Although we call it “transportation time,” this challenge also occurs when there are other delays between steps in the process. For example, instances of transportation time in job shops might occur when:
A robot transports products from one manufacturing process to another.
The application requires a minimum time for heating or cooling of a product between processes, e.g. metallurgy or food production.
Products have a minimum setting time, or chemical reactions need time to happen between operations.
In practice, almost all real-world applications include transportation time between processes. It’s a good idea to include these times in your scheduling, although you may be able to omit them if they are significantly shorter than the processing times.
2. No-wait or maximum delay
The opposite of transport time is a “no-wait” constraint, where the delay between processing operations should not exceed a maximum time. The next operation must start before the maximum wait time or the product will be ruined.
The no-wait constraint usually occurs when products must maintain a workable condition, e.g. due to heat or chemical reactions. Instances of no-wait scheduling constraints include:
Applications where operations must be finished before a chemical reaction occurs, e.g. hardening of polymers, setting of cement, etc.
Products that must maintain a certain temperature while being worked on, e.g. hot metal, cold ice cream, etc.
Applications where airborne contaminants could compromise product quality or integrity, e.g. food contamination, static interference of electronics, etc.
Products on a continuously moving conveyor belt, which means that the robot or operator doesn’t determine when the product enters or leaves the operation.
No-wait conditions are restrictive and can make scheduling extremely difficult. Try to reduce the number of no-wait conditions to only those that are absolutely necessary.
3. Blocking
The final constraint is “blocking,” which means the operation is halted until the product is passed onto the following operation. It indicates a process where there is no “buffer” between the operations – e.g. boxes or piles of Work in Progress.
Blocking is one consequence of the lean manufacturing drive to eliminate buffers between operations. For example, imagine a robot must move a part from a CNC lathe to a milling machine without putting it down. The robot must wait for the milling machine operation to finish before it can move the part from the lathe. This is a blocking constraint.
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Blocking is less restrictive than the no-wait condition. However, it can introduce some particular challenges, such as deadlocks – where each operation is waiting on the other to finish and none can continue.
How to solve job shop scheduling problems
There are many solutions to The Job Shop Problem, but they all involve somewhat lengthy and complicated mathematics that are beyond the scope of this blog post. However, it helps just to be familiar with the three common scheduling problems – transportation, no-wait, and blocking – so that you can better schedule jobs in your machine shop.
This article has been republished with permission from Robotiq.