TECHNOLOGY FOCUS

UNDERSTANDING CONTAMINATION IN METALWORKING FLUIDS

PART 3 - CAUSES AND EFFECTS OF MACHINING PARTICULATE CONTAMINATION

by Joe Eppert

March 2001:

Introduction:

Previous segments of this series discussed the identification of those substances typically responsible for MWF contamination and the cause and effect relationships of tramp oil, this installment will discuss the addition of machining particulate on the efficiency and useful life of MWFs.

Although the primary functions of MWFs are to lubricate and cool, there are secondary functions, such as chip evacuation, that must be fulfilled as well.  For the majority of machining operations it is extremely important that the chips do not re-enter the cutting region once they are formed.  Chip clogging must be prevented, and it is one of the secondary functions of the MWF to remove the chips from the cutting zone as quickly as possible. 

If we are dealing with flood cooling, the majority of any chips or small metal fines will end up in the MWF system.  Before returning to the sump, various methods are used to remove the chips from the MWF, usually some type of filtration, centrifugation, or settling.  For each of these techniques, the removal efficiency for the particles is directly related to their size.  Below some lower limit, the methods are not capable of removing a certain quantity of the particulate, and this quantity circulates through the MWF system and back to the machining region.  It is generally accepted that all machining operations produce some size distribution of chips, including those in the micrometer range, and that the workpiece material, operation, and use of MWF will impact the overall particle size distribution and shape(s) observed (2, 3, and 6).

Assuming that some sort of chip removal technique is being used, we can focus our attention on the causes and effects of metal fines in the MWF system.  Fines that are allowed to recirculate will generally be smaller than 10 micrometers.

Once in the system, the fines have ample opportunity to interact with the MWF.  The primary effects of the fines can be broadly categorized into chemical effects, interacting effects, machining performance effects, and health and safety effects (the same as for tramp oil contamination).

Chemical Effects of Machining Particulate Contamination:

The primary chemical effect of metal fines in a MWF is the stripping out of particular ingredients of the fluid.  A single fine has a small surface area, but the ratio of the surface area to diameter (or volume) is much higher than for a larger particle.  These fines, which have just undergone machining, have highly reactive surfaces and there are generally a large number of fines in the MWF, leading to a large amount of available surface area (8).  With such a surface exposed to the MWF, it is possible for components such as emulsifiers and biocides of the MWF to react with the fines and become “tied up”, depleting their effective concentration in the MWF (8).

Additionally, larger chips are responsible for carry off of the MWF.  In a similar manner as above for smaller metal fines, carry off on chips removes those ingredients in the MWF that are surface-active, most notably emulsifiers and biocides (7).  A method for prediction of carry off volume on chips has been developed (4).

Interacting Effects of Machining Particulate Contamination:

The primary interaction effect of metal chips and fines pertains to the growth of microorganisms (2).  The effect of microorganisms on the MWF will be discussed in the future, but for now it is worthy to note that machining particulate can be responsible for providing food to the microorganisms and can act as a “breeding ground” in the sump (9).  The existence of metal chips is common when trying to grow microorganisms in laboratory experiments, as it is understood that they play a significant role in the growth of bacteria (1).

Machining Performance Effects of Machining Particulate Contamination:

One of the most significant direct effects of chip and fine contamination in MWFs is the deterioration of machining performance with increased concentration of fines.  Performance measures such as workpiece surface finish, corrosion inhibition, and tool life are all affected by the amount of fines in the MWF (2, 3).  In particular, it is thought that fines a few micrometers in size are able to enter the cutting zone, with those larger being too big to enter, and those smaller passing through with little effect (3).  If able to enter the cutting zone, the fine acts as an abrasive between the tool and workpiece.  Inclusions in the workpiece material that are extremely hard have ability to affect the wear rate of the tool and workpiece surface finish significantly.

In addition to the tool wear and workpiece surface finish effects mentioned above, it is supported that if small metal fines are allowed to deposit on machined parts, they can contribute to the formation of rust (2).

Health and Safety Effects of Machining Particulate Contamination:

With respect to the machine tool system, the most common health and safety effects of chips and fines in MWFs pertain to the accumulation of dirt on machine tools and the clogging of coolant delivery lines.  In addition, small metal fines have the ability to cause skin irritation due to abrasion of the skin when the MWF comes in contact with the worker (2, 5).

Summary:

Machining particulate contamination –
        cannot be avoided, as it is a function of the MWF to remove metal fines and chips.
        leads to the reduction of various ingredients in the MWF, thus impacting machining performance.
        can act as a food source for microorganisms.
        can act as a breeding ground for microbial growth.
        contributes to accelerated tool wear and deteriorated surface finish.
        can cause skin irritation, dirty machine tools, and overall rancidity of the MWF.

References:

Bennett, E.O., “The Deterioration of Metal Cutting Fluids”.
Byers, J.P. ed., 1994, Metalworking Fluids, J.P.B., ed., Marcel Dekker, Inc, New York.
Marano, R.S., G.S. Cole, and K.R. Carduner, 1991, “Particulate in Cutting Fluids: Analysis and Implications in Machining Performance,” Lubrication Engineering, Vol. 47, No. 5, pp. 376-382.
Munoz, A.A., and P. Sheng, 1995, “An Analytical Approach for Determining the Environmental Impact of Machining Processes,” Journal of Materials Processing Technology, Vol. 53, pp. 736-758.
Nachtman, E.S., and S. Kalpakjian, 1985, Lubricants and Lubrication in Metalworking Fluids, Marcel Dekker, Inc, New York.
Nehls, B.L., 1977, “Particulate Contamination in Metalworking Fluids,” Lubrication Engineering, Vol. 33, No. 4, pp. 179-183.
Organization Resources Counselors, Inc., Management of the Metal Removal Fluid Environment, 2^nd Edition.
Organization Resources Counselors, Inc., Metal Removal Fluids – A Guide to their Management and Control.
Sluhan, W.A., 1993, “Don’t Recycle – Keep Your Coolant,” American Machinist.