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Instrumentation at its Best
K Venkata Subramanian, CEO, Surag Instruments If instruments are the life of a company, then calibration maintains and improves life standards with enhanced precision, quality and output of the company. This ar ticle highlights the necessity and importance of calibration of instruments along with the factors and machiner y involved in calibration.

Measuring instruments are the 'heartbeat' of your company as they check and measure your production processes. They are control the quality of your products, and in the end are responsible for the success and profitability of your business. But, how can the quality of the measurement are assured? A regular check of your 'heartbeat' with traceable calibration equipment is essential.

First, it ensures your quality always matches the customer’s expectations, which is vital where ISO 9000 certification is involved. Second, regular calibration pays dividends.

Calibration
'Calibration' is a comparison of results from a measuring device (or source) of an unknown accuracy with a standard device of known accuracy. This standard device is universally accepted as a 'reference' and is traceable to international standards. Calibration records and corrects any deviation from this standard. Each instrument has a specific calibration procedure, which indicates exactly how and what must be checked. Regular and traceable calibration combines the forces of accuracy and certainty and provides you with the key aspect of ISO 9000 registration. There are three general types of calibration measurement: • Verifying the performance of the test instrument.
• Adjusting the response of the test instrument.
• Obtaining correction factors for the test instrument.

When Instruments may Need to be Calibrated?
Equipments used in the monitoring and control of suitable process parameters and product characteristics (ISO 9000 series, Clause 4.9 d) may need to be calibrated. Such equipments can be identified by a common sense assessment of the risks (which is not calibration) versus the cost of calibration. Questions are:
• Would it matter if the instrument is out of calibration or is faulty? If not, then why is the measurement being made at all?
• Is the instrument simply indicating an on-off condition? If so, a simple functional check may be adequate.
• Is the instrument used only as a comparator or transfer a measurement from one place to another? While calibration may not strictly be necessary, it may still be the most cost- effective and may to ensure the instrument is functioning correctly, and is stable and repeatable for the duration of the transfer.
• Can faulty product be detected by calibration instruments at intermediate or final inspection, and the process remedied without costly loss of product? If so, calibration of process monitoring instruments may not be necessary.

Traceability
Traceability to procedures and records are used and kept to demonstrate that calibrations made in a local laboratory accurately represent the qualities of interest. The scientific aspects of traceability involve principles of metrology that used locally and independently by scientists, engineers, and technicians. The legal aspects of traceability involve a governmentally established and maintained infrastructure within which the measurements are performed. The infrastructure that is the general use today has three major constituents:
• Worldwide legal adoption of the International system of units (SI) as the basic system of units of weights and measure.
• The establishment of national laboratories such as National Institute of Standards and Technology (NIST), chartered to maintain representations of the SI units (standards) and to disseminate their values to calibrated laboratories.
• Definition, implementation, and use of methods and procedures that allow individual calibration laboratories to compare their local standards with those of the national laboratories.

What is Involved in a Traceable Calibration?
Years ago, in an effort to ensure properly calibration of measuring equipment, the US department of Defence issued MIL-C-45662 as a standard document to govern the calibration of instruments used in qualification of products purchased by the military. MIL-C-45662 has since evolved into the widely used MIL-STD- 45662A. One requirement of this and similar standards is that measurements must be traceable to national standards.

In MIL-STD-45662A, traceability is defined as: “The ability to relate individual measurement results to national standards or nationally accepted measurement systems through an unbroken chain of comparisons.” In broader terms, a measurement can be said to be traceable to designated standard within stated uncertainty limits, U, with a coverage factor k, if convincing scientific evidence can be produced which shows that a direct comparison to those standards would reduce a result that falls between the stated uncertainly limits with a confidence determined by k. Both definitions are clearly intended to guarantee quality and compatibility of measurements by ensuring that they are properly expressed in terms of standards maintained by NIST, or to be accepted measuring systems, maintained by others.

Calibration reports are essential part of traceable calibrations. These provide auditors in the MIL-STD or ISO 9000 environment with evidence of traceability. In addition, metrologists, test engineers, and quality personnel can use the data in calibration reports to improve the accuracy of measurement in a particular facility.

Right Machine & Tool for Right Job
In a multipurpose and multi-product machine shop normally working on small batch production, the job of a methods engineer is to assign proper machine and recommend right tooling for the same on almost day-to-day basis. For this, the methods engineer has to consider variety of factors:

1. Identify and Study the Components to be Manufactured: Here two primary considerations come into play:

(a) The size and shape has to be studied with reference to the drawing. This has its practical problems. The drawing normally available is of the finished components. But if the raw material undergoes 20 operations like turning, milling, drilling, etc, its shape at first operation and the shape at the 17th operation are entirely different from that at the 20th operation. The holding method, selection of tool and setting parameters like height, tool reach, etc depend very much on this. Normally, the product design department does not supply such stage drawings. At least intermediate sketches have to be made for clear understanding (visualisation).

(b)Basic material and its state: Tool geometry, tool material and speed, feed parameters can be decided only after ensuring the information whether the component is non-metallic (plastic, etc) or phosphor bronze or mild steel or hardened above Rc 60. Also, one must know whether the components, which are soft at this stage, need to be hardened later as some allowances will have to be kept to clean them to take care of scaling, deformation, etc. Also, all dimensions at soft stage will have to be done with proper subtraction or additional of the grinding allowance as the case may be.

2. Scale of Production and Repetition of Work Order: Selection of tooling depends quite much on this aspect. For a small batch, normally the tendency is to manage with standard tooling used on a universal type of machine. A one time medium size batch (to go for three/four days) can be handled with provisional/ workable type of jigs/fixtures and cutters, etc with clear instructions to the worker/ supervisor to take care of the limitations of the set up. But if hundreds of pieces are to be completed per month and the order is to continue for many months or permanently, one has to make a perfect tool set up including proper cutters, jigs fixtures, etc with proper care to avoid human errors by fool-proofing methods. All components of the fixture should be of proper material specification and hardness, etc. Also, speed, feed rate fixing will have to be done. Even proper grade & concentration of coolant has to be recommended. This is called as a dedicated system.

3. Suitable Machine Group for Each Operation: This has to be properly decided and requires sound knowledge:

(a) Smallest details and parameters also have to be matched between the tooling and the machine. Thus, tool holders, arbors, collets and collet-holders have to be matching specifications. Over some years, difference arises even within two machines of exactly same original specifications. Some facility is detached for simplicity while some attachments are added. Some parts are badly damaged but still clinging to the machine and there are many other possibilities. For all these, all the data should be available apart from the basic essential machine manual. But, this data may not come automatically to the methods engineer’s desk. For this, he has to keep going around making mental notes of all the developments in the shop and should be eager to help out people with data references, solving their difficulties, etc. Also, he will get some novel ideas being practiced there, which are not found in textbooks and handbooks.

b)There is one more aspect to this. The most suitable machine group may be currently or always busy with some other more prestigious jobs and hence, the second best choice has to be availed of. Thus, the tooling system has to be flexible enough to suit the available machine after making "known and definite changes" (like using different sets of tenons and clamping bolts). This has to happen with minimum trial and errors. It is standard practice to preserve some high accuracy machines only for important jobs to get better life. As such, the machine is occasionally kept idle instead of running sundry jobs on it and spoiling it permanently with reference to the superior work.

(c) Also, jobs not required in big quantities and in urgency may be run on slower machines. But these machines may be less sturdy, giving vibrations, hence causing chatter marks and needing compromise on quality or quantity or both.

Many engineering companies prefer not to have a full-fledged tool manufacturing set up (Tool Room). They would establish it very judiciously. They may critically identify their tooling requirements and develop vendors and buy from them regularly. Some may prefer to remain satisfied with only tool re-sharpening (Tool Club) facilities. Items like drills, taps, reamers, milling, cutters and turning tools, etc are always purchased as standard items from renowned companies, which are highly specialised in their fields and buying from them is ultimately more economical and trouble - free. Nowadays, throwaway tungsten carbide inserts are available for all these toolings. These are available in very wide ranges so that you have a choice very close to your machining conditions.

4) Outsourcing: Nowadays, there is a growing tendency to outsource large varieties of items. This also has to be often initiated by the methods engineer for which, naturally, he is expected to know the capacities of vendor companies. Make-or-buy decision is a critical techno-commercial exercise for any engineering industry. General criteria for make-or-buy decisions are as below:

Generally, companies will make the items under following circumstances:
• If they can make it cheaper than outside or if they have to rely upon limited number of vendors.

The process is vital for company's product and requires extremely close quality control.
• If the item is readily done with the company's existing facilities and is similar to other items running in the shop.
• If the item requires extensive investment in facilities that is not possible at supplier’s plant, like wire -cut, CNC machines.
• The item has stable and large demand.

Companies will prefer to buy items from outside suppliers, if:
(a) If they have no facility existing for these items.
(b) Existing facilities can be used more economically to make other items.
(c) I f persons with particular skills like die-makers are not available.
(d) Demands are very temporary or seasonal.
(e) Extensive investment in terms of sophisticated machinery, quality control equipments, etc will have to be made by the company. If these items are to be made in-house, then there are more profitable options for investing capital.

Factors considered for Oursourcing Items:
1. Generally, aggressively growing companies are interested in capturing new products and markets, and would like to throw open their routine traditional items to vendors. They use their own capital more profitably through horizontal diversification, whereas, declining or stabilised company will prefer integrating backward, forward and sideways, thus making more share of its own in the final value of output. Company can easily keep loading or shifting items to various suppliers depending on changes in requirements and designs.

2. Some items must be kept in-house to guard trade secrets of the company.

3. Periodical review of “make-orbuy” decisions is necessary as technologies are ever changing.

4. There are some illusions about overhead costs. For example, the tool room attached to the production shop may be less loaded but supposed to have higher overheads because of expensive machinery, measuring instruments, skilled workers and supervisors, etc. But if a small batch of production is occasionally diverted to the tool room for few days, there will be expenses only in the electricity, labour (if not idling), some consumable tools and coolant, etc.

5. You should always develop two or more suppliers and keep internal facility in working condition or else, a labour or taxation problem at the supplier may jeopardise your entire production.

6. While complicated operations like multi-drilling, multi-strip or follow-on type sheet metal working, etc are possible in-house with the help of sophisticated machinery, you have to break the operation into simple ones to make it possible for small vendors.

7. Some companies give their own space and some basic facilities for vendors. For example, a company manufacturing handling equipments can get hand operated trolleys, hand trucks and light HOT cranes assembled by the contractor placed in their own premises and try to concentrate more on EOT cranes of 50 tonne or higher capacities. Also, a foundry - cum - heavy engineering shop can make heavy casting of weights above on tonne and get lighter casting of about 10 kg by another small supplier.

8. In some cases, getting work done cheaper from outside serves as an eye opener and a good replacement of the complacent in-house production people, who may be working lethargically on equipped machines at higher costs and lower productivity.

9. About several decades ago, the trend was to have family groups of companies who buy and sell within or another to retain the profit in family. Now, that scenario is vanishing and you have to make the decision purely on the basis of techno-commercial feasibility and soundness.

Dialogue with Other Functions Methods Engineering department is a typical service department which has to act intermediate to many functions. In fact, quite often, it is taken for granted unless they are assertive. There are situations where they have to raise their voice and ask product design people to be practical and realize the limitations of manufacturing functions in general. This may need explanations.

In general and particularly in India, where practical shop floor experience is not considered as a must for design engineers, designers have a tendency to over-design to keep their side safe. This may lead to witting very close tolerances with a stroke of the pen. Accordingly, the tolls, jigs, and fixtures are to be made to close tolerances and even after making them so perfect, the products may not come out perfect as it depends on many factors. Rejection rate may rise, thereby disturbing the smooth flow of production.