Thread forming taps are also known as fluteless taps, form taps, roll taps or cold forming taps. They form threads by displacing material without producing chips. Form taps are used on aluminum, brass, copper, lead, stainless steel, carbon steel, cast steel, leaded steel and zinc as well as other mild steels and medium alloys.
Diagram of a Thread Forming Tap
Thread Forming Advantages:
- Chipless tapping eliminates problems in blind holes.
- Greater thread strength when grains follow the contours of the thread in steel and stainless steel.
- Better thread gaging and reduced oversized thread productivity.
- Constructed without flutes for a stronger, more solid tap
- Lasts 3 to 20 times longer than fluted taps with cutting edge.
- Less tap breakage, faster tapping speeds and well suited for machines without lead screws.
- Thread forming taps require a larger pre-tap hole size than a cutting tap. Tapping with too small of a pre-tap hole size results in excessive tap wear, torque and possible tap breakage.
- Forming taps require good lubrication. Cutting oils are generally preferred because of their lubricity compared to water soluble coolants
| Hardness | Size and Pitch Range |
|---|---|
| 16 Rc | 1" and smaller - 8 pitch and finer up to 1-1/ 2" - 10 pitch and finer |
| 17 - 23 Rc | 1" and smaller - 10 pitch and finer |
| 24 - 35 Rc | All machine screw sizes and miniature sizes |
| 30 - 35 Rc | Extreme care – miniature sizes and machine screw sizes w/ 56 T.P.I. |
Calculating Tap Drill Sizes for Forming Taps
Tap Drill Size = Tap Basic Major Diameter - (Pitch ÷ 2)
Drill Size = Major Diameter - [(0.0068 x desired % of thread) ÷Threads per Inch]
Drill Size (mm) = Major Diameter - [0.0068 x 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.
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.
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 that 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.
More Thread Forming Taps From MSC:
Thread forming STI taps are used to form internal threads by displacing the metal without producing chips in large holes. They are ideal for aluminum, brass, copper, steel and soft metals. For use in tapping holes according to screw thread standards
Thread forming pipe taps are used to form internal threads by displacing the metal without producing chips in pipes and pipe fittings. They are ideal for aluminum, brass, copper, steel and soft metals.