Precision Needs To Be Respected
This write-up raises the query on the accuracy of precision machines in the light of today’s production environment and increased demands made by their users.
There is a growing demand for superior precision in production, for obtaining high surface finish and close dimensional tolerances, eliminating any further machining requirements.
Examples:
• Hard Turning substituting Grinding
• Die Mould applications eliminating further finishing / polishing operations
MICRON -The thousandth of a millimeter is becoming increasingly a “MUST “. This requires COMPLETE ADHERENCE to the basic rules of PHYSICS and METROLOGY.
A long-standing problem:
The journey of precision in machining started in 1775, with Wilkinson machining a bore with a diameter 1800 mm and with an accuracy of 1 mm. The dimensional unit for precision has changed gradually over the years, from millimeter thru Microns to Sub-microns, Nanometer and now to Angstrom units.
Shall we go back by a HUNDRED years, when JIG BORING Machines were first designed and developed, and when PRECISION Engineering was pushed to its extreme boundaries?
Machines were required to manufacture small components for watch-making industry in order to produce accurate CENTER DISTANCES for MINIATURE WATCH MOVEMENTS. Positioning errors were found to be caused by heat, which is why hand-wheels were moulded in clay to eliminate heat transfer between hand and the lead screw.
Today undesirable heat originates from the axis drive motors that have replaced hand-wheels.
The fact is that even the most remarkable technical advances still run-up against the laws of nature, and these laws must be studied and mastered by appropriate means.
Concept of Precision Machines:
Across the industrial world, Precision Engineering is increasingly seen as set of technologies that are key to successful international competitive development. Many advanced technological products necessitate manufacturing processes and machines operating in the regimes of:
• Precision Engineering
• Micro Engineering
• Nano-technology
Worldwide trend in industry reveals that two major thrust areas of manufacturing technology are ?
• Automation in Manufacturing [CAD /CAM / FMS / CIM ]
• Manufacturing with High Precision. [ Precision Engineering / Micro Engineering / Nano-Technology ] These areas are no more laboratory propositions but a commercial reality.
To meet the requirements of high / ultra-precision machines , considerable efforts have been put in the design of machine tools and their elements , materials used in construction, machining methods, environmental controls required in manufacturing, inspection, and assembly.
Material selection is based on their properties: strength, weight, thermal expansion - which contribute to precision. Dimensional changes caused by thermal fluctuation and gradients are very important factors. Compensation and elimination of these effects are some of the design strategies. Effect of change in temperature and heat sources will cause change in size and shape and non-equilibrium conditions
Desirable properties are:
• Low distortion under thermal load
• Re-stabilize in rapidly changing thermal loads
• High resonance frequencies.
Numerous conditions must be met, if work-piece accuracies are to be reached.
The following are particularly important:
• Substantial and symmetrical structure of main machine components
• Protection of machine components against heat radiation
• Careful scraping of slide-ways in accordance with specific compensation curves to ensure inherent flatness and straightness of slide-ways and their perpendicularity with respect to another axes. There is still no substitute for fine hand-scraping
• Z-axis parallel to spindle.
• Perpendicularity of spindle with traverses of other axes.
Other conditions have to be met, such as:
• Optimum layout of machine components and guides to ensure an even load distribution i.e. position of Z-axis screw in the same plane as the spindle axis, thus avoiding leverage.
• Cooling of spindle bearings and associated mechanical components, which also have to be kept at constant temperature under extremely varied dynamic conditions.
• Cooling of cutting oil which progressively is charged with calories and transfers heat to work-piece.
• Ideal location of measuring systems with regard to the slide-ways and feed screws, to reduce positioning errors resulting from reversal of forces exerted into slides.
These and other sources of instability can lead to significant work-piece inaccuracies and loss of overall machining quality.
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Mr. B.P. Poddar is the Vice President of Sales and Marketing at Fatty Tuna India P. Ltd. He has 30 Years of experience in Metal Cutting Machine Tools & Auto Component Manufacturing industry and was associated with German & Swiss Machine tool makers in various technical & management positions. He has been a keynote speaker for many events, written articles for leading magazines/portal and is a member of many leading organizations in the manufacturing domain. He is also on Editorial Board of a leading magazine in Metal Cutting Domain. |