Opting for straight oil coolant in grinding operations

Figure 1: G-Ratio

Many different types of grinding coolants, including water-soluble oils, synthetic, and semi-synthetic, are available from a broad range of manufacturers. These options are used 80 to 90 per cent of the time.

Straight oil often is not considered at all because of the potential for fire and the higher initial cost. However, in some processes, the benefit potential is so great that oil is well worth the risk as well as the cost when it is introduced properly.

Without question, straight oil can increase productivity and wheel life in many grinding operations.

For example, when a Norton Targa ceramic-grain wheel for creep-feed grinding INCONEL® alloy 718 was tested, a six to 11 times effective G-ratio (volume of metal removed in.3/volume of wheel dressed in.3) increase resulted when using straight oil over water-soluble oil (Figure 1). Also, if superabrasive single layer-plated products are being used, the abrasive will prematurely strip without the lubricity of oil, which makes straight oil coolant highly advisable in these cases.

The risk of fire is the most important consideration when using straight oil. When straight oil coolant is in a tank or even in a puddle, sparks and heat from a grinding operation are unlikely to ignite it. When oil is in a mist, sparks and heat can ignite the fuel/air mixture more easily.

PRECAUTIONS

The following precautions can be taken to gain the benefits of straight oil coolant with minimal risk:

Enclosing the Guarding. This may seem obvious to some, but fully enclosed guarding is necessary to contain the mist and splash of the oil, as is the use of a fire-suppression system (Figure 2).

Directing the Initial Fire. If an oil fire starts, the air is heated and expands rapidly pushing out of the machine guarding. To prevent this superheated air from endangering personnel in the shop, blast gates should be installed, usually at the top of the machine, to allow the expansion to escape safely (Figure 3). These gates may not always be necessary, so it is advisable to talk to the fire-suppression system installers and the machine manufacturer to determine the need for them.

Extinguishing the Fire. A fire-suppression system is an absolute must. Though it is not the first line of defence, it will save very expensive precision grinding equipment from serious damage, not to mention the danger to personnel and potential damage to the building.

Several different types of fire-suppression systems are available, such as dry chemical, foam, and CO2 systems. These are highly recommended because they do not contaminate and the gas reaches every nook and cranny of the machine, including any wire ways (Figure 4). A fire-suppression designer should be consulted for each specific case.

Figure 2: Enclosed Guarding

Avoiding the Fire Altogether and Reaping the Benefits. The key to enjoying the clear benefits of using straight oil in grinding operations is avoiding the fire altogether. Some of the steps outlined below are not only for fire avoidance, but also to avoid heat damage to parts, while increasing metal removal rates and wheel life. In fact, most of these steps can benefit any grinding operation, even when using water-based coolants.

1. Flow Sensor: If the oil coolant flow decreases, the risk of a fire and potential for part damage actually increases. A flow sensor is much more effective than a pressure sensor because if a blockage occurs, the flow will decrease but the pressure may not.

A flow sensor should be installed in an area receiving the correct flow. It should be installed in a straight pipe at a distance equal to 15 to 30 times the diameter of the pipe after any elbows or tees. Elbows and tees cause turbulence in the flow, which will affect the measurement. The flow sensor should be tied into the control and cause a cycle interrupt condition when the flow drops below a reasonable preset value equalling about 15 to 20 per cent.

2. Pressure Gauge: Install a good pressure gauge as close as possible to the nozzle. Pressure drops often occur between the pump and the nozzle.

3. Control Over the Program: This is a basic skill that cannot be ignored in any operation regardless of what other type of coolant is being used.

Before any grinding starts, prove out the program to be sure there are no physical interferences with parts, fixtures, coolant nozzles, and so forth.

Check that the desired feed rates, wheel/work speeds, and coolant pressure/flow rates are correctly programmed.

Ensure that the program has safeguards that will trigger a cycle interrupt, and be aware of what corrective action takes place when it is triggered. The cycle interrupt program should be triggered when a wheel interference is detected (power spike during rapid moves). It should also be triggered when the coolant flow drops and of course when the cycle interrupt button is pushed by the operator.

4. Coherent Coolant Jet: Check if the coolant nozzle can cover the total area of contact. Often a simple nozzle is sufficient, but in some cases a customdesigned nozzle is necessary. Round nozzles are generally more efficient than rectangular ones, but rectangular nozzles can work quite well if designed properly. A coherent jet is one without air entrapped in the coolant (Figure 5). This is achieved by:

• A coolant nozzle with a smooth and balanced internal geometry.
• Avoiding elbows too close to the nozzle.
• Sharp, clean edges on the nozzle exit.

5. Targeting the Jet: Target the main jet just before the grind zone. A properly targeted jet should create a "rooster tail" on the opposite side of the wheel (Figure 6). Check that the coolant is entering the grind zone and not being redirected by fixtures or the part itself.

Figure 3: Blast Gate Inside Top of Guarding

6. Coolant Ramp: With creep-feed grinding, use a coolant ramp to guide the coolant into the grind zone at the exit of the grind. Burn during creep-feed grind operations often occurs at the exit because the coolant is being deflected by the part or fixture.

7. Match Coolant Speed With Wheel Speed: An air barrier is formed when the rotating grinding wheel moves the air around it at the same speed. This air barrier can be amplified by the roughness of the wheel. Coolant flowing at a lower speed is pushed away by this barrier. Matching coolant speed to wheel speed allows the coolant to penetrate the air barrier and deliver the coolant into the grind zone. Calculate the wheel speed in surface feet per minute (SFPM), and calculate the coolant pressure needed to match the wheel speed in SFPM.

Wheel speed SFPM = RPM x (Pi12) x Wheel OD

PSI = [Specific Gravity of Coolant x (SFPM2)] / 535,824 (Note: For the specific gravity of oil use 0.93 for ester oil [synthetic] and 0.87 for mineral oil [natural]. For water, 1.0 is recommended.)

8. Coolant Volume: Volume is just as important as pressure in any type of coolant application. It takes flow to remove heat from any grinding operation. The more power that is being consumed, the more heat is being generated. Find out how much power the spindle is drawing during the grind by using a power meter on the spindle drive. Use 1.5 to 2.0 gallons per minute (GPM) for each horsepower of grinding power. If the grinding power is unknown, use 25 GPM per inch width of contact.

9. Nozzle Exit Aperture Size: Once the desired pressure and flow are known, the correct nozzle aperture size can be calculated. No coolant nozzle or system is perfect; therefore, a discharge coefficient (Cd) is used. The discharge coefficient adjusts the size of the aperture so that the volume at the correct speed and coherence is closer to theoretical. Use 0.7 to 0.9 for Cd. A highly efficient nozzle like the Rouse design would use 0.9.

Nozzle area (in.2) = [(19.25 × Flow GPM)/Jet velocity SFPM]/Cd

10. Coolant Tank Volume: Any coolant needs five to 10 minutes of settling time to allow trapped air to escape. Oil needs more time than water because of its higher viscosity, so 10 minutes should be targeted. If the whole coolant tank is turning over in less than five minutes, air will most definitely be trapped in the coolant. Aerated coolant cannot be pumped efficiently and causes pump cavitation. Furthermore, air will adversely affect the coherency of the jet, not to mention the fact that air does not make a good coolant (Figure 7).

11. Scrubber Nozzle: If grinding chips are loading the wheel or chips/sparks follow the wheel periphery, scrubber nozzles can be installed. The feed pressure should be 600 to 1,000 PSI and 1 to 2 GPM per inch width of contact. Scrubber nozzles are equally or more important with water-based coolants, especially when grinding stainless steels, aerospace alloys, or other difficult-to-machine materials.

12. Coolant Pressures: Don’t unnecessarily increase the pressure assuming that if a little is good, a lot is better. Pressure should be increased no more than 10 to 15 per cent than calculated. The higher the pressure, the more mist will be created, which will increase the potential for fire.

Figure 4: Fire Suppression System

13. Mist Collector: Always use a mist collector when using oil. Again, oil mist only acts as fuel for a potential fire. Use a recycling mist collector if at all possible so that the collector can run at all times without losing precious oil. Don’t be tempted to run the mist collector only at the end of the cycle. Also, have an automatic gate installed to close the mist collector duct when a fire is detected to prevent the fire from spreading to the mist collector (Figures 8 and 9).

14. Spark Suppression: Sparks are unwelcome when grinding with oil, but when sparks are created, it is advisable to use an extinguishing nozzle where the sparks exit the grind zone. This should be at a lower pressure than the main nozzle so it doesn’t interrupt the flow through the grind zone, while still having enough flow to immediately cool the sparks.

Oil coolant can have an enormous impact on wheel life, surface finish, and cycle time, which makes the initial cost and care over the process and fire-suppression efforts worth the investment. In addition, the service life of oil is much longer than that of water-based coolants (greater than five to 10 years in many cases), making the initial investment pay for itself in a reasonable amount of time.

Straight oil coolant is not the answer for every grinding operation, but in many cases, it is the best answer for a controlled process that involves hard-to-machine materials or when high metal removal rates are needed.

Mark J. Martin is an application engineer, Advanced Application Engineering Group, Norton|Saint-Gobain Abrasives, One New Bond St., Worcester, Mass. 01616, 254-918-2313, www.nortonabrasives.com.

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