| Should you practice dry metal cutting? |
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Although dry metal cutting has become more than just a passing trend in some automotive industry and in manufacturing plants, you will still find machine shops using coolants and lubricants. It's not hard to understand why a shop is reluctant to give up its metal cutting fluids. After all, fluids lubricate and cool the point of cut, flush chips out of the cutting area, and control built-up edge (BUE) on the tool. With higher production rates, lower manufacturing costs, and all the other benefits associated with cutting fluids, why would a shop want to practice dry metal cutting?
There are environmental factors why dry metal cutting should be considered. It eliminates the hazards associated with coolant mist and wet shop floors; circumvent the health effects attributable to long-term exposure to straight-oil, soluble-oil, and synthetic metal cutting fluids; and concerns regarding cutting-fluid disposal. Rather than wet chips that require processing and filtration, you get clean, dry chips that are ready to be sold or recycled.
Economical factors also provide compelling reasons to try dry metal cutting. Once you take into account the costs for fluid procurement, maintenance, and disposal, along with any fines that may be incurred for violating governmental regulations, cutting fluid can amount to a sizable percentage of your total manufacturing cost.
 | | Dry cut metal saw by Northern Tool + Equipment |
Whether or not a shop adopts dry metal cutting as a common practice depends on how the costs of doing without the benefits of cutting fluids compare to the costs of buying cutting fluids and maintaining and disposing of those fluids in compliance with environmental regulations. To calculate these costs, a shop must determine how well the workpiece, machine tool, and cutting tool can tolerate the heat and chips generated in a given dry-cutting operation.
Workpiece
Without the cooling effects of fluid, a metal cutting process may produce excessive heat that subjects the workpiece material to high stress and the danger of thermal expansion. However, for many work materials, properly performed dry cutting can expel the heat with the chip to avoid these adverse thermal effects on the workpiece. BUE is the main problem in the dry cutting of aluminum. By transferring all the heat generated in the cutting process to the chip, higher speeds can help eliminate aluminum's tendency to weld to the tool.
Machine Tool
The workpiece material dictates whether or not a shop needs to purchase a machine tool specifically designed for dry cutting. To achieve the higher speeds and feeds that are typically used for the dry cutting of aluminum, the complete structure of the machine has to be designed for rigidity to accommodate the higher spindle-speed requirements. Cast iron and steel, on the other hand, can be dry milled or drilled with the machines already installed in most shops. Unlike aluminum, the dry cutting of cast iron or steel doesn't necessarily require higher spindle horsepower or greater machine rigidity. Since chip loads are about the same cutting dry as they are cutting wet, torque requirements and tool forces are also about the same. Although it isn't necessary to increase cutting parameters for cast iron or steel, higher spindle speeds and feeds may allow the chips to be ejected from the cutting zone before the heat can penetrate the workpiece or tool.
Compressed-air or vacuum systems can blow or suck ferrous chips out of the cutting zone, but their effectiveness depends on controlled chip formation at the cutting tool. The configuration of the machine must enable proper chip collection and evacuation to prevent dry chips from accumulating and building up heat, which may cause thermal growth of the machine. Chip removal may be aided by a machine's chip-auger system. As long as you don't have large piles of chips sitting in any area of the machine, you shouldn't have a problem with thermal displacement.
The chip augers take care of the heavy particles, but small, airborne particulates must be vacuumed out. In extreme cases, shop vents may have to be relocated to provide adequate ventilation. Additional precautions may or may not be necessary, depending on how well the cutting process lends itself to being performed dry.
Operation
Dry drilling is complicated because there is constant heat in the cutting zone due to the continuous engagement of the tool in the cut. The drill's constant contact with the workpiece may cause some thermal growth, which can create tolerance problems for the hole. In drilling, the greater the hole's depth-to-diameter ratio is, the more problems there are with heat and chip flushing.
When drilling a shallow hole, heat and chip evacuation aren誸 as significant. However, drilling often requires some type of coolant or lubricant to produce exacting finishes and rigid tolerances at high production rates. In fact, some dry-drilling applications are performed with minimal lubrication; they are technically dry because the small amount of fluid used can be completely dissipated through heat.
 | | Dry cutting high performance drill |
Cutting Tool
You may be able to cut dry using a tool with a standard geometry, but you probably won't be able to run it as aggressively as you could if you were using cutting fluid. The tool material you select for a dry-metal cutting application is just as important as the tool geometry you choose. Not all tool materials have the properties required for dry cutting with the same parameters used for wet cutting. Due to the complex thermal- and mechanical-load conditions, the cutting tool's hot hardness and toughness are crucial in dry cutting.
Cutting dry with ceramic and cermet tools typically isn't a problem. In fact, tool manufacturers often recommend cutting dry when using ceramics and cermets due to the danger of thermal shock when using cutting fluid. Improper fluid application can result in irregular distribution of fluid in the cut, which creates an unstable heat zone for the cutter. These temperature variations can cause premature tool failure. Standard HSS and carbide, however, generally aren't suitable tool materials for dry milling, drilling, or turning. HSS has poor deformation resistance, and carbide lacks the necessary toughness. With the proper coating, however, a standard HSS or carbide tool may be suitable for the dry cutting of cast iron, steel, or aluminum. In a dry operation, a coated carbide tool generally performs better in terms of tool wear than an uncoated carbide tool.
Most titanium-aluminum-nitride (TiAlN) with higher aluminum content is harder, but they all have about the same oxidation temperature. TiAlN has a higher oxidation temperature than titanium nitride (TiN) or titanium carbonitride (TiCN). It is preferred for many dry-metal cutting applications. It can withstand higher temperatures without breaking down and causing degradation of the tool substrate.
TiAlN can be combined with a soft coating based on molybdenum disulfide (MoS2) to form a multilayer coating with high wear resistance and a low coefficient of friction. The MoS2-base coating can also be used on its own to reduce BUE and speed chip evacuation. Tungsten-carbide/carbon and diamond-like carbon (DLC) coatings also show very good potential for replicating the effects of cutting fluid.
In the dry cutting of aluminum and nonferrous metals, diamond coatings perform much like DLC or MoS2-base coatings, with the additional benefit of high hardness. Diamond composites and CBN are under development for coating tools designed to cut cast iron, hypereutectic aluminum alloys, and various steels and titanium alloys without cutting fluid.
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