Cryogenic machining

How many times have you asked or been asked, “Are we doing our best to manufacture this part in the most efficient and economical way?”

In some instances, parts have been machined the same way for years, and changing the methodology is often met with internal resistance. The company that makes the successful change can be the one that gets a jump on the competition and saves money.

Will the benefits of changing make you more competitive and help you bring more value to your products?

Engineers at Air Products and Chemicals, Cleveland, developed a low-flow nitrogen delivery system for CNC machining. Termed ICEFLY technology, and licensed to Advanced Research Systems, Macungie, Pennsylvania, it is now available through its subsidiary, Industrial Cryogenic Technologies (ICT).

Nitrogen is already used in the food industry for flash freezing raw and processed food to preserve freshness. However, there are also applications for machining.

An inert gas, nitrogen is approximately 80% of the air you are breathing as you read this article. Oxygen roughly makes up the other 20%. At -321°F, liquid nitrogen can affect the material being machined or add considerable durability to the insert used to machine hardened materials.

Here are two distinct ways nitrogen can help your machining productivity.

Machining polymers

The machining of medical polymers – including polyether ether ketone (PEEK), ultra-high-molecular-weight polyethylene (UHMWPE), and rubber – presents some unique obstacles. The machining is straightforward, but the polymer is soft when compared to metals. This soft condition is referred to as the modulus of the material or Tg (glass transition temperature). When polymers of any type are chilled, they become more rigid. The more rigid a polymer becomes, the more machineable it will be (see Figure 1). In this application, the material to be machined is chilled but the cutting tool is not.

Machining medical polymers forms burrs, and extensive effort is needed to remove the burrs afterwards. An acceptable part needs to be burr free – requiring manual labor, soda blasting, and cryogenic tumbling – all steps that can result in an increased percentage of lost and scrap parts.

When cold nitrogen gas is introduced to the machining process, the modulus of the polymer is stiffened and it becomes easier to control the burrs. In many cases, the burrs no longer exist when the part comes off the machine.

In almost all cases, medical polymers must remain clean, so machining of implantable polymers typically occurs in a temperature-controlled cleanroom. This is true even if the shop air used for blow off is oil- and contaminant-free. When cold nitrogen is introduced to this machining process, the part is machined burr-free and the nitrogen is clean and without contaminants.

ICEFLY technology can be used in the CNC machine with a programmable nozzle directed at the material/cutting tool interface in the same manner that programmable coolant is used. The nitrogen can be turned on and off, just as a user would turn coolant or air on and off.

To study the effects of machining PEEK at different temperatures, engineers machined some samples of PEEK (see Figure 2). Typically, most PEEK is machined with the introduction of compressed shop air to blow chips off the part and for cooling. When machining with shop air, the cutting tool gets hot, as does the PEEK. Burrs are hard to remove because they melt back onto the part. When cryo-machined, the modulus of the material is moved more to the glassy/ductile range and becomes easier to machine. The cutting tool still creates heat but it is offset by the cooling of the nitrogen gas. While there may still be some small burrs, they are not attached with the same resilience as the room-temperature burrs.

All polymers have their own distinct Tg and the ICEFLY technology allows users to dial in the correct machining temperature. If the temperature is too cold, the polymer will become too glassy and can crack. On the opposite end, if the temperature is too warm, burr formation increases. Studies also indicate that when machining parts at a consistent temperature, part dimensions are controllable with tool offsets, just as they are when machining at room temperature.

Machining metals

The second way the ICEFLY technology can be beneficial involves machining metal parts made from hardened steel and iron (A2, D2), powdered metals, hard metal matrix composites (carbides, ceramic thermal spray coatings, weld overlays), and medical materials (CoCr, Ti). In machining hardened metals, machinists still need the heat generated at the material/insert interface since the removal of metal by an insert is due to the heat generated by friction of the insert against the metal. In addition, many factors affect this process, such as spindle speed (SFM), feed rate (ipr, ipm), depth of cut (DOC), and insert geometry (rake, tool nose radius, lead, coating, chip breaker).

The application of ICEFLY nitrogen technology in this machining process involves cooling the insert, not the material. In one test, nitrogen was introduced through the cutting tool in a lathe application in much the same way as high-pressure coolant is used. The goal is to preserve insert integrity and keep it from wearing out sooner than necessary. By keeping the insert cold, tests showed the ability to increase DOC, increase feed rate, increase SFM, and in some instances, use less-expensive ceramic inserts instead of PCBN inserts (see Figure 3).

Since cryo-machining enhances the insert’s strength and durability, many components can be machined in their hardened state, eliminating intermediate manufacturing stages and often reducing process time when grinding hardened parts. Since a part can be machined to near finish size in a hardened state, the time to final grind can be substantially reduced. In one test, grinding time went from 8 hours to 1.5 hours.

Nitrogen safety, availability

Nitrogen is an inert, colorless, odorless, tasteless, non-flammable, and non-irritating gas. It is used a range of industries and is readily available from Air Products Inc. or any industrial gas supplier. As with any industrial gas, proper use and safety regulations must be followed. In its liquid form, nitrogen is -321°F and can create frostbite very quickly. When the liquid is exposed to room temperature air, it instantly vaporizes and becomes part of the atmosphere. In a high-bay-machining environment, the nitrogen/oxygen ratio changes very little. In a small clean room with a few CNC machines, the nitrogen/oxygen ratio can change and needs to be monitored. ICT systems include oxygen-monitoring devices that can be either wall-mounted or worn on the operator’s belt as a safety precaution.

Contact Rick Knopf at 908.413.4890 to discuss cryogenic machining with ICEFLY technology.

Industrial Cryogenic Technologies LLC
http://industrialcryotech.com/