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FMS strategy for small-lot tool production.

It isn't possible to guarantee successful installation of an FMS. Careful planning, however, will at least ensure that all the elements are considered and no options overlooked. This was the concept of Toshiba Tungaloy Co, Ltd, Kawasaki City, Japan, during their development of an integrated manufacturing system for producing cutting-tool bodies.

Realizing that there is a pressing need to improve manufacturing efficiency while producing produces of the highest quality, the company developed a plan to build a system that also would incorporate the flexibility to handle small-lot sizes of approximately 4000 different types of workpieces--face milling and single-point cutting tool bodies, Figure 1. Because of the needed attiributes of high-quality products coupled with flexibility, the system was called a programmable precision manufacturing system (PPMS).

The company began by developing a model of the system designed to meet the above objectives. They set goals that included:

Both standard and special products must be handled simultaneously.

Higher machining accuracy than previously achieved must be attained--specifically accuracy in the IT6 class.

Untended operation during lunch breaks and nightime hours must be possible.

It also was established that the system be economically justifiable, practical, and effective. For this reason, it was decided not to automate any process for which ROI would be small.

Assessing the work

Once goals were established, company engineers reviewed manufacturing drawings of approximately 3000 workpieces, classifying them according to similarity of machining processes, geometry, and other factors, into approximately 10 types. It was determined that the range of workpieces included:

Roudn workpieces (face-milling bodies)--more than 1500 types--measuring from 50 mm to 300 mm in diameter, with lengths ranging from 30 mm to 200 mm.

Square workpieces (single-point tool bodies)--more than 2100 types--measuring in cross section from 16 mm to 50 mm, with lengths ranging from 50 mm to 300 mm.

Maximum weight for automatic loading--50 kg

Maximum weight for manual loading--200 Kg

This study of was based on a long-term manufacturing plan, and included an analysis of orders received and changes in technical specs over a period of time. Also, Toshiba Tungaloy's engineers investigated the possibility of changing the range of products offered, based on economic conditions, so the system could accommodate future changes.

The next step was to investigate various machining processes used and calculate respective machining times based on the preceding data. This resulted in establishing a table of machining times for various product ranges, Table 1.

Machining times were obtained for two cases: A-type general-purpose NC machines, and B-type special-purpose NC machine with automatic workhandling devices. Further investigation determined labor and equipment costs (including development costs) for various combinations of machine tools to perform each machining sequence for each workpiece type, Table 2.

Studying both tables reveals that the B-type operation is more effective, even though the expense for installing this type of equipment is high. Table 1 shows dramatic savings in machining times in each product range when B-type machining center is used. When using A-type, general-purpose machines, cycle times are increased because of setting up jigs to control complex cutter paths. Also, under Case 1 in Table 2, using special-purpose milling machines along with special-purpose NC grinders minimizes operating labor.

The company reports that the special-purpose grinder was newly developed at great technological risk and high development costs, Figure 2. Based on various system configurations shown in Table 2, comparative machining costs for Cases 1, 2, and 3 are highlighted in Figure 3. Obviously, Case 1 optimal for achieving lowest finished product cost.

Looking at production control

Material flow for each workpiece type is shown in Figure 4. Various paths of material flow are provided, based on processing characteristics of each product type.

Toshiba Tungaloy engineers point out that, for this system to operate effectively, it's critical that the firm's machining schedule be planned. It was decided to introduce a program of computerized job-shop control as a means to control manufacturing; however, this isn't without problems. To adopt the scheduling program for of small-lot production requires adequate prep. The reason? It is hard to determine accurate cycle times at the production scheduling stage.

Moreover, it's difficult to determine cycle time, tool preparation time, and tool service life. Because an NC tape is produced at the preparation stage for machining--and the tape also is produced with an automatic programming system--cycle time for each process and for each tool can be obtained by analyzing the NC program. Therefore, if a toollife file is prepared, it's possible to introduce job-shop scheduling into the system. Machining schedule and corresponding operation sheet, product delivery time, machine capacities, tool life, and other factors, are output automatically.

System makeup

The machining system is shows in Figure 5, along with a block diagram of its control system. The NC lathe and vertical machining center are equipped with garter-type autoloaders, workpiece magazine, and workpiece discriminator. Each magazine accommodates up to 10 workpices. Workpiece diameter is measured by the discriminator mounted on the magazine. Measurement takes place before loading the workpiece into the machine. Measured diameters are coded, the data are verified by a minicomputer with the codes on the NC tape, and it's confirmed if the workpiece is correct.

If so, the workpiece is carried to the machine by the autoloader and the first operation is performed. Next, the part is turned over and the second operation is performed, after which the workpiece is returned to the magazine. The simultaneously-controlled five-axis horizontal machining center also is equipped with a magazine accommodating up to 10 workpieces. In this case, a general-purpose fixture or handling arbor is used. Various ring configurations fixed to the arbor discriminate between workpieces. The rings are scanned by a code reader mounted on the magazine. When the code is verified, the workpiece is transported to the machine.

The part-handling method for the NC grinder is the same as the five-axis machining center. Each grinding wheel is prepared for each grinding process, i.e., the grinder has two wheel heads in the left head: the first to grind the setting surface, the second to grind the reference surface of face milling cutter bodies. The right head is equipped with an internal face grinding wheel. The internal wheel head has an NC automatic sizing device for in-process measurement.

Special system features

In PPMS, both in-process and post-process tool breakage detection are performed. The in-process method detects changes in cutting-tool load by reading the value of spindle motor current. The trigger current is either the maximum current permissible or the initial current multipled by a specified coefficient immediately after a tool change.

If breakage is detected, cutting stops and a warning signal starts. This is effective for tools larger than 6-mm dia, but is unstable for smaller tools.

In post-process detection, effective for smaller tools, the tool is moved to a prescribed position by an NC command, and difference in tool length is checked with a limit switch to identify tool breakage.

Tool wear detection generates a warning to change a tool when wear exceeds a prescribed value. If the motor current multipled by a specified coefficient reaches a predetermined value, it is judged that the tool has reached the wear limit, and cutting is stopped.

As a means to prevent accidents because of tool breakage, tool life for each cutter is controlled. When cutting time with a tool reaches a programmed value, the tool is changed at the next program stage. If a substitute tool isn't prepared, a warning signal is generated. Here, cumulative cutting time is counted, and if the remaining tool life is short compared to the cycle time of the next workpiece, it is automatically changed to the substitute tool. Tool-life control is critical to achieving untended production. Data from tool breakage and tool wear are used for updating the tool file.

Accuracy of the five-axis horizontal machining center significantly influences accuracy of finished products. For this reason, an attempt was made to minimize the effect of heat generation at the spindle bearing by calculated compensation in the NC system. Accuracy also is enhanced by using an NC automatic sizing device on the internal grinder. With diameters ranging from 19.050 mm to 80.000 mm, constant accuracy is assured up to IT6.

There also are several peripheral techniques used to enhance automation. One is a jaw checking method on the NC lathe. Workpiece diameters handled by an automatic workpiece chucking/unchucking device range from 50-mm to 200-mm dia. The chuck's stroke, however, is approximately 20 mm, making it necessary to confirm whether the workpiece diameter is within the range of the jaw set. This can be confirmed automatically.

Chuck jaw sets are stored on jaw-control shelves. The weight of each set is registered, indicating its presence. If the weight of a specified set isn't registered on the jaw control shelf, then the supposition is made that the set is mounted on the lathe.

System operation and performance

The company felt it important to test run the system after installation. This was done to check machining capabilities--including machining accuracy--and train operators, disclose initial troubles, and determine counter measures for potential problems. In the sixth month after start of the testing--when 24-hr operation was realized--the ratio of machine downtime was less than 2.1 percent, indicating the feasibility of untended night-shift operation.

Engineers at Toshiba Tungaloy indicate that to effectively use this system, not only must new operation technology be mastered, but inherent characteristics peculiar to individual machines must be grasped.

Table 3 shows the gains made by using the new system vis-a-vis conventional machining processes.

The company has pegged product yield at 96 percent, and production index at 63 percent. Improvements are expected as adjustments are made to the system.

Planning tips for FMS

The need for mutual reliance between system users and equipment suppliers must be emphasized. This has proved to be the key to success in this case.

Toshiba Tungaloy is careful to point out that each machine-tool maker possesses proprietary knowledge and experience, but it's seldom possible to match these to specific user needs. An FMS should be implemented by assessing the backgrounds of machine suppliers to see what each offers.

For example, various aspects of whether or not to adopt automatic material transfer to the system were considered. Eventually, a manual material transfer system was adopted by systematically analyzing workpiece characteristics, inherent complexity of automatic material-transfer hardware, investment cost, etc. In the end, a manual material-transfer system was selected because of its greter flexibility.

In FMSs, precise, punctual input of workpiece material and information is essential. To achieve this, NC programming, computerized job-shop scheduling, and automatic cycle-time estimating must be prepared in advance. Systematic compiling of historical data concerning machine accuracy and system malfunction also is essential for safe, reliable system operation.

For more information on Toshiba Tungaloy cutting tools, circle E64.
COPYRIGHT 1985 Nelson Publishing
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Title Annotation:flexible manufacturing system
Author:Green, Richard G.
Publication:Tooling & Production
Date:Apr 1, 1985
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