Understanding Today's Machine Coolants
By Larry Carley
Many types of grinding and honing operations require the use of a "process fluid" for cooling and lubrication. Remanufacturing procedures such as crankshaft grinding and surface grinding generate a tremendous amount of heat and require a fluid primarily for cooling. Heat control is absolutely essential for a good finish and accurate tolerances. Use of a coolant also helps prolong the life of the grinding wheel.
With honing, the situation is a little different. Some type of process fluid is also required, but primarily to lubricate the honing stones as they cut the cylinder bore. Lubrication reduces friction so less rotational force and pressure are needed to hone the cylinder, and it allows the abrasives to cut more cleanly. The fluid also provides cooling, but heat buildup is less of a factor in honing because the rate at which the stones travel across the metal in surface feet per minute (sfpm) is only about 85 to 150 sfpm, compared to 5,000 to 6,000 sfpm for crankshaft grinding.
The ability of a fluid to provide lubrication is especially important when honing with superabrasives such as polycrystaline diamond (PCD) and cubic boron nitride (CBN). Superabrasives are much harder and longer lived than traditional vitrified abrasives such as aluminum oxide and silicon carbide, but the superabrasive particles are duller and have more rounded edges. This requires a stronger metal bond to hold and support the superabrasive particles, as well as more force to hone a cylinder bore. Because of this, superabrasives typically generate more heat than vitrified abrasives. So to limit bore distortion, a superabrasive honing fluid must also provide cooling as well as lubrication.
A third function that a coolant provides is to rinse away metal and abrasive particles from the work surface. Removing debris keeps the pores in grinding wheels and honing stones open so the abrasive doesn’t load up and stop cutting. Flushing away debris keeps the stones cutting efficiently and reduces heat buildup. This also reduces stone wear so grinding wheels don’t have to be dressed as often.
So what kind of coolant works best? The one that generates the least amount of friction and provides the best cutting efficiency and abrasive life. Which particular coolant that happens to be, though, will vary according to the type of grinding or honing you’re doing, the equipment you’re using, the speed and pressure settings you’re using, the type of abrasive and who you’re asking for advice.
So many choices
There are four basic types of coolants from which to choose: oil-based (petroleum oil or animal fat); emulsion-based (water soluble oils); semi-synthetic (water-based); and synthetic (water-based). Within each class of coolant you can find a variety of products because it’s difficult to have one single coolant that satisfies the needs of every possible grinding or honing process and abrasive combination. Consequently, many equipment manufacturers and chemical suppliers offer a number of different coolants. Most are sold in one or five gallon containers.
For example, one equipment supplier offers its customers a choice of two basic coolants for grinding (one water-based, one petroleum-based), and a different petroleum or water-based coolant for honing. Another equipment manufacturer sells a single grade of oil for all types of honing operations, and a water-based synthetic coolant for all types of grinding.
Browsing through the catalog of one leading shop supply company, we found a huge variety of color-coded coolants from which to choose. The company has an "all-purpose" green grinding coolant, a special amber coolant for crankshaft and camshaft grinding, another amber coolant for general resurfacing, a pink coolant for belt sanding, a blue coolant for flywheel grinding, a dark blue coolant for resurfacing aluminum, a light yellow oil for cylinder honing, a dark brown oil for rod honing, and a light yellow oil for valve grinding.
Is such a variety of products overkill? According to one engineer who develops coolants and lubricants, it’s easier to formulate a coolant for a very specific application than it is to develop a general purpose product for a wide variety of applications. The advantage of having different products for specific applications is that fewer compromises are necessary. You have a product that is ideally suited for a particular application and that application primarily. That doesn’t mean the same coolant can’t be used for other purposes. But it may not do as good a job as another product which is better tailored for that purpose.
Even so, from what we’ve seen, many shops still use a single "general purpose" mineral oil for all their honing and grinding operations, while others use a honing oil for honing and a water-based synthetic coolant for grinding. Petroleum-based honing oil has long been used for cylinder boring with vitrified abrasives because it provides the best lubrication. Next would come emulsions followed by semi-synthetics and synthetics.
One leading supplier of honing equipment said honing oil will almost always give the highest quality finish when used with vitrified honing stones. Water-based coolants are generally not considered acceptable for use with vitrified honing stones because water weakens the ceramic that supports the abrasive causing the stones to break down more rapidly. Switching to a water-based synthetic coolant, they say, will usually result in accelerated stone wear with conventional vitrified honing stones.
Even so, some users say they’ve switched to a water-based synthetic and have not experienced much of an increase in wear with vitrified honing stones. In fact, some of today’s synthetics have lubricity properties that are almost on par with an oil-water emulsion (though still not as good as a straight oil-based honing oil). Where water-based synthetics work best, most agree, is grinding and diamond honing.
"What some machinists don’t understand," said the sales manager of another equipment company, "is that you can’t just switch from a petroleum-based honing oil to a water-based synthetic coolant without affecting the honing process itself. If you change the type of coolant you’re using, you’ll probably have to make some adjustments to the feed and pressure settings on your equipment, and maybe change the type of stones your using, too.
Emulsion-based coolants are a mixture of oil and water. Oil and water do not naturally mix, so a dispersant chemical (detergent or soap) is used to dissolve (emulsify) the oil in the water.
Water-based synthetic and semi-synthetic coolants, by comparison, contain no oil. What they do contain are various man-made chemicals that provide lubricity, suppress foaming and rust. The ingredients, which are supplied as a concentrate, are then mixed with water to form a highly diluted solution. One gallon of concentrate typically yields about 20 to 30 gallons of coolant. But as much as 90 to 99% of the coolant mixture may be water.
Because water makes up such a large portion of the coolant, water quality is an important consideration when using a water-based synthetic. Some products are formulated for use with ordinary tap water while others work best with water that has been demineralized to reduce its hardness.
Water hardness comes from dissolved minerals in the water. Rain water averages about 88 parts per million of dissolved minerals. Well water typically starts at about 180 ppm and can easily go to 250 ppm or more. Hard water is undesirable because it can react with the chemicals in a synthetic coolant to form a soap-like scum on the surface. Hard water can also cause the oil and water to separate in emulsion type coolants.
According to one chemical supplier, water used for a coolant mixture should have a hardness of 80 to 125. Softened water may seem like a good choice (hardness as low as 50 ppm), but water softeners substitute calcium chloride (salt) for dissolved calcium and magnesium to reduce hardness. Chloride you don’t want in the water because it causes rust.
When water is relatively soft (100 or less), a foam suppressant is usually required to prevent foaming during grinding operations. Foaming is usually not a problem with honing because the speeds are so much slower. But with grinding, you don’t want any foam because it reduces the cooling provided by the fluid (air bubbles carry away less heat than liquid). Foam can also obscure the workpiece and interfere with the settling out of contaminants in the coolant reservoir.
A really bad case of foaming may even cause the coolant to overflow the reservoir tank, creating a mess in the shop. Foam is mostly a problem when a fresh batch of coolant is mixed. After a few days of use, the tendency to foam will diminish as the water picks up contaminants and dissolved minerals that increase its hardness.
Water that contains high levels of phosphate should also be avoided because phosphate promotes the growth of bacteria as does sulfate. Phosphates also make the water more alkaline, which can be irritating to the skin. The best recommendation is to use water that has been demineralized by deionization, distillation or reverse osmosis. Ordinary filtration does not remove hardness, only solid particles. If your tap water is too hard or contains phosphates, sulfates or chloride and you do not have your own means of demineralizing your coolant water, use distilled water.
Advantages and disadvantages
One of the main advantages of petroleum-based honing oil is its low price ($4 to $14 a gallon depending on the source and type of oil) compared to water-based synthetic coolants ($16 to $40 a gallon). But honing oil also has some drawbacks.
Because it is an oil, it will leave an oily film on work surfaces and equipment. Oil helps prevent rust on the hone heads and engine parts, but it also attracts dust and dirt and may have to be cleaned off prior to subsequent machining or assembly operations.
With cylinder bores and connecting rods, washing is usually not a problem because the bores have to be washed to remove metal swarf and honing residue anyway after they’ve been honed. But for surface grinding operations, using an oil-based fluid might create the need for an extra and unwanted cleaning step.
Honing oil also has a characteristic odor, which most machinists have learned to accept. But some don’t like it, and given a choice they’d just as soon be rid of it. The mist created by oil can also harbor bacteria and may create some health concerns or sensitivity for those who have had long term exposure to it. For these reasons, many machinists today would rather work with water-based coolants.
The environmental benefits of water-based coolants is often cited as a major advantage over oil-based products. Disposal is less of an issue with water-based coolants because most of the coolant can be evaporated away to reduce its bulk. The coolant will still pick up heavy metals, which must be disposed of properly, but the contaminants can be concentrated into a very small volume of waste material.
According to one coolant supplier, disposal should not even be an issue with a water-based synthetic as long as the coolant mixture is properly maintained. Adding make-up water and chemical as needed, and filtering the coolant to remove solid metal and abrasive particles can prolong the life of the coolant almost indefinitely. But others say water-based coolants should still be replaced yearly to eliminate a buildup of metallic compounds that accumulate in the liquid. Dissolved metallic salts can’t be removed by physical filtration, and may pose a health risk to machinists.
Maintenance is the key to coolant longevity and performance, but it requires more diligence with water-based products. Checking the concentration of chemicals in the coolant regularly with a refractometer should be a regular part of daily routine in a high volume shop.
In a low volume shop, the coolant can be checked less often depending on how much work is being done and the type of grinding or honing operations the coolant is being used for. The coolant may only have to be checked one every couple of weeks. During hot weather, though, more frequent checks are usually necessary to compensate for higher evaporation rates.
Keeping the coolant relatively clean is also an important aspect of coolant maintenance. Some type of separation or filtration is necessary to remove contaminants. Grinding produces a tremendous amount of debris, which accumulates rapidly in the coolant unless it is removed by settling and/or filtration.
Within a very short period of time, the coolant may contain millions of microscopic particles per cubic centimeter. The contaminants rapidly turn the coolant itself into an abrasive, which can cause a variety of finishing problems including wheel loading, wheel wear and poor surface finishes.
Many of the particles are fragments of abrasive grit, strain-hardened metal fragments and metal oxides — all of which can cut through the wheel bond and gouge out individual grains of abrasive. As the wheel wears, the surface becomes uneven and tolerances are harder to control. The wheel must be dressed more often, which shortens its life even more. Abrasives in the coolant can also splash onto and enter the shaft bushings that support the grinding wheel. Over time, this can cause wear and wobble that makes the grinding wheel run out of round.
Particles that build up on the surface of a grinding wheel fill in the voids between abrasive grains. This causes the wheel to lose its cutting edge, so to speak, and rub rather than cut the work surface. Rubbing slows the rate of metal removal and increases friction and heat. The result can be a burned surface. Heat will also cause thermal distortion that upsets tolerances even more. To remove the debris the wheel must be dressed more often, which reduces wheel life.
Larger contaminants in the coolant can be especially troublesome because they can produce deep scratches and pits in the work surface resulting in a rougher then normal finish. This, in turn, may require additional grinding to remove the flaws.
A buildup of contaminants in a coolant can also have a detrimental effect on the chemicals in the coolant. Chemical reactions can cause the fluid to break down, requiring more make-up chemicals or replacement of the coolant more frequently.
Bacterial growth is another concern with both water and oil-based coolants, though most say it is less of a problem with today’s synthetics. Aeration can help suppress bacterial growth and odor, as can the use of anti-bacterial additives. But some bug-killing additives can be irritating to the skin. The greatest concern is preventing bacterial growth that can be spread throughout the shop in droplets of mist created by a grinding operation.
Choosing the "right" coolant, as we’ve seen, involves a lot more than personal preference. You have to consider the requirements of the machining process (cooling versus lubrication), the comparative benefits and costs of different coolants, then choose the one you think will work best for you and your employees.