Metalworking Fluids Aren't Commodities
Although inexpensive, fluids have a major effect upon plant operational costs
By Dave Moon
Manager, Strategic Alliances,
Castrol Industrial Americas
Downers Grove, IL
The lifeblood of manufacturing plants that employ metalcutting is the metalworking fluid itself. It impacts nearly all of a plant's operational aspects. Metalworking fluids influence final part quality, plant waste treatment and disposal, plant tooling, in-plant environmental air quality and mist control with HVAC equipment, machine maintenance and repair, central system metalworking fluid maintenance, and electrical energy power consumption. While the cost of metalworking fluids represents less than 5% of total plant expenditures, these products can impact more than 40% of the plant operational budget. Yet metalworking fluid, an item that can help to increase productivity, has yet to be leveraged within major manufacturing plants. In spite of the productivity potential metalworking fluids can provide, they are still relegated by major mass producers to the commodity classification within the Maintenance Repair and Operations budget (MRO), listed somewhere between the gloves and work rags.
An example of industry's disregard for the role of metalworking fluids involves machine tool sump size. By reducing sump size, manufacturers reduce the retention time of the cutting oil in the sump, where heat can be dissipated. Shortening retention time and using a smaller sump means that manufacturers reintroduce fluid into the cut zone more frequently. Cutting oils dissipate heat three times more slowly than water. So the longer they machine parts, machine tool users, in effect, add heat back into the process.
The source and amount of heat is directly related to the machine tool's spindle speed. Therefore to decrease the heat delivered into the process, operators must either increase sump capacity (to increase metalworking fluid retention time and thus heat dissipation), or reduce spindle speed. Failure to do one or the other ultimately leads to heat buildup in the part, causing metal deformation in the form of heat checks or heat-stress-related metallurgical cracks. How much the spindle speed is reduced depends on variables such as the size of the part, the workpiece material, and tool geometry.
Over the years, as machine tool technology evolved, machining fluid chemistry was enhanced to meet the requirements of users. However, the role of machining fluids was not yet fully understood. Some machine manufacturers continued to design their equipment so that the fluid sumps were placed out of the way under the machine, and nearly impossible to reach for service and fluid sampling. Worse yet, some machine tools were designed with DC motors next to or directly above the sump, thus creating an environment perfectly designed to retain heat in the machining fluid. This setup also allows microbiological activity to flourish, causing further operator disdain for machining fluids.
Today, the factors limiting machining productivity have dramatically changed. CNC machines have replaced the human art of machining. Machine and plant productivity is measured in cycle time per unit of production. Cycle times in turn are limited by such factors as machine rigidity, machine spindle speeds, and heat-transfer rates. While these limiting characteristics are interrelated in achieving success, the least understood is the importance of heat transfer.
Heat transfer is the greatest challenge to attaining maximum sustainable productivity. Degradation caused by heat impacts the quality of the part being machined, and damages the tool used to machine the part. Heat is not the result of the depth of cut--rather it is the result of the speed of the cutting process and the friction and deformation that occurs within the process. The heat generated in the cut zone increases in a direct linear progression with the speed of the cutting process. Left unabated, heat creates heat stress and heat checks that reduce the strength of the part, and can cause the part to fail while in use.
Combining a HPHV system and the correct fluid can significantly improve productivity.
Heat also reduces tool life. It has been documented that reducing the heat at the point of the cut by 50ºF (18ºC) can increase tool life 30%. Extending tool life reduces costly downtime, and increases machine productivity. Therefore, the value of heat transfer can be directly measured through better part quality, increased machine productivity, and lower tooling costs. Additional associated savings are measurable in lower tool inventory costs, reduced electrical consumption, and less machining fluid evaporation, misting, and atomization.
Introducing high-pressure high-volume (HPHV) machining fluid flow has proven to dramatically help reduce heat buildup in the machining process. Generally speaking, high pressure designates a pressure exceeding 720 psi (5 MPa). But the industry standard is based upon 1000 psi (6.9 MPa) and higher, and HPHV OEM manufacturers use 1000 psi as the baseline for their machine model specifications. The industry standard for high volume is 8 gpm (30 L/min) at 1000 psi, but applications determine flow rate versus pressure. High-pressure high-volume fluid makes possible higher spindle speeds, increased feed and speed settings, and better chip control. The principle behind HPHV fluid flow is very simple. It's to create more cooling effect with the machining fluid volume and pressure than the machining process can generate from tool friction and the deformation of the metal part. Removing heat from the system makes it possible to increase spindle speeds to a level that reduces cycle time by as much as 50%.
Critical to the successful application of HPHV machining fluids is the ability of the machining fluid to withstand the compression pressures and shear generated within the high-pressure pump. The machining fluid must have the ability to withstand the entrainment of air without generating foam. It must remain in a liquid phase in order to eliminate heat, because foam (which is mostly air) can't absorb heat.
Of the machining fluid classifications, true synthetic fluids are typically the least likely to generate foam, due to absence of surfactant soaps that act as the coupling agents normally found in semisynthetic and soluble-oil machining fluids. They are also the least likely to split in the presence of high-pressure shearing. Conversely, soluble oils with their larger oil-droplet size and high surfactant level are (generally speaking) more susceptible to foaming and splitting in HPHV applications.
Filterability of the machining fluid must also be considered in HPHV applications. High-pressure pumps are susceptible to degradation due to high dirt load in the machining fluid. Dirt levels are determined by an automatic particle count based on ISO standard 4402. Typically HPHV units use five-micron filters. A five-micron filter provides filtration to dirt loads typically in a 3-D range classification based upon ISO 4402. Piston pumps are subject to degradation by dirt, but hydraulically balanced diaphragm pumps are usually not affected by dirt levels when used in conjunction with five-micron filtration. Positive filters are best suited to reduce dirt load counts--provided the high volume flow requirements of HPHV are not restricted. Again, the performance characteristics of synthetic fluids would generally be considered the most likely to meet this prerequisite.
Coupling the appropriate machining fluids with HPHV applications has proven to radically improve productivity when compared to conventional machining applications employing traditional machining-fluid delivery systems.
This article was first published in the May 2004 edition of Manufacturing Engineering magazine.