Because of its properties, thin-wall aluminum can be problematic to hold during a machining operation. If there is too much clamping pressure, the workpiece distorts; not enough clamping pressure, the workpiece slips.
Although slippage is unacceptable in any application, in precision machining, distortion is even worse. The workpiece may at first seem to be acceptable, but when inspected, the distortion from the force of clamping the workpiece can clearly be seen. In some cases, the inspection data will define the exact location where the workpiece is being clamped, simply because of the excessive distortion in the location where the clamping device was engaging the workpiece.
Another issue to consider is repeatability. In production machining, it is important that a machining operation be consistent from one workpiece to the next. A key element to this success is to get consistent enough clamping pressure on a workpiece in order to hold it rigid for the machining operation, but without distortion or slippage.
One common resolution is to reduce the speeds and feeds of the operation down to a value where the clamping pressure can be reduced to not show distortion. However, this is a costly alternative in the production environment where machine time is extremely valuable.
Workholding manufacturer, Emuge, has created a solution for these types of workpieces called the System SG expanding bush. This system uses as much of the workpiece surface as possible, thus spreading out the clamping force into a larger surface area of the workpiece. It is able to still maintain the required rigidity in the operation, but also achieves higher transferable torque values, enabling speeds and feeds to be optimized for tool life and production quotas.
In one particular thin-walled aluminum workpiece application, the customer has had very good results using the System SG expanding bush design. Its workpieces are generally from 6061-T6, have a wall thickness ranging from 0.070 inch to 0.050 inch and have a unique geometry that often requires two distinctly different clamping diameters. Two clamping diameters can be problematic because of different clamping diameter tolerances or even slightly different wall thicknesses. Also, inconsistencies can develop in the finished product because of over clamping in one area and under clamping in another.
The design of the expanding bush system, in this case, has one clamping cycle pressure provided by the machine tool and then a second clamping cycle pressure applied by an integrated spring clamping package. This two-pronged approach to clamping pressure can be custom applied to the workpiece, as the integrated spring package can be adjusted with shims and/or different spring package orientation. Then the actual machine side clamping pressure can be programmed into the machine. This two-pronged approach ensures an accurate and repeatable clamping cycle pressure for volume production, all while minimizing or eliminating any distortion in the workpiece.
The correct approach for clamping thin walled aluminum workpieces can yield exceptional results.