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.
BA stands for British Association Thread and denotes a British thread system for small screwed connections. A familiar application is the 2BA thread used on darts. BA threads differ from metric ISO threads in characteristics including their thread profile and flank angle, so they must not be treated as equivalent solely because their diameters are similar.
A blind hole ends inside the workpiece and has no open exit. The drilling depth, chip space, chamfer and chip flow must therefore be considered. Spiral-flute taps with a right-hand spiral are frequently used, for example Form C / 35° RSP.
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.
Dimensional table: Bolt diameters for thread cutting (PDF)
In a multi-piece hand tap set, the bottoming tap is the final cutting stage. The taper tap and second tap 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. VÖLKEL explains a practical application in Fishing thread BSF 3/8 × 20 explained.
BSP stands for British Standard Pipe and denotes British pipe threads with a 55° flank angle. The BSP family includes G threads, R threads, Rp threads and Rc threads. BSP threads must be distinguished from American pipe threads such as NPT and NPTF. VÖLKEL explains more in Pipe thread types explained.
BSW stands for British Standard Whitworth and denotes a British inch-thread system with a 55° flank angle. The pitch is typically specified in TPI.
A buttress thread has an asymmetrical thread profile. Its steep load-bearing flank is designed for high axial forces in one preferred direction. It belongs to the power screw thread family and is used where the principal load acts mainly in one direction.
The chamfer is the tapered, ground entry section of a thread-cutting tool, such as a tap or die. Its teeth remove material progressively until the full thread profile is reached. Its length and design affect how the tool starts cutting and how much clearance is needed before the full thread profile is produced.
The chamfer angle describes the angular geometry of the chamfer, for example on a tap or die. In the chamfer, the cutting edges progressively remove material before the full thread profile is reached. The angle influences how the tool starts cutting, how cutting forces are distributed across the cutting edges and how smoothly the thread is started. It must be distinguished from the rake angle and spiral-point angle.
Chamfer length is the axial length of the section in which the cutting teeth progressively generate the thread. A longer chamfer distributes the cutting work across more teeth but requires a longer travel before the full thread profile is reached.
Chip evacuation is the controlled removal of chips produced during thread cutting. Chip breaking deliberately divides long chips into shorter sections so that they can be removed more easily. Unfavourable chip formation or chip compaction can increase torque, tool wear and the risk of breakage.
A chip hole is a clearance space in a die that provides room for chips produced during thread cutting and assists their removal. 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. Chip holes must be distinguished from the holes or seating 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 particularly important in blind holes.
The clearance angle is the angle between the flank face of a cutting edge and the machined workpiece surface; DIN 6581 designates it α (alpha). It prevents unnecessary rubbing between the tool and the workpiece and influences cutting forces, heat generation and tool wear.
Co5 denotes a high-speed steel containing approximately five per cent cobalt and is commonly associated with HSSE tooling. Compared with plain HSS, the cobalt content particularly improves hot hardness and wear resistance.
Within a metric nominal size, the coarse thread is the standard, coarser pitch assigned to that size. In contrast, 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.
Depending on the application, cutting fluid reduces friction and heat and assists chip evacuation. The appropriate fluid depends on the tool, workpiece material and machining process. For manual thread cutting, see also cutting oil.
Cutting oil reduces friction and heat during thread cutting and supports surface quality and tool life. Cutting paste is a paste-like cutting lubricant that adheres particularly well to the tool and machining area. The product selected 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. During thread cutting, the cutting data must be matched to factors including workpiece material, tool, coating and lubrication.
Additional tables: Cutting-speed conversion table (PDF) and Cutting speeds (PDF)
The term dart thread usually refers to the 2BA (BA2) thread. On many darts, it connects the barrel to the shaft. The 2BA thread has a nominal diameter of approximately 4.7 mm, a pitch of approximately 0.81 mm and a flank angle of 47.5°. Suitable taps can be used to repair damaged internal threads or cut new ones; corresponding dies are used for external threads. VÖLKEL explains more in What is a dart thread?
A die is a cutting tool used to produce or recut external threads on bolts, rods or pipes. Round dies are the typical choice for producing new external threads; hexagon die nuts are particularly suitable for recutting and repairing existing external threads. Selection depends on the thread system, size, material and application. For manual use, round dies are guided in a die holder. VÖLKEL provides detailed practical instructions in Cut external threads cleanly by hand using a die; suitable tools are available under Round dies / hexagon die nuts.
Technical overview: Dies (PDF)
A die holder securely holds and manually guides a round die. The die's outside diameter and thickness must match the holder. Clamping or retaining screws secure the tool; on suitable split dies (Form A), they can also be used for slight adjustment. VÖLKEL explains why the dimensions must match in Choosing the right die holder.
A die stock guide helps position the die as squarely as possible to the workpiece axis. The first thread turns are particularly important: if the tool is started at an angle, the resulting external thread will also continue at an angle. The guide sits on the bolt or workpiece and stabilises the tool until the first thread turns engage securely. VÖLKEL offers die sets with guides.
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. The thread size alone is therefore insufficient for selecting the holder. VÖLKEL explains more in Choosing the right die holder.
DIN 371 specifies machine taps with a reinforced shank for certain size ranges of metric threads. VÖLKEL supplies machine taps to standards including DIN 371, DIN 374 and DIN 376.
DIN 374 is used for machine taps for metric fine threads. Clearly specifying the thread pitch is particularly 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 and denotes a hard, carbon-based coating with a low coefficient of friction. On thread gauges, DLC can increase the wear resistance of the working inspection surfaces and thereby contribute to a longer service life.
On conventional wire thread inserts, the drive tang is the part engaged by the inserting tool to drive the insert during installation. After installation, it is removed from the relevant insert types using a tang break-off tool.
An external thread is formed on the outside of a bolt, pipe or other cylindrical workpiece. It mates with a corresponding internal thread. Dies, for example, are used for manual thread cutting.
External thread repair restores the usability of damaged or deformed thread turns. Depending on the design, minor damage can be reworked using a thread file, a hexagon die nut or a suitable die. Severe damage may require replacement of the component. VÖLKEL explains more in External thread repair: recut or replace?
An extraction tool is used to remove an installed wire thread insert. It is needed when an insert must be replaced, corrected or removed as part of another thread repair. VÖLKEL supplies extraction tools in its V-COIL installation-tool range.
Feed is the axial movement of the tool. During tapping, the feed per revolution must match the thread lead: the tool advances by exactly one lead per revolution. For single-start threads, lead and pitch are equal.
For the same nominal diameter, a fine thread has a smaller pitch than the corresponding coarse thread. VÖLKEL explains more in Fine thread vs. coarse thread: what is the difference?
Many fishing accessories use the BSF 3/8" × 20 thread. This 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. VÖLKEL explains its use in Fishing thread BSF 3/8 × 20: dimensions, applications and tools.
The flank angle is the angle between the two flanks of a thread profile. It is a characteristic feature of a thread system, for example 60° for an ISO metric thread and 55° for a Whitworth thread.
The flank radius is a defined rounding at the transition between a thread flank, root or crest. Its form depends on the relevant thread profile and thread standard.
For round dies, Form A denotes the slotted or split design. The slot distinguishes this design from the solid Form B die. In a suitable die holder, the split version can be adjusted slightly using the screws provided for this purpose. This use of “Form A” must be distinguished from Form A for taps, as the two terms describe different tool geometries. VÖLKEL provides an overview of round dies under Round dies / hexagon die nuts.
Form A denotes a long chamfer extending over six to eight thread turns. This distributes the cutting work across several cutting teeth. This form is particularly suitable for short through holes.
Technical drawing: Form A ↗
Form A – long chamfer
For round dies, Form B denotes the solid design. Unlike the split Form A die, it has no continuous adjustment slot, which makes it particularly rigid. Form B is the standard design for many VÖLKEL round dies. This designation must not be confused with Form B for taps, where “Form B” denotes a spiral point that directs chips forwards.
Form B has a chamfer with a spiral point that directs chips forwards in the cutting direction. This design is particularly suitable for through holes. More information: Form B or Form C: which tap suits which hole?
Form B-AZ combines the chamfer of Form B with interrupted teeth. The interrupted tooth arrangement can offer advantages in suitable tough, long-chipping materials.
Form C denotes a short chamfer and is frequently used for blind holes. Chip flow also depends on the flute geometry.
Form C / 35° RSP combines a short chamfer with a 35° right-hand spiral flute. The spiral flutes carry chips back towards the hole opening, making this design particularly suitable for blind holes.
Form D has a longer chamfer without the spiral point characteristic of Form B. VÖLKEL supplies Form D on short machine taps, among other tools.
The G thread is a parallel pipe thread from the BSP family. Internationally, it is often referred to as BSPP. It does not normally seal through the thread flanks, so an additional sealing face or sealing element is required. VÖLKEL explains more in Pipe thread types explained.
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 meets specified limiting conditions. Unlike measurement, it normally does not provide the user with a numerical value.
GO and NO-GO denote the two inspection functions of a limit gauge. The GO side must pass through or along the thread being inspected in accordance with the applicable inspection rule. The NO-GO side may engage only to the permitted extent. For internal threads, the GO thread plug gauge and NO-GO thread plug gauge perform these functions.
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.
Hand taps are intended for manual thread cutting and are typically guided with a tap wrench.
Technical overview: Hand taps (PDF)
Hand tap No. 1 is the taper tap in a traditional three-piece hand tap set. Its longer chamfer performs the first part of the cutting work and prepares the thread for the following tools.
Hand tap No. 2 is the second tap in a three-piece set. It follows the taper tap and deepens or enlarges the thread that has already been started.
Hand tap No. 3 is the bottoming tap in a three-piece set. It finishes the thread to the intended full profile.
A hexagon die nut is a tool for recutting and repairing existing external threads. Its external hexagon allows it to be operated with a suitable open-ended or ring spanner, providing access where there is insufficient space for a die holder. For cutting a complete new external thread, a round die is the more precise standard choice. VÖLKEL discusses both tool types in Cut external threads cleanly by hand using a die.
Technical overview: Hexagon die nuts (PDF)
The VS hexagon die nut is VÖLKEL's HSS version for general manual use. It is particularly suitable for recutting and repairing damaged external threads and for difficult-to-reach working areas. Its external hexagon allows operation with a suitable spanner. Its function therefore differs from that of the VS round die, which is used particularly for the precise production of 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 on machine taps and in the TwinBox, among other products.
HexTap is VÖLKEL's designation for double-ended taps with a hex drive. The taper and bottoming stages are combined in one tool, providing a compact solution for manual tapping and repair work. Depending on the kit, V-COIL rapid and V-COIL automotive also use HexTap versions.
Hole type mainly describes whether the tap drill hole is a through hole or a blind hole. This has a major influence on the appropriate chip control 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 method.
HSSE is cobalt-alloyed high-speed steel, frequently containing approximately five per cent cobalt (Co5). VÖLKEL explains these materials in Tap materials: HSS, HSSE and HSSE-PM.
HSSE-PM denotes cobalt-alloyed high-speed steel produced by powder metallurgy. It is used for threading tools intended for more demanding applications. VÖLKEL supplies machine taps in HSSE-PM, among other materials.
Inch thread is a collective term for thread systems with inch-based dimensions or pitch designations. Examples 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 inserting tool is used to install a wire thread insert in the prepared STI receiving thread. Its design depends on factors including the thread size and insert type.
Inspection length describes the section of a workpiece or thread considered during an inspection. In thread inspection, the permitted use of GO and NO-GO gauges depends on the applicable inspection rule and thread type.
An internal thread is located inside a hole or nut. It can, for example, be cut with a tap or produced without chips with a thread forming tap.
JIS B 4430 is a Japanese standard for taps. In addition to DIN and ISO versions, VÖLKEL supplies machine taps to JIS B 4430.
Land width is often designated a in technical drawings of a die. It is the width of a land between the clearance spaces or chip holes and is one of the die's design parameters. Land width is related to cutting-edge stability and the available chip space, but must not be confused with die thickness.
A left-hand thread tightens in the direction opposite to a conventional right-hand thread, that is, anticlockwise. VÖLKEL explains more in Identifying and cutting left-hand threads 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 the permitted tolerance but does not provide a numerical value. For threads, this principle is applied with limit thread gauges and the GO and NO-GO inspection functions.
A limit size is a permitted maximum or minimum value of a dimension. For threads, limit sizes define the permitted 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 inspected with a GO thread plug gauge and a NO-GO thread plug gauge; corresponding thread ring gauges are used for external threads. The result is pass or fail; no numerical measurement is displayed.
Machinability describes how a material behaves during cutting operations. It influences characteristics including 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.
Technical overview: Machine taps (PDF)
Measurement determines a physical quantity as a numerical measured value, for example a diameter in millimetres. This differs from gauging, which establishes whether specified limiting conditions are met.
For an ISO metric thread, nominal diameter and pitch are specified in millimetres, and the flank angle is 60°. The designation M10, for example, denotes a metric coarse thread with a nominal diameter of 10 mm.
The minor diameter is the smallest diameter of a thread profile. Different notation is used for internal and external threads. It must not be equated with the tap drill diameter produced before thread cutting.
A multi-start thread has two or more parallel helical thread paths. Its axial advance per revolution is therefore greater than that of a single-start thread of the same pitch. The lead equals the pitch, meaning the distance between adjacent thread turns, multiplied by the number of starts.
NO-GO is the international designation for the rejection side of a limit gauge. For an internal thread, the NO-GO inspection is performed with a NO-GO thread plug gauge.
The NO-GO side of a limit thread gauge checks the rejection limit of a thread. For an internal thread, a NO-GO thread plug gauge is used. The NO-GO side checks the limiting condition opposite to that checked by the GO side and may screw in only to the extent permitted by the applicable inspection rule.
A NO-GO thread plug gauge is the NO-GO side used for limit inspection of an internal thread. It checks the tolerance limit opposite to that checked by the GO side and may screw in only to the extent permitted by the applicable inspection rule.
Nominal size is the designated size used to identify a thread. Actual dimensions may deviate within the specified tolerances.
NPS denotes American parallel pipe threads. These must not be confused with tapered NPT threads. VÖLKEL supplies NPS tools in its short machine tap range, among others.
NPT stands for National Pipe Taper and denotes an American tapered pipe thread with a 60° flank angle. It differs in profile and standardisation from European BSP systems.
NPTF is an American tapered pipe thread for applications with higher requirements for the sealing performance of the threaded connection. NPT and NPTF must not be treated as generally equivalent despite their similar names. VÖLKEL supplies NPTF tools in its short machine tap range.
The outside diameter of a round die is often designated D in technical drawings. Together with the die thickness, it is an important fitting dimension when selecting a suitable die holder. The holder must therefore be selected according to the actual external dimensions of the die, not solely the thread size to be cut. VÖLKEL explains how to match the dimensions in Choosing the right die holder.
PG stands for Panzerrohrgewinde (armoured-conduit thread), also known as Stahlpanzerrohrgewinde (steel armoured-conduit thread). This thread system was used mainly for PG conduit, PG fittings and cable glands in electrical installations. VÖLKEL supplies suitable PG threading tools, including short machine taps.
The pitch diameter is an important dimension for thread fit. It has a major influence on how the internal and external threads mate at the thread 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 therefore does not automatically mean a thread gauge.
A power screw thread converts rotary motion into linear motion or transmits forces along an axis. Typical designs include trapezoidal threads, buttress threads and square threads.
The R thread is a tapered external Whitworth pipe thread to ISO 7-1 or EN 10226-1 with a 55° flank angle. It belongs to the system of pipe threads that seal in the thread, often referred to as BSPT, and is typically paired with an Rp internal thread. VÖLKEL explains more in Pipe thread types explained.
The rake angle describes the angular orientation of the rake face at a cutting edge. DIN 6581 designates it γ (gamma); the symbol α denotes the clearance angle in that standard. The rake angle influences how the material deforms and separates as a chip during cutting, thereby affecting cutting forces, chip formation and cutting behaviour. The appropriate geometry depends on the workpiece material and tool design. 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 clockwise and anticlockwise operation, as well as short and extended versions; see VÖLKEL ratcheting tap holders.
Rc denotes a tapered internal pipe thread from the BSPT family. The tool and mating component must both be specifically intended for this thread system. VÖLKEL supplies Rc tools in its machine tap and short machine tap ranges.
The receiving thread is the specially prepared thread into which a thread insert is installed. In the V-COIL system, this is an STI thread with geometry matched to the wire thread insert.
A right-hand spiral describes helically formed flutes on a tap. With suitable tool geometry, they help transport chips against the cutting direction towards the hole opening.
A right-hand thread is the conventional thread direction. It screws in or tightens when turned clockwise. The opposite is a left-hand thread.
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 finish, spiral point and design. VÖLKEL versions include VS, VE, VX and MS. VÖLKEL provides practical instructions in Cut external threads cleanly by hand using a die.
MS is VÖLKEL's designation for round dies designed for short-chipping materials. These tools are made from HSS, are lapped and have a spiral point. They are intended particularly for brass and other short-chipping materials. Their special design accounts for the fact that the chip formation and cutting behaviour of these materials differ from those of long-chipping steels.
VE is VÖLKEL's designation for round dies made from HSSE with a spiral point. This version is intended for more demanding and difficult-to-machine materials, such as stainless or tough steels. Compared with the general-purpose VS version, the emphasis is on the greater load-bearing capability 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. Versions including VE, VX and MS are available for more demanding materials or particular chip-formation characteristics.
VX is VÖLKEL's high-performance die made from HSSE. It is lapped, steam-treated and equipped with a spiral point. The VAP surface promotes lubricant retention and favourable sliding behaviour. VX is designed for more demanding applications and for both hand and machine use. It is particularly suitable for long-chipping materials or materials that generate higher cutting loads.
A round thread has a strongly rounded thread profile and is used, for example, where contamination or mechanical loading is a consideration. VÖLKEL includes Rd round threads in its machine tap range.
Rp denotes a parallel internal pipe thread from the BSP system, typically paired with a tapered R external thread. VÖLKEL supplies Rp tools in its machine tap range, among others.
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.
Scrap recovery means restoring the usability of a component that 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 often allow the component to remain in service.
In a three-piece hand tap set, the second tap is the second cutting stage between the taper tap and the bottoming tap.
A self-locking thread insert also exerts a clamping effect on the installed screw. On V-COIL Type SL, one or more polygon-shaped coils provide this function and help resist loosening under vibration.
The hole or recess 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 suitable split Form A dies, adjustment using the retaining screws can also permit a small change in the tool's cutting diameter.
A short machine tap cuts the internal thread using a single tool. Depending on the design, it can be used by hand or by machine. VÖLKEL supplies versions for M, Mf, BSW, UNC, UNF, G, Rc, PG, NPT, NPTF and NPS, among others; see VÖLKEL short machine taps.
Technical overview: Short machine taps (PDF)
VHM is the German abbreviation for solid carbide. In a solid carbide tool, the cutting body is made entirely from carbide. The material offers high hardness and wear resistance but requires a rigid machine and tool guidance that is as stable as possible.
A spiral flute is a helically shaped chip flute. With a suitable helix, for example a right-hand spiral, it can carry chips out of a blind hole towards the hole opening.
A spiral point is a tool geometry that directs chips forwards in the cutting direction. It is characteristic of Form B and is particularly suitable for through holes. Many round dies also feature a spiral point.
The spiral-point angle describes the angular geometry of the spiral point, for example on a die. The spiral point influences how the tool engages the material and how chips are formed and directed during cutting. The spiral-point angle is therefore a separate geometric parameter and must not be equated 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 has been installed, the intended nominal thread is available again.
An STI twist drill prepares the hole required for producing an STI receiving thread. The required drill diameter depends on the thread size and repair system.
Dimensional table: Tap drill sizes for V-COIL (PDF)
A surface treatment modifies the tool surface without replacing the base material. Examples include VAP and coatings such as TiN, TiCN and TiAlN. VÖLKEL supplies machine taps with these surface finishes.
Technical overview: Surface treatments (PDF)
A tang break-off tool is used to remove the drive tang of the relevant wire thread insert after installation. This step is unnecessary for tangless inserts such as V-COIL Type SN.
A tangless thread insert has no 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 tool for cutting an internal thread in a prepared hole, known as the tap drill hole. Types include hand taps, machine taps and short machine taps. The VÖLKEL threading tools page provides an overview.
A tap and die set contains matched tools for several thread sizes. Depending on the version, it may include taps, dies, tap drills and suitable holders. Terms such as “threading tool set” and “tap set” are often used for these assortments. VÖLKEL offers them under Threading tool sets and tap sets.
Tap breakage is the failure of a tap during machining. Possible contributing factors include an unsuitable tap drill hole, inadequate lubrication, chip problems, misalignment, wear or excessive torque.
In a blind hole, the tap drill depth is greater than the usable thread depth. Additional space is required for the drill point, chamfer, thread runout and accumulated chips.
The tap drill diameter, also called the core hole diameter, is the hole diameter before an internal thread is produced. A tap drill hole that is too small increases tool load and torque. A hole that is too large reduces thread engagement. The required hole diameter depends on the thread, pitch and manufacturing process.
Dimensional table: Tap drill sizes (PDF)
The tap drill hole is the hole in which the internal thread is subsequently produced. Its diameter must match 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 under VÖLKEL holding tools.
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.
In a traditional multi-piece hand tap set, the taper tap is the first cutting stage. Depending on the set, it is followed by the second tap and the bottoming tap.
In a tapered or conical thread, the diameter changes along the thread length. Examples include R threads, Rc threads, NPT and NPTF. These systems differ in their standards, profiles and sealing principles and must not be combined solely because their dimensions are similar.
A tapping chuck is a tool holder for machine tapping. Depending on the design, it can provide axial or torque compensation.
A thread is a helical profile on or inside a component. Characteristics including thread type, nominal size, pitch, direction and thread standard are important for unambiguous identification.
In thread cutting, material is removed with a cutting tool. An internal thread is cut with a tap, while an external thread is cut with a die. In contrast, a thread forming tap works without creating chips.
Thread depth describes the axial length of the thread produced. In blind holes, 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 influences 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, particularly on external threads. Its profile and pitch must match the existing thread. A thread file is used for external thread repair and is not intended to produce a complete new thread.
Thread fit describes the intended relationship between internal and external threads within their tolerances.
Thread flanks are the side surfaces of the thread profile. In a screwed connection, they transmit significant forces between the 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, larger than that used for thread cutting, is required.
Technical overview: Machine forming taps (PDF)
Thread gauge is the general term for inspection tools used to check specified characteristics of a thread. A limit thread gauge is a particular type: it uses the limit gauging principle to check whether the manufactured thread lies within the permitted limits. Unlike measurement, this does not determine a numerical measured value.
Further technical information and dimensions: Precision thread gauges catalogue (PDF, German/English)
Thread identification starts by determining whether the thread is internal or external and establishing its nominal diameter, pitch, profile and direction. These characteristics then allow it to be assigned to a thread standard. For inch threads, pitch is often specified as threads per inch.
A thread insert is installed in a suitably prepared receiving 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 lead is the axial distance a thread advances in one complete revolution. Pitch is the axial distance between corresponding points on adjacent thread turns. For a single-start thread, lead and pitch are equal. For a multi-start thread, the lead equals the pitch multiplied by the number of starts. Lead and pitch are important identifying characteristics 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 means strengthening an internal thread with a thread insert. Particularly in materials with lower shear strength, it can provide a thread with high load capacity and low wear.
A thread relief is a deliberately machined recess at the end of an external thread or internal thread. On an internal thread, it is referred to as an internal relief. The relief provides clearance for tool runout and allows a mating component to reach a shoulder or seating face. Thread reliefs are frequently made to DIN 76.
Thread repair restores the function of a damaged, worn or stripped thread. With the V-COIL system, an STI receiving thread is produced and a suitable wire thread insert is then installed. VÖLKEL offers products and kits under VÖLKEL thread repair.
Application instructions: V-COIL thread repair (PDF)
A thread repair kit contains matched tools and thread inserts for repairing a particular thread size. Depending on the kit, the V-COIL system includes an STI tap, an inserting tool, a tang break-off tool, inserts and, frequently for sizes up to M12, the matching twist drill.
A thread ring gauge or limit ring gauge is used to inspect external threads. The GO ring gauge checks the GO limit and must screw on in accordance with the applicable inspection rule. The NO-GO ring gauge checks the opposite limiting condition and may engage the external thread only to the permitted extent.
Thread runout is the area where the full thread profile ends and the thread turns become progressively incomplete.
Thread standards define characteristics including 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 internal and external threads. 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.
Threading tool is often used as a general term for tools used to cut threads. A tap produces an internal thread; a die cuts an external thread. When selecting a tool, the specific tool name is therefore clearer than the general term threading tool.
TPI stands for “Threads per Inch” and specifies how many thread turns occur over 25.4 mm. The higher the TPI value, the smaller the pitch. The relationship is: pitch in mm = 25.4 ÷ TPI.
A through hole passes completely through the workpiece. During thread cutting, a suitable tool design allows chips to exit forwards through the hole. 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. This gold-coloured hard coating is used on cutting tools to reduce wear and friction in suitable applications.
A tolerance specifies the permitted dimensional deviations from the target dimension. For threads, the tolerance class influences how the internal and external threads fit together. VÖLKEL explains more in Tap tolerances at a glance.
The tolerance class specifies the position and size of a thread's tolerance zone. Designations such as 6H for metric internal threads or 6g for metric external threads combine the tolerance grade and tolerance position.
The tolerance zone is the permitted range between the specified limits for a dimension. For threads, its position and size are defined by the relevant tolerance class.
The tool holder connects the tap to a hand holder, machine or clamping system. It must transmit the required torque securely and keep the tool correctly aligned with the axis. Ratcheting tap holders, for example, are suitable for confined working areas.
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 including workpiece material, tap drill diameter, thread size, tool condition, lubrication and chip formation.
A trapezoidal thread has a trapezoidal profile. It belongs to the power screw thread family and is often used to transmit motion and forces, for example in leadscrews. VÖLKEL supplies tools for trapezoidal threads, among other types.
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 workpiece materials and as a HexDrive version; see VÖLKEL TwinBox and threading tool sets.
Product overview: VÖLKEL TwinBox (PDF, German)
UNC stands for Unified National Coarse and denotes the coarse-pitch series of the American Unified thread system, with a 60° flank angle. The 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 includes 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-COIL is VÖLKEL's 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; see V-COIL thread repair.
V-COIL automotive comprises thread repair solutions for typical automotive applications, such as spark-plug, oil-drain and oxygen-sensor threads. Depending on the kit, STI HexTap taps, matching inserts and inserting tools are used; see V-COIL automotive.
V-COIL notch is the tangless version of the V-COIL system. The tangless thread inserts used are also called Notch Coils. Type SN inserts have a notch for the inserting tool. This eliminates the need to break off a drive tang; see V-COIL notch.
Installation instructions: V-COIL notch instructions (PDF)
V-COIL rapid is a V-COIL version designed for fast thread repair using compact drives. The repair kits use HexTap taps, among other tools, and are designed for an efficient sequence of preparation, tapping and installation of the thread insert.
V-COIL Type S denotes a free-running wire thread insert. After installation, the matching screw can be screwed in and out without an additional clamping effect from the insert.
V-COIL Type SL denotes 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 denotes 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 a steam treatment of the tool surface. In suitable applications, it can improve lubricant retention and friction behaviour. VÖLKEL explains more in HSS-G, HSSE or HSSE VAP?
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 helical insert made from spring wire that is installed in a prepared STI receiving thread. Once installed, it provides the intended nominal internal thread. Wire thread inserts are used both for thread repair and for thread reinforcement.
Technical information: Wire thread inserts to DIN 8140 (PDF)
