Better Broaching through Process Control

Those concerned with streamlining their broaching process will appreciate the following step-by-step description of aspects to be examined, understood, and improved.

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Is your broaching process capable and predictable? Many are not. This article will discuss why and how performance metric based management can help bring your broaching operation into control. Predictability and reliability are desirable in every operation in the factory to create sustained value. Find an unstable process and you’ve found heartburn for managers. This article is aimed at high production broaching processes, but many points can help plants that are broaching lower volumes of parts, even though the savings in these low volume situations occur over months and years, not hourly like manufacturers running high volume broaching operations.

In every operation certain critical process variables exist. Defining, understanding, and controlling them are key to the success of the organization. Many companies have tweaked and tracked operations for years, and if your company has perfected its broaching operations you can stop reading now. If your broaching operation is the cause of manager heartburn, maybe this article can help. If you are a fireman, too busy putting out fires to take the time to stop the source of the fire, then you are not alone. Figure 1

For tool related process development I suggest that you ask for help from your broaching tool vendor’s expert or tool engineer. What you likely want would ideally be for the broaching expert to provide a one-visit, five-minute solution to all your problems with broaching tools, or for that matter all of your manufacturing problems. I’m sorry to tell you that it rarely happens that way, and achieving success takes time and some testing of tool design, materials, and coatings. Many manufacturers have a tough time with the cost of process development and would like the broach tool vendor to absorb the cost of process development. It’s a tough way to go for both sides, and one that requires trust and loyalty.
If after process development you will insist on staying with the vendor who helped you develop your process, I suggest that you share that with the vendor, and you will see a deeper commitment to your process improvement. Broach tools, while important, are the problem only about 40 percent of the time, in my experience. For best capability and predictability, all the variables need to be addressed. Figure 2

Let’s take a look at methods for measuring. Companies shouldn’t fall into the trap of saying “if you can’t measure what you want, want what you can measure.” Quality measures are not solely internal measures. Incoming part quality, delivery, and perishables—including tooling supplies maintained—are also relevant and eventually effect internal processes and quality. Some important measures and key indicators, of course, are parts per hour, cycle times, tool wear, force requirements, scrappage, and “machine up” or running time.
Innovative features incorporated in American Broach’s machines can help manufacturers collect a lot of this data, and we have systems designed for just that purpose, using PLC and CNC controls that measure track relevant process data. These key indicator measures should be collected accurately and used as an alarm in monitoring process control.

They must give management a quick, accurate overview of the current status and should be defined by needs of the end user or customer. When a measure is out of control, the process is out of control, and a variable is hard at work creating heartburn. Defining, measuring, and controlling all the relevant variables is the answer to success. Many of us know these things, but having the time and budget approved to gain process control and achieve a competitive advantage is often the challenge. I don’t have the answer, but I will say that companies I see finding a way are busy and seem profitable, and the ones that don’t are a combination of hopeful and fearful, and in trouble for the most part.

Most of us have seen situations where bad parts have been shipped inadvertently, only to have the customer insist on 100-percent inspection until the process can be evaluated and proven capable. This is expensive, and it proves another fact: that quality cannot be inspected into the process. Manufacturers generally cannot inspect every incoming and outgoing part in most manufacturing processes. Planning, experience, and paying attention to statistical process control results can help define the variation.

There are two types of variation: common cause, and special cause. Common cause variation is naturally occurring inherent to the process. Special cause variations are caused by problems or special/abnormal occurrences. It is management’s job to define and reduce common cause variation to its lowest point and attempt to eliminate special cause variation, since both affect the broaching process. Knee-jerk reactions to common cause variations should be avoided, while swift action to control special cause variations is recommended.
The following 10 points should be considered when seeking process improvement:

1) Machine stability, repeatability, and new machine innovations.

• Under load force, does the machine move accurately, or are the ways and guides worn? Of course this can be measured under no-load, but accurately measuring during the broaching process is difficult and subjective.

• Is the machine design capable of consistent repeatability over time?

• Does your company need a new machine to achieve best process capability? Then you have to ask, will a capable process reduce overall cost?

• An example of machine innovation is a software and load-forces sensor system designed to maximize the number of parts broached with each tool between tool resharpening, while eliminating costly tool overrunning or part scrappage. This system measures the force required to broach a part and can be dialed in to notify the operator when the cutting tool is ready for resharpening, thus maximizing the number of parts while protecting the tool from overrun. This is a huge cost savings, and this option is available on all American Broach machines.

• Do you have a relevant in-use maintenance program for your broaching machine? Have you rebuilt your pullers lately? American Broach offers regular preventive maintenance service calls, or emergency service calls in the event that you’ve postponed regular maintenance, or have an emergency critical machine breakdown. American Broach will also rebuild your pullers or supply new ones for improved tool alignment.

2) Part nesting fixture stability, alignment, and perpendicularity under load force.

• Under load force, does the fixture support the part?

• Is the fixtures performance reliable and repeatable in design and function?

3) Force, vibration, and speed.

• Is the drive system providing consistent force (power)? New technology using drive systems that reduce or eliminate the vibration are available using mechanical forces or advanced metered flow hydraulics. These new offerings in our industry offer improved part surface finish and improved tool life netting lower cost. Feel free to contact me at the number or e-mail address listed at the end of this article for a detailed explanation of the theory and physics involved. In some cases broach tool design can reduce required forces. Grandma used to say, “there’s more than one way to skin a cat,” and although it’s a gruesome visual the truth is that several different material removal approaches will produce an acceptable part. With some testing and an open mind, innovative tool designs often improve the process.

• Is excessive vibrations causing part quality, tolerance, or even measuring capability issues? In some cases broach tool or part nest design can reduce vibrations as well.

• What is the “right speed” to broach your parts? Is your machine capable of that speed?

4) Coolant volume, type, mix, and contaminates (especially bad is hydraulic oil that has leaked into the coolant). Coolant should be capable if four functions for best broaching results.

• First, reduce heat fast, heat is our enemy in broaching, the consistent and proper volume delivered at the right point is necessary. Sometimes a coolant chiller is required to stabilize the temperature. Second, lubricate the tool during the cutting process, including being “slippery” throughout the heat ranges of the broaching operation. Third, flush away chips form the part nest and out of the cutting tool teeth. Fourth, protect the part surface finish.

• Good coolant management is fundamental and often underestimated. I regularly recommend one of Master Chemicals specially formulated broaching coolants coupled with a coolant control plan for best process results. If your company is using the same coolant for grinding, turning, milling, and broaching, you can drive immediate process improvements by going to a coolant formulated for broaching.

5) Broach tool design and geometry, material, coatings and, most important, correct resharpening of the tool.

• The design of the tool is normally a cookbook formula based on decades-old rules of thumb introduced by Lapointe in the early 1900s and further developed by Colonial Broach in the thirties through the fifties. Some clever engineers have made a few innovations in design that have been quickly adapted through reverse engineering by many companies. The only point I would make here is that the “more than one way to skin a cat” rule applies, and likewise there is more than one approach to removing metal during the broaching process. It makes sense to try at least two to three different designs when yearly tool cost and performance is critical to reduce cost and maximize tool life.

• The steel and, of course, the heat treat of the steel that the broach tool is made of, to produce the optimum hardness for and toughness for the application. PM steel is widely used, and the recent improvements in steel with regard to grain size of the powder has lead to improved tool life. Tool steel should be specified to the part being cut and the needs of your process. High-end steel can cost double or triple the price of wrought M-2 HHS, and the cost to heat treat special alloys can be double that of wrought M-2 HSS.

• Coatings and surface treatment are surely a way to improve broach tool life and add another variable since results are often unpredictable and provide a process variation of their own. Because broaching is a niche industry, little research and development has been done on coatings for broaching. Our industry has mostly borrowed coatings developed for other metal cutting applications and tested them based on ROI compared to not coating at all. Consistent coating thickness and quality often cause problems with sizes when tight tolerances are required. Broach makers try to recommend surface treatments that are matched to the application, providing lubricity and hardness without the brittleness that can lead to micro chipping and unpredictable premature tool wear. Based on experience with the material to be broached, quantity of parts, and the broaching machine, I often recommend testing two or more coatings and documenting the results to find the best coating for the application. Many engineers have given up on coatings as too much trouble to justify the cost, but I almost always encourage re-thinking this because of the benefits I have seen over the years. Customers are often really happy with the improvements from adding a specialty coating. For months the new coating is predictable, and then a set of tools mysteriously don’t work. The answer is often found in the coating application, and if the tools can be stripped and the coating re-applied and then the tool resharpened, it will often work properly.

• Broach tool resharpening is a huge variable. I’d be wealthy if I had a dollar for every time I went into a factory to try to help diagnose a broaching problem and found the main problem was incorrect tool resharpening. To bring a broaching process into control, the resharpening must be consistent, and to achieve maximum tool life CNC resharpening under coolant is the only way. I would be glad to discuss all the advantages of CNC broach tool resharpening with a machine designed specifically for broach tool resharpening. Again, just send your question to the e-mail address found at the end of this article.

6) Incoming part characteristic composition should be specified not only for end product best performance, but also for best broaching properties if possible. Variation in incoming part composition must be controlled. I often see variation in hardness that adversely affects tool life and part quality. General hardness should not be above 32 R/C, generally. Very soft parts are a problem, too. We recommend 16-32R/C for broaching, with 28-32 R/C as the best. Of course part condition, temperature, geometry, pre-broach machining accuracy, and stock to be removed are also important, but it’s the hardness variation that most often affects process predictability. I want to note that parts can be broached dead soft and above 32 R/C. My hardness recommendations are for better or best results in broaching.

7) Gage quality, maintenance, and gauging procedures must be designed and carried out to accurately measure what is important, and these measures should be cleverly monitored and used as a quality tools to monitor the process. I often see the easiest attributes to measure tracked when they are not as important, or at times even relevant, and the difficult to measure but critical attributes not measured.

8) Part handling and in-plant variables are sometimes overlooked. The ambient temperature in the plant or pre-broach part storage condition or temperatures can be a variable as well.

9) Operator introduced variables can be addressed with training and visual process plans. Many manufacturers have been very proactive in eliminating most of these variables, but a second look is worthwhile. Process improvement is continuous!

10) Has the process engineer or operator with historic development knowledge passed on this process-specific knowledge so that the critical detail can be monitored?  

I suggest dedicating some time with your in-house or vendor broaching expert defining, tracking, and interpreting the results of performance metrics aimed at eliminating or controlling variables. Many times the first step I use to help a customer is to talk to the operator. Good, concise, unfettered communication is an important and desired quality in a company culture, but many operators will share facts with me as a result of simple pointed questions that can solve problems and identify root cause variables within minutes of my visit. They are normally more talkative if I am alone and not accompanied by their boss or the department engineer. The next question I ask is how they think we could fix, control, or eliminate the problem, and surprisingly to their boss or the engineer on the job—but not to me—they are right on the money many times.

The goal for me is increasing throughput and profit potential for our customers. I have come to believe that the best tool lies in optimizing the entire broaching process itself to be capable and predictable, and then using metrics to visually display the process controls and results. The way I see it, helping broaching customers prosper will help the industry in which I have earned my living for the last 30 years. If you need help or advice, the team at American Broach & Machine Company are ready to help.   

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MBA, also known as the “Broachman,” is a card-carrying journeyman broach-maker who specializes in sales and quality process management. Nemec is president of Ever Sharp Tools, Inc. (EST), “where the best broaches on Earth are born, including annulus ring gear, turbine, and hard broaching tools.” To observe the CNC broach tool manufacturing or re-sharpening process or to schedule a visit to your plant, contact Nemec at ken@eversharp.us or go to www.eversharp.us.