written, compiled and edited by Bernard Martin

Class of fit for a cutting tap refers to the specific tolerance or fit that is desired between the threads of the tap and the threads of the hole it is being used to create. In other words, it defines how tightly or loosely the threads should mesh together.

The class of fit is typically expressed using a combination of letters and numbers. The most commonly used standards for class of fit are the Unified Thread Standard (UTS) and the ISO metric thread standard. In the UTS, the class of fit is denoted by a combination of a letter and a number, such as 2B, 3A, etc. In the ISO metric thread standard, it is represented by a combination of a letter and a number, such as 6g, 4h, etc.

For cutting taps, the class of fit is usually specified based on the intended application and the level of precision required. The class of fit can affect factors like the ease of assembly, the strength of the threaded connection, and the ability to engage the threads smoothly during tapping.

A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most, but certainly not all, threaded items are made to a UTS classification standard. This system is analogous to the fits used with assembled parts.

• Class 1 threads are loose fit, intended for ease of assembly or use in a dirty environment.
• Class 2 threads are free fit, and the most common. They are designed to maximize strength considering typical machine shop capability and machine practice.
• Class 3 threads are medium fit, still quite common and used for closer tolerances on high quality work.
• Class 4 threads previously designated a close fit for even tighter tolerances, but this classification is now obsolete.
• Class 5 fit is an interference thread, requiring the use of a wrench for turning. These can be seen in applications like spring shackles on an automobile.

The letter suffix "A" or "B" denotes whether the threads are external or internal, respectively. Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads

Here are some common classes of fit for cutting taps:

1. Class 2B (UTS) or 6H (ISO metric): This is a standard fit for most general-purpose applications. It provides a balance between ease of assembly and thread engagement.
2. Class 3B (UTS) or 6HX (ISO metric): This is a tighter fit than Class 2B or 6H and is used when a higher level of thread precision and engagement is required.
3. Class 2A (UTS) or 6g (ISO metric): This is a loose fit that is often used for applications where ease of assembly and disassembly are important, such as with interchangeable parts.
4. Class 3A (UTS) or 6HX (ISO metric): This is a tighter fit than Class 2A or 6g and is used in applications where a higher level of precision is required.

The choice of class of fit should be made based on the specific requirements of the project, taking into consideration factors like material type, thread size, intended use, and the desired level of thread engagement. It's important to consult relevant standards and guidelines to ensure that the chosen class of fit is appropriate for the application.
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The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch. For diameters smaller than 1⁄4 inch, the diameter is indicated by an integer number defined in the standard; for all other diameters, the inch figure is given.

This number pair is optionally followed by the letters UNC, UNF or UNEF (Unified) if the diameter-pitch combination is from the coarse, fine, or extra fineseries, and may also be followed by a tolerance class.
Example: #6-32 UNC 2B (major diameter: 0.1380 inch, pitch: 32 tpi)

Browne & Co has been appointed as Regaional Managers for Regal Cutting Tools.  They will be responsibilve for distributor and technical support for the Kentucky, Ohio, and Western Pennsylvania market areas.

Regal Cutting Tools has been manufacturing world-class cutting tools since 1955  Regal offers a high performance SuperTuf series of taps for the demanding difficult to machine materials.

Regal also manufactures special taps from blanks with as little as 24 hour notice.

According to Dave Browne, President of Browne & Co, "Regal Cutting Tools. is a well established and trusted brand within the metalcutting industry. Their full line of taps as well as their tap specials capabilities really compliment our product mix and fills an area we have pretty deep experience in but didn't have a product to recommend.  We're really looking forward to working with our customers and running these tools!"

How can you optimize a tap's chamfer based on your application?

Although North American Tool/ GWS  stocks many common special taps with standard chamfer lengths, they can design and manufacture a special tap for your application.

​Optimizing the chamfer results in, longer tap life, reduced tapping torque, better finish, and make the difference between success and failure.

As most of you know, the chamfer is the tapered section on the front of the tap. It includes the length, angle, radial relief, and point diameter.

​As the tap rotates and advances forward, each succeeding chamfered tooth enters the drilled hole and takes deeper and deeper cuts until the first full thread on the tap completes producing a full thread in the part. The balance of the tap’s threads within the length beyond the chamfer, do not do any cutting and just goes for the ride.
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Increasing the total number of chamfered teeth cutting, can increase tap life exponentially.

​The example above shows that a standard plug chamfer (3 to 5 threads) length, so on a 4 fluted tap, it will range between 12 to 20 cutting teeth. Because North American Tool/ GWS understands more is better, they make it a point to manufacture our taps with a chamfer length closer to maximum length, in this case, 5 threads. This is also true for the other standard chamfer lengths Bottom, Semi Bottom, and Taper. 

Although more is better, you may be limited to the length of chamfer due to the job requirements. We should also note that the incomplete threads created and left in the part by the chamfer are not too full thread height and will cause assembly interference, therefore they are not considered part of the required thread length.

As for manufactured specials, knowing your application requirements is necessary for us to design a tap that optimizes performance. For chamfer design, North American Tool/ GWS would need to know, tap drill size, tap drill depth, and full thread length requirement.

Knowing the tap drill size allows us to grind a chamfer with a point diameter that permits the tap to start cutting within the first half thread of entry. Because there are many factors that go into determining a tap drill size, there can be a relatively wide range of diameters. If the chamfer point diameter is smaller than the tap drill diameter, then the tap may not start cutting until the second chamfer tooth or beyond.

Using the same 4 flute, plug tap from the example above, in an application with a tap drill size larger than the chamfer point diameter, such that the tap does not start cutting till the 2nd chamfer tooth, will have a reduction in cutting teeth by 6 (1.5 threads X 4 flutes), or 30%. If the application is such that only a bottom chamfer (1 to 2 threads) can be used, and it is ground to the maximum length of 2 threads it will result in a reduction of cutting teeth from 8 (2 threads X 4 flutes) to 6 (1.5 threads X 4 flutes) or 75%.

Knowing the tap drill depth and full thread length requirement also allows us to design the maximum length chamfer for your application. This may take into consideration the overspin of the machine spindle, or room at the bottom of a blind hole so the tap does not run into any chips that may have made their way to the bottom.

Although the information presented may be confusing, hopefully we have explained the importance of the chamfer, and the many considerations that go into its proper design.

So, if you are ready to increase tap life, reduced tapping torque, improve the finish, and make the difference between success and failure, give us a call with your application requirements.

edited by Bernard Martin

Getting a good understanding of the definitions of the parts of a tap will help you to better understand the functions of  tap designs.  Special thanks to North American Tool for letting us share their short and simple explanations!

Have you ever had a tapping job that was so troublesome that it caused heartburn or acid indigestion due to broken taps, bad finish, short tap life, over or undersized threads, etc.?

One way of avoiding or alleviating such a condition is accomplished with the use of a tap feature called “relief”. The definition of “relief” according to Marian Webster, is removal or lightening of something oppressive, painful, or distressing.

For a tap, “relief” is the reducing of surface contact between the tap/tap feature and the part being tapped. Surface contact generates unwanted heat causing the issues mentioned above. Depending on the tap feature, relief is applied in a direction that is, radially, around the tap, or axially, along the axis of the tap.

All taps require a minimum number of features to have relief for it to cut, other reliefs are applied when the tapping application requires it. There are always tradeoffs when designing a tap, if a relief is applied or it’s amount is greater than necessary, it can cause the tap to run free or loose to a point it will cause heartburn or acid indigestion by producing issues mentioned above.
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Relieved features that are always necessary on a tap are:
               
Chamfer, the tapered threads at the front of the tap. The crests or major diameter of the chamfered threads are radially relieved from the cutting edge to the heel of the land. Without this relief it would be like cutting a tomato with the non-sharp side of a knife, you can imagine the results of that. When looking at a taps chamfer, relief results in the crest width being wider at the cutting edge and narrowing towards the heel;

Back Taper, a slight gradual reduction of the taps thread form including it’s major, pitch and minor diameters. It starts at the chamfered end of the tap and continues axially for the length of thread towards the shank end.

Additional features that can be relieved

Thread Relief, a radial reduction of the taps major and pitch diameters from the cutting edge to the heel. Relieving of the pitch diameter results in the minor diameter being relieved as well due to the manufacturing process whereas the major diameter is relieved separately. The application of the major or pitch diameter relief is normally applied separately but both can be done in combination. Relief of pitch diameter is the most common followed by the major diameter. Thread relief is applied when Back Taper alone is not enough to prevent surface contact when tapping materials that close in and squeezes the tap like stainless steel. The rate of reduction from the cutting edge to the heel is based on the material being tapped and, in some cases, the tapping application.

There are two common types of Thread Reliefs:

• Eccentric, a radial relief in the thread form starting at the cutting edge and continuing to the heel.
  
• Con-Eccentric, Radial relief in the thread form starting back of a concentric margin.

The reliefs we have discussed so far are applied during the tap manufacturing and other than the chamfer relief cannot be added or changed. If you are in a bind and must ship parts but can’t wait for us to design, manufacture and ship the appropriate tap, there are additional types of relief that can be applied that may work in a pinch. Sometimes referred to as a poor man’s relief, something you may be capable of doing in your shop without too much trouble to get you through a quick job, or until properly designed tools arrive.

The application of relief types and amounts are dependent on many factors such as material properties being tapped, style and size of tap, how the tap is being used (hand, machine, etc.) and application requirements, etc. By providing us with as much information about your tapping application, it will enable our engineers to design a tap with the proper relief. This will help alleviate troublesome heartburn or acid indigestion.
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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...

​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.
 

• NEF, or National Extra Fine, is a thread-form symbol that refers to a National Standard for this combination of diameter and threads per inch (TPI). Others are UNC (National Coarse), UNF (National Fine), and UNS (National Special). These are Unified 60-degree thread forms. If the description is for another form, like Acme, Trapezoidal, or Buttress, to name a few, it would be shown in place of NEF.
• HSS refers to the material composition of the tap. (HSS for High-Speed Steel or HSSE for Premium Steel) If the taps are Carbide, they should be marked as such.
• 2B is a reference to Class of Fit. - 2B applies to "internal" thread, like seen in a nut. 2A would refer to the same Class, but "external" thread, as seen on a bolt or screw. 
• LH refers to Left-Hand thread. Right-hand thread is most common, and rarely referenced in the description.
• GH3, sometimes shortened to H-3, applies a measurement of "Pitch-diameter". GH is short for "Ground High". "3" is number of 0.0005 increments applied over "basic" tap pitch diameter defined by the Class-of-Fit assigned. Pitch-diameter adjustments fine-tune the fit with the mating part. Occasionally, you may see GL instead of GH. GL means "Ground Low".
• 6-FL to identify the number of flutes (longitudinal, or spiral channels to form cutting edges, and allow room for evacuation of chips created as the tap cuts). SPFL indicates a spiral-fluted tool. SPPT following the number of flutes would indicate spiral-point.
• PL refers to the length of chamfer. In this case, a Plug chamfer of 3-5 threads. Other common markers might include BT (Bottoming chamfer of 1-2 threads), MB (modified-bottoming of 2-2.5 threads, also commonly referred to a semi-bottoming) and TPR (Taper chamfer of 7-10 threads).

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.