Spring 2016 provided a trip to Boston for 2 lucky engineers from Eley Metrology. Having shipped its latest long-bore measurement (LBM) CNC CMM to an aerospace supplier near Boston, US – it was service time.
The LBM offers a solution to a challenging branch of metrology: the precision measurement of features down long, deep bores. Applications include aerospace components such as main engine shafts, hydraulically operated under-carriage landing gear, nuclear reactor items, oil industry components and line-bored bearing housings. One of the best examples is highlighted by the critical requirements of a jet engine main shaft.
Such a component will be required to rotate in use possibly at 30,000 rev/min; the centrifugal forces are immense. It is vital that the geometry and size of such a component is in compliance with the engine designers’ exacting specifications.
The concentricity of the inner and outer shaft bearing diameters is critical, as is the blend radii of internal and external diameters. All features must be measured in relation to the centreline of the shaft. Any out-of-tolerance features will cause serious vibration problems with possibly catastrophic consequences if not identified before build of the final engine assembly.
Measure of Certainty
The LBM can measure the size, wall thickness, shape and geometry of a part without the need to redatum. This is very important as any need for redatuming will have an effect upon the associated ’uncertainties of measurement’.
With previous measurement procedures, where a composite measurement approach is employed using dial indicators and gauging techniques, the budget for uncertainties of measurement alone would often take the component specification way outside of the available tolerance band.
This is why the LBM was conceived. Greater demand for precision by designers, along with the availability of superior manufacturing machines, prompts the need for more accurate measurement.
The LBM can access and measure holes as small as 20mm in diameter. It can measure bores up to 3m in length and resolves to 1µm. The latest LBM leaving Eley Metrology has been passed to ISO 10360 and meets a volumetric measurement capacity of better than 10µm at the end of the measuring lance of 2m long.
No Error Mapping Required
With no error mapping involved, Jeff Eley explained how this was achieved: ’The LBM weighs nearly 14 tons; it is extremely strong and very stable. It has a main granite guide of slightly more than 4.5m in length. All manufactured items including the guide are produced entirely at our facility in Derby. The main guide is hand finished by our highly skilled team of engineers. The specification over the entire length of the guide is 4µm in total; this covers parallelism and straightness. The other two axes are manufactured to comply within 2µm of straightness and parallelism. We are, in effect, producing measurement masters in all four axes: X, Y and the two Zs. The principal guidance system is that of a vee-flat with pre-loaded bearings throughout. This system is not new but is unique to CMMs. Vee-flat systems are employed by nearly all the major machine tool builders.’
Eley is convinced that this is the best method of guiding precision co-ordinate movement and also claims a unique bridge system. The main bridge guidance members are kinematically mounted on three spheres within a cradle described as a ’shear plate’ — all the axial forces are dissipated through this, allowing stress-free and stable movement of the bridge and the vertical Z-axis members.
The primary Z axis can measure as a conventional CMM, with or without the carbon-fibre lance in place. No error mapping is employed in either case in order to meet its specification. But what would the LBM achieve with error mapping included? Eley said: ’The lance has two Z-axis vertical members controlling stability and squareness.’ As part of a total turnkey system to its US customer, Eley Metrology has supplied universal fixturing that will accept any size and configuration of component within the LBM’s volume, plus metrology masters to assist with certification and day-to-day compliances. The analysis software used is Eley’s True Measure 4 (TM4).