Doubling Accuracy Of Manual Grinding Machines With CNC Wheel Dressers


Case Study From: 6/1/1997 Modern Machine Shop

Retrofitting older grinding machines with a CNC wheel dresser makes it possible to reduce the total tolerance band on broaching tools produced with the new dressers from 0.0004- to 0.0002-inch. These broaching tools are used to machine slots in jet engine disks that hold turbine and compressor blades in place. The pantograph-type single point dressing systems used in the past were dependent on the skill of the operator to follow the template in shaping the wheel. The CNC dresser generates precisely the same form every time on the wheel, regardless of the operator's skill or attention level.

Southeast Broach, using a template dresser method, produces broaching tooling used by aircraft jet engine and land turbine engine builders to produce disks that hold the engine's compression and turbine blades. An aircraft engine typically contains 8 to 14 of these disks made of high nickel alloy such as Waspalloy, IN100 or stainless steel or titanium. The slots produced by the company's tooling contain a series of two to eight lobes that match a mirror image form on the shaft of the blade that loosely holds it in place. The blades fit into the slots rather loosely at room temperature but have a higher thermal expansion coefficient than the disks so they expand to an interference fit as the engine reaches operating temperature.

The problem with template dressers was that the accuracy of the finished tool depended on the accuracy of the template, the condition of the single point diamond, the condition of the stylus used to follow the template and the diligence of the operator.

If all of these variables were carefully controlled, then the template dressing method was capable of producing tools to virtually unlimited levels of accuracy. In practice, operators have differing levels of skill, they get tired and, sometimes, they make mistakes. By carefully controlling these variables to the best of their ability, management was able to maintain tolerances within 75 percent of the allowable limits. However, customers began requesting that Southeast Broach find a way to hold even closer tolerances with even more consistency.

Management first considered replacing their manual grinding machines with new CNC grinders. The problem was that new CNC systems cost about $500,000 each. Management then investigated upgrading the manual grinders. The company decided to rebuild the manual grinders and purchase a Normac CNC wheel dresser. The cost to rebuild the grinders, add the CNC dresser including installation and accessories was said to be far less than a new system. Additionally, the CNC dresser control supported "G" code programming and featured an RS-232 interface.

The key advantage of the CNC dresser is that, at the touch of a button, it produces a precision profile on the grinding wheel that is defined by the program without concern about any of the variables that affect template dressing. As a result, Southeast Broach can produce broaching tooling to much closer tolerances than were previously possible. Where necessary, the company's tools can now hold a ±0.0001-inch tolerance band. Precision such as this helps their customers produce more accurate slots and also improves tool life.

The CNC dresser has also improved overall productivity of the manual grinding machines by about ten percent. This figure is impressive when it is considered that the machine spends the majority of its time in grinding operations upon which the CNC dresser has no impact. The productivity improvement arises from the fact that the template dresser, including setup, took 20 to 30 minutes to initially dress a square wheel while the CNC dresser, including setup, takes only ten to 15 minutes. Maintaining the wheel took three minutes for the template dresser, compared to less than one minute for the CNC dresser. In addition, the CNC dresser uses a rotary diamond wheel used by the template dresser. This saves additional time because it eliminates the need for frequent setups required as a single point diamond wears.

Southeast Broach was so pleased with the performance of the first CNC dresser that they soon bought a second. Last year, they installed a third CNC dresser on an Okamoto NC surface grinder. The advantage of using a CNC dresser on an NC grinder is that the dresser can be programmed to operate as part of the machine's cycle, allowing the machine to run nearly automatically. This makes it possible for an operator running a manual grinder to operate the NC grinder with the CNC dresser at the same time. All in all, the new dresser is said to have dramatically improved Southeast Broach's quality while boosting their productivity at the same time. MMS

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