Requirements of a Fixture

In order to maintain the workpiece stability during a machining process, an
operational fixture has to satisfy several requirements to fully perform its functions
as a workholding device. The following constraints must be observed while
designing a viable fixture:
Deterministic location
A workpiece is said to be kinematically restrained when it cannot move without
losing contact with at least one locator. The workpiece is constrained by a set of
appropriately placed locators so that it is presentable for the machining operation.
Locating errors due to locators and locating surfaces of the workpiece should be
minimised so as to accurately and uniquely position the workpiece within the
machine coordinate frame.
Total constraint
A workpiece should be fully constrained at all times to prevent any movement.
Clamps should provide locking forces to hold the workpiece in place -once it is
located. A totally restrained part should be able to remain in static equilibrium to
withstand all possible processing forces or disturbance. A necessary and sufficient
condition to warrant workpiece stability is to satisfy the condition of force closure.

Contained deflection
Workpiece deformation is unavoidable due to its elastic/plastic nature, and the
external forces impacted by the clamping actuation and machining operations.
Deformation has to be limited to an acceptable magnitude in order to achieve the
tolerance specifications.
Geometric constraint
Geometric constraint guarantees that all fixturing elements have an access to
the datum surface. They also assure that the fixture components do not interfere
with cutting tools during a machining operation.
In addition to these requirements, a fixture design should have desirable
characteristics such as quick loading and unloading, minimum number of
components, accessibility, design for multiple cutting operations, portability, low
cost, etc.