There have been countless times that I have visited machine shops and seen first-hand examples of perceived efficiencies that are not efficiencies at all. One of my biggest "pet peeves" is the "start the first operation on all the parts, then tear down and start the second operation on the same parts" approach to machining, which is often taken in the name of efficiency. In this "efficient" approach to machining, a company may spend a lot of time and effort designing fixtures to optimize the machining of each individual operation, without giving thought to how long it takes (not in minutes, but in days or weeks) to complete the entire manufacturing process.
For example, many companies will machine 100 pieces in "Position A." They may even invest in a tombstone-type fixture to line up as many parts as possible so the machine can keep running. When all 100 pieces are finished in "Position A," they will be set up for machining in "Position B." Sometimes the machining process ends here; quite often, however, these parts require more operations. The end result of this approach is many hours expended before we get our first complete, useable part.
I don't like this approach to machining because it creates an unnecessarily long cycle time (total time required to machine a part complete). When cycle times are long, parts stay in the shop taking up space. What is worse, you cannot get "anything" until you finish "everything." If a customer (either an internal customer such as an assembler, or a paying end-user) needs a few parts, you are not in a position to provide anything until you start and finish the final operation. Until that time, everything is still in work-in-process and unavailable.
Compare this approach to machining with the more effective approach of making a part complete in one setup or as few setups as possible. Edward Hoffman, a former columnist for Modern Machine Shop, says, "Pick up a part and don't put it down until its finished." This simple concept is the key to true manufacturing efficiency. There are many advantages to this concept, including the following.
- Parts are finished in a shorter timeframe. By this I don't mean the parts are machined faster; actual cutting times should be the same. What I do mean is that the parts will be ready for use sooner, as it will take less time from the start of the manufacturing process to the end.
- Quality problems are discovered sooner. In the event a part is machined "out of spec," the condition will be found earlier and therefore should be present on fewer parts. There will be enough time to adjust the machining process before more bad parts are produced. This can represent a significant cost savings because, as a rule, the cost of a defective part is directly proportional to the amount of time (and effort) that is expended before it is found.
- There will be less Work in Process (WIP) inventory. Generally, WIP inventory is a bad thing, as it represents an expense that cannot immediately be transformed into revenue. WIP inventory is not useable inventory. In multiple machining operations, WIP inventory piles up at each operation. Not only can this create a storage nightmare, as we try to cram parts into limited spaces, but it can result in misplaced parts. We all spend a considerable amount of time looking for things, but WIP inventory should be one less thing we have to look for.
- Handling and other non-value added activities are reduced. The fewer the operations, the less handling required. Picture the following scenario. Ten parts are machined in operation A. When the operation is finished, parts are removed from their fixtures and placed into a skid. The skid is then moved to another machine (or it remains by the first machine if it is to be used for the second operation as well). When it is time to start operation B, all parts are removed from the skid and loaded onto fixtures and machined. When operation B is finished, the parts are again removed from the fixtures and placed in a skid. This process is repeated until all the parts are completed.
None of these handling activities adds any value to the part, but each adds time and cost. Likewise, part transportation, queue time, paperwork processing, counting, inspecting and other activities associated with a multiple operation approach to machining, are not adding value and can all be reduced or eliminated with fewer operations.
For these reasons, you should make every effort to minimize the number of operations used to machine a part, even at the expense of what you may perceive as "efficiency."blog comments powered by Disqus