At low workspeeds, workpiece roundness levels were similar for the two machines and, in fact, higher roundness errors were measured with the stiff machine. The high roundness errors at low workspeeds are partly because there are insufficient revolutions of the workpiece to reduce the roundness errors. Higher roundness errors can result from a stiff machine in some tests due to the rounding process. This aspect is discussed below and also in Section 19.11 and Section 19.12.6 in relation to static compliance.
Results at high workspeeds are in distinct contrast to low speeds. Chatter was experienced with the compliant conventional machine at higher workspeeds, whereas with the stiff machine good accuracy was achieved at much higher speeds. This illustrates that roundness is not only a question of rounding geometry but also of the dynamic performance of the grinding machine. Since production rate is related to workspeed and wheel speed, there is an implication that a dynamically stiff machine allows much higher levels of productivity.
The important point is that vibration levels should be low. Roundness is not simply dependent on static stiffness. In fact, high static stiffness with high-vibration levels worsens roundness. Also, a slow rounding process tends to reduce errors better than a fast process. A compliant machine slows down the rounding process and can, under the right conditions, allow better roundness to be achieved. This was demonstrated by replacing the soft rubber control wheel with a cast iron control wheel. It was found that roundness errors were increased with a stiff cast iron wheel.