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Using High-Pressure Fluids

Cooling and chip removal are critical


By Robert B. Aronson
Senior Editor 
    
           

The type of cooling fluid you use is irrelevant if it doesn't get to the cutting area. Fluid force and nozzle placement are, therefore, critical "details" in many machining and grinding operations. "I bought the best machine tool, the finest coolant, then some clown doesn't remember to point the coolant onto the cut," laments one frustrated shop manager. It's estimated that about 40% of all coolant is wasted by poorly positioned nozzles.

"Dry" and minimal lubrication machining are growing in popularity, particularly in Europe where pollution-control laws are tighter. But in the US, the old standby of flood cooling is still the most common method.

Cooling fluids have the dual tasks of cooling the cutting surface and flushing chips. They also help control cutting-face temperatures and this can prolong tool life, improve cut quality, and positively influence part finish. For machining operations, high pressure is often recommended. It has the benefits of a powerful stream that can reach the cutting area, it provides strong chip removal, and in some cases enough pressure to deburr.

High-pressure coolant is most effective when introduced through the spindle or tool. Flow can be controlled by the machine tool's program with commands to turn fluid nozzles on and off and vary the pressure, or change direction. Pressure alone is meaningless, but pressure together with velocity produces the force needed to penetrate the vapor around the tool. Here's a look at some of the specific tools and research techniques that show the benefits of well-positioned, high-pressure coolant.

Fluid in the Cut.

KoolBlast is a line of products made by Advanced Industries Inc. (Chelsea, MI) that directs high-pressure fluid to the cutting area. Each unit fits around or near the cutting tool. For example, the company's end mill holder is a ring that mounts around the cutter. Holes around the periphery direct fluid to the cutting face. This flow keeps chips flushed away and minimizes thermal shock to cutter edges. Other designs are available for a variety of cutting operations including CNC lathes and screw machines.

Blasting Isn't Everything

Just blasting the fluid at the grinding wheel is no guarantee that the fluid enters the arc of the cut. The key is matching the speed of the fluid to the peripheral speed of the grinding wheel.

When monitoring a grinding fluid operation, the important instrument is the flow meter, not the pressure gage. The pressure gage will read even if the nozzle is completely blocked up by swarf, or a rag. The actual flow rate is the key parameter.

A well-designed nozzle is of little value if it is not properly placed. An ideally located nozzle will send a jet of fluid to impinge the wheel at the point immediately before the wheel enters the arc of cut. The wheel is always trying to "spin-dry" the grinding wheel. Once the fluid is injected into the wheel porosity it needs to spin out in the arc of cut and not into the air.


Dr. Stuart C. Salmon
President
Advance Manufacturing Science & Technology
Rossford, OH
             
   

Nozzle Placement

It's important to properly position coolant delivery nozzles to achieve the following:

  • Getting the fluid to the tool/workpiece interface.
  • Minimizing mist and odor problems.      
  • Controlling thermal shock. The tool will be alternately heated and cooled, if the fluid stream does not continually reach the tool.
  • Keeping the workpiece constantly in the fluid's flow. When the coolant nozzle is mounted on the machining head and the tool advances into the workpiece, fluid flow is often interrupted at the most critical time. For example, the coolant nozzle is directed at the point where the drill enters the workpiece. As the drill advances, the point moves until finally coolant is not even in contact with the drill.       
  • Moving the chips/swarf out of the cutting zone. This is one of the most important functions of the fluid, and may require positioning one or more fluid lines just to move chips out of the cutting zone.

How many nozzles do you need? For big chip-machining, you need to plan 1 to 1.5 gpm/hp (3.7 - 5.7 L/min/hp) used at the point of cut to prevent coolant from being the limiting factor in the process. In many machine tools, particularly older ones, the fluid is used to lubricate machine parts in the cutting zone, so sometimes it is worthwhile having nozzles just to move the chips out of the way. For grinding, you need between 1.5 and 2 gpm/hp (5.7 and 7.6L/min/hp) used at the point of cut. When grinding, it is important that the fluid be delivered directly into the nip between the wheel and the workpiece. An additional 1 to 1.5 gpm/hp (3.8 and 5.7 L/min/hp) is needed for each 10' (3 m) that it is necessary to move the chips or swarf.

The number of nozzles needed depends, to some extent, on the geometry of the setup. Be sure fluid reaches both the tool/workpiece and the chip/tool interface. One good method of checking cooling efficiency is to evaluate tool failure. Determine if changing the coolant characteristics has a positive or negative affect on the failure mechanisms.

Pressure and volume are important. Fluid volume, force, or weight should not deflect the part. This can be a problem with very thin-wall tubing or very long shafts. Fluid may be used to stabilize and/or reduce the vibration in open, thin-walled parts. For example, filling the part with fluid keeps a very thin wall from vibrating.

The velocity of the fluid is a key issue. Ideally, the fluid should be moving at 100 - 120% of the speed of the cutting tool, so that it gets to the point of cut on a timely basis, and the weight of the fluid does not apply drag on the cutting-tool spindle. This is particularly important in grinding, because the fpm is much higher: 6000 - 70,000 fpm vs. 200 to 1000 fpm (1829 - 21,336 m/min vs. 61 - 305 m/min) in other operations.

Fluid has to be accelerated to reach the cutting tool at the speed of the tool. In general, coolant weighs about 8.5 lb per gallon (1 kg/L), and to accelerate it from 0 - 6000 fpm (0 - 1829 m/min) requires a fair amount of energy that does not contribute anything to making a chip.

When drilling, the importance of velocity and volume are easy to see in terms of having sufficient fluid to move the chip from the bottom of the hole to the surface. The fluid, with the help of the drill flutes, lifts the chips. This should be done faster than new chips are being formed to prevent chip-packing and re-cutting the chips.

In grinding, it is critical that enough fluid reachs the wheel, so that the wheel "takes up" enough fluid to prevent running dry in the arc of contact and to ensure there is sufficient fluid retained in the wheel, so that when the wheel exits the arc of contact, the fluid is slung out by centrifugal force and cleans the wheel of grinding debris.


Andrew Nelson
Product Manager Applications
Master Chemical Corp.
Perrysburg, OH
             
   

A Big Blast Is Best

We see a trend toward more high-pressure coolant, that is fluid delivered to the cutting face at 1000 - 1500 psi (6.9 - 10.3 MPa). Almost all the new Japanese machine tools come with 1000-psi capability.

Chip removal is a big issue; you don't want to recut chips. That slows the process and damages the cutting tool. This is particularly a problem when cutting in a contained area such as a hole or slot.

Compressed air will work as a chip mover, but has less mass than liquid. You need more force. There is a pocket of vapor around the cutting edge. Because of the high temperature of the tool, if the fluid is not getting to the work area with enough force, the vapor barrier keeps it away from the tool. High-pressure coolant penetrates the vapor so that the pocket can't form. For most efficient penetration, the nozzles should be designed so that flow is as close to laminar as possible.

It's a simple calculation. One horsepower is 746 W. Of that energy approximately 90% becomes heat. The amount of heat can be calculated and so can the flow necessary to remove it. You can eliminate the vapor barrier with a fluid of known force and velocity. Usually it takes more force to eliminate the vapor than to flush the chips. For drilling, the rule of thumb is 10 gallons (38 L) of fluid per inch of drill diameter.

In general, if the chips don't change color, you know you are cooling the cutting area because the chips never had a chance to get hot. Ideally, through-the-tool cooling should be used. Adjustable "eyeball" nozzles just don't work because the coolant doesn't hit the cutting face. These nozzles don't follow tool or workpiece motion. The next best is a nozzle built into the toolholder that will deliver the fluid as close to the cutting face as possible.

One problem is that the relationship between force and velocity is not linear. Velocity is 14.7 times the square root of the pressure. If you double pressure, you get only 40% more force.

Deburring is a possible attribute of high-pressure fluids, but it usually takes more than 1000 psi. It depends on the metal. Rather than removing burrs, the cooling caused by the high-pressure flow keeps the burrs from forming in the first place.

When metal is being cut, there is a "primary shear" line that goes from the tip of the tool to the surface of the metal. The hotter the process, the flatter the shear, because the heat makes the metal softer. Chip thickness is therefore a function of the length of the primary shear line. But the cooler the process the steeper the shear. With high pressure, chip thickness is less. There is no plastic phase, so fewer burrs form.

Our systems are available in the 1000 - 5000 psi (6.9 - 34.5 MPa) range with most sales in the 1000 - 2000 psi (6.9 - 13.8 MPa) range.


Gregory S. Antoun
President
Chip Blaster Inc.
Meadville, PA
             
   

Castrol Industrial formulates, manufactures, sells, and services metalworking cutting and grinding fluids that are used in high-pressure high-volume machining applications. These products are specifically formulated with product performance to withstand the additional high shear and pump pressures associated to these applications. The product performance characteristics required of a cutting and grinding fluid used in high-pressure high-volume applications consist primarily of low foaming, low misting, liquid residue deposits, good swarf and chip settling capability, non-irritating to operators, and good stability in the presence of hard and soft water.

The most significant of these characteristics is that of low foaming. The functional value of high-pressure, high-volume fluid applications comes through the ability to reduce or eliminate heat in the metalworking process. Water provides the best cooling, provided it remains in a liquid state. Cooling is unachievable when the cutting and grinding fluid is allowed to entrain air and becomes foam-laden or frothy.

Most metalworking fluids contain surfactants and amines that are essentially soaps. Soft water combined with these soaps delivered to the cut zone under high pressure and shearing will tend to foam. To eliminate this tendency, Castrol formulation chemists developed products that resist foaming under these conditions.

Formulary candidate products are tested in Castrol developmental laboratories and evaluated for a regimen of performance characteristics that includes foaming tendencies. These results are recorded in product property reviews. Candidate products that meet the product properties review advance to Castrol in-house machining centers for further machining evaluations based upon the appropriate protocol. These machining centers employ high-pressure delivery systems capable of pressures up to 1500 psi. Through machining tests, foaming tendencies are evaluated at a full range of fluid pressures and volumes.


David Moon
Manager, Strategic Alliances
Castrol Industrial Americas
Downers Grove, IL
             
   

High Pressure on Machine Tools

No one likes fluid, but it's still the most common way to keep the working area cool and remove chips. There are some applications where dry or mist cooling works well, but when it comes to production, particularly when you are really hogging out the metal, you need fluid. Ecology concerns have forced manufacturers toward dry and mist cooling, particularly in Europe, but in the US flood or high-pressure cooling is still preferred. For example, with drilled holes and slots where chip removal is an issue, you really need fluid.

Conventional flood cooling is standard with most of our machines, and high-pressure systems, up to 1000 psi (6.9 MPa) are optional. About 30% of our customers take the high-pressure, through-the-spindle option. Our standard systems provide fluid at around 210 psi (1.5 MPa). We offer our machines with a Makino filtering system. It's a drum-type design that can filter out solids in the 35 - 50 µm range.

Conventional coolant systems are set up around the milling process, but today's mulitifunction machines offer milling, drilling, turning, and grinding. This means a single machine produces a variety of chip types, each with its own handling problems.

For ecological reasons, getting rid of filter residue is the biggest problem. Ideally, you would like a dry cake that you can just dump. But the mixture of cooling fluid and chips or grinding swarf is considered to be hazardous waste. In many cases this means costly special handling.

We prefer through-the-tool coolant for many applications. One reason is that when you are using several tools of different lengths, it's necessary to reposition the coolant nozzles to be sure the cooling reaches the cutting face. Operators may ignore this need. In the past, Makino has offered a "steerable nozzle" system. Changes in tool position are programmed in with the operating commands and the nozzles are automatically moved as needed. This system also requires more operator attention during setup.


Billy Grobe
Process Development Mgr.
Makino
Mason, OH
             
   

Fighting the Air Barrier

If you have a basic knowledge of fluid dynamics and some idea of the energy that you want to cool, you can develop a cooling package that works best for a given situation. The number of nozzles, their design and placement, fluid flow, and pressures must all be matched with the grinding machine's characteristics, the wheel, the process, and the desired end product.

The big problem in cooling grinding processes is getting the fluid under the air barrier that forms around the spinning wheel. If you don't deal with the air barrier the coolant won't be able to do its job. The kinetic energy of the fluid has to be greater than that of the air. A lot of fluid at low pressure won't get through the high-velocity air. Set up the flow so that it dominates the air and hugs the wheel's periphery. Matching the wheel speed with the jet speed is one way to do this.

The coolant flow has to be matched to the type of grinding. For example, with creep feeding you have a long contact arc and high spindle power, so it's best to have a porous wheel that can carry a lot of fluid into the grinding zone to keep things cool. But keep in mind you want to minimize the amount of fluid used for both financial and environmental reasons. Often a process needs less fluid than is commonly used. But it has to be ideally supplied. Too much flow rate can also increase the hydrodynamic push off from the part.

Wheel cleaning is another function to consider. If the fluid keeps the wheel clean, clogging is reduced. So it is not necessary to dress the wheel so frequently. Often the continuous dressing rate can be reduced when the fluid is applied at the right pressure, flow rate, and position. If there is a problem with wheel loading, a high-pressure cleaning nozzle might be added.

Design of the nozzle is also critical. We have developed a nozzle geometry that produces a coherent stream of fluid. Although the flow cannot be classified as laminar, it does have a low dispersion coefficient. It is not necessarily true that the more nozzles the better. Too many nozzles can deplete the available pressure, and the jet flows may interfere with each other. It's better to have one well-placed, well-designed nozzle in the direction of the grinding wheel. Additional low-pressure nozzles may be added to flush the chips and cool just the outside of the grinding area.

A coherent jet offers several advantages. It doesn't have to be on top of the wheel. It can be some distance away (16 - 18" [406 - 457 mm]) and not interfere with the fixturing. A main issue is to minimize the entrainment of air in the flow. Air reduces the fluid's heat removal capability and increases jet dispersion. It also promotes foam and mist creation.

Sometimes the user will slow the wheel speed to prevent burn. But if you have the right fluid flow, pressure, and direction, you can go faster. Higher wheel speed allows the user to increase throughput, or decrease abrasive wear, sometimes both.

Exact figures may vary, especially in academic circles, but we have found that generally the most effective flow rate is 1.5 - 2 gpm (5.7 - 7.6 L/min) per grinding horsepower.

Pressure is related to jet speed through Bernoulli's equation. To double the jet speed you need four times more pressure. At 6000 fpm (1829 m/min), you need water-based fluid at 67 psi (462 kPa). At 12,000 fpm (3658 m/min), you need four times more pressure or 270 psi (1862 kPa).

For operating pressure up to 250 psi (1.7 MPa) a multistage centrifugal pump can do the job. For higher pressures in the 250 - 1500 psi (1.7 - 10.3 MPa) range, you need a positive-displacement design such as a diaphragm, piston, gear, or screw pump. As the pump type changes so does the noise, power, and cost.

With three to six-axis grinding centers, or machining centers, the flow situation changes with each wheel, process, and cutting path. One answer is motorized nozzles that follow the grinding arc. Another is multiple nozzles that are programmed to turn on and off as the work area changes. This conserves flow.

As for fluid type, with a single-layer, plated, superabrasive wheel, mineral or synthetic oils give greatest wheel life. For conventional abrasive wheels, which can be dressed, water-based emulsion and synthetic fluids also work well. However, design of the grinding machine system should consider the type of fluid used, the size and type of pumps, and the filter system.


Dr. John Webster
Research Associate
Higgins Grinding Technology Center
Saint-Gobain Abrasives
Worcester, MA
             
    

 

This article was first published in the June 2004 edition of Manufacturing Engineering magazine. 
   


Published Date : 6/1/2004

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