Precision Machine Design
Cranfield Precision are specialists in Precision Machine Design. Cranfield Precision has a rich history of designing precision machines, born out of Cranfield University's Unit for Precision Engineering we fully embody the 11 principles of precision machine design as defined by Prof Pat Mckeown (former MD Cranfield Precision).
What goes in to precision machine design?
- Environmental Stability
- Error Budgeting
- Precision Metrology
- Error Compensation
- Control System Technology
- Advanced Design Software
The Principle of Determinism
Determinism is one of the fundamental principles of precision machine design. Determinism suggests that there is no such thing as a random behaviour and that every part of a machine and process obeys a cause and effect relationship. A deterministic way of thinking can't accept that there is a "random" error. It is considered that any behaviour apparent on a machine is the result of some condition that can be controlled and the list of conditions and factors is small enough to be manageable. Cranfield Precision fully embody this principle and it is the fundamental root of our precision machine design philosophy.
John Loxham, founder of the Cranfield Unit for Precision Engineering, was an astronomer, as well as an educator, precision engineer, businessman, and a pioneer in understanding thermal effects on manufacturing systems. He deserves the credit for formally introducing the Deterministic Point of View to manufacturing engineers. This view states that automatic machine tools and measuring machines are perfectly repeatable, just like the stars and the planets. Jim Bryan July 07
In order to build a precision machine it must be repeatable. By making stiff repeatable machine structures that have consistent and repeatable errors and deflections it is possible to use advanced machine design techniques to fully compensate for any errors. By using precision metrology techniques to measure the repeatable motions of a machine advanced control system technology can be used to apply full error compensation. This is one of the fundamental differences between a conventional machine and an ultra precision machine design.
Environmental stability is another key area in producing an ultra precision machine. Thermal instability is one of the largest contributors to error so high accuracy temperature control systems are typically used in precision machine design. Different materials have different thermal expansion coefficients which can cause adverse and seemingly non repeatable errors with different thermal cycles. It is of paramount importance to control the temperature to a stable level with a high degree of accuracy. Previously Cranfield Precision have produced machines with temperature control to ±0.01°C. Air conditioning systems and facility design, flooring and services connections are all considerations in precision machine design that are typically given much less importance in conventional machine design.
Error budgetting is an important technique that allows errors that are intrinsic to machine axes to be minimised at the design stage. An error budget provides an estimate of potential errors within a machine axis that lead to deviations from the desired motion. We use error budgets in advance of comprehensive design effort as a method of evaluating the ability of a proposed machine axis configuration to meet the desired specification.
After designing a machine that has miniaml errors and is stable and repeatable you have the basis for a machine with high instrinsic accuracy. At this stage you have a good base for a high precision machine but to take the machine to the next level of ultra precision accuracy must measure all of the machine axes and error motions. Typical metrology testing sequences encompass the following areas
- Straightness of Motion (horizontal and vertical)
- Tilt Error Motions (yaw, pitch & roll)
- Displacement / Position Compensation (linear & rotary)
- Spindle Error motions (Radial, Axial and Tilt - synchronous and asynchronous)
- Orthogonality & Parallelism
Typically metrology tests are considered at the design stage and are set up so as to measure the machine in the direction and at the point of the process interaction. In a lathe type machine this would be errors measured at the cutting tool tip in the direction of travel and at spindle height. This ensures that the intrinsic errors in the machine are fully understood and characterised.
Error Compensation & Error Mapping
Once you have designed and built a precision machine and measured all of its intrinsic errors through precision metrology, you now have enough data to create a full error map of the machine. Depending upon the machine configuration, its use and the drive and control systems used error mapping is applied either as individual compensations, a combined error compensation or as a machine work volume comensation. Error mapping of a machine distringuishes a good machine tool from a high precision machine tool.
Control System Technology
The control system is really the final enabler in putting all the pieces together. You have designed a stiff, stable and repeatable precision machine, you have measured all of its errors and created an error compensation map, you now need to program an advanced control system to instruct the machine to move in the required directions.
Once you have programmed the machine and applied all of the error compensation, a final metrology stage is conducted to check the final compensated accuracy of the machine.
Symetrical designs offer many benefits. If you have a symetric machine then thermal growth is more even and consistent. Symetry also helps to balance masses, equalises stifness, and therefore evens out deflections giving simplicity and predictability to a precision machine design.
Advanced Design Software
Modern precision machine design is facilitated by a host of software packages that enhance productivity and allow precision machine design to be taken to new levels.
3D CAD Solid Modelling
3D CAD is used to produce a solid model of the machine. 3D Solid Modelling allows you to improve a design and get a greater concept of scale and access. Assembly and mating conditions also allow you to use simulation to cycle the machine through its range of motions and detect any interferences or areas that may cause issues before a machine is built ensuring that the correct final solution is met.
FEA - Finite Element Analysis
Finite Element Analysis has revolutionised the industry allowing structural resonances, deflections and thermal characteristics to be easily determined for complex machine structures and systems. By using FEA software it is possible to make a machine lighter and stiffer giving optimum dynamic performance.
CFD - Computational Fluid Dynamics
Computational Fluids Dynamics software allows complex modelling of fluid flow around a machine or through a machine's services and systems. This is very important because it allows environmental and thermal control to be optimised by keeping fluid moving to the right places and effectively managing heat transfers and flows.