Mar 3 2013
From Ybry charts to work-combination charts
This is a screen shot from yesterday’s evening news on the France 2 channel, part of a story about TGV high-speed trains used on regular tracks to bring vacationers to ski areas. The TGVs, of course run at regular speeds on these single line tracks and must stop at sidings to let regular trains through in the opposite direction. In an earlier post, I discussed the charts invented by Charles Ybry in 1846 for railroad scheduling, and this newscast shows that they are still used in railroads today. Besides railroad scheduling, they are also used in the management of multiple, concurrent projects, and I believe they were the basis for Toyota’s work combination charts.
The x-axis is time; the y-axis, position along the line. On the chart, the downward lines represent trains going down the line; the upward lines, trains coming up the line. When and where the lines cross, trains cross, and there must be a siding available. The news story had the TGV pilot call in his position on a siding to a control center in Chambéry where the chart was displayed. On the high-speed TGV lines, the signalling is all electronic, and the system automatically knows where the trains are; when you run a TGV train at reduced speed on a regular line, however, it seems that the driver has to report what happens the old-fashioned way.
I learned about these charts in Edward Tufte’s Envisioning Information, where he describes them as a special case of a “narrative of space and time.” Among the examples he gave were a similar railroad scheduling application from Switzerland 80 years ago and the development of Wagner’s operas over almost 50 years in the 19th century:
Work combination charts are a tool to design and communicate about production jobs that require operators to perform a sequence of operations on multiple machines that operate automatically between visits. This is a Japanese example of such a chart:
The concept looks similar, doesn’t it? I found this chart particularly useful when you need to plan the activities of more than one operator, as in the following example:
In the Legend, “Manual In” refers to time spent by the operator on the machine with it stopped; “Manual Out,” time spent on the machine while it runs.
To this date, in the US, this powerful technique is far from enjoying the popularity it deserves. It is generally perceived as “too complicated” and I still don’t know of any software tools that fully support it. In designing jobs that involve interactions between human and machines, however, the consequence of not using it is leaving about 50% of the potential productivity improvement on the table. It may take a project team an extra day to do it, but the result is achieving a 40% productivity increase instead of 20%. Details are discussed in Chapter 7 of Working with Machines.
Mar 4 2013
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:
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
, which is obviously not workable. But 99% uptime on each machine still only gives you 
, 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.
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By Michel Baudin • Answers to reader questions 0 • Tags: Automotive, Cellular manufacturing, Lean, Machine-shop, Machining