What a machine designed to finish grind slots in turbine rotors and a CNC tool grinder reveal about the awesome potential of fine grit CBN electroplated wheels
For 35 years I worked for Norton/St Gobain, the world’s largest abrasive manufacturer. The last 13 years or so of my career there, I spent at a large jet engine manufacturer where together we helped develop a grinding process for manufacturing rotors for fighter aircraft engines.
Part of the process involved finish grinding slots in the rotors using approximately 1” diameter 170/200 grit CBN electroplated wheels. There were 72 slots per rotor and we were removing approximately .003” per slot. If that wasn’t tough enough, the wheels had to be “dressed” to maintain a 32 Ra on the part. If you know anything about plated wheels your first reaction would be; NO WAY that’s going to work! That was my reaction anyway!
I didn’t think the wheels would last for than 10 or 20 slots before they would be dull and have to be changed out. In most applications it would have been true, but there was something unique in this aerospace application that made all the difference – spindle/wheel runout and machine stiffness. Spindle and wheel runout were virtually nil and the specialty grinder made for this work had a very solid base and had the stiffness needed for the job.
The end result was that we were able to finish grind not just one rotor with 72 slots, but two!! That is a lot of metal removal for such a small wheel and it opened my eyes to the amazing potential of CBN Plated wheels. Under the right set of conditions, these wheels can perform well beyond what would normally be expected.
For many years after I left St Gobain, I just assumed that that application was unique and that it would be tough to duplicate that kind of performance in the general industrial market. That is, until recently. In the course of my consulting work with Connecticut based electroplated wheel manufacturer, Triatic Inc., we had a request from a customer who had an Anka grinder with no truing and dressing capabilities and they wanted to try CBN plated to full form grind a hardened steel punch.
Their cycle time for this part, done by single point profile grinding with a bonded CBN wheel, was 7 minutes. The kicker was that they needed an 8 Ra so we had to go to a very fine grit 30-micron CBN plated wheel. I told them not to expect much, so when they came back to me and told me they had ground 180 parts, reduced the cycle time from 7 minutes to 2 minutes, and the wheel was still going strong I was quite surprised.
They asked me to visit their facility and we reviewed the application and I immediately saw why the test was such a success. The machine they were using was an Anka CNC - a very stiff machine, and, like the Aerospace rotor engine job, the spindle ran within a few tenths. We also used a smaller wheel, 5” diameter, made to a very tight runout tolerance.
These essential components, in a nutshell, are the key to maximizing CBN plated wheels – run your job on as stiff and robust a machine as you can, minimize runout, and use a wheel made to as tight a runout tolerance as you can, (.0005” or less). Cover these bases and you’ll be amazed at the potential and performance of fine grit CBN plated wheels.