Everybody wants a “silver bullet” to solve problems. For many years, machinists have considered high-pressure coolant to be the silver bullet to solve drilling’s problems. Recent improvements in tooling and equipment have made high-pressure coolant even more practical. As a result, an increasing number of shops are turning to this solution to improve their drilling operations. It is estimated that 40% of all new CNC machines will be equipped with high-pressure coolant by the year 2003.
The need for high-pressure, high-volume coolant in drilling became apparent when gundrills were introduced more than 100 years ago. When using these drills, machinists had to keep the coolant pressure somewhere near 1000 psi or the tool would friction-weld to the workpiece. Standard low-pressure coolant systems couldn’t prevent friction-welding, because the process generated so much heat that the coolant boiled away before it reached the chip/tool interface. This boiling produced super-heated steam that formed a barrier that prevented additional coolant from reaching the cutting edge, so little lubrication was provided. The vapor barrier also allowed chips to fall back into the chip/tool interface, causing costly damage.
When machinists tried high-pressure coolant on standard drilling operations, they found that the benefits of increasing coolant pressure improved the performance of these operations as well. Properly applied high-pressure, high-volume coolant prevents the formation of a vapor barrier by causing a localized pressure increase. This forces liquid into the cutting zone, removing heat, providing lubrication, and flushing chips away from the cut. Damage from heat and chips is eliminated, and tools can cut until they wear out. High-pressure coolant discourages chip welding, prevents the damaging chemical reactions that may occur at high temperatures, and allows drills to last longer (Figure 1).
With the problems caused by heat and stray chips minimized, machinists can drill faster. Surface speed can be increased by a minimum of 30%. In some operations, the use of high-pressure coolant can result in a 300% improvement. For example, a 0.500" carbide twist drill penetrating 20 ipm with low-pressure coolant can easily penetrate 60 ipm with high-pressure coolant. Lubricity is increased by blasting lubricating fluid between the chip and the cutting edge at hundreds of miles per hour. With more lubrication and lower temperatures at the cutting edge, surface finishes are often significantly improved.
By supplying a constant supply of coolant to the cutting edge, high-pressure coolant also eliminates damaging thermal shock to the tool. With conventional coolant, the temperature of the cutting edge increases as it enters the cut and stays hot until it finishes the cut. When the tool is withdrawn from the cut and the coolant once again can reach the cutting edge, the cutting tool is exposed to an extreme thermal shock. This rapid cycling between high temperatures and quenching can damage a tool more than heat or wear, but the severity of the problem depends on the operation. A turning tool typically will not fail due to thermal shock, because it is subjected to this quenching only three or four times per minute when it is withdrawn from the cut at the end of each pass. A facemilling operation running at 1000 rpm, on the other hand, subjects every insert to 1000 damaging quenches per minute. Drilling falls somewhere in between, with thermal shock occurring every time the drill pulls out of the cut.
When using a standard low-pressure coolant system, shops have had to make a difficult choice between subjecting their tools to thermal cycling or the damage that high temperatures cause. Some manufacturers recommend running tools dry on operations that may subject the tool to high rates of thermal cycling, believing that the continued heat and chip damage will take less of a toll on the tool than thermal-shock damage. Current state-of-the-art high-pressure coolant systems make this tradeoff unnecessary.
Pressure Where Needed
Appropriate speeds and feeds are also necessary. The application of high-pressure coolant will be unsuccessful if it is used to increase the feed per revolution rather than the speed. Increasing the feed increases the chip size and a chip that is too big can’t be forced through the hole, no matter what the pressure. The gullet of the drill is the limiting factor.
Even with through-coolant tools and the proper speed and feed, an increase in coolant pressure will not automatically lead to an improved drilling operation. Coolant volume is an important consideration as well. The coolants used in drilling are considered “contained coolants,” because the machining takes place in a confined area. In these situations, it’s important to pump enough coolant through the tool to completely fill and pressurize the hole. This eliminates the formation of vapor, which leads to high cutting temperatures. The pressure required to prevent vapor is generally in the 1000 psi range, but higher pressures may be needed for particularly difficult applications. The key, however, isn’t the pressure of the coolant going into the tool, but the back pressure generated in the hole that forces the coolant and chips up the drill gullets and out of the hole.
Three factors determine the amount of back pressure generated: the coolant-hole size in the drill, the open area of the drill flutes, and the pressure of the coolant entering the back end of the drill. To see how these factors interrelate, consider two 0.375" drills: an HSS drill and a carbide drill. Because of carbide’s brittleness, carbide drills are manufactured with smaller coolant holes than HSS drills. The extra material adds strength to the carbide drill’s structure. Because the HSS drill has larger holes, more coolant will pass through it. The result will be more flow, more chip removal, and more back pressure in the hole. The difference in back pressure can be large enough to cause a coated HSS drill to work better than a poorly designed solid-carbide drill with smaller coolant holes.
Table 1 compares the coolant flow possible with two different coolant-hole diameters at various coolant pressures. According to this table, a drill with 0.030" holes will only pass 1.43 gpm at 1000 psi. But when the coolant hole size is increased to 0.060", the same size drill can pass 5.94 gpm. The large increase is due to the fact that doubling the coolant hole’s diameter quadruples its area. Thus, a coolant hole twice as wide as another can pass four times as much coolant.
Many tool users and manufacturers fail to grasp the importance of coolant flow. Believing that high pressure is all that is needed, some manufacturers recommend welding up the large coolant holes of an indexable drill and redrilling them with smaller holes when the tool doesn’t generate enough back pressure. This modification makes it more difficult for the coolant to get through the holes, causing the pressure gage to read the desired pressure, but it won’t solve the problem. A 0.001"-dia. hole will permit almost no coolant to get through. Therefore, there will be virtually no high-pressure effect. Both pressure and volume are needed to generate the back pressure that will eject chips.
There is a very easy rule of thumb to determine how much volume will be needed for a given drill. Every inch of drill diameter requires a 10 gpm flow of coolant. For example, a 0.500"-dia. drill will need 5.0 gpm of coolant; a 0.750" drill will need 7.5 gpm.
Using this formula, a machinist can see that a high-pressure coolant system that produces less than 3 gpm might be adequate for a 5/16"-dia. drill, but will fail completely if used with a 2"-dia. drill, which needs 20 gpm of coolant to perform well. In fact, a low-pressure system with a significantly higher volume capacity can outperform a high-pressure system that does not have the capacity required by the drill.
Say a high-pressure system with a 5 gpm capacity is used with a 2" drill. The optimal flow for the drill is 20 gpm, but because of the size of its coolant holes, perhaps a maximum of only 8 gpm can flow through. Because the high-pressure system’s positive-displacement pump can generate no more than 5 gpm, it will not even be able to match the capacity of the drill. A low-pressure system with a 15 gpm capacity will generate greater coolant volume. Although the tool’s limitations will permit a flow of only 8 gpm, this is still better than the 5 gpm generated by the high-pressure system. As a result, the back pressure will be greater and drill performance will be improved.
|Back pressure is needed to flush the chips from the hole as the drill penetrates the workpiece. Chips cause unpredictable damage. In general, the longer the chips, the harder they are to control and the more damage they cause. Long, stringy chips wrap around drills, fill the bottoms of holes, catch on chucks, cause mechanical problems with loaders, and in many cases require manual removal. Broken chips that can fall away—or that can be evacuated from the cutting zone and away from the part and drill with coolant force—are almost always more desirable (Figure 3).|
|Figure 3: The consistent low temperatures associated with high-pressure coolant improve chip formation. These more consistent chips are easily flushed away from the cutting edge and out of the hole.|
When chips are no longer a factor, wear, rather than damage, becomes the drill’s failure mode. Wear is a predictable part of any mechanical process; damage, on the other hand, is random. Without high-pressure coolant, almost all drills fail because of damage caused by chips. Many people who have had 20 years of experience cutting steel have never seen a worn drill. Recut chips break the drill’s cutting edges; uncontrolled heat causes built-up edge and then shock as the buildup breaks off.
Why spend half a million dollars on a horizontal machining center that still needs manual intervention? Computerized systems can automatically monitor coolant level and concentration and refill the reservoir with the appropriate amounts of coolant concentrate and water, keeping the coolant within 0.5% of a pre-programmed target level. The coolant is continuously recycled so no operator intervention is required, except for a maintenance routine every three months. Computer control of the coolant concentration reduces process variability by more than 95% and increases machine uptime.
A shop with a coolant system that provides consistent temperature control in the cutting zone can afford to use safer coolants. Most coolants are designed to leave a greasy residue when the high heat of standard machining boils the water away. Manufacturers use potentially carcinogenic additives such as chlorinated paraffins, halogens, and solvents to perform this function. When the temperature range of the process is well-controlled, none of these harmful chemicals are needed. The coolant’s chemistry can be optimized to function more efficiently, using process-control fluids that are friendlier to the operator and to the environment.
Keeping Coolant Clean
If better surface finish or longer tool life is required, clean coolant is a must. Pumping abrasive fines through a drill to the chip/tool interface damages the tool and the part. Evidence of coolant contamination frequently can be seen when small through-coolant drills are used. Many operators find it difficult to keep chips and swarf from blocking the coolant flow. Before these holes get plugged, however, it’s certain that a great deal of contamination has already passed through the tool. Damage caused by the chips that do get through is part of the reason these tools have such an unpredictable life span. The solution isn’t bigger coolant holes to avoid plugging; the solution is less junk in the coolant. It can take a chip as large as 0.050" to plug up a small drill’s coolant hole, and yet this is a common occurrence. This is much too large a chip for proper performance using today’s tooling. A chip of this size, for example, is 20 times larger than the maximum size that should come in contact with HSK toolholders. HSK manufacturers recommend coolant filtration down to 5µm for proper performance.
Less Interruption, Higher Speed
The second example assumes that the shop uses the benefits of high-pressure coolant to increase penetration rates. Because of this, no increase in tool life is assumed. As in the first example, three drills with diameters of 13.0mm, 12.0mm, and 9.5mm are used to drill 1" holes in 3000 parts. Tooling cost and downtime are the critical factors in cost savings in this example.
Shops only win in the long run with a controlled process. High-pressure coolant is the most effective tool currently available to control the drilling process. However, to achieve its full benefits, it has to be used in conjunction with tools, toolholders, coatings, and carbide grades that have been optimized for its use. The rapid growth in the use of this technology will make it a necessary part of a shop’s ability to maintain prices and profits.
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Related Glossary Terms
- Rockwell hardness number ( HR)
Rockwell hardness number ( HR)
Number derived from the net increase in the depth of impression as the load on the indenter is increased from a fixed minor load to a major load and then returned to the minor load. The Rockwell hardness number is always quoted with a scale symbol representing the indenter, load and dial used. Rockwell A scale is used in connection with carbide cutting tools. Rockwell B and C scales are used in connection with workpiece materials.
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- built-up edge ( BUE)
built-up edge ( BUE)
1. Permanently damaging a metal by heating to cause either incipient melting or intergranular oxidation. 2. In grinding, getting the workpiece hot enough to cause discoloration or to change the microstructure by tempering or hardening.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
Form of milling that produces a flat surface generally at right angles to the rotating axis of a cutter having teeth or inserts both on its periphery and on its end face.
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
Grooves and spaces in the body of a tool that permit chip removal from, and cutting-fluid application to, the point of cut.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- high-speed steels ( HSS)
high-speed steels ( HSS)
Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.
- inches per minute ( ipm)
inches per minute ( ipm)
Value that refers to how far the workpiece or cutter advances linearly in 1 minute, defined as: ipm = ipt 5 number of effective teeth 5 rpm. Also known as the table feed or machine feed.
Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.
- machining center
CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
Rapid cooling of the workpiece with an air, gas, liquid or solid medium. When applicable, more specific terms should be used to identify the quenching medium, the process and the cooling rate.
Metal fines and grinding wheel particles generated during grinding.
Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
- twist drill
Most common type of drill, having two or more cutting edges, and having helical grooves adjacent thereto for the passage of chips and for admitting coolant to the cutting edges. Twist drills are used either for originating holes or for enlarging existing holes. Standard twist drills come in fractional sizes from 1¼16" to 11¼2", wire-gage sizes from 1 to 80, letter sizes A to Z and metric sizes.