You find yourself in front of a machine tool and you start getting production scrap rejected by the testing personnel.

At the same time, the customer is calling to find out when the products will be delivered.

That’s when the nightmare starts.

If you use machine tools, at one point or another you have certainly been faced with this situation, when nothing make sense, time does not seem to exist and the stress level is sky high.

Machine tools also require periodical preventive maintenance checks, such as oil change, filter change, various steps, but too often, after having checked the basic elements, we forget about the geometric check.

HOW IS GEOMETRIC CHECK OF A MACHINE TOOL CARRIED OUT?

So, how do you get your bearings in the field of  machine tool geometric check?

Lets take a look at what the ISO standards say concerning this very delicate and sensitive topic.

The history of standards concerning machine tools, as concerns testing-related problems, is pretty specific, and it is worthy of a brief account.

Towards the end of 1927, a publication appeared in Germany which immediately stirred the interest of the industry.

The publication was written by an engineer, Mr. George Schlesinger, from the Superior Technical School of Berlin, who attempted to lay down strict foundations for the problem of testing the accuracy of machine tools.

It was the first try at defining the minimum characteristics that a machine tool had to possess in order to consider it suitable to the processing needs of the industrial production of the times.

Over the following years, this publication was revised, fine-tuned and discussed in various venues, tested by manufacturers and users of machines tools alike, until the final formulation was reached and still used today.

The concepts which inspire this procedure which is undoubtedly the mother of all standards that came after it, complied with the following basic need:  to define a set of simple and clear checks, easy and cheap to carry out, not susceptible to equivocation, that allow to classify the checked machine as belonging to a defined accuracy class.

It was basically the first serious attempt of that period.

This attempt took hold despite long discussions and heated disputes that were generated by the Schlesinger procedures.

This method for checking the accuracy of machine tools is so deep-rooted in global industry that, when talking about testing a machine tool reference is still made to these procedures, though today they are obsolete in terms of methods and tools.

However, the general principles remain, for a good part, in modern standardisation, the ones formulated by Schlesinger.

Starting from here, national and international industrial standardisation bodies have developed vast and complete technical documentation specifically referred to machine tools, the structure of which, at least as far as ISO is concerned, will be analysed here below.

Perhaps it is precisely because of this history that, when it comes to machine tools, we are accustomed to seeing the testing-related issue as a problem related to the global construction aspects.

Therefore, testing means evaluating and documenting whether or not the objectives set by the design have been reached.

Clearly, these objectives are very often contractual terms and, as such, extremely important.

IN THE SPIRIT OF ISO STANDARDS, THE QUALITY OF A MACHINE TOOL IS BASICALLY THE ABILITY OF SAID MACHINE TO MEET THE NEEDS FOR WHICH IT WAS PURCHASED

If these needs have been met, the machine is a quality machine.

Vice versa, if the machine fails to meet these needs, it will be considered of poor quality.

This is precisely the problem concerning the quality of a machine tool: “defining the needs which the machine has to meet”, and this is exactly the objective of ISO standards concerning testing that offers excellent technical support to producers and users of machine tools.

So, the set of international standards dedicated to machine tools is to be considered an extremely accurate Normative System in terms of its structure and its documents.

Now, let’s see how standards pertaining to machine tools and, specifically, to their testing, is organised:

With reference to the UNI-UCIMU publication entitled “Standards for machined tools” of 2002, we can say that the standards, among those published voluntarily and which refer to technical aspects not governed by national laws or European directives, can be considered as belonging to the following subgroups:

• testing standards
• mechanical construction standards
• other standards that may be of interest for machine tools

“Testing standards” address, in particular, the final acceptance test operations of a machine tool, and should constitute the basic contractual reference for the definition of its performance characteristics. “Mechanical construction standards” define the harmonised types of accessories and mechanical parts to be used when building machine tools.

The “other standards that may be of interest for machine tools” consist of nomenclature, designation, programming, signalling and numerical control standards. In this document, we will be examining the “Testing standards” only.

The Regulative System for the testing of machine tools is basically organised into two major orientation lines: the first consists of a series of standards to be considered procedure-related standards.

The second line of orientation consists of a set of standards that can be considered as specific type standards.

Procedural standards are collected as the ISO 230 series.

These documents provide a set of operational instructions concerning the performance of machine tool tests, a very important and delicate area, especially if you take into account the basic importance, in a metrological sphere, of the procedures adopted for carrying out a measurement.

During the contractual phase, one procedure must be chosen over another and, most importantly, the operational method to be used must be specified in a final procedure.

This set of standards constitutes a veritable tester’s manual, and all testing operations should comply with the principles and the methods expressed in said standards.

Each Specific type standard, for the testing of various types of machines, will expressly refer to said standards in order to avoid any doubts concerning the interpretation of the operational methods. One last observation on ISO 230 standards has to do with the fact that this standard fails to indicate the error limits which are identified through the checks, but only indicates, with extreme precision and detail, how to carry out the measurements.

On the other hand, Specific type standards are specific documents for the classical types of machine tools, an example of which is given by standard ISO 10791 which, after identifying the machines’ type market, specifically indicate the checks to which said machines must be subjected along with the admissible error limits in order to be declared compliant with the requirements set forth in the Standard.

Of course, these standards systematically refer to the Procedural standards previously mentioned.

We will now examine in detail how the series of ISO 230 standards are organised and, subsequently, the structure of the Specific type standards will be described by machine type, such as, for example, standard ISO 10791 pertaining to testing conditions for machining centers.

This will allow us to evaluate the completeness and validity of these Standards as well as to understand how important it is, for purposes of testing operations, to never lose sight of these documents, as they represent fundamental reference bases.

Procedure standard – ISO 230: this Standard, entitled “Test code for machine tools”, is made up of 9 parts which are specified here below:

• PART 1: GEOMETRIC ACCURACY OF MACHINES OPERATING UNDER NO-LOAD OR QUASI-STATIC CONDITIONS;
• PART 2: DETERMINATION OF ACCURACY AND REPEATABILITY OF POSITIONING NUMERICALLY CONTROLLED AXES;
• PART 3: DETERMINATION OF THERMAL EFFECTS;
• PART 4: CIRCULAR TESTS FOR NUMERICALLY CONTROLLED MACHINE TOOLS;
• PART 5: DETERMINATION OF NOISE EMISSION;
• PART 6: DETERMINATION OF POSITIONING ACCURACY ON BODY AND FACE DIAGONALS (DIAGONAL DISPLACEMENT TESTS);
• PART 7: GEOMETRIC ACCURACY OF AXES OF ROTATION;
• PART 8: DETERMINATION OF VIBRATION LEVELS [TECHNICAL REPORT];
• PART 9: ESTIMATION OF MEASUREMENT UNCERTAINTY FOR MACHINE TOOL TESTS ACCORDING TO SERIES 230, BASIC EQUATIONS.

Now it’s your turn!

Try to answer to the following 3 questions, take 10 minutes of your time and I suggest you to answer writing your responses on a sheet of paper: it’s very important to write down your thoughts and not only to think them.

Writing is more difficult because it needs concentration, the concentration that often due to lack of time we haven’t and that makes us come to BAD decisions or even worse to not making a decision.

WHAT WOULD CHANGE IN YOUR COMPANY IF YOU INSTALLED A MULTICENTER GEOMETRICALLY CHECKED ACCORDING TO STANDARD ISO-230?

…AND WHAT HAPPENS IF YOU KEEP USING THE SAME MACHINES YOU HAVE ALWAYS USED?

WHAT WOULD BE THE RISKS FOR YOUR COMPANY IF YOU DID NOTHING AND SIMPLY POSTPONE YOUR DECISIONS?

AND REMEMBER THAT…

IT IS NOT THE STRONGEST OF THE SPECIES THAT SURVIVES BUT THE MOST ADAPTABLE

www.machiningcentersbook.com

Maurizio Porta
Expert in Flexible Production