They're in nearly every plant: the coolants that lubricate and cool drilling, milling and turning operations. The problem is that the nature of the coolant changes over time. At first, it performs well, but gradually it loses effectiveness, and eventually it must be replaced.
Coolant emulsions respond well to maintenance, especially when done early, often and regularly. Add to that the benefits of in-house recycling, and the incentives for nurturing coolants along become compelling.
When a coolant is maintained properly, machine tools may operate for months on the same charge. The direct costs associated with consumption and disposal are reduced. The indirect costs associated with health, the work environment, downtime, scrap and machine damage are also lowered. What's more, a constant process capability requires fewer machine adjustments. It's not surprising that many shops maintain coolants with the discipline afforded million-dollar machine tools.
Coolants are emulsions prepared by diluting a concentrate with 85 percent to 95 percent water. The concentrate contains 30 percent to 60 percent mineral or vegetable oil, which serves as the carrier for 15 to 20 other ingredients that contribute to machining performance and the coolant's stability.
The rigors of the metalworking processes and the machine tool environment eventually take their toll on the emulsion. The temperature in the cutting zone, the agitation in the fluid system, tramp oils seeping into the sump, the bacterial activitythey all add up.
But an ounce of prevention is worth more than a pound of cure when it comes to keeping coolants stable and long-lasting, especially when the effort is readily within the ability of shop personnel to perform. Key steps are simplestart with the right fluid, properly prepared; emphasize cleanliness; monitor carefully and take corrective action when needed.
Get a good start
Machining performance is the obvious selection criterion, but also consider the fluid's ability to reject tramp oil, tolerate changes in pH and water hardness and resist bacterial proliferation.
Mixing the emulsion requires potable water, not plant process water. If the water is softwhich encourages foaminguse a coolant concentrate formulated for soft water or increase the water hardness by adding food-grade calcium. If the water is hardwhich causes most emulsions to separateinstall a deionizer to remove dissolved minerals.
Achieving emulsions having sub-micron oil droplets suspended in the water makes for a homogenous and stable fluid, minimizes concentrate consumption and promotes efficient machining. It pays to invest in good mixing equipment.
Good housekeeping is half the job. At the top of the to-do list is dealing with tramp oil.
When it mixes with the coolant emulsion, tramp oil quickly begins to interfere with machining performance and the general work environment. Once a secondary emulsion forms, which is particularly tricky to prevent on machines equipped with high-pressure coolant systems, the coolant can't be saved.
The cure is to keep seals in good condition and use compatible lube oils. Effective coolants reject tramp oil, causing it to separate out quickly and float. During idle periods, a suction device or skimmer removes floating tramp oil.
Accumulations of chips and swarf on the tank bottom can produce a highly corrosive, sheltered environment for bacteria and impede biocide treatment. These mounds also displace valuable coolant volume with adverse implications on temperature control and stability. Remove the sludge often with sump suckers, scrapers or other techniques.
Evaporation, splashing and dragout constantly change the concentration and volume in the sump. The concentration can increase by several percentage points in a single day, quickly drifting off target.
If adjusted improperly and allowed to move outside the coolant's operating range, machining performance and tool life quickly become unacceptable. Above the range, the emulsion loses cooling capacity, begins to smoke, foams and retains residues. Below the range, pH and corrosion inhibition drop.
Daily monitoring using an optical refractometor and recording the results is a must (see Figure 1). When regulating concentration and level, always add mixed coolant to the sump, not just water.
Figure 1. An optical refractometer monitors lube concentration
Check the pH
Consistent pH is a sign of a healthy and stable coolant. Changes can point to any number of ailments: high concentration of minerals and other contaminants, bacterial activity or fluctuations in concentration, to name a few. Historical pH values provide clues about appropriate remedies to coolant aging.