Machining Stainless Steel? Here are Some Ideas for Improving Machinability, Productivity and Profitability
By Raymond E. Schnell
Carpenter Technology Corporation
Machine shops might improve productivity and profitability when they work stainless steels by taking a closer look at the near mystical relationship between the material worked, machine, tooling, cutting fluid and operator.
Any one and certainly all of these variables can have a profound effect on machinability. Yet, when there's a problem, the root cause often tends to defy diagnosis and correction by all but the most circumspect troubleshooters.
We are particularly sensitive to the issue of machinability because, as both a specialty metals producer and developer, we have been privileged to work shoulder-to-shoulder with machine operators in many shops that have participated in our beta site machining trials for new free-machining stainless steels. Findings reported in this article, in fact, are offered essentially from the perspective of machine shop managers and operators who have told us what they consider important.
High on their list of priorities is the need for repeatable performance. Since shops and plants of all sizes generally bid jobs based on their experience working the material specified, they can't afford any surprises.
They're in trouble if the steel they receive is not consistent from lot to lot, or heat to heat. At the very least, they will be plagued with resetting their machines virtually every time they receive a shipment.
More often than not they will have to endure the misery of unpredictable behavior, an affliction with the potential for creating all kinds of profit-draining problems.
Unfortunately this is a common experience, one which makes the steel supplied a frequent object of suspicion when unexpected problems arise. With today's state-of-the-art steelmaking technology, however, machine shops should not have to contend with troublesome variations in material.
Now the steel producer can establish manufacturing limits for chemistry, hot rolling, annealing and cold finishing operations to optimize machinability. Processing and process controls can be employed to assure repeatable, predictable machining performance. Chemistry and properties of an alloy can be managed within tighter than industry-specified limits to meet a shop's special machinability requirements.
Properties like hardness, which affect machinability, can be managed to advantage. Sometimes a compromise can be reached recognizing, for instance, that a softer material is easy to drill, while a harder material produces a better surface finish. An alloy might even be tailor made for a specific operation if the production run is large enough to justify the expense of customizing.
The results achievable by this materials management process can be verified readily by the producer who has his own automatic screw machine, CNC lathe or other machine tool inhouse for day-to-day machinability testing. The machine shop, therefore, does not have to wonder whether it is the "guinea pig" for evaluating a new or customized alloy.
After the desired value is built into the material to be machined, the shop is then challenged to utilize that value fully. Having made a significant investment in the material, the shop owes itself the time to determine how the machining bar is performing.
The material should be evaluated methodically, one step at a time to produce an accurate assessment. Nothing can be gained by rushing or looking for instant miracles. Each variable should be examined carefully, on its own merits, against machining parameters and goals. All machining processes should be clearly understood, and the critical limiting operation or pinch point for each machine identified. Time spent eliminating or minimizing this constraint is time very well spent.
A slow tapping operation, for example, can limit output for an entire job, time and again. Eliminating such a constraint, perhaps by taking a few minutes to replace the tap, could boost productivity substantially.
What criteria are used to measure success? Reduced cycle time? Increased parts productivity? Longer tool life? Reduced downtime for adjustments, retooling, tool sharpening, maintenance, etc? Better finishes? The ability to machine difficult parts? Freedom from annoying problems?
Each machine shop needs to establish its own priorities, and act accordingly. If a shop is getting increased productivity at the expense of excessive tool wear or breakage, for example, and the tool is an expensive broach, it may be on the wrong track. It may want to moderate its productivity requirements to protect its investment in broaching tools.
While type, age and condition are certainly limiting factors, it seems that many shops tend to operate machines at less than their potential capacity. Frequently there is opportunity to increase speeds and feeds to capitalize more fully on the free-machining characteristics of some of the newer alloys, like Carpenter's Project 7000® series of stainless steels.
Trying to increase speeds and feeds is a logical first step to improving productivity. Increases can be made in small increments while holding all other machining parameters constant. Results can be measured after each cycle in terms of part quality, output, effect on tooling and trouble-free performance, until the optimum speed and feed levels are reached. Increasing machining speeds and feeds may not be a realistic goal, of course, if the shop is already close to or at the upside limit for its machines.
A shop interested in boosting output by running its machines faster may want to confer with the steel supplier. If the alloy producer is aware of the job goals and machining parameters, s/he may be in a position to advise on the best alloy choice. Even if a shop feels confident with its own choice, or limited by its customer's specification, it could lose by spending too much or too little for the wrong alloy.
There should be no cause for alarm if one machine produces more identical parts from a given material than another similar machine, unless the difference in output is unexpectedly large. The dissimilar results just verify the point that working in a machine shop is dynamic, with conditions always subject to change.
Change can cause big machinability problems, especially if the change is so small it is overlooked. We worked with one shop, for instance, that was drilling Pyromet7 alloy A-286 stock with carbide drills. The shop changed its machining parameters and, suddenly, it experienced problems with tool wear.
After evaluating all the job variables, and finding no cause or solution, the shop sent us a case report with chips and tooling data. Following extensive study and lengthy dialogue, the shop recalled that it had run out of the carbide drills it used in the beginning and was using, in their place, high speed steel drills of standard quality.
The switch in tooling materials, overlooked by the shop, was the cause of the problem. This resulted in a poor finish on the I. D. of the final part. Results were further compromised by the fact that the high speed drills came from two different manufacturers.
Design and manufacture of the tooling used on a job, of course, can have as much of a bearing on machining results as the material. The shop with an unsolved machinability problem, therefore, may want to re-evaluate how the tooling is made.
Coatings applied to the surface of some tools may need to be examined because they can cause machinability problems. There are different quality and types of coatings, all of which can affect performance of the tool. Even a very thin layer of coating can have a major influence on machinability because it is in direct contact with the work piece.
Machine shops are really put to the test when they check their cutting fluids because there are no industry standards for evaluating these products. Nevertheless, the shop should know exactly what effect its cutting fluid is having on machinability. Results can be determined only on the machine.
It is well to remember that cutting fluids are used for two reasons - to remove heat and to provide lubricity. The additives used for each purpose sometimes work in conflict with each other. In many shops, the art of making these additives compatible is considered "black magic".
Major problems can arise from using the wrong cutting fluid, some of them totally unexpected. A machinist employed by one of our customers, for instance, developed a serious rash while machining our stainless Type 446. Thinking the steel caused the problem, he threatened a lawsuit.
Up to that point, nobody had given any thought to evaluating the cutting fluid. When he did check it, the customer found that it contained no biocide to eliminate the bacteria that was causing the rash. This is an additive that the shop needed to replenish periodically (but had forgotten to do so) because it is eventually consumed by evaporation. After the biocide was added, the skin rash went away.
A machining problem can be particularly vexing when there is more than one cause. One shop we supplied with Project 7000 stainless Type 316, for instance, called us in a panic. Forming tools were tearing the surface of the material.
We talked with the tooling engineer to understand the operation and the problem. Asked whether the shop was using carbide or high speed tool steel for the tooling, the engineer replied, inserts. Since tool rigidity was essential for this forming operation - and inserts are not as rigid - the tool problem was solved by switching to a solid form tool.
While the change produced some improvement, the tearing problem persisted. If the tooling was now right, and the eight-spindle screw machine was running at optimum speed and feed rates, maybe there was a problem with the coolant. It might not be the right choice or, if it was, maybe there wasn't enough at the cutting point.
Carpenter modified the alloy slightly to make it harder, and we determined jointly that the speeds and feeds were right for the job. Still, heat buildup from poor cooling caused a built up edge (BUE) on the tool. It was this BUE that continued to cause surface tearing on the work piece.
Further investigation revealed that the shop was routinely running straight oil as a cutting fluid for all operations performed on this machine. It was a chlorine-free type cutting fluid that provided lubricity but not enough cooling at the cutting point of the tool. The shop eliminated the BUE and consequent tearing of the work piece surface completely by switching to a semi-synthetic sulphurized, chlorinated cutting fluid.
In real shop life, it's not safe to assume that operators with comparable experience have the same ability, communications skills and job attitude. The differences, great or small, can have a powerful effect on productivity and profit. This makes it imperative that shop supervisors carefully weigh the human factors in their management procedures.
Is the attitude of every machine operator in sync with that of shop management? Are there any machinists who think improving productivity means working harder instead of smarter? Do some operators run their machines at their own comfort level, at the expense of optimum performance? If the answer to the first question is "no", and the answers to the second and third questions are "yes", serious effort should be devoted to changing their mindset.
Experienced operators usually know what lubrication, tooling and setup works best, and what doesn't. Skilled machinists are aware of what chip characteristics are most desirable. They generally know, in fact, when their machine is giving peak performance or running at borderline efficiency. But do they share this information with shop management?
Operators should be involved in solving machining problems and consulted on a regular basis. They should be encouraged to do their own troubleshooting and given freedom to discuss problems and irregularities with their counterparts running the same machine(s) on other shifts. If they are unable to work out solutions to their satisfaction, they should report their concerns to their immediate supervisor without delay.
Nearly every machine shop has the operator who is so conscientious and dedicated that, ironically, s/he costs the shop money. This is the machinist who, at the end of his/her work shift, installs a new tool in his/her machine for the operator using the machine next.
However, the departing operator forgets to tell the new operator - who, being equally conscientious, also installs a new tool. This type of episode, probably repeated more often than many shops realize, can be very expensive. It proves that no shop can afford anything but the best internal communications.
In today's competitive business environment, machine shops are pretty much obliged to find ways to improve machinability and productivity. With the exception of the steel it plans to machine, the shop has complete control of its own destiny. It alone is in position to manage all the other variables in the machining process.
When it has a machining problem or seeks to improve results, the shop must be thorough in its self assessment, addressing all aspects of its machining operations. Managers need to be wary that the slightest change in machining parameters can have a surprisingly big effect on results. If the goal is consistency, then no change in the process can be tolerated. Change, of course, can be beneficial, but only if it's controlled and duly considered. If the diligent manager is confounded in his/her search for the cause of a problem, the investigation should be intensified to identify that which has been overlooked, the blindspot evading detection, the little thing forgotten or the routine which may disguise an unsuspected flaw in the process.
The shop manager who has exhausted his/her own resources in solving a machining problem may benefit by consulting with those who supplied the steel, the machine, the tooling and the cutting fluids used. They bring to the scene different disciplines, with special expertise, and just might ask the right questions. Sometimes that's all it takes.
Since the machine shop has no control over the material it plans to machine, that may be why the material is often the first element to be questioned when a machining problem arises. Certainly the question provides enough reason why the shop ought to be more discerning in its stainless steel requirements.
How can the machine shop be sure that it is getting steel that will machine well, the same way as it did the last time it was used? The shop can hardly expect a guarantee, but the odds of getting consistent, superior performance can be increased greatly if it gets "yes" answers to a few pertinent questions.
Do you know, for instance, who made the steel you are buying? If so, is it a producer that invests continually in R & D, product development and improvement, and capital equipment? Does the producer conduct variation reduction programs to achieve more precise chemical compositions, and does it work continually to improve its processes and process controls - all in the interest of providing alloys that will machine more uniformly and consistently? Does the producer constantly test machinability inhouse and field test extensively to validate results? Does your alloy producer provide in-the-shop technical service on working the material purchased? Finally, does that machining specialist counsel on working the steel alone or on the entire machining operation?