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    Why job shop scheduling starts with defining resource capacity [video]

    Elmar Karlowitsch
    November 30, 2020
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    If you run a high-mix low-volume machine shop, it is both challenging and indispensable to schedule your order backlog is a big challenge but indispensable. I've spoken with a very large number of production planners and job shop owners about scheduling. They all confirmed - without exception - that everyone is planning his shop floor operations in some way. That means: shop floor planning is seen as too important to be be left unmanaged. 

    Many people I spoke to also confirmed that they built their own "tools" themselves to cope with this challenge. In that regard, Microsoft Excel is by far the technological basis and starting point to create a home made capacity planning.

    In this blog post I show you how to best use Microsoft Excel for capacity planning - and why this is also an appropriate first step to get going with job shop scheduling.

    How many job shops start production scheduling with Microsoft Excel - and why this dangerous

    As Excel is a spreadsheet software, the structure forces the “planner” to think in timeframes. So the first step is to decide which period is represented by a column. The most preferred approach I have seen is “weeks”. Hence the capacity for work centers are defined and checked against the backlog on a weekly basis.

    Let´s make an example: there is a shop with

    • 2 saws
    • 3 surface treatment machines
    • 2 CNCs
    • 2 edge benders

    The shop runs a one shift business with eight hours per day from Monday to Friday. Following the capacity of the week approach this results in following capacity per work center and week:

    • Sawing: 2 Machines* 5 days* 8 hours = 80 hours per week
    • Surface treatment: 3 Machines* 5 days* 8 hours = 120 hours per week
    • CNC: 2 Machines* 5 days* 8 hours = 80 hours per week
    • Edge bending: 2 Machines* 5 days* 8 hours = 80 hours per week

    This is inserted into a spreadsheet accordingly. Then the backlog is “assigned” to the work centers manually while checking the overall capacity per work center.

    Job Shop Scheduling with Microsoft Excel

    Although this approach leads to a kind of plan, it has serious disadvantages compared to a real finite scheduling. Let's have a closer look the three most crucial issues among many others.

    Weakness #1: Rigidity of timeframe

    • The capacity check will always be weekly focused. This means that there is an average consideration if the capacity of one week is exceeded or not. If operations are lasting less than a week – which is the pretty normal case in job shops – the possible earlier starting date for succeeding tasks is not considered as the planning method implies that all tasks are operated simultaneously within the week bucket.
    • Also, all operations need to be divided up into “work portions” if they last longer than one week on one machine. If you would not do so, the entire work center would be loaded with one operation which – due to technical reasons – often is not possible in reality. In this case the earliest possible start date for the successor would be indicated too early.

    Weakness #2: Routing structures are not considered

    • As Excel is a spreadsheet, links of operations cannot be defined. More than 95% of high-mix low-volume production orders consist of multi-staged job routings. Another characteristic of this type of business is the high dynamic and the high volatility of data. Hence the need to adjust the schedule is a challenge that occurs several times a day, e.g. when a machine brakes down, a tasks needs to run longer than estimated or when you get an urgent order that needs to get squeezed into your schedule.
    • Repositioning an Excel-based schedule is a time consuming Sisyphus' task. Most planners gather the incidents and update the schedule on a weekly basis in one go which impressively shows the poor agility of this approach.

    Weakness #3: Limited employee capacity not considered

    • The approach focusses on the finite work center capacity. Employee capacity is only considered via taking the operators work shift for the availability of machines. This is only correct if the given staff is always able to handle any workload and never becomes a bottleneck itself.
    • The reality in the vast majority of high-mix low-volume manufacturers is different. Here the unsightly peculiarity is that both machines and employee limit the throughput regularly. Hence the machines and limited manpower capacity must be scheduled simultaneously. Furthermore every operator has an individual skill set. This means that not every member of the staff has the education to operate every machine on the shop floor. Therefore, a skill matrix that defines the given capacity of needed skills is essential for productive finite job shop scheduling.

    How high-mix low-volume manufacturers shops SHOULD leverage Excel for job shop scheduling

    The inadequacies of this approach point the way to how to do it right. Finite scheduling starts with defining capacity of individual resources in two steps:

    Step 1: Define every machines' and employees' capacity per weekday.

    This is fairly easy. Just list every machine and every employee of your business that you want to schedule. Then define the standard availability per every weekday. If every machine task must be operated by one employee the machines can get a 24/7 availability while the contractual working times define the manpower availability.

    Job Shop Scheduling with Microsoft Excel

    Step 2: Spread out a skill matrix that defines the capability of each resource.

    First, list all kind of operations where manpower is needed. Together with the list of resources of step 1 this spreads out a skill matrix, where you can define which of your worker is capable to operate which task.

    Job Shop Scheduling with Microsoft Excel

    Additionally you should also structure your machines in resources groups. Here, machine resources should be bundled that can be taken alternatively. So in the end spread out a skill matrix for machines also.

    With this exercise, the most important step for successful finite high-mix low volume scheduling is done.

    If you want to get going, download our Excel template for defining finite capacities.

    Have a look into the video to see how easy this works: 

     

    PS: Want to learn more about job shop scheduling? Read our ultimate guide to visual job shop scheduling.

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