The time taken for production is one of the critical factors to be considered in any production firm. The production time is dependent on the inherent procedures of manufacturing the product and also the machine operators’ abilities. The quantity of products directly depends on how time is managed during the production process (Bello 2012: 54). This informs the importance of critical evaluation of the lean production principles of setup reduction and setup time which apply the Six Sigma principle. Fixtures have been some of the important tools used in the production of similar parts.
Fixture Types and Applications
A fixture is a work-holding device that holds and supports a workpiece but does not guide the tool. Fixtures are used for multidimensional tasks such as milling and drilling (The Engineers Post 2020). The use of fixtures improves repeatability, interchangeability, and accuracy in the products produced in large quantities (Bello 2012). There are several types of fixtures used in the industry. They include plate fixtures, profile fixtures, indexing fixtures, multi-station fixtures, angle plate fixtures, and vise-jaw fixtures (Venkataraman 2015: 21). Besides this classification, fixtures can also be classified according to the tool used. These include milling centers, grinding fixtures, lathe fixtures, and broaching fixtures. These fixtures have different applications.
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The plate fixtures are the most common types of fixtures. They are constructed from a plate that has a variety of supports, clamps, and locators (Warfield 2019: 72). The plate fixtures can be used with many types of tools. The fixtures are made from various types of materials which are determined by the workpiece being machined and operations being carried out. The angle plate fixtures are similar to plate fixtures except that they are mounted to the mounting surface to provide a perpendicular work holding surface (Venkataraman 2015).
Vise-jaw fixtures are usually modified to accommodate vises for holding different workpieces. These fixtures can easily be modified to accommodate various sizes of work and this makes them be the least expensive (Venkataraman 2015). They are also easy to modify. The work holding capability of the vise-jaw fixtures depends on the available vises. On the other hand, indexing fixtures find their application in workpieces that require machining details at a specific spacing (Warfield 2019). The indexing fixtures should locate the workpiece and maintain the workpiece at the correct indexing position during machining (Bello 2012). The multi-station fixtures are usually used for performing several tasks in a single setup. They are also used to machine individual parts sequentially while undertaking different tasks at each station.
The Rotary Table
A rotary table is used in metalworking for precision positioning of workpieces. The table allows a machinist to cut work at exact intervals. Sometimes, the rotary table can be incorporated with index plates and dividing plates for indexing and regular work positioning. There are manually operated rotary tables and also powered tables which are used in CNC machines. To position the work accurately on the table, a machine operator uses a Vernier scale. The rotary table is usually mounted flat with the table rotating in a vertical axis. An angle plate can also be used to mount the table in such a way that it rotates on a horizontal axis.
The rotary table can be applied in performing operations such as cutting arcs, cutting straight lines at any angle, milling the flat surfaces of bolts, milling helixes, cutting complex curves, and cutting circular pieces (Prodan, et. al. 2016:31). In addition, the rotary tables are instrumental in creating large-diameter holes and also allows for cutting of arcs on the surface of the work when the combined with a compound table.
Advantages of Manual Rotary Tables
The use of rotary tables over many years has proven to be advantageous. In the metalwork field, the rotary table allows the user to move metal safely (Prodan, et. al. 2016). For instance, in welding and cutting, the user can move dangerous metal pieces safely while working on them. Again, the rotary table allows for flexibility when a machinist wants to move metal from place to place. Thus, it allows the machinist to have full control over the work. Moreover, the use of rotary tables allows machinists to achieve cutting precise curves, lines, and more complex cuts. The manual rotary tables are also more advantageous in that they are cheaper.
Disadvantages of Manual Rotary Tables
First, since there are no industry standards, the claims of accuracy on their usage are misleading (Kniel, et. al. 2020). Accuracy applies when the rotary table has been newly purchased and has not been exposed to cutting forces or even other accidental impacts. Some accuracy specifications from manufacturers do not specify what they mean although some of them specify accuracy to a certain number of degrees. Again, significant impact forces on the rotary table are enough to cause the wormgear to be offset (Kniel, et. al. 2020).
Secondly, the wormgear uses a friction brake to contain forces during cutting. However, the cutting forces are supposed to be applied directly on the centerline of the rotary spindle for the friction brakes to work efficiently (Kniel, et. al. 2020). When the cutting forces are applied far off the centerline, the friction brakes cannot withstand them and thus the wormgear can deflect (Prodan, et. al. 2016). This is the scenario in most cases when thrust forces are high during heavy cuts. With time, the rotary table becomes inefficient.
Thirdly, sometimes handling complicated works is challenging on a rotary table (Modern Machine Shop 2020). The rotary table rotating mechanism is not enough for accomplishing complex operations on workpieces that are already made. For instance, holding the propeller below in a rotary table requires a combination of a rotary table and a jig (Modern Machine Shop 2020). Sometimes, using the turning mechanism for the rotary table alone causes the machinist to undergo bodily strain (Prodan, et. al. 2016). According to engineering ergonomics, designs should consider safety for the operators. Moreover, the rotary table does not always have a sufficient work-holding surface. The incorporation of a jig, in this case, can help in increasing the work-holding space.
Jig selection for use on a rotary table
Fixtures used to depend on the type of work and the orientation it has. For example, the plating fixture can be used on the rotary table for locating and supporting workpieces that are not complex in their structure (Machine Tools 2017). Again, work that is complex in its design would require a more complex fixture such as a hydraulic-powered one to clamp and position the work on the rotary table. The indexing fixture is highly applicable to rotary tables especially for jobs that require equally spaced operations (Machine Tools 2017).
Setup Reduction
By definition, setup reduction is a load balancing technique that is critical in lean production. It involves the analysis of the activities of preparation, location, replacement, and adjustment (Harrison 2015:41). These activities are associated with the setup time. Analyzing them helps in strategizing on their reduction for lean production to be achieved. By definition, setup time is the time to change from the last item of the previous order to the first item of the next order (Harrison 2015: 43). Setup reduction involves classifying the setup steps as either external or internal, converting internal steps to external steps, time reduction for the remaining internal steps, and eliminating adjustments (Hill, et. al. 2017). Internal steps are those done when the process is inactive while external steps are undertaken when the process is operating.
Setup reduction advantages
The application of the Lean Six Sigma (LSS) framework has gained momentum in most companies globally (Tennant 2017). The LSS framework has effectively been used in the aviation industries and its advantages have proved to be worth it's undertaking (Hill, et. al. 2017). Lean production is value-based and aims at eliminating non-value activities in the production process. In lean production, the systematic approach to identifying and eliminating wastes and continuous product improvement helps in achieving customer needs (Hill, et. al. 2017).
Setup reduction and time reduction are critical in maximizing profits (Tennant 2017). With the time reduction from one set up to another, time wasted is minimal. Considering the activities in a Maintenance Repair and Overhaul (MRO) shop, reducing the time for machine setup will make it possible for such a company to attain large production units (BCG 2016). This is essential, especially where there is an integration of a supply chain that depends on products supplied from various suppliers.
Setup reduction is a powerful tool in reducing maintenance costs. Without careful consideration of production processes, businesses are prone to experiencing high maintenance costs (Harrison 2015). Reducing the time taken to work on a product in a workshop means that low energy has been used and tools have experienced minimal wear. In addition, time reduction prolongs machine life and also the human resource has been utilized optimally.
How setup reduction will benefit from time reduction
Typically, the machine setup process involves the activities of preparation, positioning, gauging, and exchanging. This involves cleaning the machine, positioning tools, and calibration (Kochan, et. al. 2018). The time is taken for these activities also depends on the machinists’ promptness. Research shows that gauging the machine is the longest activity that occurs in setup time (Anderson 2014). Gauging involves the positioning of the correct positioning of the workpiece and then conducting trial runs before the machine can start full operations. Depending on the complexity of the workpiece, the setup time can be long or short (Pawar, et. al. 2014). Therefore, there is the necessity for critical evaluations on machine setup time reduction. The benefits of setup time reduction include better quality products due to consistency of processes, reduction in production costs, less inventory, shorter lead time, better workforce utilization, increased flexibility, less process, variability, and increased capacity (Kochan, et. al. 2018).
In the aerospace industry, setup time reduction cannot be overemphasized. Low setup time translates to low turnaround time. The faster the repairs are done on an aircraft the faster the aircraft will be ready for many operations (Rijsel 2016:23). The TAT can, therefore, be improved by proper setup reduction and setup time reduction. One method of reducing setup time in machines is by the use of jigs and fixtures. They help in the easier and proper holding of similar workpieces.
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