If you are a big sports fan looking for some inspirational story on how David triumphed over Goliath, a good story is the ‘Miracle on Ice’. It depicts how the US national ice hockey team (made up of amateur and collegiate players) displayed steely resolve and teamwork to defeat the highly fancied Soviet Union national team, which had won the gold medal in six of the seven previous Olympic Games.
The underlying factor here is undoubtedly teamwork. In the world of sports, teamwork can sometimes overcome individual brilliance. In the world of metalworking, teamwork is also a strong virtue.
According to Haas Automation, using multiple, affordable CNC machine tools in a deliberate and clever way can benefit the user in terms of greater flexibility, lower investment and running costs and, ultimately, lower cost-per-part compared to using high-end, special purpose machines.
Finding The Flow
The company’s claim is supported by an independent study on lean machining at the Technische Universität (TU) Darmstadt in Germany. A recent report from the university demonstrates that the use of several, modestly priced CNC machine tools arranged in a cellular configuration can offer benefits over the use of more expensive special-purpose machines configured for ‘done-in-one’ operations.
The idea for the research was based on the perceived absence of lean manufacturing principles applied in widespread machining operations at manufacturing companies of all sizes.
Although deployed widely in assembly and process engineering environments (typically in the volume automobile manufacturing sector), the principles of flow as a central element of lean manufacturing have not often been transferred successfully to machining operations, and are rarely to be found at all in Europe.
“In German companies, job shops are usually organised in a way that similar technologies are grouped. For example, all milling machines of the factory are located next to each other and all lathes are located next to each other,” said Jörg Böllhoff, research assistant (Center for Industrial Productivity) of Technische Universität Darmstadt.
“A product that requires both milling and turning operations would for example be processed on the lathe first, then be transported to the warehouse, and after a certain period of time, finished on the mill. To achieve cost effectiveness, the products are processed in certain lot sizes which causes further transportation and storage effort. This means there is no efficient one-piece-flow from machine to machine as it is usually known from final assembly in the automotive industry.”
In order to address the issue of ‘lack of flow’, the research team set up a cellular manufacturing system, which is essentially a group of heterogeneous equipment (in this case, CNC machine tools) to manufacture a family or group of similar parts. Typically, the concept involves the arrangement of machines in a U-shape to aid flow and balance work-in-progress with ‘takt’ time (the average unit production time needed to meet customer demand).
“U-shaped lines have several advantages. For instance, one worker can operate several machines. This gives him a good overview of all machines and a short distance to walk,” he said.
Good scalability and flexibility is another advantage that he mentioned. “Depending on your customer’s demand, you could operate a U-cell with only one or more operators and therefore change your capacity (assuming the operator, and not the machine is the bottleneck),” he added.
Finally, a U-shape arrangement also means that the start and end of the line are at the same position.
“Raw materials are delivered and finished goods are picked up at the same point. This results in less effort for logistics. In addition, the short distance from the last to the first machine in line is particularly important if only one worker operates all machines in the line.”
Comparing The Two
To demonstrate the potential advantages, a reference production line was established at the PTW’s Process Learning Factory (CiP). The ‘done-in-one’ line features two, high-specification machine tools: a 3, 5 axes CNC horizontal machining centre with tombstone fixture, and a CNC multi-axis automatic turning centre.
According to Mr Böllhoff, the machines in the ‘done-in-one’ line have better individual capabilities. Some of the extra capabilities (for milling) are automatic palette change to enable parallelisation of processing and set up; 3,5 axes; and four-face tombstone for workpiece clamping with automatic rotation. For the lathe, it has two spindles and an automatic workpiece handling system.
The competing line, a machining cell, features two Haas Super Mini Mill 2 and two Emco Concept Mill 250 three-axis machining centres, as well as a Haas SL-10 and an Emco Concept Turn 250 two-axis CNC lathe.
The report notes that the investment for the two machines in the ‘done-in-one’ line totalled €780,000 (US$882,000), while that for the six CNC machine tools in the cell configuration was €340,000.
An economic comparison between the two machining line configurations (based on 2,000 parts per week, and one worker on each line) is as follows. The ‘done-in-one’ line required 15 shifts to complete the 2,000 parts. The lead-time was 35 minutes, while the unit cost of each part (without material) was calculated at €3.95. The cellular machining configuration took 12.6 shifts to finish the 2,000 components to an identical specification, lead-time was reduced to 10 minutes, and unit cost to €2.55.
As an alternative scenario, if two workers are deployed in the machining cell, the unit price climbs slightly to €3.10 as a result of the additional labour cost (but still cheaper than the €3.95 unit cost of the ‘done-in-one’ line). The number of shifts are reduced to 9.8 and lead-time is cut from 10 minutes to seven.
Using a capacity comparison based on 15 shifts, the ‘done-in-one’ line will complete 2,000 parts; the cell with one worker will finish 2,377 components (a 19 percent increase); and the cell with two workers will complete 3,064 parts (a 29 percent increase).
Quality
In all cases, the study ensured part accuracy and quality standards were maintained. The less expensive, general purpose machines were said to be able to produce to a similar standard as the special-purpose machines.
APMEN asked Mr Böllhoff just how accurate the parts produced in the U-shape line are.
“There are basically two different parts that are produced in our machining cell — the bottom and the piston rod of a pneumatic cylinder. Overall, we have two different variants of the cylinder bottom, which are made from aluminium, and then eight variants of the piston rod (stainless steel),” he said.
According to him, a process feasibility study was done for the cylinder bottom. The figures below show the evenness of the ring surface.
Evenness of the ring surface
- Tolerance of: 0.04 mm
- The results of the corresponding process feasibility indices are: cpk=1.35, cp=1.45
Deepness of the notch
- Tolerance: 1.45 ± 0.05 mm
- cpk=1.41 , cp=1.87
Diameter ring surface
- Tolerance: 39.9555 ± 0.039 mm
- cpk=2.27 , cp=3.12
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“In our cell we use two Haas Super Mini Mill 2 and two Emco Concept Mill 250 to machine the cylinder bottoms. The piston rods are produced by using a Haas ST-10 and an Emco Concept Turn 250. The numbers presented (left) are the ones for the cell with the Haas and Emco machines.
“We mainly wanted to state that it is possible to keep the quality standards of a real industrial product with such a setting (low cost machinery, multiple pneumatic clampings, high manual workload etc).
“In fact the cpk figures for our high-tech cell are worse. The main reason is the clamping situation in that cell. So at the moment we cannot do an appropriate comparison of the two concepts in terms of quality,” he said.
Future costs could also be taken into consideration when comparing the different approaches. In the case that the production line needs to be expanded, introducing a new machine to the cell will be considerably cheaper than introducing a machine to the ‘done-in-one’ line due to the large disparity in their individual purchase prices.
With a good selection of machines and a good formation, this example has proven that even in metalworking, sometimes general-purpose machines can be combined to out-perform machines of a higher grade.
