Defining the six big losses
One of the major goals of Total Productive Maintenance (TPM) and OEE programmes is to reduce and/or eliminate what are called the Six Big Losses - the most common causes of efficiency loss in manufacturing.
- Breakdowns (tooling failures, unplanned maintenance, general breakdowns equipment failure)
- Setup and Adjustments (setup, changeover, material shortages)
- Small Stops (components jams, obstructed product flow)
- Reduced Speed (under design capacity, equipment wear)
- Start up rejects (scrap, rework)
- Production Rejects (scrap, rework, in-process damages)
Mr DMA Kulasooriya
Addressing the six big losses
Now that we know what the six big losses are and some of the events that contribute to these losses, we could focus on ways to monitor and correct them. Categorising data makes loss analysis much easier, and a key goal should be fast and efficient data collection, with data put in for use throughout the day and in real-time.
Eliminating unplanned Down Time is critical to improving OEE. Other OEE factors cannot be addressed if the process is down. It is not only important to know how much Down Time your process is experiencing (and when) but also to be able to attribute the lost time to the specific source of reason for the loss.
Setup and adjustments
Setup and Adjustment time is generally measured as the time between the last good parts produced before setup to the first consistent good parts produced after setup. This often includes substantial adjustment and/or warm-up time in order to consistently produce parts that meet quality standards.
Tracking Setup Time is critical to reducing this loss, together with an active program to reduce this time. Many companies use creative methods of reducing Setup Time including assembling changeover carts with all tools and supplies necessary for the changeover in one place, pinned or marked settings so that coarse adjustments are no longer necessary, and use of prefabricated setup gauges.
Small stops and reduced speed
Small Stops and Reduced Speed are the most difficult of the Six Big Losses to monitor and record. Cycle Time Analysis should be utilized to pinpoint these loss types. In most processes recording data for Cycle Time Analysis needs to be automated since cycles are quick and repetitive events that do not leave adequate time for manual data-logging.
By comparing all completed cycles to the Ideal Cycle Time and filtering the data through a Small Stop Threshold and Reduced Speed Threshold the errant cycles can be automatically categorized for analysis. The reason for analyzing Small Stops separately from Reduced Speed is that the root causes are typically very different.
Startup rejects and Production rejects
Startup Rejects and Production Rejects are differentiated, since often the root causes are different between startup and steady-state production. Parts that require rework of any kind should be considered rejects. Tracking when rejects occur during a shift and/or job run can help pinpoint potential causes, and in many cases patterns will be discovered.
Often a Six Sigma program, where a common metric is achieving a defect rate of less than 3.4 defects per million "opportunities", is used to focus attention on a goal of achieving "near perfect" quality.