Questions from an Industrial Engineer in an Automotive Machine-Shop

I received the following questions from an Industrial Engineer (IE) who has recently moved from vehicle assembly to the machining of car engine parts, blocks, heads, crankshafts, etc., activities that all new to him:

  1. Any reading material you would recommend?
  2. Is takt based off the slowest machine or the machine in the line that makes the least parts?
  3. Knowing cycle times and uptimes of a 30 machine line how do you calculate system uptime?
  4. Should there be more overspeed for machines at the beginning half of the machine line?

Following are my answers:

Any reading material you would recommend?

Industrial Engineering, as taught in universities, is generically about how people work, and gives you no process-specific knowledge. Manufacturing Engineering (ME), on the other hand, is heavily focused on metal working, as if these were the only processes worthy of the name “manufacturing.” To be effective as an IE in a machine shop, you need some familiarity with whatever processes are performed in your shop, such as turning, milling, drilling, reaming, broaching, grinding, or heat treatment. You don’t need to master them, but you need to know them enough to have a meaningful conversation with specialists. And you also need to know about the key operational issues of the machines used to perform these processes, such as lathes, machining centers, drill presses, etc. including how parts are loaded and unloaded, jigs and fixtures, cutting tools, and NC programs.

You will find more than you need to know in books written for MEs, like Degarmo’s Materials and Processes in Manufacturing. I would not attempt to read it cover to cover but instead use it as a reference, to cram on any process you are actually working on. There are other similar books, but this one was co-authored by J.T. Black, who was quite possibly the first American academic to recognize the significance of Lean and make it central to his teachings. My own book Working with Machines is about all types of manufacturing activities that involve the interaction of people and machines which includes automotive machine shops. It includes discussions of takt time, OEE, and availability.

The two main industrial applications of machining are automotive and aerospace, and the two are quite different. In automotive, you remove small amounts of metals from many parts that have been cast or forged near their final shape, in commonly available alloys and in high volume; in aerospace, by contrast, you remove 90%+ of the metal from slabs or forging that look like caskets in exotic alloys and in low volumes. Your needs are in automotive, so don’t waste your time studying approaches that are only used in aerospace. The literature does not always make this distinction obvious, so you have to be on the lookout.

Is takt based off the slowest machine or the machine in the line that makes the least parts?

The takt time is not based on machines but on demand and net available production time. If you have a line that puts out completed parts one unit at a time at fixed intervals, the takt time is the length this interval must have in order to meet the demand within the net available production time, which is the time you can count on the machine actually processing parts. It is not a parameter of your slowest machine but a requirement that even it has to meet.

Knowing cycle times and uptimes of a 30 machine line how do you calculate system uptime?

As you know, uptime ratios are multiplicative, so that, if you have a line of 30 machines, each of which is up 85% of the time, you line is up 85\%^{30}= 0.7\% , which is obviously not workable. But 99% uptime on each machine still only gives you 99\%^{30}= 74\% , which is still too low. So what do you do?

First, you don’t put 30 machines in line. machining cells usually have 5 to 10 machines, including simple, auxiliary machines that rarely break down. And you have buffers between cells that are managed by pull. A cell of 10 machines with 99% uptime will be up 90% of the time. With 5 machines, 98% uptime on each machine is enough to give you 90% on the whole.

Second, you work on improving the machines and customizing them to your needs so that they have fewer breakdowns and can be changed over faster, and you use these improvements to increase the number of machines in line.

Should there be more overspeed for machines at the beginning half of the machine line?

The takt time is set for the entire line. The line meets this requirement if, and only if, the last machine puts out one unit of the product in question like clockwork at the end of every takt interval. For this to happen, you must not only make sure that this last machine is up and running but also that it has a part to work on, and one way to ensure this is to give all the upstream operations a modicum of slack. This strategy, however, works better in manual assembly, where much of the work can be reassigned backwards and forwards among assembly stations in minuscule increments, where you cannot ask a lathe to do milling and vice versa.

The minimum takt time a machining line can support is determined by the capacity of its bottleneck machine, which is usually not last in line.

Should an auto parts plant use “smart” part numbers?

Mumin Vatansever, from TKG Otomotiv in Turkey, asked the following question:

I am a newly graduated production control engineer in an auto factory.
We are trying to organize our system according to the SAP.  We really do not know whether we should use smart numbers or not.  We do not know what the advantages and disadvantages of using them ? Also if it is possible could you please give me a smart code example ?

55774 05020 is a part number we have, and its process steps are as follows:

  1. Scissor
  2. Press machine
  3. Weld
  4. Packing
  5. Delivery

Technically, there is no doubt that “smart” part numbers should be replaced with keys and property lists.

I am not expert in SAP, but I don’t believe it restricts you in these matters. Manufacting Part Numbers (MPN) should be unique and short, with all information stored in other fields, either standard in SAP or user-defined. Being unique is a part number’s main job, for obvious reasons. Being short matters if they ever have to read by humans. Sequences of 5 uppercase letters and numbers give you 60 million possible unique IDs, which is probably enough for your needs. Avoid case-sensitive IDs, because people will confuse items 78De5 and 78DE5. 

What worries me is your statement that you are a “newly graduated production control engineer.” I don’t want my advice to get you in trouble. If your bosses are like 99% of the manufacturing professionals I know, they have been trained to believe in “smart” part numbers and are uncomfortable with the thought that they are an obsolete legacy of the pencil-and-paper age. You may have to go along and implement one anyway.

Supermarket sizing

Bosch’s Taojie Hua (涛杰 华) asked the following question:

How do you define a maximum limit for a supermarket?
Especially when the customer withdraws less than planned, and the lot can not be formed as a production signal, how can I react to that “deviation” by setting a proper max limit?

The response covers the following topics:

Supermarkets in Lean

First we have to clarify what we mean by a supermarket in a Lean manufacturing context. As the term has become popular, some plants have started using it for their warehouses, which is clearly excessive. Often, it is used for any kind of buffer on the shop floor, provided it is used to implement pull. I prefer to reserve the term for buffers from which users withdraw items in smaller quantities than are brought in. If pallets come in and go out, I don’t call it a supermarket, but, if 27-bin pallets come in and withdrawals take place 1 bin at a time, I do.

On a shop floor, supermarkets are found on the edges of manufacturing islands containing a group of cells or a production line and contain either incoming or outgoing materials.  A supermarket for incoming materials has more in common with the refrigerator in your kitchen than with the supermarket you buy groceries in. You need one when your plant Materials or Logistics organization is unable to deliver materials in a form that is suitable for direct use at a production work station.

Water spider at Solectron in  Mexico (2005)

Water spider at Solectron in Mexico (2005)

The supermarket is owned by Production, and more specifically by the first-line manager in charge of the cells or lines it serves. It is replenished  by  Materials or Logistics through periodic milk runs, but parts are withdrawn by experienced members of the production team — cell leaders or water spiders — and move from the supermarket to production on hand carts, gravity flow racks, or by hand. The parts arrive in the supermarkets in bins that are too large for the line side, and leave in kit trays, small bins, or single units.

You need a supermarket for outgoing materials when your production runs are multiples of the quantities needed downstream. This happens, for example, if you only know how to paint parts in batches of 50 with the same color, while assembly alternates colors one unit at a time. In outgoing supermarkets, materials are replenished by Production and withdrawn by Materials/Logistics.

Supermarket capacity

For incoming supermarkets, replenishment by milk runs is essential because it makes lead times predictable. I am assuming here that the upstream supply chain does not cause shortages. Making it work is no small feat, but this question is specifically on supermarkets. On the withdrawal side, you want to have the smoothest possible consumption rate for all items, so that you don’t have large ups and downs to contend with, which you achieve with  heijunka (平準化)  sequencing of production. Little’s Law then tell you that you have, for means:

\overline{Quantity\, on\, hand}\left ( Item \right )= \overline{Consumption\, rate}\left ( Item \right )\times \overline{Replenishment\, lead\, time}\left ( Item \right )

If you take the minimum quantity that Materials can deliver to the supermarket, on the average the Quantity on hand will be half of it. You know the Consumption Rate.  The Replenishment lead time is a multiple of the milk run pitch, plus the time needed for Materials to act on the pull signal, which depends on when the need is identified and how the signal is passed to Materials.

Assume you consume 1 unit every 25 seconds, the milk run pitch is 30 minutes, and Materials delivers in bins of 100 units. You consume 72 parts/pitch = 0.72 bins/pitch. If the milk runs are used to convey pull signals, as happens with the two-bin system or with hardcopy kanbans, replenishment may take up to 2 pitches. In this example, the 2-bin system would cause shortages, but a Kanban loop with two cards wouldn’t, because you pull the card when you withdraw the first unit from the bin and it is still 99% full. If, instead of using cards, you issue an electronic signal when you withdraw the first unit, Materials can act on it in the next milk run, meaning at most 1 pitch later. You still need room for two bins, because the current bin will still hold at least 28 parts when the replacement bin arrives.

In this example, the mismatch between the size of the delivered bins and the consumption rate forces you to hold enough excess material that you don’t need to worry about safety stocks. If it were instead perfectly matched, you could receive a bin of 72 parts like clockwork every 30 minutes, except that fluctuations in consumption occasionally would cause shortages, and you would need some safety stock to protect yourself against it.  Coming up with a sensible plan for any one item in your supermarket is not a major task, but you need such a plan for every item.

The speed with which signals circulate adjusts itself with fluctuations in consumption. The real question is whether your “customer withdrawing less than planned” should be treated as a fluctuation or a permanent drop. In the first case, there is no action required; in the second, you need to recalculate.  In any case, you need to periodically validate the parameters of your pull system to make sure they still reflect reality. In auto parts, it should be done at least quarterly.

Further reading

For details on pull systems, see Lean Logistics, Part IV, pp. 197-330. See also the two posts on Safety Stocks: Beware of Formulas and Safety Stocks: More about the formula.