Precision needs to be respected – Part 2

By Niraj M Wanikar, Chief Editor, CNCTimes.com, 2016-08-05 14:51:00

This is Part-2 of my earlier Blog Post. To read the first part, click here.

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.

The "Micron"- A Dreamland or Reality:

Not so long ago, only the manufacturers of measuring instruments, jig borers, and metrology equipment talked in term of thousandths of a 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 its use in such a way that they exceed the capability of their own manufacturing & inspection facilities!!!

The micron must be treated with a good deal of consideration and respected.

Did you know?

• Steel enlarges by 0.011 mm / 1000 mm /degree Celsius. The slightest change in temperature cause variations in few microns.

• If a 500 mm long Aluminium component is only 2 degrees warmer than the assumed temperature (of 22 degrees) of the measuring machine, the 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 the shape.

• Change in temperature has different effects on various elements of a machine and measuring equipment and takes different time span to attain equilibrium.

• Slightest heat radiation onto the machine causes bending or deformation of the workpiece or the machine component.

• Along with temperature; dust, humidity, vibration, air velocity / air turbulence and noise have an effect on measurement and machining.

• The Foundation on which any machine rests is subject to the same physical laws, and as a consequence cannot be subjected to changes in temperature without exerting an effect on the machine.

Many machine tool makers’ state accuracies in their brochures and have different meanings as per different standards. It is important to understand standards and their meanings before getting carried away by numbers.

There is no doubt that numerical control has impressively improved accuracy as far as axis positioning is concerned. The basic structure of the machine, its design and dynamic performance are key factors to determine stability and accuracy.

What does high precision mean?

Association between the accuracy required on a drawing board and the 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 the range of tolerance. Otherwise, this will lead to the non-acceptance of work-pieces.

This involves not just geometrical errors in positioning or on the machine itself, but an accrual of all the minor faults connected with machining and workpiece measurement.

What are the various influences that affect machine accuracy and to what extent?

Let us consider this. If the 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:

• 0.002 mm Unpredictability of work-piece or machining process
• 0.002 mm Repeatability of tool change
• 0.002 mm Deformation of clamping the workpiece.
• 0.002 mm Loss of accuracy at second or subsequent clamping
• 0.002 mm Defect in fixture and / or workpiece resting face
• 0.002 mm Thermal / Environmental effect
• 0.001 mm Uncertainty of measurement
• 0.013 mm Total of Errors unconnected to Machine Accuracy
• 0.007 mm Left for errors originated by the machine itself, under dynamic conditions, geometry and positioning, mechanical deformation under load and internal temperature deformations.

It is decisive to consider the range of tolerances, total errors unrelated to the machine, machine accuracies to understand the domain of high precision.

It is inconceivable technically, but today’s reality is that customers require some performances from machine tool that its basic design is not able to provide. As a consequence, it will be wholly inappropriate for production problems which need to be solved.

Many times, people choose machines that are economically priced, but at the same time expect unrealistic specifications.

Following examples will throw light to illustrate these remarks:

• Spindle speeds of 20000 rpm and more are demanded
• Spindle power 15 kW and more
• Rapid feed rates 40 mt / min and more
• Positioning accuracy of 0.005 mm with repeating accuracy 0.002 mm

We ask the following 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 cannot be ignored.

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.

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