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Effective Cooling & Lubrication - by Doctor Cool

 

 

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Effective Cooling and Lubrication

Tips on Coolant nozzles

The humble coolant nozzle can have a greater impact on the grinding process than you think.

In this feature, Dr John A. Webster examines the role coolant plays in the search for a quality surface finish and micron accuracy.

Nozzles are an established way of supplying coolant into a grinding process to control the part temperature, keep the wheel clean and open for chip flow, allow the extreme pressure additives in the coolant to do their job, and flush the chips away from the cutting area and into the filter system.

Plastic modular nozzles work well at low pressure (2 bar) but can move out of position at the higher pressures that give better grinding. They are easy to aim, but need to be placed close due to the dispersed jet nozzle geometry.

Rigid metal tubes—such as copper and steel—are stiffer, but also give dispersed jets that entrain air into the coolant unless the end is accurately swaged or a nozzle tip attached to the end. Aiming and re-aiming them requires frequent bending that ultimately leads to their fracture.

Coherent-jet nozzles are very effective

Over the last 20 years the author has demonstrated laser-like coherent-jet nozzles to be more effective than modular plastic and rigid tube nozzles, especially in grinding where most of the heat goes into the part, not into the chip. With wheel speeds of 6000 sfpm or greater, the boundary layer of air travelling with the wheel surface will prevent low-pressure coolant from wetting the wheel surface. The higher impact energy associated with coherent jets, and their ability to match the wheel speed, removes the boundary layer, cleans the wheel, and allows the wheel to transport the coolant through the grinding zone.

Figure-1-smaller-(2).jpg

This twin-jet fluting nozzle system is shown fitted to an ANCA TX7 grinder. The round nozzle targets the
corner of the wheel and the flat nozzle the OD of the wheel. The 2 outer wheels are lubricated by round
swivel nozzles on ring manifolds

 Coherent jet nozzles in the medical & aerospace industry

A medical tool manufacturer with an ANCA TX7 grinder experiences five times less wheel-corner wear, elimination of burr and burn from the gummy stainless material, no nozzle damage due to the greater distance from the wheel, easier jet aiming, and 40% increase in productivity, after fitting coherent jet nozzles.

In grinding, a well-proven flow-rate model to effectively cool the process is 1.5 to 2 GPM per spindle horsepower, i.e. a 5 HP cycle requires 7 to 10 GPM of well-aimed flow rate directed into the process. An aerospace component manufacturer producing turbine blades could only run five machines in a cell of 10 from the central system as the nozzle apertures gradually increased over the years in an effort to control grinding burn.

Each machine was fitted with eight 10 mm diameter open pipes to cool two grinding wheels on the same spindle. After stripping off the original nozzles and replacing them with five 4.5 mm diameter round coherent-jet nozzles - the dressing interval doubled, surface finish improved, grinding power reduced by 40%, flowrate reduced by 66%, the burn issue went away and all ten machines could run off the central system.

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Close-up of a TX7 coolant manifold fitted with a fluting nozzle system matched to the wheel angle


Benefits of coherent jet systems

A manufacturer using coherent jets made the following comments on its benefits. “The rigidity of the coherent jet system and the clearance away from the wheel is a key benefit for us as we do not need the coherent jets nearly as close to the contact/grinding area. “Our experience shows that the wheel life, dressing frequency and surface finish are superior when using a coherent jet system over both copper and Loclines. The ability to direct a precise jet to exactly the proper position allows us to actually use less coolant banks/nozzles as opposed to the old copper/Locline style while still increasing the wheel performance.”

Figure-4.jpg

A comparison between a coherent-jet nozzle and open tube nozzle with water at 120psi


Other requirements to encourage use of coherent jet nozzles

For machine tool operators to be encouraged to use coherent jet nozzles, it is important to integrate a high degree of adjustability into the design as well as high stiffness once aimed. This can be achieved by fitting nozzles with swivel joints that are locked after aiming. Pre-soldered round and flat nozzle tubes can be bent very close to the tip of the nozzle using special tube benders and then fixed to a manifold or pipe fitting using releasable collets.

This allows axial and rotary adjustment when aiming the jet, unlike conventional compression fittings that bite permanently onto the tube. Using a combination of flat and round nozzles, even a complex wheel profile can be fully covered by coolant using the available degrees of freedom listed above.

Figure-3-(2).jpg

Push-fit round & flat nozzles can be attached to copper tubing.
They are ideal for machines such as the MX5 which mainly use copper tubes


Additional benefits of improved coolant application are:

  • Longer-lasting, harder grade grinding wheels to be used for better form holding and without the concern of thermal damage occurring
  • Better surface integrity due to direct cooling of the process, not quenching of a hot part before the next pass
  • Reduced flow rate and pumping energy associated with more effective flow, plus less filter media indexing
  • More rigid nozzles without the risk of jets moving, allowing the machine to be unmanned
  • Less loading of the wheel (or tools) with chips produced in previous revolutions, giving more consistent spindle power and forces, resulting in better part geometries

 

The author, Dr. John Webster, has studied theoretical and practical aspects of the grinding process for 20 years at universities in Connecticut and the United Kingdom. After six years of implementing his coolant strategies at Saint-Gobain he launched Cool-Grind Technologies in 2004. Technical articles relating to coolant application can be downloaded from his website www.cool-grind.com.

 

 


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