Threads, Taps, and Tapping
Before discussing tapping an understanding of thread systems must be gained. The graphic on the right explains how thread systems are defined showing the diameter, number of threads per inch, series, class, direction, length, and tap depth.
There are five basic thread series in American National and Unified Screw Thread system. They are:
2.Fine (UNF)
3.Extra Fine (UNEF)
4.Special (UNS) National
5.Pipe Thread (NPT)
ISO Metric threads have a somewhat different coding system. For example a metric thread would be noted as M 10 x 1.5-6H.
- M indicates a Metric thread.
- 10 indicates a basic size of 10 millimeters.
- 1.5 is the pitch in millimeters. Note that the M 10 taps are also available as M 10 x 1.00 - 6H and 10 x 1.25 -6H.
- 6 identifies the grade of tolerance. Six (6) is commonly used and comparable with class 2 Unified American Standard. The grade of tolerance ranging from 3 to 5 fine, 6 medium tolerances 7-9 are coarse.
- H means that it is an internal thread with no allowance. Internal threads are defined with capitals, for example G designates a tight allowance.
Thread Tolerances
Just like for machining operations, internal and external threads must be machined within a specific tolerance zone.
You must refer to Machinery's Handbook to find a table that will give you the correspondence between the desired tap size and the grind number.
In the manufacturing process the tap is ground on the High or the Low side of basic pitch diameterand shown in place of tolerance class as GH or GL.
These two letters are followed by a number showing the tolerance of pitch diameter. The "G" is normally understood and not always shown in the alpha-numeric code, for example 1/4 20 GH3 or 1/4 20 H3. The table bwlow shows you an example of recommended tap for a 1/4-20 thread.
Tap Drill Sizes
For a thread to be formed by tapping, the hole must have sufficient material left over after drilling to form at least 75% of a full thread. The remaining 25% is formed by extrusion of the material inwards during the tapping process. This phenomenon can be checked by attempting to place the pilot drill back into the hole after tapping. It wont fit!
Lab tests have shown that 50 to 65% of a full thread holds as much as 75 to 100% of the load.
To determine a tap drill size, a formula or a table is used. Tables derived from mathematical calculations using the 75% factor are readily available and are widely used. Tap drill size other than 75 percent are also published. An example is printed to the right of this text.
Please note that blueprints will often specify a countersinkor a bevel at the top of the hole to guide the tap and eliminate the possibility of getting a thread burr.
Pipe threads are unique in that they are tapered. The taper provides a method of sealing off liquids and gases. When torqued tightly the threads merge together laterally which provides the seal.
There are two tapping tools used to turn the tap and form the threads, the T-handle and the Tap
Wrenches.
The tap must be directly straight over the hole. This can be accomplished in a variety of ways. The simplest way is to use a square in (figure 2a and b) two positions 90degrees apart. The difficulty here is maintaining this upright position while turning the tap.
There are other tools that will keep the tap square and steady, a solid or spring loaded tapping center used with a drill press (figure 3a and b), a tapping block (figure 4) or a tap and reamer aligner.
Taps are manufactured with a center hole or have a bevel ground at the square end of the tool. T-handles also come with center holes at the top of its body.
Be wary of the machinist who says he has never
broken a 1/4-20 tap because he is either not a
machinist, or he is a liar. R.S
Hand tapping should be performed by having both hands on the handle and equal pressure applied to rotate the handle. The tap will then tread itself into the hole. To break the chip and to avoidloading the tap with material, the tap should be backed off about a half a turn for each complete thread formed. On completion the tap is backed out of the hole, all chips are blown clear; and the threads are checked with a thread gage.
There are three tolerance classes of fit:
1=lose, 2= medium, and 3 = tight.
The most commonly used in general manufacturing is the class 2 tolerance. However in the defense and aerospace industries the class 3 is more widely used.
An example would be 1/2 - 13 UNC-3B, a half inch diameter thread, thirteen threads per inch, coarse thread, and held to the class 3 tolerance. The B indicates that we have internal threads (A would be for external threads).
Tolerances should be looked up in the Machinery's Handbook but general formulas are available also.
An example would be 1/2 - 13 UNC-3B, a half inch diameter thread, thirteen threads per inch, coarse thread, and held to the class 3 tolerance. The B indicates that we have internal threads (A would be for external threads).
Tolerances should be looked up in the Machinery's Handbook but general formulas are available also.
Common Thread Standards
Tap Grinding Codes
As noted earlier, there are three tolerance class of fit, 1= lose, 2= medium, and 3= tight.
Taps are rarely listed in a tool catalog by the tolerance class but rather by a grind number (G).You must refer to Machinery's Handbook to find a table that will give you the correspondence between the desired tap size and the grind number.
In the manufacturing process the tap is ground on the High or the Low side of basic pitch diameterand shown in place of tolerance class as GH or GL.
These two letters are followed by a number showing the tolerance of pitch diameter. The "G" is normally understood and not always shown in the alpha-numeric code, for example 1/4 20 GH3 or 1/4 20 H3. The table bwlow shows you an example of recommended tap for a 1/4-20 thread.
Tap Drill Sizes
For a thread to be formed by tapping, the hole must have sufficient material left over after drilling to form at least 75% of a full thread. The remaining 25% is formed by extrusion of the material inwards during the tapping process. This phenomenon can be checked by attempting to place the pilot drill back into the hole after tapping. It wont fit!
Lab tests have shown that 50 to 65% of a full thread holds as much as 75 to 100% of the load.
To determine a tap drill size, a formula or a table is used. Tables derived from mathematical calculations using the 75% factor are readily available and are widely used. Tap drill size other than 75 percent are also published. An example is printed to the right of this text.
Please note that blueprints will often specify a countersinkor a bevel at the top of the hole to guide the tap and eliminate the possibility of getting a thread burr.
Taper, Plug, Bottoming, and Pipe Taps
Principle features of taps are:
The most common hand taps are called taper, plug, and bottoming taps.
All three tools are identical except for the bevel angle at the tip. The bevel at the tip serves two purposes: it guides the tap into the hole and it ramp cuts the undeveloped first threads.
Taper taps have the longest bevel angle with 8 to 10 undeveloped threads.
Plug taps are the most popular of the three and have 3 to 5 undeveloped threads on the bevel.
Bottoming taps have only 1 to 1 1/2 undeveloped threads at the tip and should be used only when full amount of threads specified on the drawing can not be achieved by the other two taps.
The number of flutes and the geometry of the cutting edge will depend on the material tapped.
With the exception of the tip, the three hand taps will form thread to specifications and can be used independently.
The bottom tap should only chase the threads originally cut by the taper or plug tap (for hand tapping) The bottom tap is only forming the last few threads. In the past this chasing of the bottom tap has caused some confusion with serial hand taps.
There is considerable difference. Serial taps come in sets of three. The first two taps cut under size and the third completes the threading.
Serial taps work well in tough metals such as stainless steel or nickel and are recommended for tapping deep holes, open or blind. Also by working the thread in three passes a smoother thread is produced.
Pipe threads are unique in that they are tapered. The taper provides a method of sealing off liquids and gases. When torqued tightly the threads merge together laterally which provides the seal.Machine Screw Taps
Machine screw taps, both hand and machine, are smaller than a 1/4 (.250) inch and designated by a number rather than a fraction nomenclature.
These numbers range from 0 to 12. For example the smallest size (0-80) diameter is about .060 and has 80 threads to the inch.
The largest size is 12-28 and the diameter is about .216 with 28 threads per inch.
With the exception of the number in place of the fraction, the Alpha Numeric Identification scheme remains the same. The tap drills are derived the same way as the fraction drills are.
Hand Tapping Tools
There are two tapping tools used to turn the tap and form the threads, the T-handle and the Tap
Wrenches.
Tap Wrenches have adjustable jaws that permit the use of a variety of tap sizes (from 1" down to 1/16" diameter. The tap wrench is often used where there is limited space between the spindle and the work piece .
Some T-handles have jaws or split-collets for holding more than one size but not as wide a range as Tap Wrenches. The largest T-handles will hold a maximum of
a 1/2 inch square-shanked tap and are used in confined areas where a Tap Wrench would not be able to rotate. For the smaller taps such as 0-80 (.060 Dia.), ratchet T-handles allow the thumb and forefinger to advance the tap into the hole.
a 1/2 inch square-shanked tap and are used in confined areas where a Tap Wrench would not be able to rotate. For the smaller taps such as 0-80 (.060 Dia.), ratchet T-handles allow the thumb and forefinger to advance the tap into the hole.
It is important to use a tapping wrench that is proportional in size to the tap. Too large of a wrench on a small tap can easily break the tap.
Hand Tapping Aids
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| Figure 1 |
 |
| Figure 2a |
Hand tapping aides will allow the tap to be manufactured properly and will avoid damaging the part. A broken tap is often difficult to remove and requires special tools like the tap extractor shown (figure 1).
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| Figure 2b |
 |
| Figure 3a |
Taps are manufactured with a center hole or have a bevel ground at the square end of the tool. T-handles also come with center holes at the top of its body.
 |
| Figure 3b |
 |
| Figure 4 |
Be wary of the machinist who says he has never
broken a 1/4-20 tap because he is either not a
machinist, or he is a liar. R.S
Hand Tapping Procedure
Before tapping the following should be determined:- The proper tap hole has been drilled.
- The tap has the correct specifications.
- The tap hole is clean and free of all chips (particularly for blind holes).
- A suitable cutting fluid has been selected for application during the tapping process.
Hand tapping should be performed by having both hands on the handle and equal pressure applied to rotate the handle. The tap will then tread itself into the hole. To break the chip and to avoidloading the tap with material, the tap should be backed off about a half a turn for each complete thread formed. On completion the tap is backed out of the hole, all chips are blown clear; and the threads are checked with a thread gage.
If the thread is too tight, "chase" the threads by running the tap through again until it meets specifications.
Article resource : www.jjjtrain.com
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