Centerless cylindrical grinding
and centerless cylindrical grinding machines 1

Centerless grinding is a machining process for removing material from cylindrical outer surfaces using centerless grinding machines. The workpieces are not clamped between centers or in a chuck, but are guided by the grinding wheel (blue, right in the image), the regulating wheel (brown, left in the image), and the guide rail (below the workpiece).

The grinding wheel and regulating wheel can be moved radially relative to the workpiece. Ideally, this is done on independent slides, so that the center of the workpiece remains unchanged laterally. Optimally, the guide rail height is adjusted via a servo axis, as is the case, for example, with the high-performance centerless cylindrical grinding machine Ebert C-3070 .

The slowly rotating control disc sets the workpiece into a rotation corresponding to its own speed. The grinding wheel rotates significantly faster and removes material. The grinding wheel, control disc, and workpiece all rotate in sync with each other.

Basically, two variants of centerless cylindrical grinding can be distinguished:

 

Through-loops

In through-feed grinding, the regulating wheel is significantly inclined, not only rotating the workpiece but also conveying it axially through the machine. This produces a continuous cylindrical contour on a single workpiece or a batch of workpieces. For machining bar stock, a set of two bar tables is required for workpiece infeed and outfeed. These tables guide the workpiece and initiate rotation even before it enters the centerless grinding machine. Short parts can be fed using step feeders, belt feeders, vibratory feeders, etc.

Centerless cylindrical grinding – through-process

 

plunge grinding wheels

In plunge grinding, the regulating wheel sets the workpiece in rotation without any axial movement. The grinding wheel then performs the plunge motion in the direction of the workpiece, creating the negative of its contour. This allows for the production of more complex, non-cylindrical contours. The process is more flexible, but generally also slower and usually requires elaborate workpiece loading devices.

Centerless cylindrical grinding – plunge grinding

 

Formation of roundness in centerless cylindrical grinding

To achieve roundness, the workpiece rests simultaneously against the grinding wheel, the regulating wheel, and the guide rail. The grinding wheel removes material, the regulating wheel drives and guides the workpiece, and the guide rail ensures the workpiece’s position within the machine. This results in a three-point support system.

The workpiece center is usually located above the spindle center.

  • The altitude increases or decreases the roundness errors of the raw parts differently for each polygon shape.
  • The most relevant polygons are those with an odd number of vertices.
  • An elevation that improves, for example, the triangles of the raw parts may have no effect or a negative effect on pentagons.
  • The diameters of the workpieces and the grinding and regulating wheel are also relevant for the calculation.

Setting parameters:

  • The most important parameter for preventing roundness errors is the height of the guide rail. It influences:
    – Stability during grinding
    – Form error order
    – Dimensional accuracy

 

  • For the same workpiece feed rate, a smaller control wheel angle must be compensated for by a higher control wheel speed.
    –  This leads to higher workpiece speeds and corresponding problems with workpieces that are unbalanced, have unsuitable mass, or are not straight.
    –  At higher speeds, the speed ratio between the workpiece and the grinding wheel decreases.
    –  This increases the theoretical thickness of the individual chip, reducing friction and potential grinding burn  .

 

  • Grinding wheel feed:
    – A  larger feed rate leads to a more productive process, but also to higher thermal and mechanical stress on the workpiece and the grinding wheel.
    – Consequently, grinding wheel wear is greater and grinding burn can occur. The geometry and surface quality of the finished part generally deteriorate.

 

  • Factors such as imbalances in the grinding wheel or workpiece, grooves or bores in the workpiece, inadequate coolant supply or faulty guide rails lead to deterioration of the geometry and surface of the finished parts.

Centerless cylindrical grinding and centerless cylindrical grinding machines 2

Centerless cylindrical grinding and centerless cylindrical grinding machines 3

Centerless cylindrical grinding and centerless cylindrical grinding machines 4

Centerless cylindrical grinding and centerless cylindrical grinding machines 5