How Meister’s vit diamond bonded wheel technology contributes to optimal HVOF grinding consistency and productivity
The conventional nickel- and chrome-plating processes used to harden or repair aerospace components produce emissions that are harmful to the environment. So nickel and chrome are rapidly giving way to HVOF (High Velocity Oxy-Fuel) coatings. The application process is also known as “flame coating.”
This high-temperature thermal spray technique is capable of depositing corrosion and wear resistant materials like Tungsten Carbide or Chromium Carbide in a layer that is dense, strong and brutally hard. HVOF coatings can be up to 90% harder than the ones they replace, so they provide superior protection to critical wear components. That’s the good news. The bad news is that HVOF coatings require aggressive machining to bring their rough irregular surfaces to final geometry and finish tolerances.
HVOF Grinding Prerequisites
To grind HVOF components productively and at optimal cost three things are needed:
- An ultra-stiff, high-end CNC grinding system with automated rotary dressing
- A carefully selected diamond abrasive grinding wheel
- An optimized grinding process
An example of a company that has obtained these HVOF grinding prerequisites is Complete Grinding Solutions of Springboro, OH. As reported in the Winter 2009 Issue of Quality Times, CGS has developed numerous prototypes and transferable grinding processes for HVOF-coated aerospace parts. The company relies on a “fully loaded” Studer S40 grinder as well as a Studer S33.
One Vit Diamond Wheel
While the specifics of the grinding processes CGS develops for its customers are closely guarded, we are at liberty to divulge this: a single Meister vitrified diamond abrasive specification has been used in all of the companyʼs HVOF grinding projects to date- regardless of the HVOF formulation, size of the part or the size of the wheel.
These grinding processes are single set-up and totally hands-off. The Meister vit diamond wheel is automatically dressed during various stages of the process. By using different dressing parameters, the same wheel can be used for both roughing and finishing. Automatic dressing at specific intervals also results in exceptionally consistent surface finish and geometry from part to part.
CGS’ co-founder Beat Maurer said, “Establishing the right dressing cycles is an important part of what we do to develop productive and cost- effective HVOF grinding processes for our customers. The ability to standardize on a single Meister vit diamond abrasive spec for our wheels has allowed us to become very proficient at using the dress cycles to alter the cutting characteristics of the wheel as needed.”
Dressing Secrets
Beat currently uses a standard Meister metal bond rotary dresser to condition the wheel, although Meister is also developing its hDD (hybrid diamond dressing) technology to address the requirements of this demanding application. Among the variables Beat manipulates to impart appropriate stage-specific wheel conditions are:
- wheel speed
- speed ratio of dresser to wheel
- direction of rotation of dresser and wheel at point of contact (uni-, or counter-directional dressing)
- speed ratio of dresser to wheel
- dresser speed
- traverse speed
Because the vit diamond wheels can be rotary dressed on the machine (as opposed to resin bonded diamond wheels which cannot), the need to have separate machines or wheels for roughing and finishing is avoided. Vit bond diamond wheels have a porous structure with bond “posts” that shatter and break down to expose new layers of diamond during the dressing process. This makes it possible to control the cutting behavior of the wheel with the dressing process and therefore achieve the desired results on the part.
For example Beat typically uses a uni-directional dressing cycle to “open up” the wheel, exposing more diamond for aggressive cutting during the roughing stage. Later in the process he switches to `counter- directional dressing to smooth it out for finish grinding.
The bottom Line: The Meister vit diamond wheel chosen for this application has allowed CGS to develop complete unattended HVOF grinding processes that result in optimal productivity along with high levels of geometry and surface finish consistency.
