This subject probably has been discussed before. I am looking for suggestions on a production schedule. We are a 3-4 man shop. We produce 1 product and most commonly 3-4 different sizes. There are random orders of 16 different sizes also.
There is also 12 different steps from start to finish. The steps range from 20 parts per hour to 45 parts per hour.
For production scheduling, people adopt various methods which include whiteboards, heijunka, kanban, CONWIP,
quick response manufacturing, Excel applications, drum-buffer-rope method of TOC, MS Project (scheduling software), finite capacity scheduling (enabled by software), etc. Some people may even make simple mental calculations for scheduling using their experience, intuition and commonsense. Each one of these methods is indeed effective in some cases but their relative merit varies with the situation.
I understand that:
1). Your job shop receives orders for different sizes and different quantities of a product probably with some uncertainty
2). Processing of any order involves 12 steps
3). Cycle time varies with processing step
I assume that a worker can perform more than one step but one at a time. I do not know whether:
1). All 12 steps are to be done in the same sequence for every order or the sequence may vary with order or some of the steps can run in parallel if required resources are available
2). Workers have different skill sets
3). Some steps need both machine and worker
4). Some steps require more than one worker simultaneously
5). Some machines are to be shared for different steps of production.
6). Material moves from one step to another either piece by piece or in terms of a single batch or a few small transfer batches
7). Two or more orders progress on shop floor simultaneously.
All these issues have an impact on production schedule. I wonder how you are able to (a) Quote a rational lead time for a new order given the existing workload and requirements and availability of meager resources and (b) Fix a right start time for the order. You may lose the order itself if you quote a long lead time. On the other hand, you and your workers may have a serious problem if you quote a lead time shorter than achievable. If you start an order too early, its production lead time may be longer and clutter the shop without any gain in the completion time due to the presence of earlier orders. On the other hand, if you start an order relatively later, you may miss its due date and lose some throughput.
Sometimes, resources may be unexpectedly unavailable due to various reasons. Changing order priorities or expediting an order can cause a major effect on delivery times of orders. Even though job shops are small in size and revenue, their production management is not easy in general. But, perpetual firefighting will hide the intensity of the problem.
I usually propose software-based finite capacity scheduling which provides prediction of workflow and bottlenecks, determines order lead times and right start times for orders and enables what-if analysis and capacity planning. Some job shop people feel it is too rigorous for them.
I fully agree with prasad. We are in the process from changing from an excel based manual schedule to shedulize which prasad developed. It is going well and will be much better as well as very scaleable. If you are a four man shop it would be easy to implement and let it grow with your shop.
Thanks for asking very relevant and right questions. Here is my response.
Our model handles the efficiency level / speed of workers to account for worker-dependent process times. It also considers preference levels for workers for any given skill. It allows a worker to have multiple skills with varying preference levels.
If some task requires more time from a worker than the estimate for any reason, let the worker continue until it is completed and select the next task in his task list that is generated as part of his schedule. We will update the schedule later as part of regular schedule revision. It is much easier to handle unexpected changes of moderate size by asking resources to implement task sequence rather than his schedule with specific start and finish times of tasks. The task sequence will absorb a lot of variation. If a task in the sequence (for a worker) is not ready at the required right time, then the worker can proceed with the following tasks in the sequence until this task is ready.
The actual mechanism for implementing this optimization is based on rigorous, finite capacity scheduling logic (my pet subject) and implemented on computer with the help of software.
You asked, "Is it theoretical optimization or does it actually drive the shop? Do you need someone carrying a clipboard to update status or is this done at the level of individual worker?".
Many real job shops have been regularly using our scheduling software, which is based on rigorous, scientific logic. As a past academician and researcher, I am happy that I could bring such rigorous, scientific scheduling logic to even small shop floors in real world.
Somebody in the shop (preferably scheduler) has to update the progress of each ongoing project (job) in our software whenever the schedule is to be revised.
So does finite capacity scheduling create a system where work orders are not released until capacity is available to perform the task?
Would this work to help keep work in process at a minimum? Anything 2nd priority projects sitting around your shop that are half done competes for resources (carts - floorspace etc) that could be brought to bear on 1st priority tasks.
There are also the unmeasurable (but nonetheless real) extra management costs associated with building something before it is needed.
Passing out tasks when they need to happen is hard enough without having to keep track of the things that didn't need to happen. Just keeping track of what has been done and what remains to be done is complex. Quite often you have to spend dollars to mop up the negative effects of over production. You certainly commit space to it and this space could instead hold a new work station or minimize transport costs navigating station to station.
The time and resources spent on these misfires could be better spent with Prasad's program. You're going to spend it anyway, you might as well get something for it.
So does finite capacity scheduling create a system where work orders are not released until capacity is available to perform the task?
Work orders will be released at right times so that they neither sit on shop floor for long time waiting for bottleneck resources nor they unnecessarily get delayed leaving a risk for late delivery, throughput reduction and resource underutilization. I am giving a link below for illustration.
Would this work to help keep work in process at a minimum?
The given link should answer this question affirmatively. The number of horizonal lines that intersect a vertical line representing a time point on either diagram represent the number of jobs in process at that time point.
The yellow color in the second diagram on the web page represents controlled waiting times of jobs before bottlenecks.
Seems like being "Lean" means eliminating unnecessary things. Start by defining the processes required. Make a flow chart for each product. Entry & exit points for processes/components. Where will a buffer be justified? Can sorting be done @ the off loading point from a machine, rather than a separate operation? When is another job started? When the current job is finishing the first operation or when the buffer is nearly empty. How do I tell when there is a problem in the flow? A buffer is either full or empty.
If that is the case you adjust resources, move labor. A properly designed system will make it obvious to employees when they need to shift. No management input required other than training.
Systems for handling small parts can work by just having tables between operations. No walking, no pushing carts.
Bigger systems can use roller conveyors & transfer cars.
The ideal system will allow visual control with little management and no computer inputs. Feeding a computer information that is not fully automated is a waste. Is the customer willing to pay for it?
Scheduling the supply of work is based on tracking production times and is based only on the limiting process.
Visual management is simple if the system has been designed correctly. You won't waste time counting, putting info into a computer. Computers are useful for the operations that take place before manufacturing.
Gantt charts are useful for setting up the manufacturing system. By analyzing flow charts you may be able to determine where product modifications will improve efficiency. KISS!
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