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Machining tests for optimization

By Moise Cummings

Executive Summary
Economic recovery brings opportunities for machine shops to take orders from new businesses that may require dealing with unfamiliar materials. In addition, environmental regulations are mandating newer, less-toxic fluids. In these cases, managers should return to machinability testing and Frederick Taylor’s tool life equation. This time-tested method can ensure that tooling costs for using these new materials don’t decimate a bid’s profit margins.

As the economy continues to recover, many small shops are receiving orders from industries that may require them to machine materials and alloys that are not in their normal scope of operations. To make an intelligent bid for these jobs, managers of these shops must have a way to rate the new material in comparison to a material that they understand and are familiar with.

Industrial engineers, cost estimators and others involved in submitting bids know that tooling is a portion of the machining cost for every job. Controlling and reducing these costs benefit the company’s bottom line. It can cost an organization a great deal of money, not to mention management headaches, to set a price for a job only to have that job wear out tooling at a faster rate than expected.

In addition, without fully understanding the proper machining parameters for a material, it is more challenging to achieve acceptable part quality in terms of features such as surface finish. What often happens is that these adjustments are made on the shop floor, which can be costly in both scrap and time wasted. A methodology of comparing the various tooling can be an effective way to evaluate the tool based on the tool life that can be expected versus the various cutting parameters.

Also, with more stringent EPA requirements calling for fluids that are more biodegradable and are safer for the operator, companies would like to determine the financial impact and the effects on quality that various cutting fluids will have. This helps them understand how changes in the chemistry of the fluids affect the cutting parameters used, the quality of the part produced and the life of the cutting tool. It generally is understood that these kinds of changes come with a trade-off in other areas.

These are the primary reasons that many shops turn to machinability testing. In the changing industry landscape, machine shops constantly are looking for ways to gain or maintain a competitive advantage. Shops that can understand quickly what it will take to machine a new material, machine a material with a modified process, rate an existing material or evaluate new cutting tools can make better decisions on how to program and process materials without using time on the shop floor. They have less waste in terms of scrap and time, can make better decisions on capital investment and work with material suppliers when there are quality issues.

All of this comes down to knowing what reasonable level of machining performance to expect. Machinability is more than knowing that one material, tooling or coolant is easier to work on than another. It’s actually an index that can be calculated, quantified and specified to a supplier, and it keeps both producer and customer on the same page. While companies have developed machinability rating tables for various materials, ratings for newer alloys or fluids may not be published.

In the most basic terms, machinability testing is nothing more than a controlled experiment. Each test includes controlled components: material, tool and coolant. These components have known characteristics, while the test components are being evaluated to determine other characteristics. It’s important that all other variables, such as depth of cut, speeds, feeds and type of coolant, that are not being evaluated remain the same during the testing. From this, a machine shop should be able to develop a clear picture of how to approach the machining process, which means that it will have a good handle on the cost to perform a particular job and how to achieve part quality.

Machinability of materials

A baseline material is required for a machinability test. Often, this is B1112. This steel alloy has been used in machinability testing for decades and is understood to have a machinability rating of 1.00, meaning that it is “the standard” material used in testing.

In order to make intelligent decisions about machining parameters, it is important to have an understanding of failure modes and wear conditions of cutting tools since tooling costs have a large influence on the price of a job. A common performance metric for machinability testing is the measurement of flank wear, as shown in Figure 1. During machining, the clearance edge of an insert rubbing against the work piece is one source of cutting wear. All other things being equal, the degree of flank wear is a good indicator of the performance of the variable that is being evaluated.

Machinability testing of materials is based on the point at which flank wear reaches the end of tool life, which is when the tool no longer performs satisfactorily. This is done by testing several materials and measuring the flank wear at specific intervals of machining to see how the relative flank wear compares. The machinability rating (MR) can be thought of as a ratio that compares the cutting speed of a specific material to a “standard” material, such as the B1112 mentioned above. A material with a machinability ratio higher than B1112’s 1.00 is easier to machine, so the tools that work on it have a longer life. MR can be expressed in the following equation:

MR = (Speed of machining the work given 60 minutes of tool life / Speed of machining the standard metal given 60 minutes of tool life) X 100

These ratings can be used in conjunction with the equation Vc × T n = C. In this equation for tool life expectancy, devised by Frederick W. Taylor, the father of scientific management, Vc is the cutting speed; T equals tool life; and n and C are constants from experimentation or handbooks, with n based on the tool material and C based on the tool and work.

Taylor’s tool life equation is meant to estimate the change in tool life as you change the cutting speed while already knowing the machinability rating. End users who deviate from test conditions still will get an idea of how this will affect their tool life.

Other machining tests also are useful for manufacturing shops. Different coolants and cutting tools can be checked for their effect on parameters that include cost savings, quality and optimization. However, the method described above still stands; the proposed coolant or tool has to be evaluated in a controlled environment to see exactly how it compares.

Cases in point

Take the case of an aerospace company that is reviewing a proposal to use a new coolant in its machining area. While some companies may perform tests in their own research and development shops, those without such facilities often hire a research company because flushing and recharging a machine tool for a coolant test holds up production. In addition, changing a coolant out in a production environment can impact the quality of the parts being made.In this case, the coolant is the variable being evaluated to determine how it will impact the cutting performance and the work material. The methodology is to compare the tool life as well as part quality traits such as surface finish with the proposed coolant versus a known coolant as a base line. The test is considered complete when the flank wear reaches 0.015 inches or 60 minutes of cut time.

The result of the test will be a side-by-side comparison of each coolant and the flank wear at specific times during the test. This gives companies a documented performance of coolants from each supplier in unbiased conditions.

In another case, a small company had developed a coating for a turning insert that was intended to be used across various materials. The company needed to attain third-party validation of its new product.

Using a lathe, the insert and four competitor inserts were used to cut three different materials (4340 steel, 4140 steel and ductile iron) at various cutting speeds. The goal was to measure the amount of tool wear on each insert over the life of the test.

The methodology of the test was to turn each material at a specific cutting speed with a specific depth of cut and feed rate. The amount of flank wear was measured at various points throughout the test. The wear was plotted versus the cut time. The test was considered complete at the max tool life (when the flank wear reached 0.015 inches) or at the end of the 60-minute run.

The result of this machinability test was that the company that developed the insert was provided with documented proof to support the claims for its product. It was clear, under the testing circumstances, what could be expected from the tools with their coating and the tools with the competitive coating. This testing method could be applied to a machine shop that was interested in evaluating the claims of a tooling salesman when comparing inserts for improved part quality and/or cost-cutting measures.

Machining tests can help when a shop plans a new cell that will be dedicated to one customer. In another case, a customer could increase a large machine shop’s business by more than $300 million per year. To accomplish this, the shop will be required to machine materials that it is not completely familiar with. Executives want to make sure that shop floor workers not only are optimizing their machining parameters, but that they also are gaining the most out of their tool life.

This is a common concern in production settings. In many cases the manufacturing engineer is relying on information provided by the tool salesman or a machinability handbook, or the engineer is forced to develop a production-friendly test to perform on the line.

In this particular case, the materials and tooling were the constant (independent variables) and the cutting speeds were being changed (dependent variables). As in all cases, the test was considered complete when the flank wear was at 0.015 inches or the cutting time was 60 minutes.

The result was a tool wear versus cutting time chart for each cutting speed that shop officials could review and use to determine the optimum parameters. Armed with this information, the machine shop was able to get a better handle on the cost of the project, as well as have a base line of the tooling cost that it could expect from the new business that this cell would work on.

Going forward

A solid machinability study is simple and straightforward, but it does require time and effort. Companies that choose to perform their own must dedicate the required resources of manpower, machines and material.

The machining house that makes the proper investment reaps several benefits and a competitive edge. Production engineers can use machining tests to help them optimize shop floor operations. They can troubleshoot quality issues that arise due to raw materials, they can plan jobs more effectively, and they can establish processes that will position the shop to accept more diverse projects and profitably provide better quality products.

Moise Cummings is a machining systems engineer and supervisor at TechSolve Inc. He previously worked for Detroit Diesel and Delphi Automotive and has worked in quality and process control, spending several years in high-volume production as a quality engineer and manufacturing engineer. He earned a bachelor’s degree and a master’s degree in mechanical engineering technology from Purdue University. He is a member of the Society for Manufacturing Engineers and the American Society for Mechanical Engineers.

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