Advancements in grinding wheels have promoted the resurgence of the continuous dress creep feed (CDCF) grinding process. Developed in the 1970's for grinding complex profiles with high stock removals in aerospace applications, CDCF is once again being touted as the optimal process for several products including turbine parts, clipper blades and gear profiles.
The materials for blades and vanes are typically high-end nickel alloys. One reason these alloys are commonly used for aircraft turbine engines is because heat transfer through these materials is very slow. Although the contoured airfoil section of a blade encounters high-temperature combustion gasses, the blade's root must remain cool to avoid softening at the point of attachment with the rotor. Slow heat transfer prevents the excessive heat from progressing from the airfoil to the root.
An example of creep feed grinding of gear profiles.
Nickel-alloy blades are commonly produced from investment castings that are then ground to final shape via creep-feed grinding. Care must be taken when grinding the blades to minimize the amount of heat that is delivered into the workpiece. Because grinding heat won't dissipate throughout the entire nickel-alloy blade, it will remain localized at the blade's surface. This can cause the material to melt and re-solidify as a recast layer that may measure 0.001" to 0.002" thick. The recasting of the material can change the alloy's physical properties, causing the blade to become more brittle, softer or harder. Blade performance and longevity could suffer.
Continuous Dress Creep Feed (CDCF)
Given the newly developed CNC grinding machines, CDCF utilizes a constantly in-feeding rotary diamond dresser that dresses the wheel constantly during the grind cycle at a pre-programmed rate. The initial CDCF process relied on heavy dressing rates up to 40 mils/ per revolution that resulted in high wheel consumption per part. In the 1990's, CDCF faded in popularity as super abrasives and CBN began to dominate grinding applications.
CDCF has experienced a rebirth for several applications for difficult to grind materials and complex geometric shapes, such as jet engine blades and vanes, which are made out of advanced, exotic materials such as single crystal alloys or nickel alloys.
It came into favor back in the 1970s or 1980s with the advent of CNC controls that allowed multiple axis movements, and like creepfeed, you take a deep heavy cut. It's for high stock removal. You have a very high horsepower machine. The Blohm and M'gerle models of the continuous dress machines start at a 65 hp spindle and go up to 150 hp. Conventional creep feed machines start at 20 hp and go up to about 55 hp. So CDCF is a magnitude higher in power.
An example of creep feed grinding of the root form of an aerospace engine blade.
Then you have the rotary driven dresser unit situated above the wheel, with a separate CNC axis, which is actually dressing into the wheel simultaneous to the grinding process. The big advantages are: one, the wheel is always sharp because you are constantly dressing it; two, you get true free cutting action; and three, you get a good form consistency because the process is constantly dressing the wheel, so you don't need to worry about the wheel breaking down.
In essence, you have sharpness, cleanliness of form, and you don't waste time having to make a cut, then travel over to a table mounted dresser and then come back and start grinding.
One interesting thing is that with the CNC control, the wheel is actually getting smaller as you grind across, and the CNC control is compensating this ever-changing wheel diameter. This process became very popular because it replaced reciprocating grinding which was the standard for grinding hard-to-grind materials like the nickel alloys in the turbine industry, or some of the deep shapes in automotive like steering racks.
Out of Favor, Then Back Again
CDCF fell out of favor in the 1990s when things like super abrasives became popular because continuous dress consumed a lot of wheel. This meant you had a high wheel cost, you had to stop and change the wheel frequently (which resulted in down time), and you also created a large amount of swarf in the form of the sand from the dressed wheel. That sand got into the bearings and seals, so it was a destructive process on the machinery and the tooling. If you combine high wheel costs and the inconvenience of changing the wheel, you had high productive grinding coupled with several nonproductive elements. So CDCF faded away, especially with the advent of CBN or plated CBN and even ceramic or what they call blended wheels which came out at the time.
Examples of creep feed ground aerospace engine components. Note the diversity of shapes and forms.
This was through the 1990s and into the 2000s. During the last five or six years, the aerospace sector (jet engines) and land-based turbines developed more high temperature alloys, which were very difficult to grind. These alloys are so sensitive to any heat input, as far as friction from the wheel or friction from the process, that heat will actually distort the metal and create an unacceptable condition.
With the arrival of these materials - single crystal alloys, mono alloys, nickel alloys - continuous dress became popular because an ever-sharp wheel gives you a much freer, cooler cutting process, thus a much more stable process.
CDCF is also very forgiving of the variations in the stock of the material you're grinding. So, you're going to grind a part, and you anticipate that you will have a layer of 0.0040" to grind off. But because these parts are castings, and they are not an exact science in that they are not always consistent, there may be from 0.0040" to 0.0050", perhaps even 0.0060" material to grind away. Now you've got roughly 50% more material than you planned on. If you have a conventional creep feed process, the control is always going to assume that it has the same amount of material to remove, and if you have this excess material it will create a situation where you overload the wheel, or create excessive forces. However, with CNC continuous dress this can be easily compensated for just by turning the dress ratio a little higher, so you're dressing a little faster, making the wheel sharper and cleaning out the material. It's a very forgiving process both for stock variation and material sensitivity. Pratt & Whitney bought four machines in the last two years, and through all their testing they have determined that continuous dress is the only process that they can grind certain alloys on the Joint Strike fighter engine they are developing.
The continuous dress creep feed process is used to grind certain alloys on this Joint Strike fighter engine.
According to Pete Umlor, applications engineer, abrasive machining, Aerospace Division, Moeller Manufacturing (Wixom, MI), "The main advantage to CDCF is high stock removal while introducing low stress levels to the part. The process is highly accurate, and it has really reduced cycle times over any other machining process. We've seen great quality and through put improvements. If done with proper coolant nozzling, there is very little heat introduced into the part, very little thermal distortion or build up. CDCF is a great process, especially when dealing with difficult-to-grind alloys."
Thus, the resurrection of continuous dress - driven mainly by customer requirements for sensitive alloys and their ability to machine them.
Another thing to keep in mind is that the grinding wheels of 15 years ago were extremely soft, so wheel consumption was very high. Wheel vendors have developed wheels that are bonded with more aggressive abrasives, so wheel consumption in the continuous dress process has greatly diminished. General Electric in its land-based turbines used to average 20 very large turbine parts per grinding wheel. Now they're running between 80 and 100 parts per wheel, so their wheel consumption is down by a factor of four. The economics of continuous dress have gotten far more attractive. So combining improving economics with the increasingly difficulty of the alloys to be ground, and you have a resurgence in the need and demand for continuous dress creep feed grinding.
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