Mar 4 2013
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:
- Any reading material you would recommend?
- Is takt based off the slowest machine or the machine in the line that makes the least parts?
- Knowing cycle times and uptimes of a 30 machine line how do you calculate system uptime?
- Should there be more overspeed for machines at the beginning half of the machine line?
Following are my answers:
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.
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.
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.
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.