone, not simply at the workpiece surface. For deep slots or cavities of some depth, use an extended nozzle or articulating air line to direct the airflow to the tool tip.
Поширені запитання
Can the surface quality achieved with dry cutting match that of wet cutting?
Under the right material and parameter conditions, dry cutting can match or even exceed the surface quality of wet cutting. POM typically yields good surface results under dry cutting. Acrylic dry cutting with compressed air can also produce a good finish, provided the tool is in good condition and the parameters are well matched. If surface quality remains unsatisfactory under dry cutting, check whether the tool is dull, the spindle speed is too high, or there is batch-to-batch variation in the material.
Does dry cutting affect tool life?
There is some impact, particularly during extended continuous machining. Without the lubrication provided by cutting fluid, there is more direct friction between the cutting edge and the material, and the wear rate is generally somewhat higher than under wet cutting. However, this difference is not pronounced when machining self-lubricating materials such as POM and PTFE. Shortening the tool-change interval and selecting tools with well-polished flutes can effectively control tooling costs under dry cutting conditions.
Is dry cutting suitable for high-volume production?
It depends on the material and the part geometry. For parts with conventional geometries made from POM or PTFE, dry cutting is entirely viable for high-volume production. For heat-sensitive materials such as PC and nylon, or for deep cavities and precision parts, at least minimum-quantity lubrication or compressed-air-assisted cooling is recommended in production to maintain process consistency.
Висновок
Whether to adopt dry cutting in plastic CNC machining is fundamentally a trade-off between cooling requirements, material characteristics, and machining efficiency. Materials such as POM and PTFE are naturally suited to dry cutting; acrylic and PEEK can also be dry cut with the right parameters; whereas low-melting-point materials such as ABS and PE, as well as glass-fiber-reinforced grades, are largely dependent on cooling or lubrication. Where conditions allow, the most reliable approach is to verify through a trial cut: machine a sample part using the same tool and the same parameters under both dry and wet conditions, then compare surface quality, dimensional accuracy, and tool wear before deciding whether a dry cutting strategy is appropriate.
