Craig, Oliver
(2019)
Conformable fixture systems with flexure pins for improved workpiece holding.
PhD thesis, University of Nottingham.
Abstract
The design of fixture systems for low stiffness freeform components is challenging. The tendency of thin components to deform under clamping, and vibrate during machining, are long-standing and persistent industrial problems. Excessive clamping-induced workpiece deformation can lead to poor dimensional accuracy, and weakening of the finished component due to residual stresses. Machining-induced vibration can affect the accuracy and surface finish of components, and also cause increased wear on the cutting tool, or even damage to the spindle bearings of the machine tool. Satisfactory fixturing of low stiffness components with high geometric complexity using conventional methods can be difficult and costly, due to the large numbers of clamps and/or dampers required to fully support and stabilise the workpiece. Fixturing of small components is particularly challenging, due to the lack of space for clamping and damping features. Furthermore, typical fixture systems cannot easily accommodate any manufacturing variability of blanks. These issues are particularly relevant to the machining of legacy components, where it is typically necessary to develop appropriate fixture solutions at low cost, and with short lead times.
This work presents an innovative concept of additive manufactured fixture, based on tuneable deformable supporting structures known as flexures, to improve the holding of delicate workpiece geometries during machining. The concept takes full advantage of design for additive manufacture to enable near full-face contact between the workpiece and fixture, which provides a conformable supporting surface for freeform components that improves fixture performance compared to existing designs. Arrays of deformable flexures occupy the space between locating features, and engage with the component when clamping is applied. They are shown to have minimal effect on the frequency response of the workpiece-fixture system relative to conventional fixtures, meaning an existing fixture can be replaced with a flexure-based solution without the need to alter any tooling or machining parameters in the manufacturing process.
Initially, the dynamic properties of flexures are studied to investigate the effect of flexure geometry on frequency response, with finite element models developed to predict flexure eigenfrequencies. Following this, a simplified clamping setup, comprising a row of flexures to simulate a thin section through a fixture, is used to investigate the ability of fixture conformability to reduce workpiece deformation, with comparisons made against a conventional fixture with a rigid clamping surface. Finally, the findings from these investigations are used to develop additively manufactured machining fixture blocks, which are used to hold a simple cantilever workpiece during end milling, and to investigate the ability of flexures to damp workpiece vibrations during machining.
Finite element models have been developed and refined, and shown to be able to consistently predict the eigenfrequencies of flexures, both individually and in large arrays within a fixture, with an error below 5%. When studying fixture conformability, the flexure configuration was found to reduce the clamping force required to hold aerofoil-type workpieces with geometric errors by up to 60% compared to a rigid clamping surface. This was observed in simulations and experiments to be due to a mechanism whereby flexures deform to accommodate geometric errors in the workpiece, while a rigid clamping surface will deform a distorted workpiece until it adopts the profile of the fixture, which therefore reduces clamping-induced deformation by more than 40% in some configurations. In machining, it is found that flexure resonance can be utilised to reduce workpiece vibration, by selecting milling parameters that excite the workpiece at the first eigenfrequency of the flexures. In good agreement with models, this frequency-specific damping effect is found to reduce vibration of the workpiece by 25% compared to n-2-1 fixtures, and 34% compared to rigid vice-type fixtures.
This thesis provides an understanding of the behaviour of flexures, and shows how this knowledge can be exploited in the design of fixtures to reduce the deformation and vibration experienced by delicate workpiece geometries.
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