Case Study - DT40 - Machine Elements
The finished design has to be able to transfer the new concepts of the enabling technologies into a viable commodity machine tool. The fabricated delta structure is small and light enough to readily form part of a number of boxed sections which connect together to occupy the compact footprint of the machine. The finished production machine was operated as part of a manufacturing cell receiving palletised workpieces at the metrology station. The pick and place robot transferred workpieces between the workzone and the metrology station.
Structure
The machine base is a steel weldment filled with epoxy granite designed to have highest
possible natural resonant modes. FEA of the un-filled structure predicted the first three
modes to be 410 Hz, 530Hz and 570Hz.
The first mode of the structure increases when
filled putting it even further above the ultimate system bandwidth. No structural
resonances had to be dealt with in the stabilisation of the servo systems.
The delta shaped weldment is supported in the machine structure by isolation mounts that
prevent high frequency vibrations disturbing the metrology system. The stiffness of the
elastomer mounts constraining the delta structure was chosen to be high enough to
ensure that the acceleration forces were reacted in the framework of the enclosure.
Machine Axes
The two linear axes use a symmetrical trapped hydrostatic design constructed from
aluminium bearing housings with precast pockets running on 80 mm diameter precision
ground steel bars. The stroke is only 50mm which enables the unsupported length of the
bars to be short.
This places their first mode above 400Hz which is consistent with the
design criterion of the base structure. The lower of the two bearings has three pockets to
form a control guide, the upper has two pockets which permit a second degree of freedom
which avoids over constraining the axis framework
The workhead spindle is mounted between the upper and lower bearing blocks. The
spindle casing forms part of the Z axis structure stiffly connecting the upper and lower
bearings. The design minimises the moving mass and ensures that the force to
accelerate the axis is as low as possible.
The X, tool slide is designed in the same way, this time using the body of the turret tool
changer to form part of the axis. The indexing turret mechanism is a proprietary assembly
Issue 1 – Mar 09
that uses a face tooth coupling to provide sub arc-second angular indexing repeatability.
Measurement of axial and radial repeatability showed a maximum uncertainty of 50 nm in
the Z direction and 75 nm in the X direction. The turret repeatability is the weakest link in
the error budget contributing to nearly 75% of the tolerance. The turret stiffness was
measured at 118 N/micron radially and 47 N/micron axially.
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Cranfield Precision operates to ISO9001 quality standard and is accredited by Lloyds Quality Register.
A Division of Cinetic Landis Ltd.
Registered in England
Registration Number 05577045
Registered Office: Cross Hills, Keighley, West Yorkshire, BD20 7SD