I especially enjoy writing articles on subjects that are important to our industry but are rarely given the attention they deserve. Maybe it&#;s because these topics are not often, if ever, discussed in schools, training seminars, or SPE functions. This month&#;s article on shoulder bolts is a good example of an important subject on which very little information is readily available. There is so much to know about this common fastener and how it directly affects the performance and longevity of your molds.
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Shoulder bolts are used in in a variety of ways and for various purposes. Their two primary functions in injection molds are limiting the distance a plate travels and pulling a plate during mold opening. Shoulder bolts are frequently used in three-plate molds and stripper-plate molds. While the generic name of this type of fastener is shoulder bolt or shoulder screw, these applications have inspired other, often more common names, such as range bolts and stripper bolts, based on their function.
Shoulder bolt terminology.
As seen in the accompanying diagram, shoulder bolts have three main sections: the head, the shoulder, and the thread. You specify a shoulder bolt by its shoulder diameter and shoulder length. For example, a &#; × 2 shoulder bolt has a shoulder diameter of &#; in. and a shoulder length of 2 in. If you purchase this &#;-in. bolt from a mold-component supplier, the thread will be &#;-18 UNC, not &#;-16 UNC. The overall length of a shoulder bolt is the height of the head, plus the length of the shoulder, plus the length of the threaded portion. This nomenclature is very different from that of a standard bolt.
Shoulder bolts are available in both standard and precision grades. The diameter of the shoulder of a standard shoulder bolt is typically 0.002 to 0.004 in. smaller than its nominal value. Precision-grade shoulder bolts are only 0.001 or 0.002 in. smaller. Precision-grade shoulder bolts are typically used in applications where the shoulder acts as a shaft for a sliding or rotating component, such as a bushing or bearing. In these instances, the shoulder is referred to as a journal.
The tolerance on the length of the shoulder is typically ±0.005 in. However, some suppliers have length variation as much as 0.020 in. Several mold-component suppliers don&#;t even specify this tolerance. That can be a major problem. If the length of the shoulders varies by a significant amount, any plate in contact with the bolts can cock and possibly bind. The more likely scenario is that the head of the shortest bolt will break prematurely due to uneven load. Therefore, it is incumbent on the toolmaker to measure the shoulders and possibly machine them to a uniform length.
Most manufacturers include a slight reduction in diameter directly under the head. This is called the shoulder neck. It allows the head of the bolt to seat flush against its mating surface. Most manufacturers add a radius between the shoulder and the head of the bolt to reduce the notch effect. If the bolt has a radius, make sure that any washer or mating surface clears the protrusion. Otherwise, the load applied to the head of the bolt will be concentrated over a very small area and can cause the head to crack.
All manufacturers include a slight reduction in diameter at the end of the threads where they meet the shoulder. This is called the thread neck. It is necessary for manufacturing and lets the shoulder seat flush against its mating component. The thread neck is the weakest point of a shoulder bolt because it has the smallest diameter. This is why the torque values for shoulder bolts are about 40% less than the torque values for standard bolts with the same thread size and pitch, as shown in Table 1. If a shoulder bolt is over-torqued, it will most likely fracture at this location.
The thread neck is the weakest point of a shoulder bolt
Properly torquing a shoulder bolt is very important. When you torque a bolt, it stretches like a spring. This spring-like tension reduces the chances of a bolt coming lose due to external forces such as impact and vibration. The amount a bolt will stretch is directly related to its &#;effective length.&#; For the sake of simplicity, the effective length of a shoulder bolt is the distance between the face of the shoulder and the closest engaged thread, as shown in the diagram. As you can see, shoulder bolts have a very short effective length. They will not stretch very much, and therefore will not have a lot of holding force. That is why shoulder bolts have the nasty habit of coming lose. The chance of one coming lose when affixed to a heat-treated component is even greater. Using a thread-locking compound can help, but properly torquing the bolt is best. So, what is the best way to torque a bolt? Table 2 lists various methods used to torque or preload bolts and the accuracy of each method.
It probably comes as no surprise that the torque accuracy of someone going by feel is very poor, but you would think that using a torque wrench would be very accurate. The reason why the torque wrench variation is as much as ±25% is due primarily to the coefficient of friction. The torque applied to a fastener must overcome frictional forces before any preloading takes place. If the assembly is not clean and lubricated, there is a considerable amount of friction at two locations&#;the bearing surface under the head (or in the case of shoulder bolts, the shoulder), and the thread-to-thread contact surfaces. Approximately 50% of the torque applied to a bolt is used to overcome the head-bearing friction, and about 35% is used to overcome the thread-contact friction. Therefore, 85% of the torque value is used strictly to overcome friction and only 15% is used to preload the bolt. The error percentages in Table 2 apply just to that 15%. If these two frictional surfaces are lubricated with cadmium plate, molybdenum disulfide, anti-seize compounds, etc., the friction is reduced by roughly 10% and thus, a substantially greater preload is obtained with the same amount of torque.
Always clean and lubricate the bearing surfaces of a bolt.
Therefore, always lubricate the threads of any bolt during assembly, as well as under the head or shoulder. Interestingly, the friction factor is at its highest value the first time you torque a new bolt. It gradually lessens with each subsequent loosening and tightening.
The turn-of-the-nut method for torquing a bolt can be as simple as rotating the bolt, or nut, by &#;, ½ or &#; of a revolution past snug. Snug is defined as the point where you can no longer loosen the bolt by hand. The amount of revolution is based in part on the effective length of the bolt. However, since there are so many variables with this method, it should be considered a rule of thumb.
There are companies that offer sophisticated equipment to measure the tension, not the torque, on a bolt, but they are fairly expensive and thus overkill for moldmakers. There are also mathematical calculations, which are very accurate, if you have a degree in engineering and nothing else to do with your time. In critical applications, you might consider using a load-indicating washer. There are several different types of these washers on the market. They are commonly known as DTI&#;s, or Direct Tension Indicators. They are not very expensive, very easy to use, and as Table 2 shows, extremely accurate.
The industry-standard shoulder bolt in the U.S. is made of medium-carbon-alloy steel, heat treated, quenched and tempered to 32-43 Rockwell C (as per ASTM A574), with Class 3A UNRC (Unified National Rolled Coarse) thread and a minimum tensile strength of 160,000 psi, which meets ANSI/ASME B18.3 specifications. Yes, that&#;s a mouthful, but if you are ordering bolts from a general supply company, be very careful that they meet these specifications.
The &#;R&#; in UNRC indicates the bolt was made using the rolling process, which dramatically increases its fatigue life. The &#;A&#; in class 3A means that it&#;s an external thread. If it was a &#;B&#; it would mean an internal thread, like for a nut.
The &#;3&#; in class 3A describes the fit of the thread. For UNRC external threads there are three standard classes of fit: 1A, 2A, and 3A. At one time, there was a class 4 thread, but it is now obsolete. There is also a class 5 thread, but this has an interference fit. Thread classes 1 through 3 are &#;clearance&#; fits. That is, they assemble without interference. The higher the class number, the tighter the fit. Class 1 threads have an extremely loose fit, with an intentionally significant amount of play. This class is used for quick and easy assembly and disassembly. Threaded rod or &#;all-thread&#; is an example of this class of thread. A Class 2 thread has the best balance in terms of performance and economy. It has enough clearance to minimize any galling or seizing, and sufficient clearance for lubricants, thread-locking compounds, platings and coatings. Nearly 90% of all commercial fasteners are this class of thread.
Moldmakers don&#;t use &#;commercial&#; fasteners. We use class 3 threads, which are for close-tolerance applications, where safety is a design consideration. This fit class has only a little clearance for lubricants or thread-locking compounds. That is why sometimes a bolt may be a little difficult to install, and you might think your tap has worn out.
Has anyone ever given any thought to the strength of the thread on a shoulder bolt? Why would you?
The length of the threaded section is a fixed value. You get what you get, but that&#;s OK because screw manufacturers use the correct thread length to achieve the maximum holding power. However, there are times when you have to make a custom shoulder bolt in order to satisfy a problem when a standard size is not available&#;typically when a very long shoulder is needed. That&#;s when knowing some additional information about thread strength is important.
Ideally, you want the bolt to break in tension before it strips the threads. This depends on two things: the strength of the material that the bolt is screwed into, and the amount of thread engagement. If the strength of the steel into which the shoulder bolt is screwed into is greater than the strength of the steel of the bolt, and there is sufficient thread engagement, the bolt will not strip the threads. It will undergo tensile failure first. But what if you have a mold with a shoulder bolt screwed into mild steel or aluminum? It&#;s not going to take a lot of force to strip those threads out, no matter what the thread engagement length is. In a case like this, try to install a hardened nut or threaded bushing that is retained in the back of the plate.
Next, what is the minimum amount of thread engagement required to prevent a bolt from stripping? There is a rule of thumb that says the amount of thread engagement for a bolt should be 1 to 1.5 times its nominal diameter. You should know by know how I feel about rules of thumb. There are some mathematical formulas available to calculate the optimal amount of thread engagement, but there is a much easier way that is sufficiently accurate. Table 3 breaks down the load distribution over the first six threads of a carbon-steel threaded fastener. As you can see, the first six threads account for 100% of the holding power. If you simply divide the number 6 by the pitch of the fastener, you get the minimum required thread-engagement length. For example, for a ¼-20 bolt:
in. of engagement.
The first six threads of a fastener account for 100% of its holding power
While this value is correct for the amount of thread engagement, it does not account for the lead-in on the tip of the thread, nor does it account for the runout, or thread neck. These two areas account for about another three threads. Therefore, to calculate the overall distance from the end of the shoulder to the tip of the thread, use 9, instead of 6, divided by the pitch. If you check a supplier&#;s catalog, you will see that the overall thread length that comes with standard shoulder bolts is very close to 9 divided by the pitch.
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Jim Fattori is a third-generation injection molder with more than 45 years of molding experience. He is the founder of Injection Mold Consulting LLC. Contact ; injectionmoldconsulting.com.
Many consider press brake tooling a minor accessory in metal forming when in fact the opposite is true. Although press brakes have evolved into multiaxes, high-precision machines with self-stabilizing features, the tooling is all that ever actually touches the part during bending (see Figure 1).
The line has blurred between RFA, New Standard, European, and American standard tooling. Many features needed for high-performance bending have migrated to all the various tooling types. Regardless of which tooling and clamping style you choose, be sure it meets at least a few minimum requirements.
High precision. The tools should be manufactured to tolerances within the 0.-inch range. This is critical to achieve part accuracy without shimming or other tweaks during setup.
Segmented sections. These allow you to build various lengths out of several precut pieces. Small pieces are safer and easier to handle too.
Self-retaining installation. You should be able to load the tools with the ram up. The toolholding system should hold multiple pieces in place until the clamping pressure is applied (see Figure 2).
Self-seating. As clamping pressure is applied, the punches are mechanically pulled up into position. This eliminates the need to bottom the punch into the die during the setup.
Front loading. You should be able to install tools from the front of the machine. This shortens setup time because you no longer need to spend time sliding tools from the end of the press brake. In most cases, front loading also eliminates the need for forklifts and overhead cranes.
Standard sizes. Common-height tools can reduce the need for machine adjustments when changing jobs. Front support arms, backgauge heights, and safety devices all remain at a common position. And because tools are made to the same heights, you can add off-the-shelf pieces and be sure they will match your existing tools.
Many high-quality press brake tools are made to metric standards. So a nominal sized 0.250-in. V opening is actually 6 mm, or 0.236 in. Moreover, bends in sheet metal have slightly elliptical corner radii, so you only have to get close to get correct. For simplicity, imperial dimensions are rounded in this article.
Note that the discussion that follows focuses on air bending, and for good reason. The trend is to abandon bottoming or coining and embrace air bending whenever possible. Be aware, however, that not all parts can be produced using classic air bending techniques.
Operators throughout the industry use very different tooling to make parts of similar or identical quality. Plenty of operators make acceptable parts with incorrect tooling because they don&#;t have access to the correct tooling. They make it work; but &#;making it work&#; isn&#;t efficient or repeatable, and it can seriously hinder work flow. Best practices in tooling selection really should have one elegantly simple goal: to achieve the best-quality parts in the least amount of time possible.
A maintenance shop will need and use different press brake tools than a custom fabricator will. So before diving into specifics, identify your needs and budgetary constraints.
For instance, you might need additional tools to shorten setup times. You might follow lean manufacturing principles and recognize the benefits of having a separate tool library for each press brake&#;and hence, be willing to invest in duplicate sets of tools stored at machines. You don&#;t lose valuable setup time walking to and from the tool crib and elsewhere looking for the correct tools. An added benefit here is that tool style compatibility from machine to machine is no longer necessary, because the tools tend to stay with their intended machine (see Figure 3).
If you need to buy additional, duplicate tools to expand each brake&#;s dedicated tool crib, choosing them is relatively straightforward. You&#;ll often find these tools located in convenient places, if not already in the press brakes. Look for the tools with the most wear and tear&#;those with shiny, bright working surfaces. The body of the tools will likely be clean and bright too. Rusty, dirty tools on the bottom of the rack are not likely candidates.
To get the biggest bang for your buck, choose a minimum number of lower dies that will cover the entire range of metal thicknesses your shop forms. Shops with little tribal knowledge, unforeseen applications, and limited budgets should try selecting lower dies using the 8×2 rule.
First, determine the range of metal thicknesses you want to bend. For example, you might need to bend material 0.030 in. through 0.250 in thick.
Second, assess the smallest V die needed by multiplying the thinnest metal by 8. In this case, 0.030-in. material would need the smallest die, hence: 0.030 × 8 = 0.24, which we&#;ll round up to 0.25.
Third, assess the largest V die needed by multiplying the thickest metal by 8. In this case, the thickest material of 0.250 in. would need the largest die: 0.250 × 8 = 2.
You&#;ve now determined the smallest and largest die you need&#;0.25 and 2 in. To fill in what you need in between, you start with the smallest V die and double its size. In this case, that gives you a 0.5-in. die (0.25 × 2 = 0.5). Next, double the 0.5-in. die to get 1.0 in., then double that to get 2.0 in. This gives you a minimum of four different V-die openings to bend 0.030- to 0.250-in. material: 0.25, 0.5, 1.0, and 2.0 in.
You also use material thickness to determine the minimum number of upper punches. For material 0.187 in. and thinner, you can use an acute offset knife punch with a 0.04-in. radius. The acute angle allows bending past 90 degrees, and the offset allows you to form J shapes. To handle the higher forces when forming material between 0.187 and 0.5 in. thick, consider a straight punch with about a 0.120-in. radius.
Note that for some applications, including those using thicker and high-tensile material, the workpiece tends to crease, crack, or even split in two when using common industry bending standards. It comes down to physics. A narrow punch tip exerts more force on the bend line; combine that with a narrow V-die opening, and the forces rise even more. For challenging applications, and especially when material thicknesses are above 0.5 in., it is best to consult your material supplier on the recommended punch tip radius.
In a perfect world, you should be able to select the V-die opening using what we call the rule of 8; that is, the V-die opening should be 8 times the material thickness. To determine this, multiply the material thickness by 8 and choose the closest available die. So if you have 0.060-in.-thick material, you need a die that&#;s 0.5 in. (0.060 × 8 = 0.48; 0.50 in. is the closest die width); for 0.125-in. material, you need a 1-in. die (0.125 × 8 = 1). This ratio gives the best angular performance, which is why many call it the &#;sweet spot&#; for V-die selection. Most published bending charts are centered around this formula.
Simple enough? Well, it would be in that perfect world, and you could live in that perfect world if the sheet metal designers always followed the rule of 8. But alas, in the real world, the exceptions abound.
When air bending mild steel, the inside bend radius forms at approximately 16 percent of the V-die opening. So if you air-bend material over a 1-in. V die, your inside bend radius will be about 0.16 in.
Say a print specifies 0.125-in. material. In a perfect world, you&#;d multiply that thickness by 8 and use a 1-in. V die. Simple enough. But many sheet metal designers like to specify a bend radius equal to the metal thickness. What if the print specifies an inside radius of 0.125 in.?
Again, material air-bends an inside radius that&#;s about 16 percent of the die opening. This means your 1-in. die can produce a radius of 0.160 in. Now what? Just use a narrower V die. A 0.75-in. die will give you an inside radius that will be close to 0.125 in. (0.75 × 0.16 = 0.12).
Similar thinking applies for prints that specify larger bend radii. Say you need to form 0.125-in.-thick mild steel to a 0.320-in. inside bend radius&#;more than double the material thickness. In this case, you&#;d choose a 2-in. die, which would produce an inside bend radius of about 0.320 in. (2 × 0.16).
There are limits to this. For instance, if you find that to achieve the specified inside bend radius you need a V-die opening that&#;s less than five times the metal thickness, you will compromise angular accuracy, possibly damage the machine and its tooling, and put yourself in a very unsafe situation.
Keep flange lengths in mind when choosing your V dies. The minimum flange a given V die can form is approximately 77 percent of its opening. So a part being formed over a 1.-in. V die will need at least a 0.77-in. flange.
Many sheet metal designers like to save metal and specify a flange that&#;s too short, like a 0.5-in. flange in 0.125-in. material thickness (see Figure 4). According to the rule of 8, 0.125-in.-thick material calls for a 1-in. V die&#;but that 1-in. V die requires the workpiece to have a flange that&#;s at least 0.77 in. Now what? Again, you can use a narrower V die. For instance, a 0.625-in. die can form parts with flanges as short as 0.5 in. (0.625 × 0.77 = 0.48, rounding up to 0.5).
This also has limits. Just as with tight inside bend radii, if a flange requires a die width that&#;s less than five times the material thickness, you&#;ll experience angular accuracy issues, cause possible damage to the machine and its tooling, and put yourself in harm&#;s way.
For L shapes the rules are &#; there are no rules. Almost any punch shape will work. So when selecting punches for a group of parts, you always should consider these L-shaped parts last, considering just about any punch shape can handle them.
When forming these L shapes, use a punch that also can form other parts, rather than adding unnecessary tools to the library. Remember, when specifying tooling, less is always best&#;not just to minimize tooling cost, but also to reduce setup time by reducing the number of tool shapes needed on the shop floor (see Figure 5).
Other shapes do require specific rules for punch selection. For instance, when forming J shapes, the rules are (see Figure 6):
As you can see, the punch selection rules deal mostly with workpiece interference, and this is where bending simulation software can play an important role. If you don&#;t have access to bend simulation software, you can use your tooling supplier&#;s drawings with grid backgrounds to check for punch-part interference manually (see Figure 7).
If you&#;re using a conventional toolset, you&#;ll need to use two ram cycles to form offsets or Z shapes. For these shapes, the rules are (see Figure 8):
Any unsupported material inside the V die is subject to deformation; in holes and other cutouts, this deformation manifests itself as blowouts (see Figure 9). When the holes near the bend lines are small, the associated blowout will be small too. Also, most applications will accept some distortion, so there is no definitive rule on the best V-die width to choose when a cutout is on or near a bend line.
When the flanges, cutouts, and miters are clearly too close to the bend line for the metal thickness, you can specify rocker-type dies. The rockers rotate and support the material throughout the entire bending process and, thus, eliminate the blowout.
Figure 9 shows identical parts with cutouts close to the bend lines; the foreground one&#;with the telltale blowout&#;was formed using a conventional V die; the background one was formed with a rocker-type die. Also note that the two ovals on the left have the same width (front to back) and are the same distance from the bend line; only their lengths are different. You can clearly see more blowout on the longer oval.
Punch height becomes critical when forming three- and four-sided boxes. In some cases, short punches can form three-sided boxes if one formed side can hang off the side of the press brake during the final (third) bend. If you&#;re forming four-sided boxes, you need to choose a punch tall enough to span the box height diagonally (see Figure 10):
Minimum punch height for box bending = (Box depth/0.7) + (Ram thickness/2)If there are no top (return) flanges, or the top flanges protrude outward, you won&#;t need much clearance between the top punch and lower die to remove the part after bending. But if you do have return flanges (top flanges that protrude inward) on all four sides, you need enough clearance to twist and remove the box after bending.
Bend-hem tools can form parts with hemmed edges in a single setup, as shown in Figure 11. Just know that if you need to hem thicknesses greater than 0.125 in., you might need custom tools to accommodate the excessive forces required.
V-die opening selection rules here are basically the same as for standard bending tools. The 30-degree prebends for the hems do require somewhat longer minimum flanges&#;at 115 percent of the selected V-die opening&#;because of the acute angles. For instance, if you&#;re forming material over a 0.375-in. V die, you&#;d need the flange to be at least 0.431 in. (0.375 × 1.15).
Almost all typical V-die bending tools leave some marks on the part, simply because the metal is being drawn into the die while bending. In most cases the marking is minimal and acceptable, and increasing the shoulder radius can reduce the markings.
For applications where even minimal marking is not acceptable, like when bending prepainted or polished materials, you can use nylon inserts to eliminate scratching (see Figure 12). Scratch-free bending is especially important for fabricating critical aircraft/aerospace parts, because it&#;s hard for inspectors to visually inspect a piece and tell the difference between a scratch and a crack.
Today&#;s precision tooling and press brakes can reach unprecedented levels of accuracy. And with the right tools and consistent material, a press brake operation can bend a flange to a specific angle with a specific inside bend radius. But again, air bending forms the inside bend radius to a percentage of the die opening&#;and having the right tools matters. Specifying a multitude of different, tightly toleranced radii will increase tooling costs. And the more tools you need, the more changeovers you&#;ll have, which increases costs even more.
That said, sheet metal part designers can make tooling selection and the overall bending operation much easier if they follow a few basic rules when designing parts:
Exceptions to these rules abound, and each comes with complications. You can use a narrower V-die opening to bend a tighter radius or a shorter flange&#;but bend too sharp a radius and you risk creasing the bend line and exceeding the tonnage rating for the tooling and press brake. You can bend a narrower offset, but again, that requires a special tool and significant forming tonnage.
If a part doesn&#;t need a short flange, a narrow offset, or a tight radius, why complicate matters? Follow these three simple rules and you&#;ll improve angular performance, shorten setup time, and reduce tool cost.
Paul LeTang is product manager, press brake/tooling at Bystronic Inc.
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