Precision Machine Design 2

Transcription

1 Precision Machine Design 2 Fundamental Principles: 1. Repeatability/Determinism.no workmanship is perfect; the design must make up for its imperfections. H.A. Rowland, 1886 A basic finding from our experience in dealing with maching accuracy is that machine tools are deterministic. By this we mean that machine tool errors obey cause and effect relationships, and do not vary randomly for no reason. Further, these causes are not esoteric and uncontrollable, but can be explained in terms of familiar engineering principles. R. Donaldson the probabilistic approach to a problem is only a tool to allow us to deal with variables that are too numerous, or expensive to properly sort out by common sense and good metrology. Bryan Random results are the result of random procedures. Portas Design for repeatability. Accuracy can be achieved by calibration or compensation.

2 2. Isolation In designing an experiment the agents and phenomena to be studied are marked off from all others and regarded as the field of investigation.. The experiment must be so arranged that the effects of disturbing agents on the phenomena to be investigated are as small as possible. James Clerk Maxwell, 1890 on a windy day the ruling was affected by the swaying of the trees in the vicinity of the house that contained the machine. W.A. Scoble, 1912 allowed us to identify thermal effects as the largest source of apparent non-repeatability J.B. Bryan, 1984 Adhesives used on the structure of the diamond carriage are however sensitive to moisture. C. Evans, 1980 ISOLATION STRATEGIES Decouple the machine from the environment (shielding) Example Vibration isolation mounts Design the machine to be insensitive to variations in the environment Example Use low CTE materials (mercury clock pendulum) Control the environment Example Temperature control (oil shower)

3 3. Kinematic Constraint The pieces of our instrument are solid, but not rigid. If a solid piece is constrained in more than six ways it will be subject to internal stress, and will become strained or distorted, and this in a manner which, without the most exact micrometrical measurements, it would be impossible to specify. James Clerk Maxwell, 1890 Design the machine so that each element is constrained in exactly the number of degrees of freedom required, but not overconstrained. When motion is desired, insure that the drive devices transmit forces only along the desired motion direction, i.e. the number of constraints equals six minus the number of required degrees of freedom. Examples: Kelvin Clamp Wiffle-tree mounting Kinematic Slideway designs 4. Alignment Principles If errors in parallax are to be avoided, the measuring system must be placed coaxially with the axis along

4 which displacement is to be measured along the workpiece. E. Abbe (1890) Design to minimize the offset between the measurement scale axis and the motion axis of the corresponding stage; make the two axis parallel and coincident if possible. Sensitive Directions: Recognize that for error motions in some directions have a small effect on the measurement; i.e. on a lathe tool point motion tangent to the surface creates a radius error on the order of ε 2 /r. Do not expend effort to correct errors which are already inconsequential. 5. Structural Loops/Measurement Loops In most machines, motion of the components causes deflections of the structural elements of the machine. If the measurement/feedback devices are attached to these structural elements measurement errors can occur. To achieve higher precision, separate the measurement system from the structural elements of the machine, i.e. use a metrology frame. 6. Materials Selection Choose the material properties relevant to the performance of a machine element or to the overall

5 machine and compute a figure of merit for those properties which can be quantified. Example of alternate strategies for minimizing thermal distortions: 1. Choose materials with low CTE, i.e. Invar, Zerodur. 2. Choose materials/structures with very high heat capacity and low thermal conductivity so it takes a lot of heat to raise the temperature and a long time for it to conduct through the structure, i.e. large granite elements. 3. Choose materials with low heat capacity and high conductivity so that the temperature field in the machine rapidly reaches equilibrium, avoiding bending of elements, i.e. aluminum. 7. Energy Flow Temperature effects are the largest single enemy to attaining maximum accuracy E.G. Loewen Identify all energy sources which input energy to the machine, the paths to locations where it is converted to other energy forms, locations where it may be stored, and means by which it mat be removed from the machine. Determine the susceptibility of the instrument performance to internal and external energy sources.

6 8. Error Budget/Uncertainty Analysis There are.two main purposes for developing an error budget: (1) Instrument/machine capability estimation; this is primarily a predictive procedure for estimating the total error due to all sources in a system under study; and (2) Risk reduction; this is primarily a management function to estimate and assign resources based on a systematic evaluation of the relative significance of all process errors. T. Charlton Error budgeting (Uncertainty analysis) is a method for predicting the machine accuracy at the design stage. Identify all error sources which can be quantified, all performance error categories of interest, and all mechanisms which connect the two. Tabulate and use to predict and/or analyze major areas needing improvement. 9. Symmetry Incorporate symmetry to the maximum extent possible into the design of each machine element, the entire machine, and into the machine environment. Any departure from symmetry should be carefully weighed against the resulting compensation which will be required.