Types of Taps
Spiral Point Taps— Also known as gun taps, they have an angular point that shears and shoots chips out ahead of the tap. This keeps chips from clogging the flutes, thus reducing tap breakage. Spiral point taps are generally stronger than hand taps and can be run faster with less power. Use them in through-holes and holes two diameters deeper than the required thread length.
Hand Taps— Have straight flutes for hand and machine threading. Although most are used in machine operations today, the traditional name "hand tap" lives on.
Spiral Flute Taps— Also called helical flute taps, the spiral flute uses a lifting action to remove stringy chips that form when tapping soft materials such as aluminum, brass, copper, and magnesium. Use in deep holes, blind holes, and holes with a keyway or gap.
Pipe Taps— Use for internal threading of pipe, pipe fittings, and holes in which you'll be assembling threaded pipe.
Pipe Tap Types
Standard: Use these taps in a variety of pipe tapping applications.
High-Hook: High hook pipe taps are a good choice for materials such as aluminum and free-machining stainless and mild steels that produce long, continuous chips.
Interrupted-Thread: Interrupted-thread taps are recommended for use on soft or stringy metals and those that tend to load the teeth with chips.
Short Projection: Short projection taps are great for shallow holes and when taping depth is limited. Shorter projection and less thread chamfer allows full threads close to the bottom of the hole.
Long-Shank: Long-shank taps are for deep holes that can't be reached with regular taps.
Roll Forming Taps
Roll form taps have neither flutes or cutting edges and therefore do not produce chips and cannot create a chip problem. They are forming tools, displacing metal without removing it in a manner comparable to external thread rolling. The resulting thread is burnished and toughened by the cold forming action. Thread forming offers several distinct advantages over thread cutting for many applications. Materials particularly well suited for thread forming include aluminum, brass, copper, lead, stainless steel, carbon steel, cast steel, leaded steel and zinc. In general, any material which produces a stringy chip is a good candidate for forming. If the chips are powdery, the material may be too brittle.
Specialty Taps— Include thread forming, extension, nut, ACME tandem, and combined taps and drills.
Materials
Most taps are made of high-speed steel in grades M-1 and M-7, which are for use in a variety of applications on ferrous and nonferrous materials. High performance taps are made of premium high-speed steels and work well on stainless steel, nickel-based alloys, and other exotic metals. Premium steels add abrasion and heat resistance for longer tap life and better tap performance. These taps also have a special cutting design for lower machine power requirements, better chip removal and coolant flow, better thread finishes, and closer tolerance pitch diameter size. Use carbide taps for highly abrasive materials such as cast iron and aluminum. Also excellent for use on cast brass, cobalt chrome alloys, copper and copper alloys, fiberglass, and soft plastics. Carbon steel taps are for jobs that don't require the accuracy of high-speed steel taps and don't need resistance to heat and abrasion (such as in some hand-tapping jobs).
Surface Coatings Coatings improve tap life by as much as 300 to 800 percent over uncoated (bright finish) taps. They also improve the part's surface finish and let you use faster cutting speeds.
TiN (Titanium Nitride)— This gold-colored coating works with a variety of ferrous and nonferrous materials. Not for wrought aluminum and titanium alloys. TiCN (Titanium Carbonitride)— Harder, tougher, and more wear resistant than TiN, TiCN is becoming increasingly popular. Works in a variety of ferrous and nonferrous materials. Color is blue-gray violet.
Surface Treatments
Chrome Plating— Adds hardness to tap threads, improves lubricity, reduces loading, and offers cool cutting. Use on a wide variety of ferrous and nonferrous materials. Silver color.
Oxide— Prevents buildup or welding of workpiece materials on tap surfaces in ferrous materials. Reduces friction between tap and workpiece while providing a porous layer to hold lubricants. Avoid use with nonferrous materials. Blue-black color.
Oxide Over Nitride— Use on ferrous materials, high-temperature alloys, titanium and titanium alloys, free-machining steels, iron, high-tensile steels, and stainless steels. Blue-black color.
Nitride— For use on both ferrous and nonferrous abrasive materials that dull taps. Improves surface toughness of the tap. Use on aluminum, cast iron, brass, bronze, die castings, magnesium, zinc, and copper. Colorless.
Tapping Fluids
The use of tapping fluids is highly recommended for best tapping results. Using tapping fluids increases tap life and production, provides more efficient control of chips, and reduces the frequency of tap resharpening. Fluids are more effective if applied under pressure. Using a top and bottom stream of fluid is desirable when tapping horizontally where the tap is stationary and the part revolves. Do not use a lubricant with hard plastics. Use compressed air to remove chips.
Types of Dies
Acorn Die
A threading die used in screw machines whose cutting portion has a shape similar to that of an acorn.
Hex
Used for rethreading and repairing damaged and rusty threads. No special die holder is necessary; a wrench will do! A thread file is also useful in the restoration of damaged screw threads.
Round
Used or threading bolts, studs, rods and other parts that require external threads. Use with any conventional die stock that has corresponding outside diameter dimensions. Includes adjustable and nonadjustable versions.
Two Piece
Two-piece dies that allow a wide range of adjustment. Each piece has two cutting edges that are easily sharpened. Ideal for threading and thread repair work. One side has 2 to 3 thread chamfer for regular threading, while the other side has 1 to 1 1/2 thread chamfer for close to shoulder.
Thread Form Symbols
ACME-C Acme Thread-Centralizing
ACME-G Acme Thread-General Purpose
AMO American Standard Microscope Objective Thread
ANPT Aeronautical National Form Taper Pipe Thread
(Ground thread tap marked NPT)
BA British Association Standard Thread
BSF British Standard Fine Thread Series
BSPP British Standard Pipe (Parallel) Thread
BSPT British Standard Taper Pipe Thread
BSW British Standard Whitworth Coarse Thread Series
M Metric ScrewThread Series
N American National 8, 12 and 16 Thread Series (8N, 12N, 16N)
N BUTT American Buttress Screw Thread
NC American National Coarse Thread Series
NEF American National Extra Fine Thread Series
NF American National Fine Thread Series
NGO National Gas Outlet Thread
NGT National Gas Taper Thread (see "SGT")
NH American National Hose Coupling and Fire Hose Coupling Threads
NPS For tap marking only (See NPSC, NPSM)
NPSC American National Standard Straight Pipe Thread in Pipe Couplings (Tap marked NPS)
NPSF Dryseal American National Standard Fuel Internal Straight Pipe Thread
NPSH American National Standard Straight Pipe Thread for Hose Couplings
NPSI Dryseal American National Standard Intermediate Internal Straight Pipe Thread
NPSL American National Standard Straight Pipe Thread for Loose Fitting Mechanical Joints with Locknuts
NPSM American National Standard Straight Pipe Threads for Free-Fitting Mechanical Joints for Fixtures (Tap marked NPS)
NPT American National Standard Taper Pipe Thread (See ANPT, NPTR)
NPTF Dryseal American National Standard Taper Pipe Thread
NPTR American National Standard Taper Pipe Thread for Railing Joints (Tap marked NPT)
NS American National Thread-Special
PTF Dryseal SAE Short Taper Pipe Thread
SGT Special Gas Taper Thread
SPL-PTF Dryseal Special Taper Pipe Thread
STI Special Thread for Helical Coil Wire Screw Thread Inserts
Stub Acme Stub Acme Thread
*UN Unified Constant Pitch Thread Series
*UNC Unified Coarse Thread Series
*UNEF Unified Extra Fine Thread Series
*UNF Unified Fine Thread Series
UNJ Unified Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJC Unified Coarse Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNJF Unified Fine Thread Series with a 0.15011P to 0.18042P Controlled Root Radius on External Thread only.
UNM Unified Miniature Thread Series
UNR Unified Constant Pitch Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRC Unified Coarse Thread Series with a 0.108P to 0.144P Controlled Root Radius; Ext. thread only.
UNRF Unified Fine Thread Series with a 0.108P to 0.144P Controlled Root Radius; External thread only.
*UNS Unified Thread-Special
V A 60¡ "V" thread with Truncated Crest and Root. The theoretical "V" Form is usually flatted to the user's specifications.
WHIT British Standard Whitworth Special Thread
*Taps are not marked with "U" but with the symbol for the corresponding American Standard thread form with which it is compatible.
Thread Rolling
Thread rolling is widely accepted as the fastest and preferred method of economically producing uniform smooth, precise threads of superior physical qualities.
This process is also capable of performing non-threading operations such as burnishing, knurling and rolling of helical and annular grooves of various forms.
Thread rolling utilizes hardened steel rolls to produce external threads. The working surfaces of the rolls have a thread form which is the reverse of the thread to be produced. In penetrating the surface of the blank, the rolls displace material to form the thread roots, and force the displaced material radically outward to form the thread crests.
NPT - National Pipe Thread
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. (The use of tape or sealant will also help to limit corrosion on the threads, which can make future disassembly nearly impossible.)