The engineers also decided to install superabrasive wheels on the CNC balancing/correction machine. The new machine checks the turbocharger wheel imbalance and removes the amount of material required to achieve balance in a single setup under CNC control. They used grinding wheels with Borazon CBN 500 from GE Superabrasives in Worthington, Ohio, eliminating the need for frequent, inaccurate wheel wear compensation.
CNC grinding with conventional abrasive wheels has rarely been used in turbocharger balancing because the turbocharger heads, made of Inconel superalloy with a hardness of 35 Rc wear the wheels rapidly. The result is frequent wheel dressing
It used to take too long to balance turbocharger wheels and shafts on a CNC balancing/correction machine. Engineers at American Hofmann (Lynchburg, Virginia) set out to develop a one-step, fully automated process to more accurately correct the imbalances in turbochargers.
Using the traditional approach, a turbine wheel and shaft is placed on a balancing machine to determine the location and amount of the imbalance. Material on the nose and back face of the part is removed using handgrinding. Often three to four attempts are needed to bring the part into tolerance. Rotors with high initial unbalance are diverted to a pre-balance operation using a semi-automated electrochemical process, after which they are returned to the balance area for final balancing. The operators doing pre-balancing are subject to exposure to hazardous chemicals and balancing operators are exposed to the dust from the hand grinding operation.
The engineers developed a measurement system that checks the balance of the wheel and shaft and downloads CNC parameters to a Milltronics VM16 milling machine. A rotary indexer is incorporated as a fourth axis and a powered 15 hp motorized spindle from Grosite, operating at 18,000 rpm, replaces the original spindle/motor combination in the milling machine. The present version of the machine now has a 20 hp 30,000 rpm spindle. Engineers married all of the above systems and designed special guarding and part holding tooling. A key design restriction was that the maximum wheel diameter not exceed 1.625 inches. and/or changes. GE engineers recommended using an electroplated wheel because it eliminates the need for wheel dressing and simplifies the wheel setup and change process. They suggested Borazon CBN 500 crystals, a tough monocrystaline abrasive with high wear resistance, because it provides optimum wheel life on superalloy materials. A 1.625-inch diameter, 0.375-inch wide electroplated tool was designed with a 0.375 inch radius form. GE engineers worked with the company to determine the proper machining speeds, motor size, coolant, and grinding conditions. They also helped determine flow rates, pressure and location of the coolant nozzles relative to the interaction between the grinding wheel and the part.
Two of the new machines have already been delivered to manufacturers of automotive turbocharges. They have achieved production rates on the order of 200 cuts per hour while tool life has averaged about 1,600 cuts per tool. Tools last approximately eight hours and require less than 1 minute to change (excluding nozzle removal). Balance tolerances are achieved in one or two attempts, without using the electrochemical pre-balance operation. Installing superabrasive wheels on the CNC balancing/correction machine has reduced turbocharger wheel and shaft balancing cycle time by 25 to 30 percent. Accuracy of the correction was improved. The operators now have a clean, non-hazardous working environment, and management has an improved work flow.