Cutting fluids are important elements in maintaining the quality of parts. Due to the progressive aging of fluids, any delays in fluid maintenance will affect the productivity of the machines. When the right fluid is used and its condition is maintained, there are significant benefits to the manufacturers. Although dry machining and mist cooling are used in some applications, flood cooling still dominates, and will be the primary focus.

Positive Payoff

The most obvious advantage of well-maintained fluid is that it lasts longer, and therefore reduces replacement costs. Longer-life fluid also means lower disposal bills. Another advantage is that the quality of parts is higher. Good lubrication and cooling provided by a cutting fluid help to control the size and finish of the parts. Also, cutting tools and wheels last longer. There is less abrasion of the cutting tools and the cutting ability of the wheels is more efficient because they are not clogged with fines from the coolant. Of course the improvements are relative to the specific operations.

Observe Temperature Changes

The temperature of the fluid is important. It should be kept as constant as possible to control the expansion and contraction of the workpiece and the machine tool. Maintaining the coolant temperature at ambient ±2 or 3°C is a significant benefit. For very precise grinding operations where size is being controlled at ±0.02 mm, a chiller can be used in the fluid circulation system. Usually, controlling the retention time in the sump is sufficient. This is based on the heat introduced by the cutting process and the ambient temperature of the factory. A rule of thumb is that the fluid should settle for 7 to 10 minutes in the sump before it recycles. For sump-based systems that pump 5 gpm (19 L/min), there should be a 50-g (189-L) tank on the machine. Separate calculations are needed for central coolant systems.

Some machine tool manufacturers cheat on sump size as a means of reducing machine tool area. With central coolant systems, where one sump and circulation system serves a number of machines, another form of cheating can occur: companies will often add machines to a production operation without increasing the size of the central coolant systems.

Stop the Foam

Foam can be a problem as it reduces the ability of the coolant to carry off heat. Foam can be caused by operating at high pressure, soft water or fluid breakdown. In some high-pressure applications when the coolant is also used for deburring, gundrilling or creep-feed grinding, it is important that the coolant can withstand the pressure. Usually, a heavy-duty synthetic product is used.

How Do You Know?

To ensure that a coolant system is working properly, periodic testing is essential. One of the most obvious tests is smell. If water-based coolants are allowed to stand without being agitated regularly, bacteria can grow, generating a rotten-egg smell. Bacterial problems can be addressed by the application of biocides, which should only be used in accordance with the instructions of the fluid supplier.

Properly controlled fluid temperature helps ensure cutting accuracy.

A simple test of coolant concentration can be done with a refractometer. The technician places a few drops of coolant on a prism that is held to a light. The light passing through the sample is refracted and gives a reading on a scale that can be converted to a concentration number. The concentration of the coolant is then adjusted accordingly. These tests are usually done on a daily basis and can be easily handled by personnel.

A plant test such as a pH test can also be carried out. This can be done with a dip strip or pH meter. Readings are usually between 8.8 and 9.2. If they are lower, there may be a potential bacterial problem.

More detailed testing will include the analysis of the solids within the coolant. Coolant additives must be matched to the cutting operation, and their breakdown must be monitored to maintain cutting efficiency.

The simplest test is to let a sample sit in a graduated vessel and measure the amount of solids that settle out. For more detailed evaluation, the sample can be sent through a filter or placed in a centrifuge. A high level of solids indicates filtration problems and can lead to finish or tooling problems.

Normally, the simple tests are done by plant personnel. The next step, if necessary, is to send a sample to a laboratory for evaluation. This is a free service offered by the fluid supplier, who can also provide on-site technical assistance.

How Many Kinds?

There are generally four kinds of metalworking fluids: straight oil and three types of water-based fluids (soluble oils, semi-synthetics and synthetics). The most commonly used water-based fluids operate at a 5% concentration or a concentration of one part to 19 parts water.

The concentration should be maintained at the recommended mix ratio. If it is too diluted, the solution won't do the intended job. If it is too rich, it will waste money and cause foaming or residue problems.

Water condition is a frequently ignored issue. It varies widely from city to city and from region to region. Chemicals within the water can adversely interfere with the coolant functions. For example, if the water has a high mineral content, solids can build up in the cooling fluid. If too many chlorine ions are present, this can cause corrosion problems. Water testing is, therefore, critical.

Water with more than 100 ppm of chlorine, 100 ppm sulfate or 50 ppm nitrates is considered as "aggressive" water. It causes corrosion by breaking down protective barriers on metal surfaces.

If the water is too bad, it is necessary to change either the metalworking fluids or the source of water, and to use treated water.

It Shouldn't Be Harmful

Compatibility between the coolant and the machine tool elements is essential. It is important that the fluid does not react adversely with the elements of the machine tool such as filters, seals, window material and paint.

These problems occur after a machine tool has been repaired or refitted when the wrong type of seal or paint was used.

Efforts are underway to establish standards among machine tool builders and coolant and lubricant suppliers.

Which Fluid to Choose?

Fluid users like to find general-purpose fluids that simply supply and maintain operations. However, it is important to match the fluids used for cutting or grinding operations and the product material.

For example, cutting aluminum requires a coolant with high lubricity and corrosion or stain protection. These requirements can vary according to the material being cut or ground. Some of the most expensive coolants require special inhibitors to protect the products. For example, when grinding coated-carbide tooling, it is essential to have inhibitors that protect the cobalt.

Another concern is what the coolant does after the part leaves the machine tool. If the part is stored, the coolant should provide protection against corrosion.