Relating Machine Accuracy and Dynamics to Low Damage Grinding

When glassy materials are grounded, stock is removed via interaction between the material of the work piece and individual grit particles. This interaction can be divided as ductile regime cutting and brittle fracture, or a combination of both.

Brittle fracture causes considerable subsurface damage, while ductile regime cutting or nearly ductile regime AKA "low damage grinding" causes minimal subsurface damage.

Low Damage Grinding

In rough grinding, large grit particles are used and material is mainly removed through brittle fracture. As users shift to smaller grit sizes and reduce the depth of cut for each grit, the cutting mechanics not only turns more ductile, but the surface also has little or no subsurface damage.

In low damage grinding, it is important to have excellent control of the depth of cut made by separate grit particles. High feed rates coupled with low damage grinding relies on the cutting mechanics shifting from brittle fracture mechanics when the grit penetrates the material to near ductile regime cutting, as and when the path of the grit nears the tangent point. This is possible only when the velocity non-uniformities, deflections, and error motions of the grinding platform are maintained extremely low.

Depth of Cut

In this article, depth of cut does not refer to the overall change in part size per grinding pass, but actually applies to the cross sectional thickness of the material removed as each grit particle sweeps via the material. Depth of cut is associated with the depth of the wheel engagement within the surface as well as the feed rate of the wheel, the RPM and surface of the wheel.

An example of depth of cut can be found in creep feed grinding. In creep feed grinding, a deep cut can be made into a surface in a single pass with minimum residual damage, provided the feed rate along the surface is extremely slow.

With a slow feed rate, the material’s cross sectional thickness removed by individual grit particle is kept low and at the same time the cutting mechanics approach the ductile regime.

When the feed rate along the surface increases, it is important to control the constancy of the feed rate and the depth of cut. When feed rates are higher, the volume of material removed at each revolution of the wheel would increase.

Non-Repeatable Error Motions

Non-repeatable error motions of the machine platform as well as vibration levels significantly impact the capacity to manage the depth of cut per grit during grinding. Such error motions raise the lower limit of subsurface damage that can arise by grinding on a specified platform. Sources of error motion and vibration modes are as follows:

  • Asynchronous error motion and static and dynamic compliance of the grinding spindle.
  • Reversal errors in slide motion.
  • Cyclical errors related to lead screws and roller or ball bearings, for example, non-uniform slide velocities and sources of non-repeatable motion from one pass to the next.
  • Reduced servo system response results in temporary motion errors when tracking a difficult tool path.
  • Dynamic and static compliance of any mechanical component, which is part of the mechanical loop joining the work piece to the grinding wheel. Here, static compliance is very critical since grinding produces variable forces across a wide frequency band. When these forces act on compliant components, they produce differences in the depth of cut and also restrict the surface quality.

Conclusion

Precitech offers high-precision machine tools that are well damped and extremely stiff. These characteristics allow for good servo response as there is no metal contact in the mechanical loop to produce non-repeatable motion errors. The spindles supplied by Precitech are characterized by low error motion, high load, and air bearing designs driven by brushless motors.

Driven by direct drive motors, rotary and linear axes float on oil hydrostatic bearings. Motion control is enabled by servo control loops that run 64 bit floating point calculations with a position feedback resolution of 0.016nm. These product features are important to determine both grinding and diamond turning of materials.

About Precitech

Precitech began operations in 1992, but continues the rich history of ultra-precision machine tool building dating back to 1962, when Pneumo Precision was founded. In October of 1997, the Pneumo ultra-precision machine tool division of Taylor Hobson (formerly Rank Taylor Hobson / Rank Pneumo) was merged with Precitech. The Precitech name was retained for this corporate entity and all offices and manufacturing facilities are now located at 44 Blackbrook Road in Keene, New Hampshire.

Our facility staffs approximately 100 talented individuals in a recently designed 60,000 Sq. Ft. building.

Precitech is a member of AMT (The Association of Manufacturing Technology) and has corporate affiliations with several professional societies and academic institutions such as Germany’s Research Community for Ultra Precision Technology at the Fraunhofer Institute, ASPE the American Society for Precision Engineering, and EUSPEN the European Society for Precision Engineering and Nanotechnology.

This information has been sourced, reviewed and adapted from materials provided by Precitech.

For more information on this source, please visit Precitech.

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