Matching Centers Versus Special Machine Tools
Some believe that standard machining centers can be used effectively in a high volume production assignment to exploit their versatility. There are no doubt cases where this approach is practical, and in fact it has been done effectively for a considerable period of time. There are several factors that will bear on the advisability of that approach.
Producers of standard machine tools and machining centers are product designers, typically not manufacturing or process engineers. These are distinctly different areas of expertise. They do their best to incorporate the features that appear to be the most desirable in their marketplace. They would like their product to be all things to all people. They then mass-produce those machine tools for distributor sales and for inventory.
There are a few machine tool companies that have both standard and special machine tool divisions. Obviously, they would like to be able to take maximum advantage of all possible synergy between the two. Presumably each group is being staffed optimally for their individual business plans for their products and would not necessarily be able to sustain a prolonged period where one side or the other would dominate without some sacrifice.
The machining center engineers typically have not specifically applied their products to production assignments on a regular basis. Their normal marketplace, predominately jobbing environment shops, have their own application groups since that is the nature of their business and will reapply the machine tools numerous times in their useful lives.
The standard machine tool marketplace includes producers of every kind of component imaginable and normally at volumes much lower than in the typical automobile context. They typically have built in numerous “nice to have” features which would never be used in a high production environment. Note: It is a contradiction to the definition of lean when machining centers are used in high volume assignments, as there are resources (features) in the machines that are not used effectively or not at all.
So an important question arises: In a high volume setting, who does that application engineering work, or more importantly, who takes ownership of all the manufacturing process sequence steps (could be hundreds for a single component) for specific assignments requiring numerous machine tools.
As discussed earlier, there are many cases where a process step in the production of a high volume component can only be done marginally on standard machine tools or maybe not at all which obviously is seriously limiting to the component product designer. This is opposed to special machine tools that can incorporate highly specialized operations giving the product designer complete freedom to do things that may have never been done before.
One of the major points in this entire effort has been to emphasize the importance of Yankee ingenuity, the “find a way” mentality and the no rules environment to facilitate them. When the use of standard machine tools is emphasized or mandated to take advantage of mass purchases or for a questionable approach to achieve flexibility, the universe of manufacturing advancing opportunities becomes severely limited. Nearly as much creative thought is required to adapt the standard machine tools but with far less opportunity to optimize the process, exploit new machining ideas and provide the product designer greater flexibility for product innovation as well.
For example, the specifics of establishing the step by step manufacturing process for a complex engine component is extremely critical and is only learned by exposure and experience. The manufacturing processes for cylinder heads, connecting rods, cylinder blocks, pistons, and crankshafts, typical engine component parts, and their response to fixturing and tooling forces and the relief or imposition of material stresses are entirely different from each other.
When material is removed in a specific process step, creating or removing material stresses, and applying very significant tooling and fixturing forces the component itself will react by, bending, twisting, shrinking or expanding. In addition the cutting tools wear and the feature being produced will vary as well. These changes are either predictable by experience or unpredictable necessitating allowing for adjustment capability of some kind. It may even be by “in process” real time monitoring and feed back to a custom and maybe automatic adjustment capability.
The operation sequence is critical, as those operations that complete a particular component feature must follow those that may create unpredictable results. Obviously the finishing operations must remove all variables from previous operation sequences. There are hundreds of process steps in the production of many engine components, many of which will cause non-text book results.
In many instances, the tolerances on feature characteristics are very challenging and can be in the two or three ten thousandth of an inch range (.0002”) or even more challenging. Add to that the statistically formulated acceptance criteria and the challenge is seriously multiplied. It’s apparent that the application of an informed process sequence plays the major role in component quality and cost.
An important complication of this concern is that some in the American auto industry have, over time, reduced their manufacturing engineering capability through attrition. This was partly because of the conscious decision to buy entire process capability from one special machine tool supplier. The cylinder block example of a fourteen transfer machine line with hundreds of individual operation steps demonstrates the reduced effort required if the selected supplier can manage it. The former method involved multiple suppliers awarded individual operations on a piecemeal basis. In that approach the process and hardware conception, coordination and control responsibility remained with the buyer. The coordination of those multiple suppliers, particularly considering the need for a well thought out process sequence over hundreds of steps, is a very serious challenge requiring significant engineering staff. That kind of capability, historically, has not been available in the standard machine tool company’s organization.
Transfer machine: Multi station in line special machines that perform two different operations in each station. Each cycle advances a full complement of work pieces one station producing a completed piece (planned operations) at the end of the machine, every cycle. These machines may either be totally dedicated or flexible.
A basic difference in component processing between the machining center flexible approach and a transfer machine approach (with or without flexibility) is that in the transfer concept the component remains in each process step for a very short period (24 seconds at 150 parts per hour) and then is transferred to the next step.
The significance is that it is located, clamped, and supported as required for one or two specific process steps to be performed. The fixture (component holding device) has only to provide clearances around the component for those particular cutting tools. It is designed to resist only the specific forces involved providing the best chance to minimize distortion. In addition any distortion or stress relief changes that may occur are immediately allowed to relax as the component is released following that operation and relocated and reclamped for the following operation minimizing any impact. The machining sequence will be planned to put the distortion producing operations in a place that can be accommodated by the process as a whole.
In the machining center approach, the component is fixtured on a machining center while as many operations as possible are performed before moving to the next machine. There are as many duplicate machines as required to make the prescribed production volume. The component locating, clamping and supporting device must be able to support the component against numerous and various types of cutting tools and provide clearance for them all around the component. This clearly compromises the basic fixture structure and the component process integrity. In addition component stresses and distortions that may occur during any or all the operations in that single fixturing remain until all are complete and the component is released. As a result those distortions are cumulative, adding to the unpredictability, and are reflected in later process steps which must be considered in the overall process formulation.
You may recall that in earlier discussion it was stated that the real product of the special machine tool company is intellectual in nature (the entire comprehensive component manufacturing process) and the hardware is simply the manifestation. The question of who takes responsibility for the entire process is a very important one. The 80 million engineering hours of experience accumulated in the 1960s, 70s, and 80s by the special machine tool industry, discussed in Chapter 3, was an invaluable experience and knowledge base, and is now greatly diminished and fading.
The product designer would like his component to be as lightweight and as small as possible to fulfill the needs of its product function in the most effective way. Typically, he will seriously resist making allowances for size or strength that could be a requirement in its production. Therefore, the manufacturing process, that is the work locating, holding and supporting devices and the cutting tools, cannot impart forces that will distort or otherwise make the outcome other than perfectly predictable on a component designed only for its own functionality.
A non-magical answer that seems to satisfy many of the concerns expressed above on when or how to apply agility in high volume production such as in an automotive context follows.
If we think of a production facility producing certain components for internal combustion engines, it is not hard to visualize a line of machine tools dedicated to production of each of its component parts. Presumably the basic engine and its current and future prospective variations will be produced for some time to come, typically five to 10 years, and will be produced at least at a rate of 400,000 to 500,000 per year. We should be able to presume also that the next generation engines in the years to come will still incorporate those same basic components although ostensibly significantly different.
In the past, specific special machine tools have been dedicated to a particular group of operations within a broad total sequence of operations and could have extensive flexibility. Presumably many of those same or similar operations will continue to be required on following generations of the component involved. It is practical to design and build custom machine tools that will do those specific operations, even those not practical or possible on standard machine tools.
It can be done in a way that provides flexibility to do those operations on the new, but corresponding features on part variations and on future generations. This is a machine designed by special machine tool engineers, manufacturing engineering and process engineering specialists, who accept responsibility not just for machine tool functionality, but also for the effectiveness of the entire production process. It will incorporate whatever capability is required for its assignment, but will not be all things to all people. The flexibility can be extensive or narrowly focused based on the particular component features to be produced and the degree of flexibility desired in anticipation of future component variations.
A good example of this concept is the crankshaft oil hole drilling machine described earlier. It is a dedicated machine and a flexible machine. With relatively minor changes, it will drill the oil holes in just about any automotive sized crankshaft. It will not make cylinder blocks or even mill surfaces nor will it drill, ream, and tap miscellaneous holes. If internal combustion engines continue to be built, they will need crankshafts with oil holes.
It is interesting that even today, the Japanese auto companies in Japan and in the U.S. use a significant number of dedicated transfer machines. Some incorporate significant flexibility. Machining center type of machines, where it makes sense, will be in the same lines (agility where it is really useful and not just agility for the sake of agility).
The machining center approach has the advantage that typically several machines are used to perform the same operation. If one is down production continues but on a reduced level. If a transfer machine is down all production in that line stops until the problem is resolved. A transfer machine uses multi-spindle heads where one stroke of a machine axis will produce all the holes in a many bolt hole pattern. The machining center has one feed axis, one tool and one set of spindle bearings, requiring many strokes, to do the same work. It has two other machine axes that must reposition its spindle for every stroke. The safety margin time, when shifting from rapid approach to avoid crashing into the work piece at high speed, is required every stroke of the machine axis. A typically high maintenance tool-changing device is required when changing to another operation during the production process.