Why is it Necessary to Filter Metalworking Coolants Serving Machining Operations?
By James J. Joseph

Figure 1. Differences in Clean Tool Edge and a Tool with Built Up Edge (BUE).

This is not a trick question. There are some process engineers who feel that since machining is a rugged application, several of which can be accomplished dry, it may not be necessary to clean the coolant serving the process. Their logic is that there is carry out of fluid on the chips and evaporation, so fresh fluid is routinely entering the system as make-up. Therefore, as long as the coolant is there to cool, why spend the money to filter it. Operate with dirty fluid.

The answer to this question covers many areas and the best way to give particulars in this article is to highlight system problems, which would develop when operating with unfiltered coolant.

Before starting, it is important to remember that the main functions of coolants are: to cool, clean, flush chips/debris, lubricate, protect from corrosion and anti-weld. Granted some of these functions can be performed with dirty coolant, but only up to a point.

Also, for this article, and in keeping with the typical industry jargon, the terms filter and filtration are loosely used to apply to any device that cleans coolant. Therefore, in addition to barrier filters these can be magnetic, retention, flotation, and centrifugal separators. Obviously each device would be selected for its applicability to the operation's needs.

Tool's Cutting Action
The actual metal cutting performance is compromised when it has to function with dirty coolant. As the particles enter the fluid they grow in concentration to where they become trapped in the interface between the tool and work piece. Since the particles are work-hardened when they are cut, two things happen:

They cause the tool to work harder to overcome the increased friction. This elevates the temperature, which affects the life of the tool's cutting edge and can throw the work piece out of tolerance.

The high concentration of work-hardened particles and the elevated heat at the point of cut will allow the accumulated debris to "weld" to the tool's cutting edge. The accumulation of welded material, called "built up edge"(BUE), affects the shear angle and the tool's normal cutting edge is not cutting the work piece. The BUE is cutting. Figure 1 shows the BUE occurrence at work. The build up edge cuts the work piece differently than the desired tool and more heat is generated, work piece surface is violated and the parts are machined out of spec. The added heat also shortens tool life.

The same work-hardened material is further crushed into finer particles, which are highly abrasive. In aluminum applications this material could be aluminum oxide, while some ferrous alloys with silicon could create silicon carbide. There are other transformations into abrasive material. All of these damage the work piece, create unwanted inclusions in the work piece, and shorten the life of the tool's cutting edge.

Coolant Chemistry
Long periods of coolant exposure to freshly cut metal particles can create other chemicals, which inhibited good performance.

Ferrous alloys create the scum-like material, which packs the interface as well. The scum plates out on the parts and surfaces.

Cast iron has its graphite and "soot-like" material causing the same problems.

The aluminum machining applications have a more significant problem with this phenomenon. Aluminum soaps are created, which if allowed to grow in concentration will change the lubrication quality of the coolant. The soaps will also attract fine particulate and jam the interface, act as a lapping compound, and plate out as residue all over the system. Another curious factor which is not always predicable is that after a period of extended exposure to freshly cut aluminum surfaces on chips and particles, the coolant begins to loose some of its normal operating characteristics and "dies" with no readily measurable warning.

Tramp oil in a system could be a major factor in the coolant's ability to perform its functions properly.

System Conditions
Dirty coolants will eventually allow material to settle in the sump of the machine. As the solids accumulate, the volume of fluid is reduced so the system is working with less coolant. This affects heat removal and as the temperature climbs other adverse chemistry changes can occur.

Dirty coolant promotes bacteria and many times a great deal of money is spent on biocides, which in turn may create unwanted risks to personnel.

Figure 2.

Test Data
Over the years some manufacturing facilities have conducted controlled tests to see the difference in machining performance with coolants filtered versus unfiltered.

One plant set up two identical machines - machine A had filtered coolant while the other machine B operated with unfiltered coolant. The test showed that the unfiltered coolant machine produced 125 work pieces per cutter re-grind. The filtered coolant machine maintained an average of 1,200 work pieces per cutter re-grind. In an effort to insure that there was no machine tool bias, the filtration system was taken off machine A and placed on machine B. After a suitable run time, machine A dropped to 150 work pieces, while machine B climbed to 1,200 work pieces.

Other data, which is summarized from a number of tests, suggests that filtered coolant can be expected to increase tool life by the percentage shown in Figure 2.

Please note that these are offered as a guide only. These are summaries of many tests with all kinds of variables involved, so the results with other operations probably may have different percentages. It is almost like posting expected miles per gallon on a new automobile. At least the summary shows the ranking and magnitude of the advantage of filtered coolant versus unfiltered coolant for various machining operations. For example, it can be understood why drilling would have a marked advantage with a chip- free coolant.

There is also the issue that unfiltered coolant operations will have more machine downtime due to more frequent tooling changes.

Figure 3. Filtered Coolant cleans the fixtures and machine components.

It is important to understand that the tests were conducted with truly unfiltered coolant. If the coolant is changed frequently because it is not filtered, the operations are working with "cleaner" coolant and the differences before and after filtration would not be as dramatic. Here, dumping costs at an undesirable frequency is the issue and another strong case for filtering.

Machine Surface's Wear
Unfiltered coolants will leave debris and abrasive material on the machine tool's surfaces. Therefore moving components such as guides and ways will be exposed to highly abrasive material and usually require major machine repair more often.

Machine Platens, Work Fixtures
When chips and debris accumulate on platens, nests and other fixtures, parts may not be positioned accurately and the result is machining the surfaces out of tolerance.

Figure 3 is a good view of a typical operation where clean coolant keeps both the machine ways and parts fixtures free of particles.

Operator Sensitivity
Dirty coolants create an atmosphere where operators who work on the machine for set up or load/unload could have a higher potential for skin problems. In some cases, the problem can occur when they wipe their hands with a shop rag and the particles in the coolant scratch the skins protective barrier. Other cases have revealed where more sensitive operators can develop skin irritations because of the hostile chemistry allowed to develop in dirty coolants. Again this is further exacerbated if there are tramp oils and biocides in the fluid.

The question of why it is necessary to filter metalworking coolants serving machining operations leads to six answers from the major fundamentals of a metal cutting operation. These include: tool life and performance, product quality, coolant longevity, operator comfort, machine performance and maintenance. All these reasons support the fact that filtering a coolant used in a machining operation is an "asset and not a liability."

James J. Joseph is a consultant who has also written a book "Coolant Filtration 2nd Edition, Additional Technologies."

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