|
Most machine shop operators think of coolant or metalworking fluids (MWF) as a necessary nuisance, something that blows chips off the part. But the most important job of coolant is keeping the tool cool so it lasts longer, and lubricating the tool edge so that it makes a faster and cleaner cut. This article will help users understand the role of coolant in band saw and how to select the right cutting fluid including the cost of coolants.
Role of coolant in band saw
One of the most common complaints from band saw owners is the “breaking blades” problem. By using high - quality, properly mixed coolant saw owners can extend blade life substantially. Too many saws are operating with coolant designed and mixed for other machines. While a particular kind of coolant may work miracles in a turning center, it could be disastrous when used in a band saw. （图片） The role of coolant is to keep the tool and the material cool. In a lathe you have a fairly large tool removing fairly small amounts of material from the workpiece. In effect, the tool is larger than the chip. The tool and the workpiece are directly exposed to flood coolant. The chips that are being removed are not as big as the tool. The instant the chip is removed it is flushed away from the cutting action. In band saw cutting, the tool is very small. Only the very tip of the tooth does much work. The rest of the tooth is designed simply to hold the tip in place. The chip remains curled up in the blade during the cut.
The function of coolant in band saw cutting is as much lubrication as it is cooling. Note that not all cutting fluids are suitable for all materials. Cutting fluid also prevents chip welding to either the blade or the parent material by chemical and/or thermal interface. When chips weld to the blade, the tooth form is changed resulting in cut deviation or lack of penetration. If the chips weld to the parent material, the usual result is a stripped blade.
Cutting fluid lubricates the blade and, more importantly, the chips as they pass up into the gullets of the blade. It tends to cool the blade and the material being cut by absorbing heat. Heat is always generated because “work” has occurred from the cutting action, as well as from friction. Note that when wide material is being cut, the blade gets much hotter than when narrow material is cut. This happens even when both materials are cut at the same rate in square inches per minute.
Coolant is so important it cannot be over stressed. A good quality cutting fluid in a band saw is one of the most important factors in straight cutting. If cutting fluid is unable to cool the blade teeth, they will soften and become dull. If the cutting fluid is distributed to only one side of the blade, the opposite side will become dull. This will cause the blade to cut toward the side that has the most cutting fluid and the cut will be crooked.
If we compare sawing to milling, we immediately see that in sawing there is much less room for the chip. The chip must lodge in a small place between the teeth and be carried smoothly out of the cut.
Selecting a cutting fluid
Coolant is categorized into four main groups, and each group is available in many different formulations. Oil Soluble: contains 60-85% mineral oil and emulsifies into water, Semi-Synthetic: contains 5-50% mineral oil and emulsifies into water, Full Synthetic: contains no mineral oil, and Straight Oils: contains 70-85% mineral oil and is not water-miscible.（图片） Without proper cutting fluid one of two things will happen. First, the chip may become welded to the tooth. This will change the form of the tooth, which, in turn, will change the amount of force required for the blade to cut. The result is an unbalanced blade that will produce a crooked cut.
The second possibility is that the chip will wedge in the cut. Since the chip is work-hardened and harder than the stock from which it came, the blade will cut into the stock beside the chip. Again, the result is a crooked cut and a dull blade.
In selecting a cutting fluid, pick one that is of high quality. Avoid thinly mixed soluble oils. Some of the new synthetic oils are highly satisfactory in difficult operations. A general rule of thumb for cutting fluid mixture is a ratio of 5 parts water to 1 part cutting fluid. Consult with the manufacturer for specific mixture ratios.
If optimum cutting and blade life are the desired result, before selecting a cutting fluid and mixture for your saws, ask yourself the question, “Would I tap this material with this fluid?” Saw owners and operators may believe that we are over stressing the importance of coolant as a factor in good blade life. But it is a fact that we have learned from more than 30 years of band saw experience, in the field, in our own shop, and in tests that we perform regularly.
Costly cooling
Cutting fluids are meant to lubricate cutting tools and reduce cutting zone temperatures to protect both tools and workpieces. They reduce cutting edge pressure. Fluids also flush away metal chips and suppresses troublesome dust. Cutting fluids nevertheless pose workplace health hazards, and they impose a constant economic burden on machine shops.
In addition to the cost of fluids and dispensers, coolants require expensive cleanup and treatment to prevent the growth of disease bacteria and satisfy environmental regulations. By one automotive industry estimate, cutting fluids account for up to 15% of the total cost of machining a part. Despite the premium paid, the benefits of fluids are questionable. At the high speeds encountered in computer numerically controlled lathes, just 15% of the coolant directed at the workpiece evaporates on the cutting edge to dissipate heat. About 84% goes down the drain, and 1% remains on the part after machining, sometimes interfering with quality control measurements.
"By using high-quality, properly mixed coolant saw owners can extend blade life substantially."
Worse, the temperature variations created by liquid coolants promote tool wear. Cemented carbide cutting inserts ordinarily fail due to flank wear. However, wide temperature fluctuations due to irregular coolant flow and heat dissipation accentuate thermal shock and accelerate cracking and insert failure.
With interrupted cuts, inserts hot from the workpiece plunge into cold coolant again and again to crack even faster. Alternatively, continuous turning cuts create sustained high temperatures that soon cause plastic deformation in conventional carbide inserts. While dry turning eliminates the expensive nuisance of cutting fluids, it takes special inserts to survive increased heat and tool pressure.
To protect insert edges in dry turning, the extra-hard substrate is covered with a thick coating of titanium carbon nitride oriented in columnar crystals. A proprietary medium temperature chemical vapor deposition process uses temperatures lower than previous processes to enhance adhesion without embrittling the zone between substrate and coating. Together, the advanced substrate and coating create a cutting tool that withstands higher pressures and temperatures.
Dry turning creates more heat and more edge pressure. Without cutting fluids, ordinary carbide inserts show rapid edge flaking.
Smart users know that well maintained cutting fluids can last for years, not weeks or months.
9/27/2009
|
