Allowance
The minimum clearance or maximum interference which is intended between mating parts.

Angle of Thread
The angle included between the flanks of a thread measured in an axial plane.

Back Taper
A slight taper on the threaded portion of the tap making the pitch diameter near the shank smaller than that at the chamfer.

Basic
The theoretical or nominal standard size from which all variations are made.

Chamfer
The tapered and relieved cutting teeth at the front end of the threaded section. Common types of chamfer are taper, 8 to 10 pitches long, plug, 3 to 5 pitches and bottoming, 1 to 2 pitches.

Crest
The top surface joining the two sides or flanks of a thread.

Cutting Face
The leading side of the land.

Flute
The longitudinal channels formed on a tap to create cutting edges on the thread profile.

Heel
The following side of the land.

Height of Thread
In profile, distance between crest and bottom section of thread measured normal to the axis.

Hook Face
A concave cutting face of the land. This may be varied for different materials and conditions.

Interrupted thread
Alternate teeth are removed in the thread helix on a tap; usually restricted to those having an odd number of flutes.

Land
One of the threaded sections between the flutes of a tap.

Lead of Thread
The distance a screw thread advances axially in one turn.

Major Diameter
The largest diameter of the screw or nut on a straight screw thread.

Minor Diameter
The smallest diameter of the screw or nut on a straight screw thread.

Neck
The reduced diameter, on some taps, between the threaded portion and the shank.

Pitch
The distance from a point on one thread to a corresponding point on the next thread, measured parallel to the axis.

Pitch Diameter
On a straight screw thread, the diameter of an imaginary cylinder where the width of the thread and the width of the space between threads is equal.

Point Diameter
The diameter at the leading end of the chamfered portion.

Radial
The straight face of a land, the plane of which passes through the axis of the tap.

Rake
The angle of the cutting face of the land in relation to an axial plane intersecting the cutting face at the major diameter.

Relief
The removal of metal behind the cutting edge to provide clearance between the part being threaded and a portion of the threaded land. Also, see back taper.

Chamfer Relief
The gradual decrease in land height from cutting edge to heel on the chamfered portion of the tap land to provide radial clearance for the cutting edge.

Con-Eccentric Relief
Radial relief in the thread form starting at the back of a concentric margin

Eccentric Thread Relief
Radial relief in the thread form starting at the cutting edge and continuing to the heel.

Root
The bottom surface joining the flanks of two adjacent threads.

Side or flank of thread
The surface of the thread which connects the crest with the root.

Shank
The portion of the tap by which it is held and driven.

Spiral Point
An oblique cutting edge ground into the lands to provide a shear cutting action on the first few threads.

Square
The squared end of the tap shank.

Thread
The helical formed tooth of the tap which produces the thread in a tapped hole.

Thread Lead Angle
The angle made by the helix of the thread at the pitch diameter, with a plane perpendicular to the axis.

Threads Per Inch
The number of threads in one inch of length.

Thread
Single
A thread in which lead is equal to pitch.

Double
A thread in which lead is equal to twice the pitch.

Triple
A thread in which lead is equal to triple the pitch.

Troubleshooting

Dimensional Accuracy a) Oversize Pitch Diameter Cause Solution Incorrect Tap Use correct H limit. Use longer chamfered taps. Consider less free cutting NR style. Chip Packing Use spiral point or spiral fluted taps. Reduce number of flutes to create extra chip space. Use larger drill size. In blind hole applications, allow deeper holes where applicable or shorten the thread length of the parts. Use recommended lubricant. Galling Apply surface treatment such as steam oxide, TiN, or chrome. Use recommended lubricant. Reduce tapping speed. Use correct tap for the material being tapped. Operating Conditions Ensure correct tapping speeds to avoid torn threads. Check alignment of tap and drilled hole. Use lead screw taper. Use tapping machine with adequate horsepower. Check misalignment of tap and drilled hole due to loose spindle or worn holder. Tool Condition Check accuracy of chamfer lead grinding. Ensure correct cutting angles. Land widths too narrow. Check burrs from regrinding not present. b) Oversize Internal Diameter Cause Solution Hole Size Use Smaller drill size. Avoid taper hole. Use taps with correct chamfer. Galling See solution prescribed under Oversize Pitch Diameter c) Undersized Pitch Diameter Cause Solution Incorrect Tap Use oversize taps. For cutting materials such as copper alloy, aluminum alloy, and cast iron. For cutting tubing which will have "spring back" action after tapping. Use taps with correct chamfer angle. Use taps with higher cutting angle. Damaged thread Use proper reversing speed to avoid damaging tapped thread on exiting the hole. Leftover Chips Improve operating conditions to eliminate leftover chips in the hole. Remove left over chips prior to gage checking. d) Undersized Internal Diameter Cause Solution Hole Size Use larger drill size

Surface Finish a) Torn or Rough Threads Cause Solution Dull Tap Resharpen Chamfer too Short Increase chamfer length Incorrect rake angle Use correct rake angle suitable for material being tapped Galling Use thread relieved taps Reduce land width Apply surface treatment such as steam oxide, TiN, or chrome Use recommended lubricant Reduce tapping speed Use larger drill size Check alignment between tap and hole Chip Packing Use spiral pointed or spiral flute taps Use larger drill size b) Chattering on Tapped Thread Cause Solution Too Free Cutting Use lower rake angle Reduce amount of thread relief - consider NR style Tool condition Use taps with wider land

Tool Life a) Breakage Cause Solution Incorrect Tap Selection Tapping too deep. Avoid chip packing in the flutes or bottom of the hole. Use spiral pointed, spiral-fluted or fluteless taps. Use correct surface treatment such as steam oxide, TiN, or chrome. Excessive tapping Torque Hole too small - use correct size drill Shorten thread length Increase rake angle Use a tap with more thread relief and reduced land width Use spiral pointed or spiral fluted taps Operating Conditions Reduced tapping speed Avoid misalignment between tap and the hole and tapered hole Use floating type of tapping holder Use tapping holder with torque adjustment Avoid hitting the bottom of the hole. Tool Condition Use taps with wider land width remove all worn sections when regrinding the flutes Regrind tool more frequently b) Chipping Cause Solution Incorrect Tap Selection Use tap with lower rake angle Consider different tool steel Reduce hardness of the tap Increase chamfer length Avoid chip packing in the flutes or in the bottom of the hole by using spiral fluted or spiral pointed taps Operating Conditions Reduce tapping speed Avoid misalignment between tap and hole Avoid sudden reverse in blind hole tapping Avoid galling Use larger drill size Ensure adequate lubricant Check for hard spots in the workpiece c) Excessive Wear Incorrect Tap Selection Consider specially designed taps Change to an Applix style of tap made from PM material Apply special surface treatment such as steam oxide, TiN, TiCN or CrN Increase chamfer length Operating Conditions Reduce tapping speed Apply adequate lubrication Avoid work hardening the material being tapped Use larger drill size Tool Condition Ensure correct rake angle Minimize heat in grinding process to avoid tempering