CUTTING FLUID MANAGEMENT

Cutting Fluid Maintenance

Metal cutting fluids are dynamic systems due to their usage and environment. Because its characteristics are widely changing during time, the same product may have a different composition after using in different systems for enough long time period. Therefore, cutting fluid maintenance is very important for its proper application in processes. In addition, keeping the fluid properties between the proposed limits will increase its life time. Many companies are running different maintenance programs, all focussed on monitoring particular fluid properties and proper addition of chemicals in order to keep fluid parameters between the proposed limits. Most important parameters that are monitored in order to keep fluids ability to perform its role in machining are:

Oil Concentration
pH level
Microbial Contamination
Corrosion inhibition

Oil Concentration Concentration of cutting fluids is essential for its performance characteristics. Proper concentration additional maximizes tool life and is a good indicator of microbial growth rate. High concentration increases fluid costs through wasted concentrate, reduces dissipation of heat, foaming, lubrication, increases risk for generating BUE, etc. Since the fluids evaporate under STP, high concentration can additionally increase its toxicity. Basic reason for high concentration in cutting fluids is water evaporation due to heat generated in machining processes. Low concentration usually causes poor lubricity, shorter tool life, increased biological activity and increased risk of rust formation on contacted metal surfaces. Many chemical and physical processes influence oil concentration decay: bacterial attack, reaction of oil molecules with elements in water or metal, lubricant adhesion to the metal parts, degradation due to temperature and pressure in the cutting region, etc. Most widely used methods of measuring fluid concentration are: Refractometry. Oil concentration measurements by using refractmeter is based on a property that fluid refraction index (how much light is bent as it passes through a liquid) depends on its density. A hand held optical instrument called refractometer is used for this measurements. Since this is an inexpensive tool ($200 to $250) with a satisfied accuracy, it is often used in the industry for fast concentration control. Additional advantage of this device is that they are applicable for measuring concentration in a wide variety of fluids. Since it is optical instrument, most important disadvantage is that the accuracy of measured results depends on fluid contamination, more contaminated fluids give less accurate results. Therefore, for a more precision measurements other techniques are recommended. Oil and water separation (oil split) The idea of this test is to destabilize cutting fluid emulsion and separate it in two layers one of which is water and other is present oily material. Mineral acid (H2SO4) is added to the mixture, and after heating it separates into two layer. Oily material is concentrated in the upper layer, while water is separated on the bottom. Relation between volume of oily material and fluid volume represents the oil concentration. Detailed description of this testing method is covered with DIN 51368. This method is as accurate as refractometry, but it requires laboratory equipment sensitive to excess oil. If the contamination from machine lubrication system is presented into the sample this will have an influence on accuracy because oil volume will reflect both oil from the emulsion and tramp oil. In addition, this method can not be used for solutions. Titration By using titration method concentration of specific chemicals or group of chemicals in the mixture can be measured. Titrant is adding into a measured fluid volume until a color change is noted. The coolant concentration is proportional to the number of titrant drops added. This method is more accurate than other two and is less affected from tramp oil or water quality.

pH Level Cutting fluid pH is a very good indicators of consistent fluid quality. It represents a concentration of hydrogen ions (pH=-loh[H+]). Practically, it is measurement of fluid alkalinity or acidity. Neutral pH value is 7. Lower values represent acidic solutions while pH values higher than 7 represent alkaline solutions. Recommended pH region for water-miscible cutting fluids is 8.8-9.2. For pH value lower than 8.0 fluid is loosing its efficiency, its corrosion prohibition properties are becoming reduced and increased microbial contamination is recognizing. If the pH level becomes more than 9.5 risk for skin irritation and dermatitis significantly increases. Litmus paper provides the cheap and fast indicator of pH. Indicator stripes are dipped into the cutting fluid and it is changing color will depend on the pH level. Since the accuracy of this method is limited and it can not predict biocontamination of the fluid, another pH indicators are using more often. Electronic pH meters are more expensive, but provide more exact measurements. Their usage is standardized according to DIN 51369. This method can be very accurate (high cost pH meters are accurate up to hundredths of a pH unit), but its degree of accuracy is of little benefit for a fluid management. Titration, as a quantitative method for determining alkalinity, is also developed. Advantage of this method is that it additionally determines the rate of change of alkalinity. This helps in estimating the cause of alteration. Microbial Contamination Most common tests for microbial contamination are plate counts and dipslide test. In the plate count test, microbial growth is allowed on the plate which is than counted. The number obtained by counting is multiplied by the dilution factor and the amount of organism per milliliter is obtained. This test is standardized by ASTM D 3946-92. Dipslides method is more simple but still very common for estimating microbial population. A plastic slide coated with a nutrive gel is dipped into the test fluid and after draining microbes will start growing. Interpretation of bacterial and yeast infection depends on individual judgment and genuine differences on reproducibility, but this method is still acceptable for estimating microbial growth in metal working fluids. If there is additional rancidity problem, observations by using dipslide method could be used as a good indicator for adding biocides before problem arise.

Microbial contamination of a cutting fluid can be additionally estimated by indicating dissolved oxygen in the mixture. At STP a circulated fluid can dissolve about 9ppm oxygen when it is exposed to the air. Since the oxygen is necessary for aerobic bacteria growing, by measuring the dissolved oxygen good estimation of biological contamination can be obtained.

Corrosion inhibition Testing corrosion properties of cutting fluids is very important for protecting metal parts that are in touch with the fluid. In addition, since the corrosion protection of cutting fluid decreases significantly when oil is completely dissolved in water, this is a good indicator for adding oil into the system. Fluid corrosion tests are standardized in DIN 51360 part 1 and 2 erwalleney, S., 1996]. DIN 51360 is a well known Herbert test. Four small piles of clean steel chips are positioned on the cleaned and polished cast iron plate and are then whetted with the test mix. Four different dilutions of the same mix are used for four different chips. Plate and chips are placed in a closed container for 24 hours, after which the formation of pits and staining is assessed. Institute of Petroleum, London applies the same test (IP 125) with steel chips on the cast iron plate [Rudson S.G. and Whitby, R.D. [1985]. DIN 51360 part II and ASTM D4627 describe a testing method for cast iron chips on filter paper. About 2 g of clean iron chips are spread onto a filter paper in a Petra dish, the fluid mixture is pipetted on to the chips and the dish is covered. After a certain period of time the chips are removed and the paper is examined for staining. Similar test is developed by Institute of Petroleum, England (IP 287 test), presented by Rudson S.G. and Whitby, R.D. [1985]. The disadvantage of all presented tests is that they are developed for one particular material. IP 329 test by the Institute of Petroleum is a multi-metal corrosion test that examine the effect of water-based fluids on steel, cast iron, copper, brass, aluminum, zinc and cadmium [Rudson S.G. and Whitby, R.D. [1985].

Recycling

Recycling is the method of eliminating cutting fluid disposal. Because of major changes in environmental regulations recyclability becomes one of the most important fluid properties. Back in the 1960Õs, people did not think how dangerous spent cutting fluids can be, and therefore any treatment before disposing it was not performed. In the present days, fluid disposal costs frequently exceed the cost of buying new fluids when dilution factors are taking into account. In addition, water-soluble organic materials that do not respond well to either chemical or mechanical (ultrafiltration) recycling techniques are often components of chemical and semi-chemical fluids. Attempts at separation result in a water phase with high BOD and COD levels which can result in hefty sewer surcharges to those manufacturers who introduce the water phase into municipal sewers after playing for the haul away of the ÒseparatedÓ concentrate. It is much easier to dispose emulsion fluids with present disposal technology, and therefore the cost of its disposing is lowest. Disposal costs are still significant no matter what fluids are employed, but the feasibility of recycling is well proven today and the number of plants with coolant recycling systems in operation continues to grow rapidly. Zelnio, L.L.[] described the improvements in John Deer Harvester plant after installing a fluid recycling system. Since the coolant price increased from $3.50/gallon to $6.00/gallonfrom 1977 to 1986, disposal costs were increased from $0.8/gallon in 1977 to $1.7/gallon in 1985, and because the Federal Government introduced new RCRA (Resource Conservation and Recovery Act) in 1979 that suggested more closely monitoring of generator wastes, they started thinking how to reduce cutting fluid disposal. At the beginning, they reduced the number of different fluids from 17 to 2. Then they installed a different recycling system for both selecting fluids. A closed-loop systems were selected as the least expensive ad most effective alternative to the construction of central systems for recycling coolant from individual machine tool. The individual machine tools are cleaned by portable sump cleaners which deliver used coolant to a centrally located recycling area. After the coolant is recycled and treated, it is pumped from the recycling area back to the machine tools for reuse, thus closing the loop. The results of installing these closed loop reclamation systems were: 40 percent reduction in fluid usage, elimination of fluid disposal, 94 percent reduction in fluid related dermatitis, elimination of machine tool rust, 45 percent reduction of coolant concentrate inventories.