What is Cryogenic Machining?
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First thing's first: what is "Cryogenic Machining" anyway? Simply put, it is using Liquid Nitrogen (LN2) as a coolant for tools while machining. Of course when you think about it, anything involving Liquid Nitrogen sounds anything but simple, right?
After all, it's a liquid whose temperature is -196 degrees Celsius which is -321 degrees Fahrenheit--it's darned cold!
The good news is that while the cold liquid has to be managed carefully due to the hazard of anything that cold, nitrogen is basically inert and readily vaporizes into a gas that is the major component of air.
Why Cryogenic Machining?
Cryogenic Machining has a number of important advantages including:
- It is more environmentally sound than other coolants. When it evaporates, Liquid Nitrogen is just Nitrogen, which is the major constituent of air. No harmful chemicals are involved.
- Higher production rates are possible through higher material removal rates.
- Greater tool life is encouraged by Cryogenic Machining
We talked about the advantages of the LN2 being inert and basically harmless other than dealing with the fact that it is very cold. The bigger question is what impact it has on your tools?
Carbide tools rapidly lose their hardness as they absorb too much heat. At room temperature, they have a hardness of 1600-1800 kg/mm^2. By 900C that's dropped to about 1/3. Once the tool softens, it rapidly loses its cutten edge and very soon thereafter is rendered useless.
In 2003, Air Products (supplier of LN2) did a joint research collaboration with Hardinge to test the effects of Cryogenic Machining. It was called project "ICEFLY," and largely focused on turning with some milling. Their results were tantalizing:
- Turning Stellite (55-58 HRc) reduced machining time from 20 min/part to just 2.5 minutes. They were able to do this with whisker reinforced ceramics rather than significantly more expensive PCBN tooling too.
- Machining A2 (62 HRc) Tool Steel resulted in 62.5% higher Material Removal Rates (MRR) and 130% increase in tool life.
- Machining of Cobalt Chrome resulted in 290% better MRR and 50% longer tool life.
- MRR increases of up to 60% in Titanium milling with 260% tool life increases were possible.
Those are some substantial improvements on some very tough materials!
How Does It Work?
The ICEFLY concept, ccourtesy of Industrial Cryo Tech.
The trick is delivering LN2 in liquid form to the cut without it evaporating along the way. Air Products' ICEFLY system uses a coaxial tube-in-tube technology to get the job done. High pressure LN2 runs through the inner tube while the outer jacket tube carries low pressure LN2 to minimize heat loss in the inner tube. This ensures liquid arrives right at the end of the tube where it can be jetted on to the cutter.
A variety of rigs are available to deliver LN2 using the ICEFLY system. For example, on mills, a Cryo-SpiderCool™ programmable coolant nozzle system is available.
Here is a video of the ICEFLY system being used for turning:
An alternate approach, pioneered by 5ME, uses a vacuum jacketed tube to deliver the LN2. Think of it as a Thermos Jug in the form of a tube. The 5ME system involves retrofitting a special spindle that can provide "Through Spindle" LN2. You use 5ME's tools to mate up with the spindle.
Here is one of their systems in action on a Horizontal Milling Center:
When to Use Cryogenic Machining?
The best economics occur when machining harder materials:
- Steel and Iron harder than 45 HRC
- Sinter-hardened or heat-treated powdered metals
- Hard metal-matrix composites
- Cobalt Chrome and other dfficult materials for medical implants, especially where it is desirable to avoid the possible contamination effects of metal cutting fluids.
- Polymers and plastics that machine better when frozen, especially those intended for medical use.
Cryogenic Machining is not for every application, but for those applications where it can shine, it offers real advantages.
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