A report published by the Manufacturing Technology Centre (MTC) and entitled ‘Effects of Centrifugal Force on Hydraulic Clamping’ highlights the influence that rotation has on the efficiency of work-holding.

The clamping equipment chosen for the study was provided by the Hitchin-based MTC member Roemheld UK.

The purpose of the experimental trials was two-fold. First, it was to understand the effects of centrifugal force on three types of hydraulic clamps (swing clamps, block cylinder clamps and work supports), to avoid potential over-engineering or under-specification of fixtures during rotational machining; second, it was to gain an idea of how to optimise the force required to hold a component for turning.

Experiments were carried out on a DMU 160 P duoBLOCK five-axis machining centre from another MTC member, DMG Mori.

Two key parameters were measured: the pressure in the hydraulic system and the clamping load at different rotational speeds.

The results showed that the centrifugal force had some effect on the work-holding systems, and that the degree of change in the pressure and load readings was dependent on the type of clamp, as well as its orientation on the fixture.

Overall, the clamps that showed the least susceptibility to centrifugal force were block cylinder clamps, recording an average change in load of 1.4% between zero and 400rev/min.

Swing clamps and work supports were affected to a greater degree, recording an average change in load of 3.14% and 18.3% respectively over the same speed range.

The report says that although the changes in load readings were minimal in some instances, it still recommends taking the effects of centrifugal force into account when designing a fixture for rotational machining; and if the component material is ‘sensitive’ to compression due to the clamping load, work-holding devices should be positioned “in a manner that lessens the increase in load due to the centrifugal force, so that the part is not compressed”.

However, where material deformation due to work-holding is not a concern, the clamps should be positioned to increase the load resulting from the centrifugal forces, thereby achieving more-secure fixturing.

A copy of the Manufacturing Technology Centre report is available as a PDF from Roemheld UK (Terry O’Neill, managing director — ).