the feed direction

000). The volume of cated in T,tble 1.

from 3000 to 20000 mm from 1000 to 1500 mm from 2000 to 3000 mm

nun. High facture of

most. Therefore, the concept of thc easy iiiachine is aiined at reaching a i

and their treatnient Working environment

specific milling stres~ is very low. unmanned shifts. This solution is linked desigri where ‘easy = reliability’, and consequence niaterials such as cast evalu,~ion of both the positive and and steel have to bear les stresses, reduce the time necessary to carry iicgative aspects ofthe vciriou\ solutions,

to better maintainability in order to

s during subsequent stop . As a describing the behaviour of a popula- automotive industry, come avail- tion of machines in order to find the

Fig. I Subdivision of the machine into macrogroups

electrical system

hydraulic 1 system

pneumatic system

accessories

transducer

devices

methods and the service organisation are also to be taken into account since they are strictly coiiiiected to the coiiipetitiveness on the market, which is a primary factor of the relationship of the company with the world. The time and way to fix this goal have to be found in the design phase so as to foresee coinpleiiientary phases of teleservice and maintenance, allowing a high availability of the product. However, it is necessary to make intervention to restore the machine functions easy, particularly by means of diagnostic instruiiiciits which can act in real time on the machine. These allow the diag- nostic information of the machine to be available to the nianufacturer in order to give instructions to solve the technical

problem remotely (teleservice), thus reducing the iiiachiiic downtimes and assuring the continuous fmtioning of the machine.

Techniques for and machine design

Lluriiig the development of the product, the critical aspects are also aiialysed. The product is critical because the loss of efficiency could compromise very expensive mechanical workpieces such as moulds and dies. A product can also be critical due to problematic accessibility in case of failure or when the product can only be reached with high cost.

The critical state of the mechanical components must be considered sepa-

rately to the electrical and software components, which present different kinds of breakdowns. The mechanical component wears out with its usage, the electrical coiiipoiient breaks down suddenly and is subject to statistical laws; the software can fail according to the testing carried out. Each product must have its own reliability because it is characterised by a cost that increases according to the qualitative level. The more a product is reliable, the more the cost for maintenance is reduced; therefore the design iiiust take into consideration all functions that can assure the availability of the product.

Sometimes the failures are deeply examined (modes, effects, critical state, frequency) in order to achieve

rly:

//-

Fig. 2 Operative methodology

I66 MANUFACTURING ENGINEER AUGUST 2002

the availability needed. There are a lot of techniques allowing evaluations on reliability’,4 (FMEA, FMECA.. .) and sometimes they are used together to obtain complemental-y results.

There are two catcgorics: instruments receiving inputs of the Eund of failure and giving outputs to tlic effects, and instruments which record the break- down as an input and search for thc causes (failure tree), Thc former examines the fuiictional aspects of the system, the latter examines the safety aspects.

Index for the calculation of availability

The concept of ‘time of reliability’ and ‘time for maintainability’ of a machine create the parameter of avail- ability in a percentage. Fig. 3 shows this parameter, where MTUF (mean time between failures) is tlie average time between the two following breakdowns (time of reliability), MTTll (mean time to repair) is the average time to repair the breakdown (time for niaiatain- ability), aiid G = number of failures.

The VDI 3423 standards determine the method to measure and calculate the availability of higli-teclinoloLy produc- tion systems, both for the automatic niachine and the production system, taking into consideration the inacliine downtimes due to technical and orga- iiisational reasons.

The causes of niachine downtimes must be found very quickly in order to elinlinate and prevent them. The calculation of availability for high speed machines is made in compliance with the iiitcriiational regulations (VI11 3423), modified according to the type of machine and in cooperation with the Italian association of machine tools (UCIMU). Furtherniore, MECOF has drawn up a handbook5 supplied to the customer, providing the instruments to analyse the machine down-times due to technical aiid organisatioiial reasons. The customers shall updatc the hand- book and send MECOF both tables for standard maintenance aiid tables for iiiilling machine downtimes in sucli a way as to detect the causes and find the remedy. The iiiacliine availability i s

calculated as follows:

availability

reliability

G

e3 maintainability G

MTTR = ztRi -

G

Fig. 3 Availability as a function of reliability and maintainability

where N = availability of the iiiilling iiiachiiic as a percentage, Tb = time foreseen (calculated in hours) for the availability of the iiiilling center 14200 hours], Ti = time iiccessary to repair the

iililliiig niachiiie (calculated in hours), and Tn=Tb-Ti = time of usage of the riiilliiig machine (in hours).

The availability time (Tb) is tlie total time calculated in hours and corrcs- pond^ to die working hours 011 three daily working s h i f t s iiiultiplied for the working days and the months where the milling center is considered available ( L I S L ~ ~ 9 months). In any case, Tb cannot be lower than 6000 hours/ year equal to 4200 hours/c) months. MECOF availability is N=93+1% and is calculated after 90 days from date of installation of the milling center and during the nine months following the above date or until the elid of time for inachiiie usage. If this value is reached

during the last six months of the warranty period, the contractual obliga- tions have been fulfilled with respect to the availability level.

Conclusions The quality of a product is based

on the fulfilment of the user’s needs. Therefore, it is necessary to use all means to assure quality during the study/design phase. Taking iiito account tlie high performance of high speed milling machines, it is also important to orient the machine dcsigii in order to obtain both reliability and availability. In fact, it would be illogical to increase the ciynaiiiic pci-foiformanccs if the machine cannot carry out the workpiece machining and has to stop.

It is iiccessary to exaiiiine tlic iiiachiiie defects, considering the machine as a whole where various technologies and system are involved (pneumatic, hydraulic, electrical, elec- tronic, data processing). As a conse- quence, it will be possible to act on the project and improve the product continuously.

References 1 MECOF S.p.A: ‘HSC handbook volunie 1 ’ 2 E.lLepetto ‘High speed cutting, compara-

tive aiialysis between milling machines’ PRIME 2001

3 FOllD R&M handhooks 4 FORI) Ilocumentation Q1 5 MECOF S.p.A ‘Handbook on the avail-

ability calculation’

0 IEE 2002

MANUFACTURING ENGl N EER AUGUST 2002 I67