About Cryogenic Machining

Cryogenic machining is an advanced machining process that utilizes cryogenic coolants, often liquid nitrogen, for improved machining performance. Cryogenic coolants provide exceptional cooling and lubrication that enables benefits like increased tool life, higher machining speeds, improved surface finish and greater dimensional accuracy. Cryogenic machining has applications across many materials including hardened steels, titanium alloys, nickel alloys, plastics, and more.

What is Cryogenic Machining?

Cryogenic machining refers to the use of cryogenic coolants, like liquid nitrogen, during machining processes like milling, turning, drilling and grinding. The cryogenic coolant is sprayed onto the cutting zone where it absorbs heat and provides lubrication between the cutting tool and workpiece. This creates a variety of benefits compared to dry or conventional wet machining.

The most common cryogenic coolant used is liquid nitrogen (LN2) which has a boiling point of -320°F (-196°C). When sprayed onto the cutting zone, the liquid nitrogen evaporates extremely quickly which absorbs a massive amount of heat. The resulting nitrogen gas also displaces oxygen from the cutting zone which prevents potential workpiece combustion.

In addition to exceptional cooling, the liquid nitrogen also provides lubrication between the cutting tool and chip. This lubrication effect significantly reduces cutting forces and built up edge which leads to benefits like reduced tool wear, higher machining speeds and better surface finishes.

Cryogenic machining is utilized across a wide range of materials, but is especially effective for difficult-to-machine materials like hardened steels, titanium alloys, nickel alloys, composites and many plastics. The benefits of cryogenic machining allow improved machining performance in these materials.

Benefits of Cryogenic Machining

Cryogenic machining provides the following benefits compared to dry and wet machining processes:

• Increased Tool Life - Reduced tool wear from improved cooling and lubrication leads to dramatically increased tool life. Tool life improvements of 300-500% are commonly reported.
• Higher Cutting Speeds - The exceptional cooling effect enables higher cutting speeds, feeds and depths of cut. Increased speeds of 100-300% are possible depending on the material.
• Improved Surface Finish - The reduced cutting forces and built up edge provides significantly improved surface finish on machined parts.
• Greater Dimensional Accuracy - Decreased tool wear allows better dimensional accuracy to be held over the tool life. The excellent chip control also improves accuracy.
• Increased Productivity - The combined improvements in tool life, cutting speed and part quality lead to substantially higher machining productivity.
• No Coolant Costs - Liquid nitrogen evapourates leaving no costly coolant to dispose of or recycle. This provides a clean shop environment.
• No Thermal Damage - The reduced heat in the cutting zone prevents potential heat damage to the workpiece material. This is vital for sensitive alloys.
• No Workpiece Oxidation - The liquid nitrogen displaces oxygen preventing oxidation of reactive materials like titanium and magnesium.
• Chip Control - The liquid nitrogen freezes machining chips allowing easy breakage and evacuation from the cutting zone.
• Environmentally Clean - The process produces no liquid wastes while the nitrogen gas dissipates into the atmosphere leaving no environmental impact.

Cryogenic Machining Process

The cryogenic machining process utilizes a system for delivering liquid nitrogen to the cutting zone while also evacuating the nitrogen gas and chips. The major components of a cryogenic machining system include:

• Liquid Nitrogen Storage - An insulated storage tank that holds liquid nitrogen at atmospheric pressure. Small Dewar flasks can be used for simple set-ups.
• Liquid Nitrogen Delivery System - A pump and piping system that delivers liquid nitrogen from the storage tank to the cutting zone. Flow control valves adjust the flowrate.
• Cryogenic Nozzle - Special nozzles distribute the liquid nitrogen precisely into the chip-tool interface. Various nozzle designs exist including radial nozzles, mist nozzles and atomizing nozzles.
• Machine Tool Enclosure - The CNC machine tool requires an enclosure system to contain the nitrogen gas during machining. Clear polycarbonate material is often used.
• Nitrogen Gas Evacuation - A ventilation system evacuates the cold nitrogen gas from the enclosure to maintain visibility and prevent chilling.
• Chip Removal - Systems for removing the frozen chips from the enclosure are also important. Vacuum systems or augers are options.
• Temperature Measurement - Thermocouples may be used to monitor the temperature of the workpiece and ensure it remains above some minimum value.
• Control System - Programmable controllers can automate the delivery of liquid nitrogen based on different input parameters like cutting temperature, fluid pressure or flow duration.

The cutting tool and workpiece preparation does not require any significant changes from conventional machining. Though more advanced cutting tool grades and coatings tailored for cryogenic applications are sometimes utilized.

Cryogenic Machining Equipment

While cryogenic machining can be retrofitted onto almost any CNC machine tool, purpose-built cryogenic machining machines are also available. These machines have integrated cryogenic systems and enclosures for the most effective delivery of liquid nitrogen.

Major machine tool builders like Okuma, Mazak and Makino offer specialized cryogenic machining centers. Typical specifications include X-Y-Z travels of 25-40" x 16-30" x 25-40" with spindle speeds from 8,000-12,000 rpm and power from 15-50 hp. These machines range in price from $100,000 to over $400,000 depending on size and features.

Lathes, mills and other CNC machine types are also available from manufacturers like MAG, Gedee Weiler and Arnold Machinery with cryogenic capabilities. Purpose-built plastic machining centers with cryogenic cooling from companies like RJG Technologies are popular for machining engineering plastics.

For retrofitting existing equipment, there are also a few companies that offer standalone cryogenic machining systems. Shop Air Systems is one company that makes self-contained cryogenic coolant systems that can be installed on nearly any CNC machine tool. Precision Cryo is another that offers both retrofit systems and custom engineered systems.

Whether you purchase a specialized cryogenic machine tool or retrofit liquid nitrogen on an existing one, the capabilities are similar. The main difference is machines designed specifically for cryogenic machining will have better containment and evacuation of the nitrogen gas.

Workpiece Materials for Cryogenic Machining

Cryogenic machining is effective for machining many difficult materials where conventional machining struggles. Common workpiece materials taking advantage of cryogenic cooling include:

• Hardened Steels - Ideally suited for machining hardened tool and die steels above 45-50 HRC. Provides longer tool life and higher cutting speeds.
• Titanium Alloys - Excellent for machining titanium alloys which are notoriously difficult to machine at high speeds. Prevents workpiece oxidation.
• Nickel Alloys - Useful for nickel alloys like Inconel which work harden rapidly under heat. Cryogenic cooling prevents this.
• Cobalt Alloys - Similar benefits as nickel alloys. The reduction in heat and tool wear significantly aids cobalt alloy machining.
• Tungsten Carbide - Cryogenic cooling allows carbide to be precision machined rather than just ground. Provides cleaner cutting with less chipping and tool wear.
• Composites - Prevents potential heat damage when machining composites. Especially useful for carbon and glass fiber composites.
• Plastics - Many engineering plastics like PEEK, Torlon, Radel and others are machined much more efficiently with cryogenic cooling.

In general, materials that are prone to work hardening, tool adhesion, rapid dulling of cutting tools or thermal sensitivity yield the greatest benefits from cryogenic machining. However, advantages can be gained when cryogenic cooling is applied to almost any material.

Cutting Tools for Cryogenic Machining

Special coated and micro-grain carbide cutting tools have been developed specifically for cryogenic machining applications. The ultra-low temperatures experienced in cryogenic machining demand tools with adequate thermal shock resistance.

For turning tools, carbide grades like KCU15 and KCU25 produced by Kennametal are ideal choices. They combine good wear resistance with higher fracture toughness to resist thermal cracking.

For milling and drilling tools, fine grain carbides in the range of 0.5 to 2.0 microns are recommended. Grades like Kennametal’s KC9025 and Sandvik’s 33025 exhibit high strength and adequately low thermal conductivity.

In terms of coatings, AlTiN has proven to be an excellent choice for cryogenic machining. The AlTiN coating provides good protection against wear and adhesion while being sufficiently thermally stable at cryogenic temperatures.

When machining plastics, uncoated carbide tools are generally used. For composites, diamond coated tools are effective. Higher pressure coolant systems around 2,000 psi help sufficiently penetrate into composite materials.

No major tool holding changes are required. Though shrink fit holders and hydraulic holders can provide better grip on cutting tools at the low operating temperatures. Reduced spindle speeds and feed rates close to the low end of the recommended range also help maximize tool life.

Cost of Cryogenic Machining

The two main costs associated with cryogenic machining are the initial capital investment and the ongoing costs for liquid nitrogen.

For a turn-key cryogenic CNC machine, expect an initial investment of $100,000 to $400,000 depending on the work envelope and features. Retrofit systems can range from $20,000 for simple setups to $150,000 for more sophisticated automated systems.

Liquid nitrogen consumption varies based on the application but typically ranges between 0.5 and 5 liters per minute. With liquid nitrogen prices around $0.15-$0.30 per liter, the operating cost can be estimated at $10-50 per hour. For high-production situations, a low-pressure nitrogen generation system can be implemented to produce nitrogen on-site for additional savings.

Factor in the substantial improvements in productivity and part quality, cryogenic machining can deliver very rapid returns on investment. Often less than 12 months and sometimes just a few months for the right applications. The elimination of cutting fluids also provides significant savings on coolant, maintenance and disposal.

Applications of Cryogenic Machining

Some of the key applications taking advantage of cryogenic machining include:

Aerospace - Titanium and nickel alloys are extensively used in aerospace components. Cryogenic machining improves production rates in these expensive alloys.

Medical - Machining cobalt chrome and titanium orthopedic implants with improved surface finish and cycle times. Also biocompatible plastics in surgical tooling.

Automotive - Machining hardened steels over 60 HRC used in molds and dies where tool wear is problematic.

Firearms - Effective for machining hardened steel gun components and barrels. Prevents bluing discoloration.

Energy - Useful for machining superalloys used in gas turbine engines. Increases production and extends tool life.

Semiconductor - Diamond turning silicon wafers benefits from cryogenic cooling for better edge finishes and tool performance.

Food and Packaging - Allows faster machining of plastics like PET, HDPE and LDPE used in food packaging.

Optics - Precision machining of glass and plastics used in lenses enhanced by cryogenic cooling.

Composites - Excellent for trimming carbon and glass filled composites. Prevents potential heat damage.

The range of materials benefiting from cryogenic machining continues to expand as the technology becomes more widely adopted. The synergy between advanced cutting tool development and cryogenic cooling systems is opening additional applications.

Getting Started with Cryogenic Machining

For shops looking to adopt cryogenic machining, here are some recommendations:

• Start with a Trial - Work with a machine tool supplier or cryogenic equipment provider to trial cryogenic machining on your existing machine tools. Test performance gains on problem materials.
• Focus on Problem Materials First - Apply cryogenic machining on materials and applications where conventional machining has proven difficult. This is where the benefits will be most substantial.
• Consider Liquid Nitrogen Infrastructure - If high production cryogenic machining is planned, investing in on-site nitrogen generation may be worthwhile for a continuous nitrogen supply.
• Use Liquid Nitrogen Safely - Develop procedures for safe handling of liquid nitrogen including storage, transfer and emergency response planning.
• Optimize Processes - The optimal machining parameters for cryogenic machining may differ from conventional methods. Take time to fine tune speeds, feeds, tooling and nitrogen flow.
• Leverage Cutting Tool Expertise - Work closely with cutting tool partners to select specific tool grades, coatings and geometries tailored for cryogenic conditions.
• Provide Operator Training - Educate machine operators on the differences of cryogenic machining and how to properly monitor the process. Stress the importance of safety.

With the right approach, equipment, tooling and training, cryogenic machining can be successfully implemented and deliver game changing performance improvements over standard wet or dry machining. The technology continues advancing rapidly making integration easier for more shops.

Cryogenic Machining FAQ

Here are answers to some frequently asked questions about cryogenic machining:

How does liquid nitrogen coolant compare to compressed air or MQL?

Liquid nitrogen provides far superior cooling compared to air or MQL. The ultra-low temperature of -320°F instantly absorbs heat while the nitrogen gas also displaces oxygen. MQL cannot match this cooling performance.

What materials does cryogenic machining work best for?

The biggest benefits are seen for alloys prone to work hardening like titanium, nickel and cobalt. It also excels on hardened steels and difficult plastics like PEEK, Ultem and composite plastics.

Is cryogenic machining suitable for high volume production?

Yes, cryogenic machining can be used for high production situations. The consistent tool life improvement over conventional machining provides reliable production rates. Having an on-site nitrogen generation system is recommended for maximum cost savings.

How difficult is it to retrofit existing machines for cryogenic machining?

Retrofitting is fairly straightforward in most cases requiring a liquid nitrogen supply system, distribution components, nozzle and enclosure. No major machine tool modifications are required beyond enclosing the work area.

What size machine tools can be used for cryogenic machining?

Cryogenic machining has been successfully adapted to small benchtop machines up to very large 5-axis machining centers and lathes. The liquid nitrogen system and enclosure needs to scale to the particular machine size.

Is operator training required for cryogenic machining?

Yes, operators will need training on cryogenic machining practices and liquid nitrogen handling precautions. Safety procedures for machine enclosure cleaning and maintenance must be understood.

How much more does a cryogenic machining system cost compared to flood coolant?

An enclosed machine with cryogenic capability may be 25-50% more in initial cost. But cryogenic systems have lower maintenance costs and eliminate coolant expenses which offset the initial price.

What cutting tools work best for cryogenic machining applications?

Micro-grain carbides with AlTiN coatings specifically designed for thermal shock resistance at low temperatures are recommended. Tool selection depends on the material being machined.

Can milling, turning, drilling and tapping all benefit from cryogenic cooling?

Yes, cryogenic cooling improves cutting tool performance for all these machining operations. The system requirements are similar - delivering liquid nitrogen accurately to the tool and evacuating the gas.

How difficult is it to see inside an enclosed cryogenic machining center?

With proper ventilation and heating, visibility can be maintained inside the enclosure. Air curtains, temperature monitoring and plastic enclosure materials ensure conditions do not become obscured.

The Future of Cryogenic Machining

Cryogenic machining has already proven itself as a game changing technology, but continues to improve and expand into new applications. Here are some exciting areas driving future cryogenic machining developments:

• Hybrid Cooling Technologies - Combining cryogenic cooling with minimum quantity lubrication (MQL) and compressed air cooling for customized cooling across different machining operations.
• Specialized Cryogenic Machines - Machine tools designed and optimized specifically for cryogenic machining rather than retrofits of conventional machines.
• On-Machine Liquid Nitrogen Generation - Producing nitrogen directly within the machine enclosure rather than supplying externally from tanks improves efficiency.
• Advanced Control Systems - More intelligent automated delivery systems which monitor multiple inputs to dynamically control nitrogen flow for maximum efficiency.
• New Cutting Tool Materials - Novel carbide grades and coatings to handle expanding range of materials machined cryogenically including carbon fiber composites and 3D printed metals.
• Growth of Micro-Machining - The precision and stability of cryogenic machining will expand applicability for micromachining delicate components.
• Integration with Automation - Combining automated material handling systems, robotic loading/unloading and inspection systems with cryogenic machining cells.
• Supply Chain Improvements - Broader availability and lower cost supply of liquid nitrogen as demand increases. On-site nitrogen generators also declining in price.
• Transition into New Applications - Continued expansion into new materials like composites, ceramics, tungsten carbide and glass which benefit from cryogenic stabilization during machining.

The future is very bright for cryogenic machining as a core mainstream manufacturing process rather than just a niche technology. The performance improvements over conventional machining are simply too significant to ignore as the next generation of machining processes emerge.