Thread Forming Taps Technical Information

Thread forming taps, also called fluteless taps, form taps, roll taps, or cold forming taps, create threads by displacing material rather than cutting it, producing no chips. They are suitable for materials such as aluminum, brass, copper, lead, zinc, stainless steel, carbon steel, cast steel, leaded steel, and other mild to medium-alloy steels.

Diagram of a Thread Forming Tap

Thread Forming Advantages

Chipless tapping eliminates chip-related issues in blind holes.
Stronger threads due to grain flow following thread contours in steel and stainless steel.
Improved thread accuracy with reduced risk of oversized threads.
Fluteless design provides a stronger, more solid tap structure.
Extended tool life — lasts 3 to 20 times longer than cutting (fluted) taps.
Reduced tap breakage and faster tapping speeds.
Ideal for machines without lead screws.

Differences Between Thread Forming Taps and STI Taps

Thread Forming Taps:

Removes material to produce strong, precise threads.
Trusted for strength in demanding industrial applications.
Chip-free threading prevents contamination, even at high speeds.

STI (Screw Thread Insert) Taps:

Creates an oversized hole to accommodate a helical screw insert.
Prepares the hole for the insert, which evenly distributes load along the thread length.
Enhances thread strength compared to standard tapped holes.

Guidelines for Using Thread Forming Taps

Thread forming taps require a larger pre-tap hole than cutting taps. Using a hole that's too small can lead to excessive wear, high torque, and increased risk of tap breakage.

Forming taps require effective lubrication, with cutting oils generally preferred over water-soluble coolants due to their superior lubricity.

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.

Imperial (Basic)

Tap Drill Size = Major Diameter - (Pitch ÷ 2)

Imperial (Thread Percentage Based)

Drill Size = Major Diameter - [(0.0068 × Desired % of Thread) ÷ TPI]

Metric

Drill Size (mm) = Major Diameter - [0.0068 × Desired % of Thread × Pitch) ÷ 147.06]

Class of Fit and Thread Limits (H & D):

Class of Fit defines the tolerance and tightness between a threaded hole and a tap.

Unified threads:

A = External threads
B = Internal threads

Metric threads:

G = External threads
H = Internal threads

Example Applications by Class of Fit:

Class 1A & 1B – Loose fit for quick or frequent assembly; allows for easy disassembly.
Class 2A & 2B – Standard fit with moderate looseness to prevent seizing; commonly used for screws, bolts, and nuts.
Class 3A & 3B – Tight, high-precision fit; used where thread accuracy is critical and gauges are required for verification.

The Thread limit refers to the tolerance range beyond the basic thread size of a tap.

Indicated by:
- "H" for inch taps
- "D" for metric taps
- Followed by a number (e.g., H3, D5)

Thread limits allow for selecting the tap size best suited to achieve the desired class of fit.

H Limit Increments and Selection Guidelines

For tap sizes up to 1″, each H limit differs by 0.0005″.
For sizes over 1″, increments are 0.001″.
Use a smaller H limit (e.g., H1 or H2) if threads are too loose.
Increase the H limit number if the threads are too tight.
Proper H limit selection ensures threads meet the part print tolerance.

Rule of thumb: Choose the largest H limit that still achieves the required class of fit—this maximizes tool life and reduces wear.

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"
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 Styles

Chamfer refers to the tapered section at the front of the tap. Both the chamfered threads and the first full thread beyond the chamfer help form the finished thread in the part.

Bottoming chamfers are used for threading blind holes to the bottom. They have 1 to 2 chamfer threads.
Modified bottoming chamfers are longer than standard bottoming chamfers and have more teeth. With 2 to 2½ chamfered threads, they are designed for threading to the bottom of blind holes.
Taper chamfers, or starter taps, feature a long chamfer with 7 to 10 threads, providing a gradual, less aggressive cutting action.
Plug chamfers, the most commonly used type, are designed for efficient threading of both through and blind holes. They have 3 to 5 chamfered threads.
Semi-bottoming chamfers, used for blind holes, have 3 to 3½ chamfered threads.

Thread Forming STI tap

Thread-forming STI taps create internal threads by displacing metal without producing chips, making them ideal for large holes in aluminum, brass, copper, steel, and soft metals. Designed for tapping holes per screw thread standards.

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Thread-Forming Pipe Taps

Thread-forming pipe taps create internal threads by displacing metal without chip formation, ideal for pipes and fittings made of aluminum, brass, copper, steel, and soft metals.