Part of the motivation in developing Cryogenic Machining technology was to eliminate conventional coolant and its host of concerns. Not only does coolant present health and safety issues, such as toxicity, bacteria, and an oily work environment, there are financial and environmental issues as well. Conventional coolant involves a significant investment in supply, filtration, and mist collection equipment, not to mention the lifecycle costs of coolant-based manufacturing systems. In addition to the costs associated with using coolant, used cutting fluid must be disposed of properly since oil and other wastes negatively impact the environment.
Even though coolant is generally effective for chip management, it is highly inefficient at cooling and lubricating the cut zone. One smart alternative was the Minimum Quantity Lubrication (MQL) technology led by MAG. MQL goes through the tool to supply the cutting edge with a small quantity of lubricant mixed with air that is less harmful to the human body as well as the environment. Not only is MQL safer and more economical, it provides these benefits:
- Eliminates coolant systems and the energy to run them
- Provides near-dry chips
- Provides clean work parts
- Increases value of reclaimed chips
- Reduces investment, operating costs
- Eliminates coolant disposal costs
- Potentially eliminates in-process washers
MAG has more than 300 machines in the field with this technology, developing significant expertise for dry and near-dry machining.
Even though MQL is excellent in adding lubrication to the cut zone, it does little to extract heat. Heat reduction became the industry’s next challenge. MAG’s vision was to evolve MQL dry machining expertise into a more productive solution for extracting heat.
Basics of Cutting Tool Heat
To appreciate Cryogenic Machining, it is necessary to review chipmaking basics. The process of cutting produces heat, and the faster the cutting speed the higher the heat. Any cooling media helps reduce cutting heat (water at +70°F and liquid nitrogen at -321°F, a difference of nearly 400°F), but each tool material has a critical temperature where it will deteriorate to failure. Different work materials bring tools to critical temperatures at different cutting speeds. The question becomes how to find the critical temperature.
If we look at the heat and speed vs. wear graph (on the following page) we see diffusion and oxidation start, and soon we are at the critical temperature that accelerates tool wear. Fredric Taylor developed his famous tool life formula in the early 1900s and suggested that, in a normal work environment, 15 minutes of tool life was the most cost effective. Today, however, different industries push parameters and shoot for longer tool life depending on overhead, floor space, and other factors.
To see how Cryo affects tool life, the thermal plot (above) of carbon steel shows us where the critical temperature will be for a carbide tool edge. If we apply a Cryogenic process and are able to extract 300°F, we reduce the tool wear and see an 80% increase at conservative speeds. If we push the limits past the critical cutting temperature, which may be the most profitable machining parameters, and apply the 300 degrees delta, we can see a large increase in tool life. In many cases, tools are cheap compared to the machine burden rates (ex. $10 edge vs. $250 per hour burden rate). The question then becomes how to gain productivity.
The thermal plot is cryogenically shifted 300 degrees depending on the efficiency of the Cryo tool, which will be discussed later. Once the critical temperature is moved, the parameters move, and significant speed and productivity are achieved. Theoretically, this is the math and logic behind the Cryogenic advantage.
Why Liquid Nitrogen (LN2)?
When it comes to health and safety, 78% of the air we breathe is nitrogen. With Cryogenic Machining, we take nitrogen from the air, cool it until it liquefies at -321°F, put it through the machine, then through the cutting tool, and vent it back into the atmosphere. Oxygen sensors safeguard oxygen depletion in case of a malfunction or spill, and appropriate gloves are used for manually handling tools. Here are some advantages to using liquid nitrogen (LN2):
- LN2 is not a greenhouse gas
- LN2 is economical
- Eliminates disposal, management, and infrastructure of flood coolants
- Provides safer, non-slip approach for large, walk-on machine tables
- Dry, reclaimed chips are easily recyclable and more valuable
- Parts are free of contamination (ex. safe for medical devices)
Here are some benefits to using LN2 over other cold alternatives:
- The small difference in bulk cost of CO2 vs. LN2 cannot make up for the higher CO2 pressurization and flow rates;
- Oxygen is a powerful oxidizing agent with flammability risks;
- Liquid nitrogen is very effective at low flow rates (0.1L/min/ cutting edge);
- LN2 creates a nitrogen atmosphere in the cut, which eliminates oxidization.
MAG’s Cryogenic Machining Evolution
Phase 1: The concept of Cryogenic Machining started in 2003, in partnership with Small Business Innovation Research (SBIR), U.S. Navy, and Bell Helicopter.
Phase 2: Cryogenic Machining was introduced at the 2010 International Manufacturing Technology Show (IMTS).
Phase 3: MAG worked in partnership with Lockheed Martin to reduce the cost of their F-35 JSF program, and was approved for F-35 titanium roughing. MAG demonstrated Cryogenics on seven new machine platforms at the 2011 Interactive Manufacturing eXperience (imX) and EMO trade shows, including horizontal machining centers, vertical machining centers, horizontal lathes, and vertical lathes. Cutting demonstrations featured titanium, composites, compacted graphite iron, steel, and Inconel.
Phase 4: Cryogenic Machining was brought to IMTS 2012. Production-ready new machine and retrofit options are now available along with a strategic range of Cryogenic CYCLO CUT cutting tools. MAG holds exclusive license from Creare as well as multiple patents beyond the original technology.
MAG is constantly moving forward by securing strategic customer sponsorships, gaining partnerships focused on tool and process testing, and expanding IP.
The Cryogenic System
The complete system consists of a liquid nitrogen source, a brain for control, a feed system to and within the machine, a spindle, and a cutting tool. The source can be a simple dewar to supply a test machine, or bulk storage for a large production installation. It can also be a micro bulk source for a cell configuration.
The spindle is the source of many of the patents issued and pending. MAG uses vacuum-insulated feed tubes and control valves to keep the LN2 in liquid form, but not allow temperature transfer to the spindle shaft or bearings. Throughout years of continuous testing, the Cryogenic spindle has provided a long seal life, no thermal cycling issues, and no effect on spindle bearings.
The tooling is unique and has multiple patents issued and pending. Tooling includes solid carbide end mills and drills, indexable end mills and face mills, turning, and boring tools. The thing that separates MAG’s process from prior attempts is the refrigeration method rather than the flood spray method. Venting through the cutting edges puts the super cooling effect of LN2 where it is most needed with little waste. As the insert creates the chip, the sources of heat converge onto the edge of the insert and are extracted as they are produced. The tools are plumbed with insulation to keep the LN2 in liquid form until it exhausts into the atmosphere.
MAG IAS LLC