David M. Rascati 2017-11-07 03:03:37
“Memory Joggers” Wire, Rod & Bar Dies by: David M. Rascati Vice President Sales Premier Wire Die 1712 Dividend Street Fort Wayne, IN 46808 USA www.premierwiredie.com Why do Draw Dies Fail? There have been many articles written by industry experts such as Dr. Roger Wright, Dr. Horace Pops and Tom Maxwell that have focused on die design and wire drawing theory, but very little has been written on why draw dies fail. Why not? Draw dies are the heart of the operation and their success or failure is tied to manufacturing performance and costs. A broken or poorly designed die is worse than no die at all if it is producing scrap! The paper on which this article is based was written to enlighten the reader on how material flow, lubrication and tooling interact and contribute to a myriad of production failures, and more importantly, what to look for when draw dies fail. The following case studies include issues with raw materials, die construction, draw stock, mechanical failure, lubrication and die design. So why do draw dies fail? Poor Quality Raw Materials Underneath every quality carbide tool is a quality Preform. Preforms are semi-formed parts made of sintered carbide powder that die companies use for making draw tooling. Die companies depend on the quality of the preform as they grind and polish the rough outer surface of the tool. A poor quality preform means wasted time and resources in finishing. Die companies take a loss when the preform requires extra work to polish it. Customers take a loss when the tool fails in production! One of the first things they teach you in the carbide industry is how critical the sintering process is to the quality of the carbide. Sintering is the process that transforms pressed carbide powder into a solid piece. Time, temperature and pressure are all important parameters that must be carefully controlled during this process. The parameters for sintering are based on the grade of material and size of the part. For example, a small R2 and large R16 inserts of different grades may have to be sintered in different processes. An inadequate sintering process may lead to various metallurgical defects such as porosity, grain growth and eta-phase. An engineer at a large rod mill provided a competitor’s experimental die that broke prematurely. She believed it broke because they were taking too heavy of a draft. I believed that it was likely a tool design issue. In this particular case, the insert had a major circumferal crack about 1/3 of the way down the approach angle. While evaluating the incoming and the exit rod diameters, I realized the insert was too small for the draft. In order to accommodate the incoming rod size through the smaller insert, the die company used a wider angle. Going with a wider angle compromised the integrity of the insert as there was less wall thickness of the insert to counter the pressure of the incoming rod. There is a grey area in understanding and selecting the proper insert for each type of material. This die design may have worked for drawing copper or aluminum, but since it was stainless steel, it failed. Size wasn’t the only problem. The insert was sent to a lab to find out the grade of material. We wanted to make sure the die had the proper grade for the application. Too hard of a grade of carbide could have allowed the die to crack. The lab’s response was that they couldn’t be definitive about the grade, because the sample was too porous. The lab tech stated that the carbide was either “under-sintered or under-pressed”, which in this case he believed it to be under-sintered. Poorly sintered carbide doesn’t allow for the matrix of tungsten and cobalt (or nickel) binder to mesh properly, thereby allowing weakness in the integrity of the part (reference Figure 1 and Figure 2, courtesy of General Carbide). Figure 1 is of poorly sintered carbide that is likely undersintered. The black dots in Figure 1 are voids (pores) in the carbide which, create weak spots in the matrix and allow for easy propagation of cracks. This porosity will likely lead to tool failure. Figure 2 is of properly sintered and etched carbide. Notice the clean, uniform nature where the tungsten particles mesh perfectly with the cobalt binder. From this case study, I concluded that the cause of failure was primarily due to poor quality raw material and further complicated by a poor die design. The thin wall die geometry was further compromised by poorly sintered carbide and the die failed under pressure during production. Poor Die Construction Poor die construction is one of the least obvious reasons for die failure. There are two basic parts to the carbide die—the case and the insert. How they are married together is critical to the performance of the die. The outward appearance is not as telling as what is going on inside. One particular die company used to drop the carbide insert into the case and peen the top edge over to secure it. These dies typically failed, because all of the heat generated in the drawing process was absorbed by the carbide insert until it reached its breaking point and failed. The case of the die is meant, not only to provide support to the insert, but also act as a heat-sink. It absorbs the heat from the insert and passes it on to the metal die block. In some cases, these die blocks are water-cooled to provide further relief from the heat. The surface contact of the insert to the case is critical. The carbide inserts should be centerless or rough ground to provide more consistent contact with the case than the insert would have in the rough-cored state. The better the marriage, the better the case absorbs heat from the insert and away from the drawing process. In larger dies (R7 inserts and up) the carbide inserts are heat shrunk into the case. There is a general ratio of the diameter of insert to the amount of heat shrink that die companies adhere to during construction. In other words, the ID of the case is smaller than the OD of the insert when both are at room temperature. The manufacturer heats the case, which expands the ID enough to allow the insert to slip into the cavity. Once cooled, the case contracts and clamps down on the insert providing significant support for the pressures of drawing. A customer provided a draw die with a major circumferal crack and the top half of the insert protruding from the case. Upon extracting the insert from the case, we measured the OD of the insert and the ID of the case. We discovered that the case had a 0.003" taper from bottom to top. The diameter of the insert was approximately 1.185". The amount of necessary heat-shrink was roughly the same as the taper in the die case. Therefore, the insert broke where the support was inadequate. By just looking at a die it would be difficult to ascertain how the insert was installed in the case and most people don’t know to look for it. By dissecting the tool and taking some measurements a conclusion was made that the die failed because of poor construction. Mechanical Failure When a draw bench operator has problems drawing material, it is always the die’s fault. Sometimes it is, but most often it isn’t! A wire drawing customer complained that its dies weren’t lasting very long. Upon inspecting the cavity of a die there was a worn-smooth path at the entrance on one side of the die and a heavy wear ring down low in the approach angle near the bearing on the exact opposite side. This condition is indicative of an alignment issue. In effort to establish the direction of the misalignment, the customer said he had not established a 12:00 position on the die. We took a look at the draw block and it was easy to see there was little chance of the die being out of alignment as long as the block was clear of debris before installing the die. We then looked at the last pulley prior to the wire entering the block. It was clear there was no way to adjust the pulley to align the wire traveling into the block. There was a deep 1/4" groove in the pulley, which had never been adjusted or replaced. From this case study, we concluded that the 0.100" wire diameter was hitting at the lowest part of the groove, thus entering the die at an angle. It was apparent to us that the wire was making contact at the entrance radius of the die at the 12:00 position, creating the smooth surface finish thru the cavity on the top side and creating a deep wear ring at the 6:00 position down near the bearing where the wire was making its most direct contact. The customer changed and adjusted the pulley and the problem was solved. Marking a 12:00 position on the die when installing it into a block is essential to understanding the direction and flow of the metal. Wear rings inside the die help to paint a clear picture of what is going on during the drawing process. This is critical information for determining tool failure. Poor Quality Draw Stock Variations in hot rolled material are killers for draw dies but, they don’t have to be. A customer was taking a 1/16" draft on a rectangular hotrolled bar that was to finish at 1" x 1.250". The corner radius in the approach angle and bearing were about 0.020", with each corner being right at 90°. However, the hot rolled bar stock had only one corner that was close to 90°. Therefore, only one corner would be a good fit with the die and the other three would be hitting on the flats and not in the corners. The corners of the bar were contacting the flats in three spots, essentially digging carbide particles out of the die. The subsequent carbide particle pullout followed through the die, further degrading its structural integrity, enabling cracks in the corners. A better option was a die design utilizing a recessed conical corner. This design delayed the point of contact of the bar into the corner of the die until it was nearly on top of the bearing. The bar essentially hit on the flats and had no initial contact in the corners until the last point of entry into the bearing land. The flow of material through the recessed conical corner of the die helped to blunt the sharp corner of the bar and minimize the damage in the corner of the die. In turn, the customer went from drawing approximately 400 bars to almost 1200 bars. Die Design “Don’t be penny wise and dollar foolish!” This metaphor is priceless when buying shaped carbide tooling. This next case study deals with the issue of buying on the cheap. A customer provided an opportunity to quote on dies for drawing a Ski-Goggle shaped rod for the jewelry industry. Our price was US$495.00 and our competitor’s price was US$350.00. Instead of looking into why there was such a price difference, the customer decided to go with the least expensive bid. Upon hearing that we lost the order and by how much, I knew right away that my competitor was using an R-series insert for drawing round wire instead of an EDM blank. Not only is there a difference in price, but there is also a difference in the machine time to make the die using an EDM blank, so naturally our price would be marginally higher. However, there may also be a difference in performance! Typical of some die companies, which make use of round inserts for shaped dies, this customer wire cut the shape into the bearing area of a round die and then polished it from there. This approach may work in some cases, but there is more of a chance of die failure. The die bearing land has undulations in the length because of the un-transitioning approach and/ or back angles. Therefore, tension is relaxed in parts of the wire where it passed through the bearing, while other in-line particles are still under tension. This action may distort the wire from its desired form. The proper way to make a shaped die is to start with an EDM blank, which is a carbide disc with a small throughhole. The wire EDM machine utilizes multi-axis slides to transition the approach angle from a round to a shape or a shape to a shape. In wire-cutting an EDM blank, the approach angle and back angle work with the desired shape to maintain the bearing length. The transition of the wire from round-to-shape or shape-to-shape is done in the approach angle, not in the bearing. This is critical for filling all corners of the part during the drawing process and maintaining the desired shape of the wire. In this particular case, the customer called three months later to see if we could fix the die. The price-tag to fix the die was US$400. All in all, they spent US$750.00 instead of the US$495.00 they would have spent had they not been “Penny Wise and Dollar Foolish!” On top of that they lost another two weeks production time! The difference in cost between the US$495 and the US$350 die was US$145. The production cost difference was a fraction of a penny. The savings of US$145 cost them with the amount of lost production time and scrap produced through a poorly designed die. Add to that the additional cost of remaking the die. Proper Lubrication The next case study involves a customer who was having difficulty with drawing hex-to-hex bar. The customer was experiencing cracking dies and scratches on the bar. The company was taking about a 1/16" draft, bringing the bar to 1.250" flat to flat. The customer sent the die and a length of bar stock to us for evaluation. There were two issues found to be at play in causing these problems. First, the customer’s hot-rolled bar stock had two sharp opposing corners and four dull ones. The die that the company was utilizing had six sharp corners. The other issue was that the height of the carbide insert was approximately 1/2" short of what should typically be used for such an application. This was a pretty easy assessment and fix for us to make. The assessment was that the two sharp corners of the hotrolled bar were making contact with the die just prior to the dull corners. The sharp corners of the bar were digging into the corners of the die, which allowed cracks and particle pullout. The 1/2" difference in the carbide insert height compromised the die’s ability to provide proper lubrication. The poor lubrication allowed metal-to-metal contact, which caused the die to prematurely scratch out. With insufficient lubrication, there will be metal-to-metal contact and the die will scratch out leaving surface scratches on the bar. To correct the problems, we provided a die that was about 1/2" taller in height and provided countersunk conical corners throughout the die. The poor lubrication issue was accounted for with the extra 1/2" of insert height, providing a better wedge-gap in the approach angle. Utilizing countersunk conical corners, the bar hit on all the flats first, efficiently spreading out the pressure inside the die cavity, while minimizing the contact of the bar’s sharp corners in the die. Keep in mind that the dies will always fatigue and fail over time, but by providing a better die design you can maximize the die life and lower your production costs. Some die companies will cut corners in order to be cost competitive. One way to do that is to compromise the height of the insert—where less carbide equals lower cost. Often times this reduced cost will be just enough to influence the buyer. However, the company ends up spending more money in the long-run. Conclusion The primary objective of this article on why draw dies fail is to help the customer improve productivity. The draw die is at the heart of that objective. When cold-draw mills have issues with raw materials, die construction, draw stock, mechanical failure, lubrication and die design, the customer’s production will go down. However, these issues can be corrected with some attention to details. Just like there are a myriad of production failure possibilities centered around the draw die, there are also a myriad of ways to correct them! For further discussion, contact the author at Premier Wire Die, in Fort Wayne, IN, USA, or to learn more about the products and services available from Premier Wire Die, visit the company’s website listed below. www.premierwiredie.com Credits: • General Carbide; Denis Pasay, Drew Elhassid, for providing photographs and editorial work on properly sintered carbide. • Nick Munger of Premier Wire Die, for his images of rectangular dies and hot rolled bar. • Krish Singh, for his concept of “Memory Joggers” used to remind workers of past failures and successes in drawing wire. Company Profile: Premier Wire Die was established in 2003 to provide the wire industry with new diamond wire dies as well as recut services for diamond wire drawing dies. Premier Wire Die’s Fort Wayne, IN, USA facility opened in the spring of 2004. The management and manufacturing staff at Premier Wire Die offers the wire industry a unique blend of wire die and wire drawing experience. It is this blend of experience that allows the company to provide its customers with the appropriate wire die design to best meet their complete wire drawing requirements. www.premierwiredie.com
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