The essence of production scheduling and work flow control can be boiled down to this one imperative: each facility should be working on the right job.
The challenge, of course, is trying to figure out which job to work on next. When a workstation completes a job and passes it on to the next operation or location, the operator selects the next job to work on. In everything but a continuous flow or synchronized flow (production line) situation, the next job is selected from a pool of jobs waiting at the workstation (in the queue, as they say).
“The real problem is the backlog of work at each and every workstation in the typical factory”
Production scheduling and control systems exist to address this question using various priority schemes. The shop floor scheduling system presents the operator with the list of jobs, each with a priority assigned: The operator can select the job that has been waiting the longest (first in-first out), the one with the closest operation or order due date, the one with the most (or least) work to be completed, or more sophisticated priority calculation like slack time, slack time per operation, or critical ratio might be used.
Priority schemes, however, are band aids trying to stem the bleeding from a severed artery. The real problem is the backlog of work at each and every workstation in the typical factory. Wait time in queue makes up 90% or more of the total lead time to complete a job in a typical non-flow factory. And that amount of work-in-process in the plant takes up space, ties up money and resources, and is the reason we have to have these complex production scheduling systems in the first place.
We can’t completely eliminate work-in-process queues in a discontinuous production plant. They are necessary to support efficient machine loading for good utilization rates and to allow some flexibility for minimizing set-ups and changeovers, among other reasons. But we can and should minimize work queues. The simplest way to do that is to reduce the input of new work to the floor below the output rate of the plant. But that’s not the smart way – it indiscriminately lowers WIP with no consideration of load/capacity balance or manufacturing schedules and priorities for the work flowing through the plant.
The smart way to reduce WIP is to release jobs only when the time is right
The smart way to reduce WIP, which also reduces overall lead time for work flowing through the plant, is to release jobs only when the time is right. And the time is right when there is just enough time, including protective buffer, to complete the job by its due date. Releasing jobs any earlier than that only serves to clog up the shop floor with WIP and add to the confusion about which job to work on next.
After all, a work center can’t work on a job that’s not really needed yet if the job has not been released to the floor. Intelligent release of work to the plant makes work sequencing and production scheduling a lot less challenging and a lot more effective. Work flows through the plant quicker, lead times shrink, and there’s a lot less company money tied up in unnecessary WIP. Needless to say, proper release of work is one of the guiding principles of Protected Flow Manufacturing.
The Lilly family has been innovating software for manufacturing companies since 1960. LillyWorks serves only manufacturing companies, putting Manufacturing Back in ERP™. LillyWorks is headquartered at One Liberty Lane, Suite 210, Hampton, NH 03842. Telephone 603-926-9696 or visit www.lillyworks.com.
Frustrated by having to tell customers you’re going to be late? Watch A Better Way to Schedule (video)