Centerless Grinding Solutions Bring all Processes under One Roof
A supplier of high performance engine components used by major racing teams, including endurance racing, NASCAR and NHRA, SCCA, Le Mans and marine racing, needed to reduce lead times and the cost of grinding its components.
The company turned to Glebar Co. since it needed a partner that could not only grind these parts, but also propose equipment that could take the mystery out of centerless grinding and allow the company to grind these parts internally. It also sought a vendor partner that could train an operator to set up and run these parts in small lots with complete flexibility.
Tool steel rocker shafts, valve lifters, lightweight rollers and needle bearings are among the key valve-train components that must be ground to custom sizes. As an engineering and CNC shop equipped with the latest high precision machine tools, the company was highly capable technically to machine these challenging components. However, it chose to outsource all grinding processes, as centerless grinding is highly specialized, requiring a large machine investment and a skill set not commonly found in the machinist workforce.
Valve-train components are expected to perform under punishing conditions, operating endless hours at higher than 10,000 rpm in turbocharged or nitrous engine applications. The rollers, followers and lifter bodies, mostly less than 1 inch, must be delivered in a short time to racing teams.
The first element to tackle was through-feed grinding a valve lifter. By design, this is a lightweight, thin-walled, unbalanced cylindrical part requiring finesse to setup, as compression on one end of the part produces a different grinding pressure than the solid end. It’s a challenge to hold 0.0002-inch tolerance on the OD of the valve lifter. Glebar’s applications group solved the problem by selecting the appropriate tooling and abrasive grinding wheels, reducing the pressure of the grind while ensuring a consistent material removal with minimal wheel loading.
With experience of grinding materials as soft as foam and as hard as polycrystalline diamond, Glebar knew that grinding wheel selection is paramount in producing an efficient process where uptime is the first priority. Part of the challenge involves balancing abrasive grit size with wheel hardness and composition and matching these with the material to be ground.
Physical forces affecting setup longevity and part performance are another part of the challenge in selecting a grinding wheel. These wheels load up, meaning they do not cut efficiently. The wheels break down on hard materials, requiring frequent truing. In addition, they heat up when processing high performance steel parts, changing the dimensions of the component being ground. Maintenance such as grinding wheel change-over, wheel dressing and size compensation are disruptive to production and compound costs when lot sizes are small and diameters processed vary. Glebar’s objective was to reduce maintenance time and increase value-added production time.
The small footprint centerless grinding machine Glebar used for this application, GT-610, uses a 10-inch diameter by 8 5/8-inch wide grinding wheel allowing the grind to spread over a larger abrasive surface compared with other machines. Additionally, the dual-slide design with independent control of work rest spacing to the grinding wheel and regulating wheel spacing to the work rest makes setup flexible. A touchscreen control simplifies diameter adjustments. Hard plated dovetail slides provide the necessary rigidity of the system, which in turn, provides slide position repeatability. Accompanied by 0.1-micron linear glass scales on the slides, the motion control system constantly maintains that gap. The operator is able to maintain position and make finite 0.0001-inch incremental position changes. For diameter compensation, a built-in handheld micrometer gage is the simplest interface for this application. Since the machine has the ability to interface to a multitude of laser gages, air gages and vision systems, the simple micrometer is intuitive and seamless for diameter adjustments.
Rocker pins and shafts are processed on the same centerless grinder. Since change-over is rapid and setup is simplified, lead times were reduced. Gage feedback to the machine from a hand-held micrometer ensures that diameters are maintained. The tooling selection guarantees surface finish and straightness, and the simplicity and reliability of the machine allows for prompt component delivery.
Rollers are another problem to tackle. They have a tight concentric specification, and any runout will degrade the lifter performance. OD grinding on a CNC is the conventional method of grinding rollers, completed one at a time and supported on centers. Approached with this challenge, Glebar reviewed common features across its product line and proposed a plunge grind operation for a group of rollers on the modified PG-9DOD Glebar grinder.
Using an OD fixture, a set of rollers is loaded by the operator and placed in the machine. Infeed grinding the parts on a high precision slide assembly controlled from a touchscreen user interface simplified the process for the operator. Roundness better than 10 millionths are achieved with little effort. An oscillating fixture ensures a smooth surface finish, and a template tracing wheel dresser adds an additional level of simplicity to the setup. Now, rollers can be ground to a high degree of accuracy and provide smooth surface finish on a small machine with no programming.
With the rapid change-over advantages of the through-feed setup and similar control, the supplier company for engine components is now able to bring all centerless grinding into its facility using only one operator. It was also able to reduce lead time and control costs and quality by processing all components under one roof.
John Bannayan, president, Glebar Co., is an accomplished senior electrical engineer. A member of SME and AMT, Mr. Bannayan has earned six patents relating to machine control.