 |
 |
 |
|||
 |
|
||||
 |
 |
 |
|
||
 |
|
||||
 |
|
||||
 |
|
||||
|
Vol 30 - No 08 - December 2004/January 2005 Letter Dear Editor, I think there may have been some inaccuracies in Issue 10 of the sKnowledge Bank in CONTROL Volume 30 No. 5 concerning Cellular Manufacturing. Whilst I agreed with much of the article, I was surprised on two points. Firstly, the second statement in the ‘definition’ paragraph: ‘The cell is organised with a repetitive flow layout’, and secondly the omission of what I believed was the important and distinctive characteristic of cells: that they contain dissimilar machinery and/or processes located close together (the latter was barely implied but not stated). My understanding of cellular manufacturing is that the key objectives are to achieve process simplification and flow layout outside of the cell, and then to exploit and product similarities which defined the cell to reduce set-up or processing times, improve material handling and possibly to take advantage of local (cell) operator initiative to manually sequence in-cell work. Inside the cell there may, ideally, be flow layout, but this is not an absolute requirement, or in some cases even desirable. Also in the ideal situation the piece part or assembly is completed within one cell, but again this is not an absolute requirement. It is the dissimilarity of machinery or processes within the cell which permits more processing steps (compared to functional group layout) to be completed with less material movement for any individual item or batch. It is also the flexibility within the cell which may permit operations to be completed out of sequence at the discretion of the cell operators, who may be free to organise the work within the cell according to the current mix and priorities. The flexibility of processing may also permit multiple familiesof products to be produced in a cell – not just one. There may be family-change set-ups, but these may be outweighed by reduced or eliminated in-family item changeover times, or by having the flexibility to run a family on multiple cells. If the main point of cells, as I suggest here, is to achieve simplified flow layout outside of cells, then planning may actually be reduced, at the one extreme, to a cell-level (ie. single cell or inter-cell) just-in-time or pull system, with cell operators informally managing the more complex intra-cell sequencing and scheduling. However, using modern advanced planning and scheduling systems (APS) all operations between and within cells might be finitely planned, scheduled and sequenced automatically. The purpose of the cellular model in this case may not be to simplify planning, but may be to reduce material movement, reduce set-ups, to simplify process costing or better allocate costs to products. This contrasts with the article’s assumption that cells must be flow-lines for which obviously little or no in-cell planning, scheduling or sequencing is necessary. The emphasis in the article on dedicated tooling, FMSs, CIM and flow layout within the cell, which are all good systems – essential even – if appropriate or possible, seems to miss the point, which, I believe, is this: that rapid changes in product designs and product mix along with low batch sizes and where dedicated tooling (including FMSs) and pure flow lines cannot be justified, do not preclude the use of flow planning methods between cells. Conventional machines can still be used in cells, because the emphasis is on the ability to perform dissimilar processes within them. And because material movements are localised as much as possible, better material planning and control should be achievable than for functional group layouts. Anyway, a potential problem with dedicated tooling, including FMSs is that it is often very expensive, so alternative routing may not be possible or may be too expensive to justify switching. This effect can be made worse with CIM, because re-programming may be required to switch to another FMS/cell. Cells of conventional machines may be more flexible allowing operations sequence changes or alternative machines to be used, or alternative cells may be used instead. I am not advocating the wholesale return to conventional machinery, or deny that a modern machine tool may be considerably more flexible than several conventional ones. Neither do I deny that by rationalising piece products into part families with similar processing characteristics and designing new items to further rationalise these, you may facilitate the adoption of CIM- and FMS-based cells (within the broader flow layout of a cell-based shop floor), and I agree with most of the article where this applies. As a friend pointed out, for many FMCG verticals the article would be spot-on. However, I do believe the article’s emphasis and some of its assertions are wrong for the reasons I have mentioned. I also think the reference to ‘McDonald’s’ as an FMS is unhelpful and I question the validity of any manual production line (with zero changeover times or not) as being an FMS: most definitions I’ve seen – including the APICS Dictionary – specify computer controlled machine tools and automated material handling. I think anyone advised to invest in an FMS would be dismayed if they got anything less. Finally, I would like to point out that my knowledge and experience on this subject may be out of date. I studied Group Technology (obviously including cellular manufacturing) at university back in the late eighties and again during my spell as a manufacturing specialist consultant for an ERP vendor in the mid-90s when I used as reference a late eighties APICS publication on cellular manufacturing concepts and practices. I don’t recall seeing any significant changes to the concepts in the manufacturing or IOM journals since then, but if there has been a shift in the accepted principles I would very much like to learn more. Are my thoughts simply out-of-date, or have I misunderstood what was being said 15 years ago or perhaps in the Knowledge Bank article? Yours faithfully Paul Bird, MIOM Cachinnaire Limited Related Topics:
|
