Preface
The investment in SMT equipment is relatively large, especially the placement machine equipment is expensive. Effective use of equipment has always been the key to improving investment efficiency. Because Overall Equipment Effectiveness (OEE) takes into account all operating conditions of the equipment and is convenient for statistical calculation, it has been widely used in electronic manufacturing companies in recent years and has become the main means of implementing TPM.
1. Production efficiency indicators of placement machines
(1) Equipment usage rate
Equipment utilization rate (Operating Rate/Availability), also known as equipment operating rate and time operating rate, reflects the operating status of the equipment as a percentage of operating time to available working time. The main purpose of counting time usage is to strictly distinguish between planned equipment downtime and equipment unplanned downtime, and to improve utilization by reducing and controlling equipment unplanned downtime.
Calculation of equipment usage EA
EA =Ta/Tt
In the formula, Ta is the equipment use (start-up) time (Scheduled Running Time - Downtime), that is, the actual working time of the equipment, which is equal to the equipment working time minus unplanned (such as equipment failure/rework/unexpected waiting for materials) time; Tt is Equipment working time (Scheduled Running), also called load time, is working day time minus planned downtime.
(2) Performance efficiency
Performance efficiency, also known as performance utilization rate or equipment performance index, reflects the operating status of the equipment based on its performance and speed. Low performance efficiency means that the equipment has long downtime due to minor faults and the equipment reliability is poor; the speed efficiency is low, or the operator level is poor, and the performance of the equipment cannot be fully grasped and the potential of the equipment is fully utilized, or the equipment has been running for a long time and its performance has deteriorated. Deterioration and inability to work at full capacity.
The statistical performance start-up rate should accurately determine the theoretical cycle time of the equipment and the actual cycle time of the equipment. The theoretical cycle time is an important indicator of enterprise production efficiency. Only when the actual cycle time is accurately calculated can the production plan be accurately arranged and a zero-inventory production framework be achieved. Only by narrowing the gap between theoretical cycle time and actual cycle time can equipment performance efficiency be improved.
Performance efficiency EP is calculated as follows
EP=EO·ER
In the formula, EO is production efficiency; ER is speed efficiency, see below.
1) Speed and efficiency
Speed efficiency, also known as deceleration loss, reflects the difference between design speed and actual speed. Speed loss has a greater impact on equipment efficiency, and improvements should be carefully studied to reduce the difference between design speed and actual speed.
There are various reasons why the actual speed of the equipment is lower than the design speed or ideal speed, such as mechanical wear problems and machine quality defects, or equipment overload.
The speed efficiency ER is calculated as follows
ER=Ct/Ca
In the formula, Ct is the theoretical processing cycle of each product, or design cycle, set cycle; Ca is the actual processing cycle.
2) Production efficiency
Production efficiency, also called operating efficiency, is the ratio of effective operating time to available time. The main factor leading to the decline in production efficiency is various short-term shutdowns (downtime) in production.
The production efficiency EO is calculated as follows
EO=N·Ct/Ta
In the formula, N is the processing quantity; Ct is the theoretical processing cycle; Ta is the equipment start-up time.
3) Qualification rate
Qualification rate is the abbreviation of qualified product rate, also known as quality index, which reflects quality defects and rework in the production process caused by equipment failure. Usually, accidental defects are obvious and can be easily eliminated by resetting the equipment to normal conditions, such as a sudden increase in the number of defects or other obvious phenomena. Other defects are similar to certain chronic diseases, and their manifestations are not outstanding. The causes of the defects are difficult to find and are often missed or ignored.
The qualification rate EQ is calculated as follows
EQ=NQ/NT
In the formula, NQ is the quantity of qualified products; NT is the total processing quantity.
2. Overall equipment efficiency (OEE)
1. OEE and its calculation
Overall Equipment Effectiveness (OEE), also known as global equipment efficiency, is a simple and practical production management tool that has been widely used in the manufacturing industry. The global equipment efficiency index has become an important standard for measuring enterprise production efficiency. It is also one of the important methods for the implementation of TPM (Total Productive Maintenance).
Generally, each production equipment has its own theoretical production capacity. To achieve this theoretical production capacity, it must be guaranteed to be free of any interference and quality loss. OEE is used to express the ratio of actual production capacity to theoretical production capacity. It is an independent measurement tool that measures the performance of the entire manufacturing process.
OEE consists of three basic elements: utilization efficiency (EA), performance efficiency (EP) and qualified product rate (EQ).
OEE is calculated as follows
OEE=EA·EP·EQ
Calculation example of equipment efficiency of placement machine: Assume that the working time of a placement machine is 8 hours per day, and the planned shutdown is 15 minutes. In actual operation, it is shut down for 15 minutes due to failure, and the equipment is adjusted for 30 minutes when the product model is changed. The theoretical processing cycle of the product is 0.5 min/piece, a total of 800 products are processed in one day, and 20 pieces are returned for repair. Find the OEE of this equipment.
The calculation is as follows.
• Loading time: Tt=480−15=465min;
• Starting time: Ta=465−15−30=420min;
• Equipment utilization rate: E=420/465=0.9 (90%)
• The actual processing cycle of the product: 420/800=0.525min/piece; • Speed efficiency: ER=0.5/ 0.525=0.95 (95%);
• Production efficiency: EO=800×0.5/420=0.92 (92%);
• Performance efficiency: EP=95%×92%=87%;
• Qualification rate: EQ = (800−20)/800 =0.975 (97.5%);
• Overall equipment efficiency: OEE=90%×87%×97.5% =76.3%.
2. The essence of OEE
This is the ratio of actual output to theoretical output during load time.
3. Use OEE for loss analysis
OEE can be used to conduct major equipment loss analysis. First of all, through the analysis of each sub-item of the OEE model, we can easily find out the reasons that affect the equipment efficiency, and then solve the problems in a targeted manner to achieve the purpose of improving equipment efficiency. OEE classifies the main losses in equipment efficiency into 6 categories based on its 3 categories of elements, as shown in the table below.
4. Advantages and disadvantages of OEE
Compared with traditional methods of calculating equipment utilization, OEE has clear advantages. The traditional method only focuses on available efficiency. OEE also takes into account efficiency and quality factors. It enables decision-makers to pay attention to multiple aspects of the equipment at the same time and take corresponding adjustment measures. However, OEE also has its own limitations. It targets a single device and lacks a global optimization perspective. Although it is convenient to calculate OEE, a large amount of accurate data is needed to analyze its corresponding parameters. Due to the dynamic nature of production and changes in the environment, these data are not easy to obtain. To improve OEE requires a lot of manpower and material resources, which may involve every aspect of the organization, such as production, maintenance and process departments.
If OEE is only used as an internal efficiency indicator, its contribution to the production system will be meager. Therefore, OEE needs to be combined with a systematic perspective to achieve overall improvement of the production system.
5. TOC and OEE applications based on TOC
(1) Introduction to TOC
Theory of Constraints (TOC) is also called the production management chain effect. In the entire business process of an enterprise, any link that prevents the enterprise from increasing effective output or reducing inventory and operating expenses to a greater extent is a "constraint", often also called a "bottleneck."
TOC theory holds that for any multi-stage system, if the production of one stage (or process) depends on the output of one or several previous stages (or processes), then the one with the lowest output rate will The link (or process) determines the entire output level. To use a vivid metaphor, the strength of a chain is determined by its weakest link. This is consistent with the philosophical thinking method of grasping the main contradiction.
The core of TOC's management thinking is to improve efficiency and reduce losses. We must first grasp bottlenecks, rather than comprehensively grasping them and "one size fits all." Through analysis, the most serious constraints are highlighted, thereby technically eliminating the possibility of management shortcomings such as "grabbing eyebrows and beards". The short-term effect of TOC management thinking is to "grasp the big and let go of the small", and the long-term effect is that both big and small problems are not ignored, so that the overall production level and management level of the enterprise are increasingly improved.
(2) OEE application based on TOC
From the above discussion of the advantages and disadvantages of OEE and TOC, it can be seen that the two can learn from each other and combine with each other to improve the overall efficiency of the system. Specifically, TOC provides OEE with a systematic perspective, making OEE a process-oriented performance indicator; OEE provides TOC with quantitative analysis methods to enhance TOC's improvement capabilities. The TOC-based OEE method takes into account the advantages of both and meets the systematic and quantitative requirements for production system improvement.
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