Productivity Science of Machining - 2016 Update - Stephenson - Agapiou

Lesson 49A of Industrial Engineering ONLINE Course

Lesson 50. Industrial Engineering and Productivity Aspects of Metal Cutting Processes

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To do machine work study of machine tools, the industrial engineer needs to have good knowledge of metal cutting theory and operation of machine tools. This knowledge has to be supplemented by knowledge of process and operation planning. Then only machine work study, that is operation analysis of the operation being done on a machine tool can be carried out.

Productivity analysis is based on productivity standards, that the productivity that is achievable by ideal machine tool, ideal cutting tool, ideal cutting fluid, and ideal cutting conditions. The current productivity being achieved and the working facilities  and cutting parameters are to be compared with ideal conditions and efforts are to be done to redesign the operation to the extent economically beneficial. Productivity improvement is achieved through productivity engineering: facilities industrial engineering and process industrial engineering.

Productivity Science of Machining - F.W. Taylor

Stephenson - Agapiou Book "Metal Cutting Theory and Practice."

Number of productivity related ideas in the area of metal cutting were given by Stephenson and Agapiou in their book "Metal Cutting Theory and Practice." (Third Edition, 2016) These relationships are to be used in developing ideal machining set up and cutting conditions for the operation under study and they are to be compared with the current set up and conditions in use.

About Dr. Agapiou

Dr. Agapiou’s research and teaching interests include modeling and optimization of metal-cutting operations, including cutting tools and machining systems, and modeling manufacturing part quality for machining lines to improve part quality, process, and productivity.

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He (Taylor) felt that shop productivity could be greatly increased if standard best practices were dictated by a central planning department.

As a result of these experiments, Taylor was able to increase machine shop productivity at Midvale by as much as a factor of 5.

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Also, the complexity of the tool has inhibited the introduction of new tool materials, so that productivity gains in drilling have lagged those made in turning and milling over the past 30 years. A number of manufacturers have developed diamond or CBN tipped or coated drills to attempt to address this limitation.

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The number of tools was reduced from 17 to 12. The positioning time (of tools) was found to be the largest contributor to productivity improvement, with the cutting time the second contributor.

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                    Conv. Mc.              Std. HSMC        Adv. HSMC
Productivity   100%                    137%                  179%           (High speed machining gave 179%)

In high speed machining centers (HSMCs), the machining time for a  feature may be significantly lower compared to a conventional machine due to high spindle speed and the resulting feed high feed. But, the machining time is typically one-third of the total time, with the remainder being used for machine travel, tool changes, spindle acceleration/deceleration, and pallet changes and rotations. To increase production rate, reducing the noncutting time has to be attempted. In the HSMCs, fast acceleration capabilities of axis are being used to   permit reduction of noncutting time. A focus on raising only the cutting speed proves cost-effective in just a few applications, such as aerospace applications in which parts are machined from billets or rough forgings.

In  an example, time study comparison of a standard CNC machine, a conventional (STD) HSMC, and an advanced HSMC in machining an aluminum automotive part were given in the book.  The cycle time for the advanced HSMC is lower than that of the standard STD HSMC. The reduction is facilitated by  the process change such that different diameter holes are machined with a single end mill and multifunctional tools. That way, the number of tools was reduced from 17 to 12. The corresponding reduction in positioning time was found to be the largest contributor to productivity improvement, with the cutting time the second contributor.

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Their traveling speed and positional accuracy determine machine tool productivity and part quality in many applications.

Slides are machine components, which move a workpiece or tool on guideways (or ways) to a specified position and hold it in position under machining loads. Their traveling speed and positional
accuracy determine machine tool productivity and part quality in many applications. Therefore, in addition to stiffness and damping, parameters considered in the selection and design of guideways are machine speed and acceleration/deceleration requirements.

Currently, slide speeds are generally around  20 m/min, although machines with slide speeds between 40 and 50 m/min are not uncommon. Speeds between 70 and 100 m/min have been achieved on some high-speed machines. The maximum feed rate is typically between 40% and 50% of the maximum rapid travel rate.

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A pallet changing system improves productivity because a part is loaded while another is being machined. 

The pallet structure is a component of a machining system contributing to machine performance. The workpiece is clamped to a pallet that is usually located on an indexing table, is clamped in the desired orientation prior to cutting.  A multiple pallet changing system improves productivity because
a part is loaded while another is being machined. A standard pallet may utilize four conic couplings. The clamping unit engages at the center of the conic coupling. This arrangement of four conical couplings has been proven accurate and rigid for up to 1.5 × 1.5 m pallets with load capacity up to 12 tons.

Application example:
2/20/2009
Shop Reduces Setup Time With Multi-Pallet Machines
MAC Machine Company.
Multi-pallet systems from Matsuura contain anywhere between 2 and 32 pallets. With 32 pallet system  the shop can machine as many as 32 different parts, one right after the other, on a single milling machine.
https://www.mmsonline.com/articles/shop-reduces-setup-time-with-multi-pallet-machines

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152. R. L. Rickert, AC servos increase machine tool productivity, Mach. Des. 57:4 (February 21, 1985) 135–139.

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An improperly designed tool may wear or chip rapidly or unpredictably, reducing productivity, increasing costs, and producing parts of deteriorating quality. Tooling thus has a major influence on the productivity and economics of a process.

Cutting-tool material and design have a strong impact on machining performance. Properly designed tools produce parts of consistent quality at higher cutting speeds and feeds and also have long and predictable useful lives. An improperly designed tool may wear quickly or chip rapidly or unpredictably, reducing productivity, increasing costs, and producing parts of deteriorating quality. Tooling has a major influence on the productivity and economics of a process. Therefore it is important to consider all material options and tooling geometries for a given application with associated range of speeds and feeds and their typical failure modes. In high-volume applications, perishable tooling costs are typically 3% of the component cost.  Using higher cost tool capable of running at higher material removal rates (MRRs) normally is profitable  since a 20% increase in the MRR could reduce the total cost per component by 15%. Modifying tooling to increase tool life can also be attempted as 50% increase in tool life reduces the total cost per component by 1%–2%.

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 For nonferrous work materials, WC tools will exhibit 2–3 times the productivity and 10 times the life of HSS tools. In steels, they give 2 times the productivity and 5 times the life.

Sintered Tungsten Carbide (WC)

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Basic guidelines for selecting carbide grades are (1) use the lowest Co content and finest grain
size, provided edge chipping and tool breakage do not occur; (2) use straight WC grades when
abrasive edge wear is of concern; (3) use TiC grades to prevent crater wear and/or both crater and
abrasive wear; (4) use TaC grades for heavy cuts in steels.


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FIGURE 4.8 Comparison of microhardness (VHN-1 kg) at 1000°C and toughness of ceramic tool materials  using P01 carbide as a baseline. (After Mehrotra, P.K., Productivity Improvement in Machining by Applying Ceramic Cutting Tool Materials, A Systems Approach to Machining, ASM, Materials Park, OH, 1993, pp. 15–20.)

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Even though the range of rake angles is generally −5° to +15°, higher positive angles are slowly being used to improve productivity.

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Multipoint boring bars are often used to improve stability and to increase productivity in high volume applications.

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Rotary milling cutters can be used in some applications to improve productivity.

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Significant improvements in productivity have resulted from the acceptance of solid carbide drills. Compared to HSS drills, carbide drills permit an increase in productivity by a factor of 2–10, and/or increase in hole quality. Solid carbide drills are especially well suited for high throughput precision hole manufacturing.

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Internal coolant is more effective; this method cools and lubricates the cutting edge and reduces thermal shock in addition to removing chips, and therefore often results in improved tool life and productivity.

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26. P. K. Mehrotra, Productivity improvement in machining by applying ceramic cutting tool materials, A Systems Approach to Machining, ASM, Materials Park, OH, 1993, pp. 15–20.

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119. J. Skoglund, Carbide drills: The answer to high-productivity steel drilling, Cutting Tool Eng. (February 1990) 35–37.

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An important outgrowth of modular tooling has been quick-change tooling, which has led to
significant improvements in productivity.

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57. T. L. Schmitz and K. S. Smith, Machining Dynamics: Frequency response to Improved Productivity, Springer, New York, 2009.

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140. Increasing machine tool productivity with high pressure cryogenic coolant flow, Manufacturing Technology Directorate, Wright Laboratory, Air Force Systems Command Report WL-TR-92-8014, May 1992.

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8.7 PROCESS SIMULATION APPLICATION EXAMPLES
Simulation in current processes helps to increase productivity, part quality, and reduce tooling
and production costs.

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It should be noted, however, that reducing the cutting speed has no impact on productivity in many transfer machining operations, specifically those in which the cutting time is substantially lower than the cycle time of the line. In transfer machine operations, the cycle time should be determined by optimizing the speed and feed in critical operations; in the other operations, the cutting speed should be reduced to use all of the available cutting time and increase tool life.

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Since fracture results when the tool is loaded beyond its structural limit, it can be prevented either by reducing the cutting load or by increasing the structural strength of the tool. Since reducing the cutting load usually requires reducing the metal removal rate and thus the productivity of
the process, increasing the structural strength of the tool should be attempted first.

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Tool wear is one important factor contributing to the variation of cutting forces and surface finish.
In transfer line applications, worn tools are often changed on a statistical basis at a rate dictated
by the shortest life expectancy for multi-tool operations. In such cases, significant useful life of the
tools may be wasted and system productivity may be reduced.

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Many parts are machined to produce surfaces with consistent dimensions, forms, and finishes for locating, sealing, or similar applications. In many cases, especially finishing operations, surface flatness, and finish requirements restrict the range of tool sizes, geometries, and feed rates that can be used. Moreover, since the machined surface finish becomes rougher and less consistent as the tool wears, stringent finish requirements may also limit tool life and thus strongly influence machining productivity and tooling costs.

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Oil-based fluids reduce heat build-up because they have better lubricity  and are also typically used at lower wheel speeds. Burn limits and residual stress predictions can also be computed using grinding simulation programs and used to suitably adjust infeed cycles, the number of passes, and wheel characteristics to maximize productivity.

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64. J. D. Christopher, The influence of high pressure cryogenic coolant on tool life and productivity in turn ing, SME Technical Paper MR90-249, 1990.

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FIGURE 12.5 Polar curve of the flexibility of three different milling machines in three directions. (From Schmitz, T.L., and Smith, K.S., Machining Dynamics Frequency Response to Improved Productivity, Springer, 2009.)

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Although a decrease in rigidity in a machine tool is generally undesirable, it may be tolerated when it leads to a desirable shift in natural frequencies (especially in high speed machining) or is accompanied by a large increase in damping or by a beneficial change in the ratio of stiffnesses along two orthogonal axes, which can result in improved nonregenerative chatter stability. However, reducing the tool–workpiece compliance is not always possible in practice, and other approaches, discussed next, should be examined before productivity is sacrificed by reducing the depth of cut to ensure stable operation.

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173. P. Vanherck, Increasing milling machine productivity by use of cutter with non-constant cutting-edge pitch, Proceedings of the Advanced MTDR Conference, Vol. 8, Pergamon Press, London, U.K., 1967, pp. 947–960.

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The selection of optimum process parameters directly affects the productivity, manufacturing cost, and part quality.

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The second level of a COS interacts with the first level. It uses detailed part geometry, machine motion, and tooling data files to calculate individual machine motions and cutting times based on the optimum cutting conditions identified by the first level of the COS. In this case, the CAD model is imported to be machined, the size of the starting part is specified, the user selects the machine to be used, and whether high speed or conventional conditions are used. The software will select the sequence of operations, cutting tools, tool path, cutting conditions, etc. This analysis can provide information on gross productivity, and estimate of the required investment, and detailed data on individual machine stations and station performance. Such computer programs analyze both the whole system and individual machining stations to balance work loads, increase productivity, and decrease investment. Compared to manual methods, such machine and part processing simulations can significantly reduce the total time required to develop and analyze manufacturing systems. Several variations of the machining system, the processing sequence, and tooling designs can be evaluated with minimal effort once the initial analysis of the system is completed.

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Noncutting time; the part process sequence for the various operations, which significantly affects the
part cycle time, should be considered in the optimization. There are three stages to planning part
processes for such systems: (1) determining the cutter trajectory from geometric models of machining processes, (2) developing mechanistic models for the machining operations, and (3) applying optimization methodologies for the machining system.

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The productivity of a conventional transfer line, FTL, or FMS, is affected by the cutting conditions, the rapid machine travel rate, and the speed of the transfer mechanism.

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Reducing idle time by reducing the cutting speed and feed combinations prolongs tool life, reduces tool failure, and reduces the probability of machine failure. Therefore, the use of all or a substantial portion of the idle time should result in higher system uptime and higher productivity.

There are two approaches to the solution of the tool magazine system reliability problem: (1) the
use of cutting tools of high reliability, and (2) the use of redundant tools. The use of redundant tools
in the machine’s magazine results in an increase in production tooling costs, but tends to increase
productivity and reduce production costs. The mathematical solution for the optimum number of
redundant tools in a magazine can be determined when the reliabilities of the individual tools are
known.

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The majority of tools have a single function and are used for only one operation or are made to
machine a single part feature. However, the use of multifunctional cutting tools has become necessary in single spindle manufacturing systems in order to increase productivity and/or quality and
to reduce cost.

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 Even though vegetable oils are more expensive than mineral oil–based formulations, they may be cost effective in some applications due to reduced consumption (less dragout than mineral oils) and improved tool life and productivity.

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In appropriate applications, MQL implementation may be justified by increased productivity,
capital cost avoidance, or variable cost avoidance. In many drilling operations, such as crankshaft
oil hole or large structural applications, penetration rates are significantly higher for MQL
than for alternative wet or dry processes, leading to an increase in throughput. Capital cost avoidance is a significant driver for applying MQL in high volume machining systems such as those
used in automotive powertrain manufacture, since in these systems the capital associated with a
coolant system (including piping, pumps, filters, and chip driers) may be a significant fraction of the
total system cost. Increased throughput in MQL operations may also contribute to capital
cost avoidance. MQL also permits the avoidance of variable costs for filter media and chip drying,
and often a reduction in costs for maintenance and coolant management resources, but these savings
may be offset to some degree by increased tool costs.

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14. Anon., Less lubricant, more productivity, Aerosp. Manuf., July 2010, 27.

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High-speed dry machining (Powercutting) was introduced by Gleason in the 1990s, and dry machining has become standard for completing cutter systems on contemporary bevel gear cutting machines due to its increased productivity over lower-speed wet processes.

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Recently introduced wheels containing “Cubitron,” a controlled shape alumina sol–gel, have a more efficient cutting action than randomly oriented abrasives and have enabled further gains in productivity.

Advances in fabrication of ceramic corundum abrasives based on sol–gel process

Chinese Journal of Aeronautics

Volume 34, Issue 6, June 2021, Pages 1-17

Open access

https://www.sciencedirect.com/science/article/pii/S1000936120303228

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96. F. Klocke and A. Klein, Tool life and productivity improvement through cutting parameter setting and tool design in dry high-speed bevel gear tooth cutting, Gear Technol., May/June 2006, 41–48.

Ud. 18.7.2022,  26.5.2022

Pub. 22.4.2020