The root of a tap is the surface at the bottom of the thread-form that connects adjacent thread flanks and is expressed as a width or as a diameter. The term root diameter is also called minor diameter, it’s one of those things, you say rain and I say precipitation, meaning the same thing...
First things first, what is the root of a tap?
The root of a tap is the surface at the bottom of the thread-form that connects adjacent thread flanks and is expressed as a width or as a diameter. The term root diameter is also called minor diameter, it’s one of those things, you say rain and I say precipitation, meaning the same thing...
0 Comments
Recently we had an application question at North American Tool. A customer had opened up the drilled hole to 55 percent of thread in hopes of reducing wear and increasing tool life. Instead, it lead to the cut tap breaking at the first full thread of engagement. That prompted a engineering discussion and is the basis of this technical article to address this very common misperception. If a tap doesn’t create the hole, why is the hole size so important?
This leaves the good machinist to select a drill that is more or less than 75%. If 75% is good, then a higher number is better, right? WRONG (in many cases). On page 115 of the current North American Tool catalog a range for percentage of threads from 50% to 85%, with the recommended range between 65% and 75%. This guideline range to use is a reflection of several factors:
In higher tensile steels, larger hole sizes (less percentage of thread) may work better to reduce torque in tapping. When using oversize hole diameters, the tap should also have an oversize chamfer point diameter so all threads in chamfer are cutting and not just dropping in a hole. Such was the case with the application porblem. As was stated above the operator thought that by increasing the drill diameter size would lead to better tool life. Instead, because the diameter was of the hole was so large the cut tap did not engage the threads along the chamfer length and instead engaged at the first full thread at the end of the chamfer. The caused premature wear and stress on the that thread in the flutes and eventually to catastrophic failure. The solution was a modified tap from North American Tool. Summary: You've got to use the correct tap for the application! At the end of the day, taps are made to very strict tolerances, often much more so than the piece parts they are called on to machine, and the first place one might want to find blame is with the tap.
Why? Because;
What if one manufacturer makes a tap that works and another one made by a different company doesn’t work at all? In all likelihood, the “general purpose” of one is closer to the correct geometry for the application. One might get a completely different result if another material were being machined. Therefore it is always best to order a tap for the specific application. Once Upon a Time There Was a Magical Form TapIf you follow any industry long enough, history will become a useful tool to apply to your understanding of modern developments and designs. One such piece of history, as related by someone who was there in the 1950’s, will create an illustration of the importance of learning from errors, and creating opportunities. Once upon a Time, not so far away, a tap manufacturer with a long history in the industry made an error processing a manufacturing order. We all do from time to time. Where it went from there is the story. A cutting tap left the manufacturer missing a feature. The customer complained that they had received a tap without any flutes. Being desperate, they used it anyway, and reported that it “kind of” worked. The manufacturer quickly got the customer the finished tap as originally intended, but decided to research the performance of the “unfinished” tap. They saw the obvious benefits of a “chip-less“ tap, but identified multiple downsides. As the material is not being cut, but pushed out of the way, friction would likely be an issue. On top of that, threading a blind hole would create hydraulic pressure in the hole (with 100% thread engagement, there would be no escape route for air or liquid beneath the tap). Both issues would be addressed with “lube-grooves” and a geometry utilizing “lobes” (high points on the circumference of the tap) to reduce the surface contact between the tap and the material being threaded. These features would facilitate lubrication, by allowing some clearance between the surface of the tap and the material being threaded. They would also allow an escape route for any building pressure at the bottom of the hole. They realized that the material being formed “flows” in multiple directions, both into and away from the thread-form of the tap. One very important detail was quickly recognized. The material being formed must be malleable. No flow, no go! Ferrous and glass-filled materials are not a candidate for forming. A different hole size would be recommended to reduce the percentage of thread produced to a more acceptable level, and allow the same clearance for the minor diameter of the mating part allowed by a “cutting” tap. Further tweaking followed. Method and form of “chamfer” was changed, to more efficiently begin the threading process. Lobe geometry tweaked for better performance in different materials.
Additional lube grooves were employed in applications that benefited. Recommended speeds were increased. Coatings were added to reduce friction and improve wear resistance. Coolant-holes became an option for those with the machine capability. Base materials were developed and improved. Like anything else, improvements continue as application demands evolve. According to the trusted source, this is the way it happened. The real point of the story remains the same whether or not it is completely true. The “error” and solutions that followed illustrate most of the differences between the two styles of producing thread, and the taps that do the job.
Download your new copy today at the button below
Here’s another example of legacy term confusion. Sometimes it is necessary to use a tap with extended length. The added length may be used to add “reach” to the tool, or to allow tapping to a depth longer than the thread length of the standard blank. Maybe it’s necessary to accomplish both! The confusion lies in what to call that style of tap. “Pulley” taps “Pulley” taps were originally designed to tap threads in a pulley assembly. Length was extended, but shank diameter was approximately the same as the thread major diameter. The shank was used as an alignment guide to reach through the pulley hub. Thread depth could not exceed the thread length of the tap. Today, the term is rarely recognized by recent generations. “Extension” taps “Extension” taps provide additional length for reach just as pulley taps do. However, the shank diameter is reduced to allow tapping beyond the ground thread length of the tap. The shank diameter “clears” the drilled hole. All this seems pretty straightforward, but somewhere the industry decided to add a twist. In thread diameters 3/8” and smaller, Pulley and Extension taps use the same Industry-Standard blank. If additional reach is needed, with the desire to tap beyond the thread length of the blank, the terminology changes to Extension/Small Shank. Above 3/8” in diameter, the standard Extension blank has a smaller shank diameter, and is defined only as “Extension”. When in doubt, specify “small-shank” when tap use is intended to exceed standard thread length. As with any tool, these extended-reach taps can be made to custom lengths. Most manufacturers have standardized on 6, 8, 10, and 12 inches in blank length (varies by diameter) for stock tools. Maintaining Industry-Standard shank and square dimensions allows the use of standard tool holders. Allen Benjamin color codes HSSE taps to simplify the selection of the right tap for specific materials. Each of the six color-ring taps are manufactured with application-specific design criteria based on characteristics and hardness range of various part materials. The table below illustrates which color ring tap is best suited to the material being threaded. *NOTE: For material or material group not shown (such as No Lead Brass), please consult Customer Service here. ** BHN hardness range may vary depending on steel/workpiece condition. SURFACE TREATMENT ABBREVIATIONS
Many people will first be exposed to threading tool features in the form of a quote request, or order, from a customer. Example: Please quote 6 pieces of a 1"-20 NEF HSS 2B LH GH-3 6-FL PL tap. Dissected, the customer has defined this tap as one with a "major" or "outside diameter" measuring 1 inch, with 20 threads per inch.
Additional information provided in a description may also include the need for a surface treatment, or coating, or a specific geometry for cutting edge or flutes. Extended length to suit the application may also be noted. If additional information provided does not fit the common ones described above, please share it with Browne Sales. The experts here can interpret the information to assign it's importance to the completion of the finished tool as quoted, or ordered. *Be aware that all of the details of the description provided may not be marked on the finished tap, usually because of space constraints on the shank of the tool. Important pieces of information including; Material being tapped, depth of thread being produced, and type of hole (through or blind) being tapped, may also be included in the description. If these details are not included, the questions should be asked. This information will go a long way towards providing the you with a tool applicable to the job at hand. North American Tool is proud to announce the release of Thread Tap App Cloud®, which is available at http://tapapp.natool.com Thread Tap App Cloud® replaces the Desktop version and offers a completely updated look and feel, powerful new features, and does not require any software installation.
Thread Tap App® is also available for the iPad and Android Tablets. The companion standalone Tap Hole Size Calculator App®, which is available for iPhone and Android phones, is a 3-in-one calculator for recommended tap hole size, an easy to use trigonometry calculator, and a standard calculator for quick calculations.
All North American Tool Apps can be downloaded free of charge. ABOUT NORTH AMERICAN TOOL: North American Tool is an ISO 9001:2008 registered manufacturer of premium cutting tools – special taps, dies, and gages – for the industrial marketplace—aerospace, medical, automotive, energy, agriculture, general manufacturing, and more. For more information about North American Tool Corporation, visit the website at www.natool.com. Let's talk Spirals - not the ones quarterbacks throw for touchdowns or the swirling stream down your kitchen sink, but spirals when it comes to tapping. The most common use of the term "spiral" is either when describing the flute configuration or the point geometry of the tap. It is important not to confuse the two.
Using this same reasoning, you should rarely - if ever - use spiral flutes of any kind in materials with fine or powdery chips, i.e. free machining brass, cast iron, etc. Spiral pointed taps, on the other hand, are not even closely related since they produce an effect quite the opposite from spiral flutes. Spiral point taps, also known as "gun taps" because they "shoot" chips forward (clever, huh?), are very effective at clearing chips ahead of the cutting edge of the tap and pushing them out the other end of the hole. This is especially effective for deep hole tapping. The hole being tapped should be a thru hole, or have plenty of clearance to allow for chip collection. Spiral point taps are also very popular because of their versatility. They work well in many types of materials due to the shearing action of the spiral grind, and the fact that chips exiting through the bottom of the hole virtually eliminates the issue of backing out over broken chips on reversal. So, next time you set up that tapping application, selecting the correct spiral will help assure that your job won't "spiral" out of control! Allen Benjamin Inc. has introduced a line of high performance, application-specific H.S.S.E. spiral flute pipe taps.
|
AuthorWe've compiled the latest news and technical information about our principals and our market that we hope you find informative! Archives
April 2024
Categories
All
|