This write-up raises the query of the accuracy of precision machines, in the light of today’s production environment and increased demands made by their users. There is 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 more and more a “ MUST “. This requires COMPLETE ADHERENCE to the basic rules of PHYSICS and METROLOGY.
An Long-standing Problem:
Journey towards the precision in machining started in 1775 with Wilkinson , machining a bore of diameter 1800 mm with an accuracy of 1 mm. The dimensional unit for precision has changed gradually from millimeter thru Microns to Sub-microns and Nanometer and Angstrom units.
Shall we go back by HUNDRED years ,when JIG BORING Machines were first designed and developed ..?? PRECISION Engineering was pushed to it’s extreme boundaries.
Machines were required to manufacture small components for watch-making industry to produce accurate CENTER DISTANCES for MINIATURE WATCH MOVEMENTS.
Positioning errors were also found to be caused by heat which is why hand-wheels were molded 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 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 ]
- Manufacture with High Precision. [ Precision Engineering / Micro Engineering / Nano-Technology ] These areas are no more laboratory propositions but commercial reality.
To meet 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 charged with calories and transfer 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.
The “ Micron “- Dreamland or Reality:
Not so very long ago , only the manufacturers of measuring instruments , jig borers and metrology equipment talked in term of thousandths of millimeter and could claim to have some degree of control over how they were used.
Today nearly all manufacturers and users of machine tools claim to be familiar with this microscopic unit of measurement and even specify it’s use in such a way they exceed the capability of their own manufacturing & inspection facilities !!!
The micron must be treated with good deal of consideration and respected.
Do you know, for example:
- Steel enlarges by 0.011 mm / 1000 mm /degree Celsius . Slightest change in temperature cause variations in few microns.
- If 500 mm long aluminium component is only 2 degrees warmer than assumed temperature of 22 degrees of measurement machine , longitudinal measurement error will be nearly 0.030 to 0.040 mm.
- Thermal effect on any object is significant not only in linear dimension but also in shape.
Change in temperature has different effect on various elements of machine and measuring equipment and take different time span to attain equilibrium. - Slightest heat radiation onto machine causes bending or deformation of work-piece or the machine component.
Along with temperature ; dust, humidity, vibration, air velocity / air turbulence and noise have effect on measurement and machining. - Foundation on which machine rests is subject to the same physical laws ,and as a consequence can not be subjected to changes in temperature without exerting an effect on machine
- Many machine tool makers state accuracies in their brochures are having different meanings as per different standards. Standards and their meanings must be properly understood before getting carried away by Numerical Values or Numbers.
- There is no doubt that numerical control has impressively improved accuracy as far as axis positioning is concerned. Basic structure of machine, it’s design and dynamic performance are key factors determining stability and accuracy.
What High Precision Means..??
Association between the accuracy required on a drawing and indicated accuracy of machine tool provides confirmation that machine needs sufficient accuracy in reserve to allow for the inevitable geometrical variations that occur, without upsetting into range of tolerance. Otherwise this will lead to the non-acceptance of work-pieces .
This involves not just geometrical errors in positioning or on machine itself, but an accrual of all the minor faults connected in one way or other way with machining and work-piece measurement.
What Are These Influences??
What are these influences and to what extend do they affect the machines basic accuracy ??
Let’s consider ,if mean accuracy of 0.020 mm must be obtained for work-piece on machining center on 1000 mm cube, the sum of errors that can be imagined is as under:
It is decisive to consider the range of tolerances , total errors unrelated to machine , machine accuracies to understand the domain of high precision.
It is inconceivable technically , but today’s reality that customers require some performances from machine tool that it’s basic design does not enable to give. As a consequence it will be wholly inappropriate for production problems which need to be solved.
Many times choice falls on machines those are foremost economical but at the same time based on unrealistic specifications. Following examples will throw light to illustrate these remarks :
- Spindle speeds of 20000 rpm and more are demanded
- Spindle power 10 kW and more
- Rapid feed rates 40 mtrs. / min. and more
- Positioning accuracy of 0.005 mm with repeating accuracy 0.002 mm
We ask questions:
- How to dissipate all excess heat that is unnecessarily generated ??
- How do we control acceleration and deceleration ??
- How do we control inertia moments ?
- Do high speed spindles and their bearings compatible with high accuracy under heavy duty conditions??
Machines cannot be used for every conceivable job without certain limitations. There will always be some constraints that can not be ignored.
