A tap removes material from a pre-drilled or punched hole. The result of this cutting action produces threads within the drilled hole. These threads are used in conjunction with screws or bolts to hold two pieces of material together. The cutting edges at the front of a tap remove material from the workpiece. The chips are stored in the flutes, or pushed forward in front of the tap, or are drawn up along the flutes, removing the chips and cutting fluids from the hole.
Diagram of a Tap
Essential Tips:
- Taps typically have several flutes constructed in its design. They are the grooves or valleys cut into the body of the tap. Higher numbers of flutes increase the strength of the tap and reduce space or chip flow.
- Thread size is the number of Threads Per Inch (TPI) which is measured along the length of the tap. Metric sizes are referred to as thread pitch.
- Chamfers are the length of the tapering threads at the front of the tap. Both the chamfered portion of the tap and the first full thread beyond the chamfer produce the finished thread of the part.
Types of Taps:
Acme taps produce a trapezoid-shaped thread used for rotary and transversing motions on machines. TThey are constructed with a 29° thread angle and are used to manufacture valves, jacks and other mechanisms that move heavy loads.
Combination Drill & Taps creates holes and cuts threads in a single pass at high speeds. They are self-centering and are used for through holes of up to twice the depth of the tap diameter. Ideally suited for use on construction, maintenance and repair, assembly applications.
Straight flute taps are used to cut threads in blind or through holes. They are used by hand or in general purpose machine operations.
Pipe taps are used to cut internal threads in parts or fittings that will be mated with threaded pipe or fittings to make a pressure tight joint. Pipe threads require higher cutting forces than regular machine thread tapping because the threads require 100% thread depth. All threads on tapered pipe taps are cutting. For more information please see Pipe Taps Tech Essentials.
Extension taps are used for deep hole drilling and where a longer reach is required. They are designed with standard threads and an extended shank.
Nut taps are used for the internal threading of nuts in small holes. They are designed with long threads and shanks for reaching into hard-to-reach holes.
Pulley taps are used for tapping hard-to-reach areas, pulley and set screw holes. They are similar to hand taps and have the same basic thread dimensions, but have longer shanks. They are also used for general purpose applications where an extra-long reach is required.
Spiral flute taps are used for threading blind holes in aluminum, brass and softer steels. The flute geometry draws the chips away from the hole where chip disposal may be an issue.
Spiral point taps are used for tapping through holes. They have a flute geometry that shoots chips ahead of the cutting action to reduce loading and clogging within the flute.
Thread Forming taps Thread forming taps do not cut threads; rather, they form threads by displacing material. They generate threads without producing chips. They are used on mild steels, carbon steels and low to medium alloys. For more information please see Thread Forming Taps Tech Essentials.
Screw Thread Inserts (STI) taps are used for cutting oversized threads in through holes for screw thread inserts. For more information please see Screw Thread Inserts (STI) Tech Essentials.
Class of Fit is the standard identification system describing the tolerance and closeness of fit between the threaded hole and the tap. Unified threads are defined with an A (external) or B (internal). Metric H (internal) or G (external).
Example Applications:
- Class 1A & 1B - For frequent quick assembly, loose assembly.
- Class 2A & 2B - Fit is medium loose to eliminate seizure in assembly. Used for screws, bolts and nuts.
- Class 3A & 3B - Accuracy of thread is required, gages are used to ensure a tight fit.
Thread Limit is a standard notation system indicating a level of tolerance for the thread outside the basic thread size of the tap. The limits are identified by a letter "H" for inch or a "D" for metric, followed by a number. Thread limits have been established to provide a choice in the selection of the tap size best suited to produce the class of thread desired.
The difference in size from one H limit to the next is 0.0005″ increments for taps through 1″ diameter. Sizes over 1″ diameter are separated by .001″ diameter increments. If the threads in the part are too loose, smaller numbers such as H1 or H2 are used. If the threads are too tight, the H limit number is increased. Proper selection of the H limit number ensures that the threads are within the tolerance required by the part print. Best rule of thumb: always select the largest "H" limit possible to achieve proper class of fit and maximum tool life.
Thread Limit (H & D) Cross Reference Guide | |
|---|---|
| H1 /D1 | Basic plus .0005" - .0010" |
| H2 /D2 | Basic plus .0005" - .0010" |
| H3 /D3 | Basic plus .0010" - .0015" |
| H4 /D4 | Basic plus .0015" - .0020" |
| H5 /D5 | Basic plus .0020" - .0025" |
| H6 /D6 | Basic plus .0025" - .0030" |
| H7/D7 | Basic plus .0030" - .0035" |
| H8/D8 | Basic plus .0035" - .0040" |
| H9/D9 | Basic plus .0040" - .0045" |
| H10/D10 | Basic plus .0045" - .0050" |
| H11/D11 | Basic plus .0050" - .0055" |
| H12/D12 | Basic plus .0055" - .0060" |
Chamfer refers to the length of the tapering threads at the front of the tap. Both the chamfered portion of the tap and the first full thread beyond the chamfer produce the finished thread of the part.
Bottoming chamfers are used for threading blind holes to the bottom. They have 1 to 2 chamfer threads.
Modified Bottoming chamfers are similar to bottoming chamfers, but they are longer and have more teeth. They are used for threading to the bottom of blind holes and have 2 to 2-1/2 chamfer threads.
Taper chamfers, also known as starter taps, have a longer chamfer and require a less aggressive cutting action. They have 7 to 10 chamfer threads.
Plug chamfers are the most commonly used chamfer and are designed for efficiently threading through and blind holes. They have 3 to 5 chamfer threads.
Semi-Bottoming chamfers are used for blind holes. They have 3 to 3-1/2 chamfer threads.
Spiral Point Plugs chamfers are used for general purpose applications. They are 4 to 5 chamfer threads
Bright provides a smooth, polished finish on the tool. It increases chip flow in softer materials such as aluminum, wood and plastic.
Titanium Nitride (TiN) is a multi-purpose coating which increases chip flow in softer materials. The heat and hardness resistance allows the tool to run at higher speeds than uncoated tools.
Titanium Carbonitride (TiCN) is harder and more wear resistant than TiN. It is used on stainless steels, cast iron and aluminum alloys.
Oxide, also known as black oxide or steam oxide, is a surface treatment that prevents chip building, galling and welding on the workpiece. It is commonly used on low carbons, stainless steel and ferrous metals.
Chrome Plate is a bright electroplated coating that offer excellent anti-friction properties. It is commonly used on steel, aluminum, brass, copper or other non-chromium materials
Nitride is a thin, hard-shell coating that supports surface hardness of the tool. It is used where abrasive or wearing conditions exist.
Aluminum Chromium Nitride (AlCrN) has higher-heat resistance than AlTiN. It is commonly used for machining aircraft and aerospace material, nickel alloys, stainless steel, titanium, cast iron and carbon steel.
Aluminum Chromium Titanium Nitride (AlCrTiN) is a high heat and wear resistant multilayered PVD coating. It is designed for enhanced tool life and superior thread finish.
TiCN PLUS Titanium Carbon-Nitride (TiCN) plus Titanium Nitride (TiN) is an all-purpose finish designed to increase tool life by two to four times more than TiN coated tools. The heat and hardness resistance allows the tool to run at higher speeds than uncoated tools.
Uncoated tools do not feature supportive treatments on the cutting edge. They are used at reduced speeds in general applications on nonferrous metals.
Cobalt is harder than high speed steel and provides better wear resistance. It is commonly used on high tensile alloys.
High Speed Steel (HSS) provides good wear resistance and can be used in general purpose applications for both ferrous and nonferrous materials.
Solid Carbide provides better rigidity than high speed steel. It is extremely heat resistant and used for high speed applications on cast iron, nonferrous materials, plastics and other tough-to-machine materials.
Vanadium High Speed Steel (HSSE) is made of high speed steel, carbon, vanadium carbide and other alloys to increase abrasive wear resistance and toughness. It is commonly used in general applications on stainless steels and high silicon aluminums.
Powdered Metal (PM) is tougher and more cost effective than solid carbide. It is commonly used on highly abrasive materials including high silicon aluminums.
For Cutting Taps
Tap Drill Size = Tap Basic Major Diameter - Pitch
Drill Size = Major Diameter - [(.01299 x desired % of thread) ÷ Threads per Inch]
Drill Size (mm) = Major Diameter - (desired % of thread x pitch (mm) ÷ 76.98)
For Forming Taps
Tap Drill Size = Major Diameter - (Pitch ÷ 2)
Drill Size = Major Diameter - [(0.0068 x desired % of thread) ÷ Threads per Inch]
Drill Size (mm) = Major Diameter - (desired % of thread x pitch (mm) ÷ 147.06)
Formulas produce threads depending on the workpiece material. Verify findings with a ″Machinery Handbook″, MSC Order No. 85605756.
Calculating Percentage of Thread
Percentage of Thread is calculated by determining one-half the difference between the major diameter and the minor diameter of an internal thread and dividing it by the thread height. Formula: % of Full Thread = Threads per Inch x (Major Dia. - Selected Drill Dia.) ÷ .01299
How to Read a Tap Size
Example: 1/4 - 20NC The 1/4 represents the diameter of the thread in inches. The 20 represents the number of threads per inch or TPI. Standard taps are either standard coarse series threads NC (1/4-20), fine series threads NF (1/4-28) or extra fine series NEF (1/4-32). There are other standard tap designations such as NPT or NPTF for tapered pipe threads. Special taps are usually designated NS, indicating a special thread size.
Inch - Tap & Drill Chart 75% Thread | |||||||
|---|---|---|---|---|---|---|---|
| Thread Size | Drill Size | Thread Size | Drill Size | Thread Size | Drill Size | Thread Size | Drill Size |
| 0-80 | 3/64 | 5-44 | 5-44 | 1/4-28 | 3 | 9/16-18 | 33-64 |
| 1-64 | 53 | 6-32 | 36 | 5/16-18 | F | 5/8-11 | 17-32 |
| 1-72 | 53 | 6-40 | 33 | 5/16-24 | I | 5/8-18 | 37-64 |
| 2-56 | 50 | 8-32 | 29 | 3/8-16 | 5/16 | 3/4-16 | 21-32 |
| 2-64 | 50 | 8-36 | 29 | 3/8-24 | Q | 3/4-10 | 11-16 |
| 3-48 | 47 | 10-24 | 25 | 7/16-14 | U | 7/8-9 | 49/64 |
| 3-56 | 46 | 10-32 | 21 | 7/16-20 | W | 7/8-14 | 13/16 |
| 4-40 | 43 | 12-24 | 17 | 1/2-13 | 27/64 | 1-8 | 7/8 |
| 4-48 | 42 | 12-28 | 15 | 1/2-20 | 29/64 | 1-12 | 59-64 |
| 5-40 | 39 | 1/4-20 | 7 | 9/16-12 | 31/64 | 1-14 | 15-16 |
Metric - Tap & Drill Size 70-75% | |||||||
|---|---|---|---|---|---|---|---|
| Thread Size | Drill Size | Thread Size | Drill Size | Thread Size | Drill Size | ||
| M1.6 x .35 | 1.25 | M4.5 x .75 | 3.75 | M12 x 1.75 | Y | ||
| M2 x 0.4 | 1.6 | M5 x .8 | 4.2 | M14 x 2 | 12 | ||
| M2.5 x .45 | 2.05 | M6 x 1 | 5 | M16 x 2 | 14 | ||
| M3 x .5 | 2.5 | M7 x 1 | 6 | M18 x 2.5 | 15.5 | ||
| M3 x .6 | 2.9 | M8 x 1.25 | 6.75 | M20 x 2.5 | 17.5 | ||
| M4 x .7 | 3.3 | M10 x 1.5 | 8.5 | - | - | ||
Inch - Tap Thread Length | |||||||
|---|---|---|---|---|---|---|---|
| Thread Size | Thread Length | Thread Size | Thread Length | Thread Size | Thread Length | Thread Size | Thread Length |
| 0 | 5/16 | 8 | 3/4 | 1/2 | 1-21/32 | 1-1/8 | 2-9/16 |
| 1 | 3/8 | 10 | 7/8 | 9/16 | 1-21/32 | 1-1/4 | 2-9/16 |
| 2 | 7/16 | !2 | 15/16 | 5/8 | 1-13/16 | 1-3/8 | 3 |
| 3 | 1/2 | 1/4 | 1 | 11/16 | 1-13/16 | 1-1/2 | 3 |
| 4 | 9/16 | 5/16 | 1-1/8 | 3/4 | 2 | - | - |
| 5 | 5/8 | 3/8 | 1-1/4 | 7/8 | 27/32 | - | - |
| 6 | 11/16 | 7/8 | 1-7/16 | 1 | 21/2 | - | - |
Metric - Tap Thread Length | |||||||
|---|---|---|---|---|---|---|---|
| Thread Size | Thread Length | Thread Size | Thread Length | Thread Size | Thread Length | ||
| M1.6 x .35 | 5/16 | M5 x .8 | 7/8 | M16 x 2 | 1-13/16 | ||
| M2 x 0.4 | 7/16 | M6x1 | 1 | M18 x 2.5 | 1-13/16 | ||
| M2.5 x .45 | 1/2 | M7 x 1 | 1-1/8 | M20 x 2.5 | 2 | ||
| M3 x .5 | 5/8 | M8 x 1.25 | 1-1/8 | M24 x 3 | 2-7/32 | ||
| M3 x .6 | 1-1/16 | M10 x 1.5 | 1-1/4 | M30 x 3.5 | 2-9/16 | ||
| M4 x .7 | 3/4 | M12 x 1.75 | 1-21/32 | M36 x 4 | 3 | ||
| M4.5 x .75 | 7/8 | M14 x 1 | 1-21/32 | - | - | ||
American National Standards Institute (ANSI): Sets industry standards and product guidelines to insure conformity and to maintain the highest product standards.
Body: Made up of the flutes, land, core, diameter, thread length and chamfer. These elements produce the threaded hole.
Base of Thread: The bottom section of the thread.
Blind Hole: Hole that does not go all the way through the part, but the threads must come as close as possible to the bottom of the drilled hole.
Coolant Fed/Thru Coolant: Fluid prevents the accumulation of chips in blind holes, reducing tap breakage.
Core: The center portion of the tap that separates the flutes and provides strength to the tap. As the number of flutes increases, the core becomes larger, increasing the strength of the tap.
Crest: The top surface joining two sides of a thread. In an internal thread the crest is at its minor diameter. In an external thread the crest is at the major diameter.
Deutsche Industries Norm (DIN): German standards organization responsible for setting industrial standards of products.
Diameter: The diameters of the threads of a tap are largest at the front, behind the chamfer. The thread diameter decreases slightly toward the shank, known as the back taper. The back taper creates clearance between the workpiece and the tap.
Ground Thread: More accurate threads than "cut" taps. Held to much closer limits and tolerances.
Length of Engagement: The length of contact between two mating threaded parts.
Major Diameter: The largest diameter of the thread. Also known as the outside diameter.
Minor Diameter: The smallest diameter of the thread. Also known as the root diameter.
Pitch: The distance between a point on a screw thread and a corresponding point on the next thread. The pitch is equivalent to one divided by the number of threads per inch.
Square (Flats): The squared end of the tap shank.
Square Length: Length of the square.
Size of Square: Thickness of the square.
Shank: The part of the tap that fits into the tap holder. The end is made square for driving and rotating the tap. The square surface is known as flats.
Thread Height: The radial distance between the crest and the base of thread.
Through Hole: Hole that goes all the way through the part.