Which tap is suitable for a through hole and which for a blind hole? What is the difference between thread cutting and thread forming, and why is the tap drill hole so important? This glossary explains technical terms relating to taps, internal and external threads, thread systems, tool materials, thread repair and thread inspection.
B
BA stands for British Association Thread and denotes a British thread system for small screw connections. A well-known example is the 2BA dart thread. BA threads differ from metric ISO threads in profile and flank angle and should not be matched solely on the basis of similar diameter.
A blind hole ends inside the workpiece and has no open exit. Tap drill depth, chip space, lead and chip evacuation must therefore be considered. Spiral-flute taps with a right-hand spiral are frequently used.
Bolt diameter is the diameter of the cylindrical workpiece before an external thread is produced. It must match the thread and the die being used. An unsuitable diameter can make cutting difficult or produce an incomplete thread profile. More information: Bolt diameters for thread cutting (PDF).
In a multi-piece hand tap set, the bottoming tap is the final cutting stage. The taper and second taps have already prepared the thread; the bottoming tap produces the intended full profile.
BSF stands for British Standard Fine and denotes a British Whitworth fine thread with a 55° flank angle. More information: Fishing thread BSF 3/8 x 20: size, use & tools.
BSP stands for British Standard Pipe and denotes British pipe threads with a 55° flank angle. The BSP family includes G, R, Rp and Rc threads. BSP threads must be distinguished from American pipe threads such as NPT and NPTF. More information: Thread types explained: G, R and NPT systems.
BSW stands for British Standard Whitworth and denotes a British inch thread system with a 55° flank angle. Pitch is normally specified in threads per inch (TPI).
A buttress thread has an asymmetric profile. Its steep load flank is designed for high axial forces acting primarily in one direction. It is a power screw thread used where the main load acts predominantly from one side.
C
The chamfer angle describes the angular geometry of the entry section of a die. In this area, the cutting edges progressively remove material before the full thread profile is reached. It influences tool entry and the distribution of cutting forces and must be distinguished from the rake angle and spiral-point angle.
Chip evacuation is the controlled removal of chips produced during thread cutting. Chip breaking divides long chips into shorter sections so they can be removed more easily. Poor chip formation or chip packing can increase torque, tool wear and the risk of breakage.
A chip hole is a clearance opening in a die that provides space for chips and assists their removal during thread cutting. The number, size and shape of the chip holes are part of the tool geometry and influence chip formation, cutting-edge stability and chip evacuation. They must be distinguished from recesses or contact points for retaining screws.
Chip space is the space available on the tool or in the tap drill hole for collecting or guiding chips. Sufficient chip space is especially important in blind holes.
The clearance angle is the angle between the clearance face of a cutting edge and the machined workpiece surface. It prevents unnecessary rubbing and influences cutting forces, heat generation and tool wear.
Co5 denotes high-speed steel containing approximately five percent cobalt and is commonly associated with HSSE tooling. The cobalt content improves hot hardness and wear resistance compared with standard HSS.
Within a metric nominal size, the coarse thread is the standard coarser pitch. A fine thread has a smaller pitch.
A coating is an additional layer applied to a tool or gauge. Depending on the application, it can influence wear, friction and surface properties. Examples include TiN, TiCN, TiAlN and DLC.
Cutting fluid reduces friction and heat depending on the application and supports chip evacuation. The suitable medium depends on the tool, workpiece material and machining process.
Cutting oil reduces friction and heat during thread cutting and supports surface quality and tool life. Cutting paste is a more viscous cutting medium that adheres particularly well to the tool and machining area. The lubricant must suit the workpiece material and tool.
Cutting speed is the relative speed between the cutting edge and the workpiece surface. Together with spindle speed and feed, it is part of the cutting data. In thread cutting, these values must be matched to factors such as workpiece material, tool, coating and lubrication. More information: Cutting-speed conversion table (PDF) · Cutting speeds (PDF).
D
The thread commonly used on darts is 2BA (BA2). On many darts it connects the barrel to the shaft. The nominal diameter is approximately 4.7 mm, the pitch approximately 0.81 mm and the flank angle 47.5°. Suitable taps can be used for internal threads and corresponding dies for external threads. More information: What is a dart thread?.
A die is a cutting tool for producing or re-cutting external threads on bolts, rods or pipes. Round dies are the standard solution for cutting new external threads, while hexagon die nuts are particularly useful for re-cutting and repairing existing external threads. Selection depends on thread system, size, material and application. For manual use, round dies are guided in a die holder. More information: Cut external threads cleanly by hand using a die · Round dies / hexagon die nuts · Dies (PDF).
A die holder securely holds and manually guides a round die. The die outside diameter and die thickness must match the holder. Retaining screws secure the tool and, on suitable split dies, can also be used for slight adjustment. More information: Choosing the right die stock.
A die stock guide helps align a die as squarely as possible to the workpiece axis. The first thread turns are critical: if the tool starts at an angle, the resulting external thread will continue at an angle. The guide sits on the bolt or workpiece and stabilises the tool until the first turns engage securely. More information: Die sets with guides (German category).
Die thickness, often designated E in technical drawings, is the axial thickness of a round die. Together with the outside diameter it determines which die holder can be used. Thread size alone is therefore not sufficient for selecting the holder. More information: Choosing the right die stock.
DIN 371 specifies machine taps with a reinforced shank for certain size ranges of metric threads. VÖLKEL supplies machine taps to DIN 371 as well as DIN 374 and DIN 376.
DIN 374 is used for machine taps for metric fine threads. Clear specification of the thread pitch is especially important.
DIN 376 specifies machine taps with a reduced shank and is typically used for larger metric coarse threads.
DLC stands for Diamond-Like Carbon, a hard carbon-based coating with a low coefficient of friction. On thread gauges, DLC can increase wear resistance on heavily loaded gauging surfaces and thereby contribute to longer service life.
On conventional wire thread inserts, the drive tang is the section engaged by the inserting tool during installation. After installation, it is removed on the relevant insert types with a tang break-off tool.
E
An external thread is formed on the outside of a bolt, pipe or other cylindrical workpiece and mates with a corresponding internal thread. Round dies are commonly used for manual cutting.
External thread repair restores damaged or deformed thread turns to usable condition. Depending on the damage, light defects can be reworked with a thread file, hexagon die nut or suitable die. Severe damage may require replacement of the component. More information: Damaged external thread: re-cut or replace?.
An extraction tool is used to remove an installed wire thread insert. It is used when an insert must be replaced, corrected or removed as part of a renewed thread repair. VÖLKEL offers extraction tools within the V-COIL installation-tool range.
F
Feed is the axial movement of the tool. During tapping, it is directly linked to the thread pitch.
For the same nominal diameter, a fine thread has a smaller pitch than the corresponding coarse thread. More information: Fine thread vs. coarse thread.
For many fishing accessories, the thread commonly used is BSF 3/8 x 20. It is a British Whitworth fine thread with 20 threads per inch (TPI) and a 55° flank angle. Typical applications include bite alarms, banksticks, rod pods, rod rests, landing nets, adapters and quick-release connectors. More information: Fishing thread BSF 3/8 x 20: size, use & tools.
The flank angle is the included angle between the two flanks of a thread profile. It is one of the defining characteristics of a thread system.
The flank radius is a defined rounding at the transition between thread flank, root or crest. Its form depends on the relevant thread profile and standard.
For round dies, Form A denotes the slotted or split design. The slot distinguishes it from the solid Form B die. In a suitable die holder, the split version can be adjusted slightly using the retaining screws. This use of “Form A” is different from Form A for taps. More information: Round dies / hexagon die nuts.
Form A denotes a long lead of six to eight threads. This distributes the cutting load across several teeth. The form is particularly suitable for short through holes Technical drawing Form A ↗
Form A – long lead
For round dies, Form B denotes the solid design. Unlike the slotted Form A die, it has no continuous adjustment slot and is therefore particularly dimensionally stable. At VÖLKEL, Form B is the standard design for many round dies. This designation should not be confused with Form B taps, where it describes chip control with a spiral point.
Form B has a lead with a spiral point that pushes chips forward in the cutting direction. It is particularly suitable for through holes. More information: Form B or Form C: which tap for which hole?.
Form B-AZ combines the lead geometry of Form B with interrupted teeth. The interrupted tooth arrangement can offer advantages in suitable tough, long-chipping materials.
Form C denotes a short lead and is frequently used for blind holes. Chip evacuation also depends on the flute geometry.
Form C with a 35° right-hand spiral combines a short lead with 35° spiral flutes. The flutes transport chips back towards the hole opening, making this design particularly suitable for blind holes.
Form D has a longer lead without the spiral point characteristic of Form B. VÖLKEL uses Form D, among other applications, on short machine taps.
G
The G thread is a parallel pipe thread from the BSP family and is often referred to internationally as BSPP. It does not normally seal through the thread flanks, so an additional sealing face or sealing element is required. More information: Thread types explained: G, R and NPT systems.
Gauge conformity describes whether a manufactured thread lies within the specified dimensional and tolerance limits. A conforming thread passes inspection with the appropriate limit thread gauge.
Gauging checks whether a workpiece complies with specified limits. Unlike measurement, the user normally does not obtain a numerical value.
GO and NO-GO are the two inspection functions of a limit gauge. The GO side must pass or engage the thread in accordance with the applicable inspection rule. The NO-GO side may only engage to the permitted extent. For internal threads, these functions are provided by GO and NO-GO thread plug gauges.
A GO thread plug gauge represents the GO side when inspecting an internal thread. For a conforming thread, it must screw in over the required inspection length in accordance with the applicable inspection rule.
H
Hand taps are designed for manual thread cutting and are typically driven with a tap wrench. More information: Hand taps (PDF).
Hand tap No. 1 is the taper tap in a traditional three-piece hand tap set. Its longer lead performs the first part of the cutting work and prepares the thread for the following tools.
Hand tap No. 2 is the second, or intermediate, tap in a three-piece set. It follows the taper tap and deepens and extends the partially cut thread.
Hand tap No. 3 is the bottoming tap in a three-piece hand tap set. It finishes the thread to the intended full profile.
A hexagon die nut is used to re-cut and repair existing external threads. Its external hexagon allows operation with a suitable open-ended or ring spanner, making it useful where there is insufficient room for a die holder. For producing a complete new external thread, a round die is the standard precision solution. More information: Cut external threads cleanly by hand using a die · Hexagon die nuts (PDF).
The VÖLKEL VS hexagon die nut is an HSS version for general manual use. It is particularly suitable for re-cutting and repairing damaged external threads and for hard-to-reach locations. The external hexagon is operated with a suitable spanner. Functionally, it differs from the round die VS, which is primarily intended for accurately producing new external threads.
HexDrive is VÖLKEL’s designation for threading tools with a 1/4-inch hex shank. The common hex drive allows fast tool changes and is suitable for compact drives used in assembly, repair and service work. HexDrive is used, among other products, on machine taps and in the TwinBox.
HexTap is VÖLKEL’s designation for double-ended taps with a hex drive. The preliminary and finishing cutting stages are combined in one tool, allowing compact manual threading and repair work. Depending on the kit, V-COIL rapid and V-COIL automotive also use HexTap versions.
Hole type mainly distinguishes whether the tap drill hole is a through hole or a blind hole. This has a major influence on the appropriate chip-control strategy and tool geometry.
HSS stands for High Speed Steel and is a widely used cutting-tool material for threading tools.
HSS-G denotes ground high-speed steel. The “G” indicates that the tool is ground and describes its manufacturing finish.
HSSE is cobalt-alloyed high-speed steel and is used for threading tools that require greater hot hardness and wear resistance than standard HSS. More information: Tap materials: HSS, HSSE and HSSE-PM.
HSSE-PM is powder-metallurgically produced cobalt-alloyed high-speed steel. It is used for more demanding threading tools, including selected VÖLKEL machine taps.
I
Inch thread is a collective term for thread systems with inch-based dimensions or pitch specifications. These include UNC, UNF, UNEF, BSW, BSF and various pipe threads. VÖLKEL covers these systems with different threading tools.
Insert length describes the length of a thread insert relative to the nominal thread diameter. Values such as 1.0 D, 1.5 D, 2.0 D or 3.0 D indicate the corresponding multiple of the nominal diameter.
An insert thread is the specially prepared thread into which a thread insert is installed. In the V-COIL system, this is an STI thread whose geometry is matched to the wire thread insert.
An inserting tool is used to install a wire thread insert into the prepared STI thread. The design depends, among other factors, on thread size and insert type.
Inspection length is the section of a workpiece or thread considered during inspection. For thread inspection, the permitted engagement of GO and NO-GO gauges depends on the applicable inspection specification and thread type.
An internal thread is located inside a hole or nut. It can be cut with a tap or produced without chips with a thread forming tap.
J
JIS B 4430 is a Japanese standard for taps. In addition to DIN and ISO designs, VÖLKEL also supplies machine taps to JIS B 4430.
L
Land width is often designated a in technical drawings of a die. It describes the width of the land between clearance areas or chip holes and is one of the die’s design parameters. It is related to cutting-edge stability and available chip space but must not be confused with die thickness.
The lead, or chamfer, is the front section of a cutting tap. Its teeth remove material progressively until the full thread profile is reached. Its length and design influence how the tool starts and how much space is required before the complete thread is formed.
Lead length is the axial length over which the cutting teeth progressively generate the thread. A longer lead distributes the cutting load across more teeth but requires more travel before the full thread profile is reached.
A left-hand thread tightens in the opposite direction to the usual right-hand thread. More information: Left-hand threads: how to identify and cut them correctly.
Under the limit gauging principle, a workpiece is compared with a GO limit and a NO-GO limit. The inspection determines whether the feature lies within tolerance but does not provide a numerical value. For threads, this principle is applied with limit thread gauges and GO/NO-GO functions.
A limit size is a permitted maximum or minimum dimension. For threads, limit sizes define the permissible range within which the manufactured thread must lie.
A limit thread gauge checks whether a manufactured thread lies within the permitted dimensional limits. Internal threads are checked with GO and NO-GO thread plug gauges; external threads are checked with corresponding thread ring gauges. The result is acceptance or rejection rather than a numerical measurement.
M
Machinability describes how a material behaves during cutting operations. It influences cutting forces, chip form, heat generation, surface quality and tool wear.
A machine tap is designed for machine use. VÖLKEL supplies machine taps to standards including DIN 371, DIN 374 and DIN 376 and for numerous metric, Unified, Whitworth and pipe threads. More information: Machine taps (PDF).
Measurement determines a physical quantity as a numerical value, for example a diameter in millimetres. This differs from gauging, which determines whether specified limits are met.
For an ISO metric thread, nominal diameter and pitch are specified in millimetres. M10, for example, denotes a metric thread with a nominal diameter of 10 mm.
The minor diameter is the smallest diameter of a thread profile. It is defined differently for internal and external threads and must not be confused with the tap drill diameter produced before thread cutting.
A multi-start thread has two or more parallel thread starts. The axial advance per revolution is therefore greater than on a single-start thread. Lead equals thread pitch multiplied by the number of starts.
N
NO-GO is the international designation for the reject side of a limit gauge. For an internal thread, the NO-GO check is performed with a NO-GO thread plug gauge.
The NO-GO side of a limit thread gauge checks the reject limit of a thread. For an internal thread, a NO-GO thread plug gauge is used. It checks the opposite limit to the GO side and may only enter the thread to the extent permitted by the applicable inspection specification.
A NO-GO thread plug gauge is the NO-GO side used for limit inspection of an internal thread. It checks the opposite tolerance limit to the GO side and may only enter the thread to the extent permitted by the applicable inspection specification.
Nominal size is the designated size used to identify a thread. Actual dimensions may deviate within the specified tolerances.
NPS denotes parallel American pipe threads. It must not be confused with tapered NPT. VÖLKEL supplies NPS, among other systems, in the short machine tap range.
NPT stands for National Pipe Taper and denotes an American tapered pipe thread. It differs in profile and standardisation from European BSP systems.
NPTF is an American tapered pipe thread for applications with increased requirements for sealing through the thread connection. Despite their similar names, NPT and NPTF should not simply be treated as identical. VÖLKEL supplies NPTF in the short machine tap range.
O
The outside diameter of a round die is often designated D in technical drawings. Together with the die thickness, it is an important connection dimension for selecting the correct die holder. The holder must therefore be chosen according to the actual external dimensions of the die, not just the thread size. More information: Choosing the right die stock.
P
PG denotes the German armoured-conduit thread system historically used for PG conduit, fittings and cable glands in electrical installations. VÖLKEL supplies suitable PG threading tools, including short machine taps.
Pitch diameter is an important dimension for thread fit. It has a major influence on how the internal and external threads mate at their flanks.
A plug gauge is an inspection tool for holes or other internal contours. A thread plug gauge additionally has the matching thread profile and checks an internal thread. As a limit thread gauge it is used with GO and NO-GO functions; the general term plug gauge does not automatically mean a thread gauge.
A power screw thread converts rotary motion into linear motion or transmits forces along an axis. Common forms include trapezoidal, buttress and square threads.
R
The R thread is a tapered Whitworth external pipe thread to ISO 7-1 or EN 10226-1 with a 55° flank angle. It belongs to the thread-sealing pipe-thread system commonly referred to as BSPT and is typically combined with an Rp internal thread. More information: Thread types explained: G, R and NPT systems.
The rake angle describes the angular position of the rake face at a cutting edge and is often designated α in technical illustrations. It influences how the material deforms and forms a chip, and therefore affects cutting forces, chip formation and cutting behaviour. The appropriate geometry depends on workpiece material and tool concept. On a die, the rake angle must be distinguished from the chamfer angle and spiral-point angle.
A ratcheting tap holder grips the square of a tap and allows controlled work where swing clearance is limited. VÖLKEL supplies versions for right- and left-hand operation as well as short and extended designs. More information: VÖLKEL ratcheting tap holders.
Rc denotes a tapered internal pipe thread from the BSPT family. Tool and mating part must both be designed for this thread system. VÖLKEL supplies Rc in machine-tap and short-machine-tap ranges.
A right-hand spiral describes helically formed flutes on a tap. With suitable tool geometry, they transport chips against the cutting direction towards the hole opening.
A right-hand thread is the standard thread direction. It screws in or tightens when turned clockwise.
A round die is a cutting tool used to produce an external thread. For manual use it is held in a die holder and started as squarely as possible to the workpiece axis. Round dies differ by thread system and size as well as tool material, surface treatment, cutting geometry and design. VÖLKEL versions include VS, VE, VX and MS. More information: Cut external threads cleanly by hand using a die.
MS is a VÖLKEL round-die version designed for short-chipping materials. The tools are made of HSS, are lapped and feature a spiral-point cutting geometry. They are intended particularly for brass and other short-chipping materials.
VE is a VÖLKEL round die made of HSSE with spiral-point cutting geometry. It is intended for more demanding and difficult-to-machine materials such as stainless and tough steels. Compared with the general-purpose VS version, the focus is on the greater load capacity of cobalt-alloyed high-speed steel.
VS is VÖLKEL’s general-purpose HSS round die for manual use. It is the standard universal version for many readily machinable materials and is used to produce external threads. For more demanding materials or special chip formation, VE, VX and MS versions are available.
VX is a high-performance VÖLKEL round die made of HSSE. It is lapped, vapour treated and equipped with spiral-point cutting geometry. The VAP surface supports lubricant adhesion and sliding behaviour. VX is designed for demanding applications and for both manual and machine use, especially with long-chipping or more highly loaded materials.
A round thread has a strongly rounded thread profile and is used, for example, where contamination or mechanical loading is a factor. VÖLKEL supplies Rd round-thread tools in its machine-tap range.
Rp denotes a parallel internal pipe thread in the BSP system. VÖLKEL supplies Rp, among other thread systems, on machine taps.
Runout describes the deviation of a rotating tool from its ideal axis of rotation. Excessive runout can load cutting edges unevenly and adversely affect tool life and thread quality.
S
Scrap recovery means making a component usable again when it would otherwise be rejected because of a damaged or faulty thread. Suitable thread repair, for example with V-COIL, can restore the thread function and allow the component to remain in service.
In a three-piece hand tap set, the second tap is the intermediate cutting stage between the taper tap and the bottoming tap.
A self-locking thread insert additionally applies a clamping effect to the installed screw. On V-COIL Type SL, one or more polygon-shaped coils provide this function and help resist loosening under vibration.
The recess or seating point for retaining screws is a geometric feature of a round die and is often designated b in technical drawings. The screws secure the die in the die holder. On slotted Form A dies, they may also permit a small adjustment of the die opening.
A short machine tap cuts the complete internal thread with a single tool. Depending on the design, it can be used by hand or by machine. VÖLKEL supplies these taps for thread systems including M, Mf, BSW, UNC, UNF, G, Rc, PG, NPT, NPTF and NPS. More information: VÖLKEL short machine taps · Short machine taps (PDF).
Solid carbide describes a tool whose cutting body is made entirely of carbide. The material provides high hardness and wear resistance but requires a rigid machine and stable tool guidance.
A spiral flute is a helically formed chip flute. With suitable helix geometry, it can transport chips out of a blind hole towards the opening.
A spiral point is a cutting geometry that pushes chips forward in the cutting direction. It is characteristic of Form B and is particularly suitable for through holes.
The spiral-point angle describes the angular geometry of the spiral point on a die. It influences how the tool engages the material and how chips are formed and directed during cutting. It is a separate geometric parameter and must not be confused with the chamfer angle or rake angle.
A square thread has a rectangular thread profile and is used as a power screw thread to transmit motion and forces. It offers high efficiency but is more difficult to manufacture than a trapezoidal thread.
An STI tap produces the STI receiving thread for a wire thread insert. Its geometry is matched to the larger receiving thread; a standard tap of the original nominal size cannot replace it.
STI stands for Screw Thread Insert. An STI thread is a specially dimensioned receiving thread for a wire thread insert. Once the insert is installed, the intended nominal thread is restored.
An STI twist drill prepares the required hole for producing an STI receiving thread. The required drill diameter depends on the thread size and repair system. More information: Tap drill sizes for V-COIL (PDF).
A surface treatment modifies the tool surface without replacing the base material. Examples include VAP treatment and coatings such as TiN, TiCN and TiAlN. VÖLKEL supplies machine taps with these surface finishes. More information: Surface treatments (PDF).
T
A tang break-off tool removes the drive tang from a corresponding wire thread insert after installation. This step is not required for tangless inserts such as V-COIL Type SN.
A tangless thread insert does not have a drive tang that must be broken off. This eliminates the tang-breaking operation after installation. V-COIL Type SN is the tangless version within the V-COIL system.
A tap is a cutting tool used to produce an internal thread in a prepared hole. More information: VÖLKEL threading tools.
A tap & die set contains matched threading tools for several thread sizes. Depending on the set, it can include taps, dies, tap drills and suitable holders. Search terms such as “tap set”, “die set” and “tap and die set” are commonly used for these assortments. More information: VÖLKEL tap & die sets.
Tap breakage is failure of the tool during machining. Possible causes include an unsuitable tap drill hole, insufficient lubrication, chip problems, misalignment, wear or excessive torque.
In a blind hole, the tap drill depth is greater than the usable thread depth. Additional depth is required for the drill point, tap lead, thread runout and accumulated chips.
The tap drill diameter is the hole diameter before an internal thread is produced. A tap drill hole that is too small increases tool load and torque; one that is too large reduces thread engagement. The required drill diameter depends on thread size, pitch and manufacturing process. More information: Tap drill sizes (PDF).
The tap drill hole is the hole in which an internal thread is subsequently produced. Its diameter must suit the thread and the manufacturing process.
A tap extension increases the distance between the holder and the tap so that deep-set or difficult-to-reach threads can be machined. VÖLKEL supplies tap extensions to DIN 377 for different square sizes. More information: VÖLKEL tap wrenches, die holders and extensions.
A tap wrench holds a suitable hand tap by its square and transmits the rotary motion during manual thread cutting. Where swing clearance is limited, ratcheting tap holders can be an alternative.
The taper tap is the first cutting stage in a traditional multi-piece hand tap set. Depending on the set, it is followed by the second tap and bottoming tap.
In a tapered thread, the diameter changes along the thread length. This group includes R, Rc, NPT and NPTF threads. These systems differ in standard, profile and sealing principle and must not be combined solely because their dimensions appear similar.
A tapping chuck is a tool holder for machine tapping. Depending on the design, it may provide axial compensation or torque control.
A thread is a helical profile on or inside a component. Correct identification requires factors such as thread type, nominal size, pitch, direction and thread standard.
In thread cutting, material is removed with a cutting tool. For an internal thread, this operation is performed by a tap.
Thread depth describes the axial length of the finished thread. In a blind hole, it must be distinguished from the total tap drill depth.
Thread engagement length is the axial length over which the internal and external threads engage. It affects the number of load-bearing thread turns and must suit the load, material and intended screwed connection.
A thread file is a hand tool for reworking damaged thread flanks, especially on external threads. The profile and pitch of the file must match the existing thread. It is intended for external-thread repair, not for producing a complete new thread.
Thread fit describes the intended relationship between internal and external threads within their specified tolerances.
Thread flanks are the side surfaces of the thread profile. In a screwed connection, they transmit a substantial part of the load between internal and external threads.
A thread forming tap produces an internal thread by plastically displacing the material and therefore works without creating chips. A specifically matched tap drill diameter is required. More information: Machine forming taps (PDF).
Thread gauge is the general term for gauges used to check defined thread characteristics. A limit thread gauge is a specific type that applies the limit-gauging principle to determine whether a manufactured thread lies within the permitted limits. Unlike measurement, gauging does not provide a numerical value. More information: Precision thread gauges catalogue (PDF).
Thread identification starts by determining whether the thread is internal or external and establishing nominal diameter, pitch, profile and direction. These characteristics then allow assignment to a thread standard. For inch threads, pitch is often specified as threads per inch.
A thread insert is installed in a suitably prepared internal thread and then provides the required internal thread for the screwed connection. Thread inserts can be used to repair damaged threads or to reinforce threads.
Thread pitch is the axial distance between corresponding points on two adjacent thread turns. It is a fundamental identifying characteristic of a thread.
The thread profile is the geometric form of a thread viewed in axial section. It includes features such as flanks, flank angle, crest and root.
Thread reinforcement is the strengthening of an internal thread by means of a thread insert. It can provide a durable, wear-resistant thread, particularly in materials with lower shear strength.
A thread relief is a deliberately machined recess at the end of an external or internal thread. On an internal thread it may also be referred to as an internal relief. It provides clearance for tool runout and allows the mating part to run up to a shoulder or seating face. Thread reliefs are often produced to DIN 76.
Thread repair restores the function of a damaged, worn or stripped thread. In the V-COIL system, an STI receiving thread is produced and a matching wire thread insert is then installed. More information: VÖLKEL thread repair · V-COIL thread repair (PDF).
A thread repair kit contains matched tools and thread inserts for repairing a specific thread size. Depending on the V-COIL kit, it can include an STI tap, inserting tool, tang break-off tool, inserts and, often up to M12, the matching twist drill.
A thread ring gauge, or limit ring gauge, is used to inspect external threads. The GO ring checks the acceptance limit and must screw onto the thread as specified. The NO-GO ring checks the opposite limit and may only engage to the permitted extent.
Thread runout is the area where the complete thread profile ends and the thread turns become progressively incomplete.
Thread standards define characteristics such as profile, dimensions, pitch, tolerances and designation. Common systems include metric DIN and ISO threads, UNC, UNF, BSW, BSP and NPT. Threads of similar size from different standards are not automatically compatible.
Thread types can be classified by measurement system, profile, pitch, direction and function. Examples include metric threads, inch threads, pipe threads, fine threads, left-hand threads and power screw threads.
A threaded bushing is a solid thread insert with an internal and an external thread. It is installed in a suitably prepared hole or receiving thread and then provides the required internal thread. Its solid construction distinguishes it from a wire thread insert.
The German term “Gewindeschneider” is often used informally as a general term for thread-cutting tools. In technical English, a tap cuts an internal thread and a die cuts an external thread, so the specific tool name is preferable.
TPI indicates how many thread turns occur within 25.4 mm. A higher TPI value means a smaller pitch. The relationship is: pitch in mm = 25.4 ÷ TPI.
A through hole passes completely through the workpiece. During tapping, chips can be directed forward and out of the hole with suitable tool geometry. Form B with a spiral point is frequently used for this purpose.
TiAlN stands for titanium aluminium nitride. The coating is used on cutting tools to improve wear and temperature resistance in suitable applications. VÖLKEL also supplies machine taps with TiAlN coatings.
TiCN stands for titanium carbonitride. In suitable applications, the coating can improve wear resistance and friction behaviour. VÖLKEL also supplies machine taps with TiCN coatings.
TiN stands for titanium nitride. The gold-coloured hard coating is used on cutting tools to reduce wear and friction in suitable applications.
A tolerance defines the permissible dimensional deviation from the target size. For threads, the tolerance class influences how internal and external threads fit together. More information: Thread tap tolerances at a glance.
The tolerance class identifies the position and width of a thread’s tolerance zone. Designations such as 6H for metric internal threads or 6g for metric external threads combine information about tolerance grade and fundamental deviation.
The tolerance zone is the permitted range between the specified limits for a dimension. For threads, its position and width are defined by the relevant tolerance class.
The tool holder connects the tap to the hand holder, machine or clamping system. It must transmit the required torque safely and guide the tool concentrically. Ratcheting holders can be useful in confined work areas. More information: VÖLKEL ratcheting tap holders.
Tool life describes the usable operating period or machining performance of a tool until a specified wear criterion is reached. It is influenced by workpiece material, cutting data, lubrication, tool geometry and process stability.
Torque is the rotational load that occurs during thread cutting. It is influenced by factors such as workpiece material, tap drill diameter, thread size, tool condition, lubrication and chip formation.
A trapezoidal thread has a trapezoidal profile. It is a power screw thread commonly used to transmit motion and force, for example in lead screws. VÖLKEL supplies tools for trapezoidal threads.
The VÖLKEL TwinBox combines a machine tap with the exactly matching twist drill for the tap drill hole in one box. Depending on the version, it is available for different materials and as a HexDrive version. More information: VÖLKEL tap & die sets · VÖLKEL TwinBox (PDF).
U
UNC stands for Unified National Coarse and denotes the coarse-pitch series of the American Unified thread system. Pitch is specified in threads per inch.
UNEF stands for Unified National Extra Fine and denotes a particularly fine pitch series within the Unified thread system. VÖLKEL supplies UNEF in its machine-tap range.
UNF stands for Unified National Fine and denotes the fine-pitch series of the Unified thread system. Compared with UNC at a similar nominal diameter, UNF has a finer pitch.
V
V-COIL is the VÖLKEL system for repairing and reinforcing internal threads. A wire thread insert is installed in a specially prepared STI receiving thread. The range includes thread inserts, STI threading tools, inserting tools and repair kits. More information: VÖLKEL thread repair.
V-COIL automotive covers thread-repair solutions for typical vehicle applications such as spark-plug, oil-drain and lambda-sensor threads. Depending on the kit, STI HexTap taps, matching inserts and inserting tools are used. More information: V-COIL automotive.
V-COIL notch is the tangless version of the V-COIL system. The tangless inserts, also called Notch Coils, use a notch that is engaged by the inserting tool. This eliminates the need to break off a drive tang after installation. More information: V-COIL notch · V-COIL notch instructions (PDF).
V-COIL rapid is a V-COIL version designed for fast thread repair with compact drives. The repair kits use HexTap taps and are designed for an efficient sequence of preparation, tapping and insert installation.
V-COIL Type S is a free-running wire thread insert. After installation, the matching screw can be inserted and removed without an additional locking effect from the insert.
V-COIL Type SL is a self-locking wire thread insert. One or more polygon-shaped coils create a clamping effect on the screw and help prevent unintended loosening under vibration.
V-COIL Type SN is a tangless wire thread insert. Instead of a drive tang that must be broken off, the insert has a notch for the inserting tool. Type SN is used in the V-COIL notch system.
VAP denotes vapour treatment or steam tempering of the tool surface. In suitable applications, it can improve lubricant adhesion and friction behaviour. More information: HSSG, HSSE or HSSE VAP? Choosing the right hand tap.
W
The wear limit is the condition beyond which the wear of a tool or gauge is no longer within the permitted limits specified for its intended use.
Whitworth threads have a characteristic profile with a 55° flank angle. They include BSW and BSF as well as pipe-thread systems based on the Whitworth profile.
A wire thread insert is a helically formed spring-wire insert installed in a prepared STI thread. After installation it recreates the intended nominal internal thread. Wire thread inserts are used both for thread repair and for thread reinforcement. More information: Wire thread inserts to DIN 8140 (PDF).
