Form tap and method of making such

A form tap includes a shank having a shank length and a thread portion that extends along at least a portion of the shank length. The thread portion includes a finishing portion and a chamfer portion. Each of the finishing portion and the chamfer portion include a plurality of threads and each thread has a crest and a root. The plurality of threads of both the finishing portion and the chamfer portion are spaced apart at a constant pitch, such that a crest-to-crest distance between adjacent threads remains constant along the entire thread portion.

FIELD

The described embodiments relate to form taps and to methods for making form taps.

BACKGROUND

The process of forming threads on the inner surfaces of holes is generally known as tapping. The tools used to form the threads are generally referred to as taps. Taps are generally classified into two categories “cut taps” and “form taps” based on the method used to produce the threads in the finished part being tapped.

A form tap creates threads on the inner surface of a hole by shaping and forming the material of the hole into the desired configuration. This process is also referred to as flowing the metal, cold forming, cold working or thread rolling. The leading tip of a form tap is narrowed or angled (chamfered) below the size (diameter) of the hole to be tapped, to allow the tap to be properly aligned with a hole, and to begin the forming process gradually when the form tap is used. The threads on the angled portion of a form tap are not ground or sheared into partial/truncated threads. Instead, the threads in the angled or chamfered portion of a form tap are generally formed as full threads having a crest and root configuration that is similar to the rest of the threads on the form tap. Traditional methods of manufacturing form taps tend to produce an error in the spacing, or pitch, of the threads between the crests of the threads of the finishing portion of the form tap and the crests of the threads of the angled or chamfered portion of the tap. This thread shaping error is generally known as lead error.

When a form tap has a lead error it may have a shorter lifespan and require increased torque to operate. One known attempt to correct lead error is the use of advanced CNC thread forming machines that allow for precise, computer control of the thread grinding process. However, form taps produced using CNC machines still tend to have a lead error between the crests of the threads on the finished form tap.

SUMMARY

The following introduction is provided to introduce the reader to a more detailed discussion to follow. The introduction is not intended to limit or define the claims.

Examples disclosed herein provide a form tap having no lead error and a grinding tool for creating the form tap having no lead error. The form tap comprises a plurality of threads for creating formed threads inside a hole or article being tapped. Each thread on the form tap has a crest and a root. The threads on the form tap are separated into a finishing portion and chamfer portion. The form tap is described as having no lead error (or having a lead error correction) because the crest-to-crest spacing between threads on the form tap remains constant along both the finishing portion and the chamfer portion as well as across the transition between the finishing portion and the chamfer portion.

The form tap having no lead error is created using a unitary grinding tool that can be installed on a traditional grinding machine. The grinding tool comprises two spaced apart ribs that extend from its periphery surface. The ribs and the spacing between them are configured such that the threads on the chamfer portion and the finishing portion of the form tap can both be formed using the unitary grinding tool, and that the grinding tool can be used to shape both the chamfer portion and finishing portion threads on each continuous machine pass.

In a first aspect, some embodiments of the invention provide a form tap that includes a shank having a shank length and a thread portion that extends along at least a portion of the shank length. The thread portion includes a finishing portion and a chamfer portion. Each of the finishing portion and the chamfer portion include a plurality of threads and each thread has a crest and a root. The plurality of threads of both the finishing portion and the chamfer portion are spaced apart at a constant pitch, such that a crest-to-crest distance between adjacent threads remains constant along the entire thread portion.

According to another broad aspect, a grinding tool for forming threads on a form tap is provided which comprises a grinding wheel, first and second ribs and a trough therebetween. The grinding wheel has a peripheral surface and the first and second ribs project radially from the peripheral surface of the grinding wheel for grinding the threads. The first rib is sized and shaped to grind the roots of threads on a finishing portion of the form tap. The first rib comprises a first peak and defines a first outer diameter, a first minor diameter and a first rib width. The second rib is shaped to grind roots of threads on a chamfer portion of the form tap. The second rib comprises a second peak and defines a second outer diameter that is smaller than the first outer diameter, a second minor diameter and a second rib width that is smaller than the first rib width. The trough has a grinding surface for grinding crests of the threads on both the finishing and chamfer portions of the form tap. The first and second peaks are spaced apart by a peak spacing distance, the peak spacing distance being different than a pitch of the threads.

In some examples, the first rib further comprises a first rib leading face and a first rib trailing face; and the second rib further comprises a second rib leading face that is substantially parallel with the first rib leading face, and a second rib trailing face.

In some examples, the first rib further comprises a first peak radius and the second rib further comprises a second peak radius that is smaller than the first peak radius.

In some examples, the first and second ribs are formed on the grinding wheel.

In some examples, the first rib is formed on the grinding wheel and the second rib is formed on a second grinding wheel.

In some examples, the peak spacing distance is based on the pitch and a chamfer angle.

In some examples, the peak spacing distance is less than the pitch.

In some examples, the trough further comprises a grinding surface for forming crests on the threads.

In some examples, the rib spacing distance is between n and n+1 pitches, wherein n is integer greater than or equal to zero.

In some examples, the second peak forms a sharp edge.

According to another broad aspect, a method of forming the thread portion of a form tap using a unitary grinding tool is provided, the method comprises the step of providing a blank. The blank has a first longitudinal axis defining a first axis of rotation and an outer surface. The method also comprises the steps of rotating the blank about the first longitudinal axis and providing a grinding tool. The grinding tool defines a second axis of rotation and has a peripheral surface that comprises first and second ribs projecting radially therefrom. The method also comprises the steps of rotating the grinding tool about the second axis of rotation. The second axis of rotation being substantially parallel to and offset from the first axis of rotation. The method also comprises the steps of positioning the first and second axes of rotation at a first distance so that at least one of the first and second ribs interfere with the outer surface of a chamfer portion of the blank, imparting relative axial movement between the blank and the grinding tool at an axial feed rate and simultaneously increasing the distance between the first and second axes of rotation, thereby at least partially forming a plurality of threads on the chamfer portion. The threads have crests spaced at a constant crest-to-crest spacing. The method also comprises the step of maintaining the relative axial motion between the blank and the grinding tool at the axial feed rate while maintaining the second axis of rotation at a second distance from the first axis thereby at least partially forming a plurality of threads on a finishing portion of the form tap. The threads on the finishing portion are continuous with, and have a constant crest-to-crest spacing with the threads on the chamfer portion.

In some examples, the threads on the finishing portion and the threads on the chamfer portion are formed by a single pass of the grinding tool.

In some examples, the threads on the finishing portion and the threads on the chamfer portion are formed by the combination of at least two passes of the grinding tool.

In some examples, wherein the grinding tool partially forms the threads of both the chamfer portion and the finishing portion on each pass.

In some examples, the threads on the finishing portion are only partially formed by the second rib.

In some examples, the partially formed threads on the finishing portion are completed by the first rib.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference is first made toFIGS. 1 and 2, which show an example of a form tap100having a shank106and a thread length or thread portion109. The thread portion109includes a plurality of threads110and defines a finishing portion140and a chamfer portion150that are separated by a transition thread148. For the purposes of this description, the form tap100is understood to include a plurality of threads110, each thread110representing one revolution, or turn around the shank106.

The term “thread” is also commonly used in the art to describe the single, continuous helical structure formed by the cooperation of each thread110as defined in this specification. In this context, each thread110may be understood as forming a portion or segment of the larger helical structure, but for the clarity, in this description the term “thread110” is a single turn around the shank106.

The term “threads” is also commonly used in the art to describe the features formed on the inner surface of a hole being tapped by the form tap100. For clarity, in this description the term “formed threads” is used when referring to the “threads” created by the use of the form tap100. In this description, the term “threads” is not limited to threads of a specific size, shape or configuration, but includes any thread forms and tap dimensions.

Each thread110of the form tap100has a crest112and a root116. In this description a distance measured from a first crest to an adjacent crest or from a first root to an adjacent root is called a pitch120. The pitch120may also be understood and described as a lead or thread spacing. The distance between opposing crests112on the form tap100is referred to as the crest diameter CD (or major diameter or outer diameter), and the distance between opposing roots116is referred to as the root diameter RD (or minor diameter or inner diameter). The difference between the crest diameter CD and the root diameter RD defines a thread height H.

As exemplified inFIG. 1, in some examples of a form tap100the chamfer portion150of the form tap100tapers at a chamfer angle αctoward the tip (or end) of the form tap100, to the right as shown inFIG. 1. Within the chamfer portion150, the crest diameter CD and the root diameter RD both decrease as the chamfer portion150tapers, but the relative distance between the crest diameter CD and the root diameter RD (i.e. the thread height H) remains generally constant. At the tip of the form tap100, shown at the right ofFIG. 1, the crest diameter CD is smaller than the crest diameter CD of the finishing portion140. In use, the tip of the chamfer portion150is inserted into the hole to be tapped, and the relatively smaller crest diameter CD may enable a user to easily insert and align the form tap100within the hole. The threads110of the chamfer portion150also serve to partially form the formed threads in the hole being tapped. As the form tap100is advanced into the hole being tapped, the threads110of finishing portion140complete, or finish, the formed threads that had been partially formed by the chamfer portion150by forming them into their final, usable or finished configuration. Optionally, the surface of the form tap100may comprise a plurality of lobes spaced around the surface of the form tap containing the threads110, separated by lubrication grooves that do not contain threads. It is understood that the number, shape and configuration of the lobes formed on the form tap100may be selected based on the expected amount of heat and pressure exerted on the form tap during the form tapping process.

In existing form taps, it is common for the pitch or lead between adjacent thread crests to change, or be distorted, at the transition point (for example transition thread148) between the finishing portion and the chamfer portion of the form tap. That is, in known form taps, the crest-to-crest distance between the transition thread and the first adjacent thread in the chamfer portion of the form tap is different than the desired pitch or lead. This change in the pitch or lead is generally referred to as lead error. In other words, in previously known form tap designs, the pitch (or lead or thread spacing), at the transition from finishing portion to chamfer portion is different than the pitch within the finishing portion and/or the chamfer portion. This type of lead error can result in increased form tap wear, increased torque requirements when using the form tap and irregular or improper formed threads in the article being tapped. Lead error may also increase the amount of heat and pressure exerted on the threads of a form tap during the form tapping process. Reducing or correcting the lead error may allow the lobes of a form tap to be larger which may extend the useful life of the form tap, as the heat and pressure acting on the form tap threads is reduced.

The effects of lead error described above are generally associated with a form tap having the lead error between the crests of its threads, as opposed to the roots of its threads. The effects of a lead error across the root-to-root distance of the transition thread (for example transition thread148) may be less of a problem when the form tap is in use because the roots of the threads on the chamfer portion (for example chamfer portion150) of a form tap do not engage or contact the formed threads in the hole being tapped. When a form tap is selected to tap a hole in an article, the characteristics of the form tap may be selected based on the desired final characteristics of the formed threads in the hole being tapped. For example, if the hole being tapped requires finished threads with a thread height of 2 mm, then the thread height H (i.e. the distance between the CD and the RD) along the finishing portion of the form tap may be set at 2.2 mm.

When the form tap is first inserted into the hole to be tapped, the crests of the chamfer portion may engage a portion of the hole wall because the crest diameter CD of the threads in the chamfer portion may be greater than the root diameter RD of the finishing portion. As the form tap is advanced into the hole, each thread in the chamfer portion may engage progressively more material in the hole wall, due to the increasing crest diameter along the length of chamfer portion. However, because the root diameter(s) RD along the chamfer portion are less than the root diameter RD of the finishing portion, the roots116of the threads110in the chamfer portion150do not contact the formed threads (i.e. the material of the hole wall) during the tapping process. Because the roots116do not contact the formed threads, the shape and configuration of the roots116of the chamfer portion150do not affect the formed threads in the article being tapped. Accordingly, errors in the spacing between the roots116of the chamfer portion150and the roots116of the finishing portion140may not create the lead error problems described above.

Therefore, it is understood that a form tap that is described as having “no lead error” or a “lead error correction” is a form tap having a constant crest-to-crest pitch (or lead), but that the form tap may have a lead error between the roots116of the chamfer section150and the roots116of the finishing portion140. A lead error between roots116may not cause the lead error problems described above.

The form tap100described in this specification has no lead error between thread crests; i.e. the thread pitch120or lead of the form tap100remains constant along the entire thread portion109. The form tap100is formed using a grinding tool that produces threads110that have a lead error correction or that are considered lead error free, as defined above. The crest-to-crest spacing between adjacent threads110of the form tap100remains constant along the length of the finishing portion140, across the transition thread148and along the length of the chamfer portion150. In this description, the term “constant” is understood to mean that the pitch120of the threads110remains essentially equal between adjoining threads for useful or practical purposes, subject to the manufacturing dimensional tolerances known in the art, and should not be strictly interpreted as meaning exactly identical.

Each thread110on the form tap100can be formed into a variety of known configurations based on the user's requirements. Examples of some possible thread110configurations are shown inFIG. 2. In one example, a thread110may be shaped so that the crest112comprises a crest flat114, as shown on the left ofFIG. 2. Or, as shown on the right ofFIG. 2, a thread110may be shaped so that the crest112comprises a crest radius113. In a preferred embodiment, the threads110are shaped so as to have a crest radius113as the crest radius113may help flow the material of the article being tapped, which may reduce the force required to tap the material and may prolong the useful life of the form tap100. The shape of the crest112on the form tap100determines the shape of the major diameter created in the formed threads of the article being tapped.

It is understood that the specific details of the thread110features, for example the size of the crest flat114or crest radius113, may be set by the form tap110manufacturer or they may be based on customer or user requirements. While the configuration of the thread110features may vary between form taps100, the pitch120of the threads110on a given form tap100remains constant regardless of the thread configuration.

The form tap100also defines a first axis of rotation104that extends in the longitudinal direction of the form tap100. In use, the form tap100is rotated about the first axis of rotation104to create the formed threads in an article that is being tapped. The form tap100may also be rotated about the first axis of rotation104during the form tap100manufacturing process described in detail below.

In most examples of the form tap100, the shank106is generally round or cylindrical with flattened mounting portions (square driver)108to enable the form tap100to be securely mounted and gripped within a chuck, handle or other tool holding device. The shank106may contain any desired number of mounting portions108as needed to fit into a particular grinding machine (during manufacture) or a particular tool holder or handle (during use). Optionally, the shank106may be of a non-circular cross section, for example hexagonal, octagonal or any other suitable shape. In such a configuration, the shank106may not comprise discrete mounting portions108because the surfaces of the shank106itself may provide adequate mounting surfaces.

In the examples shown, the chamfer portion150has been shown having3threads110, however; it is understood that the chamfer portion150may have a greater or fewer number of threads110(and optionally a longer or shorter length). In the figures, the size of the chamfer portion150relative to the finishing portion140may be exaggerated for clarity, but it is understood that the finishing portion140may be substantially larger than the chamfer portion150in some form taps100. Also, it is understood that if the number of threads110in the chamfer portion150is changed, the chamfer angle αcmay also change.

While the form tap100is shown having continuous threads110, it is understood that the threads110of the form tap100may be separated into a plurality of lobes. Also, portions of the form tap100may be treated using any known treatment process to produce desired mechanical properties. For example, the form tap100may be heat-treated, surface hardened, plated or coated with any desired coating such as chrome plating, TiN (Titanium nitride), TiCN (Titanium carbonitride) and layered TiAlN (Titanium aluminum nitride).

To create the form tap100having no lead error (i.e. a constant crest-to-crest pitch or lead along the entire thread portion109) an operator may grind the form tap100using a unique grinding tool.FIG. 3shows an example of a grinding tool that may be used to create the form tap100having no lead error, as described above. The grinding tool200can create form taps100having a lead error correction or a lead error adjustment that eliminates the lead error in the completed form tap, for all practical purposes, as described above. The grinding tool200is capable of creating constant crest-to-crest thread leads or pitches because its design ensures that any lead error introduced into the threads110is located on the roots116of the chamfer portion150which, as described in detail above, have no practical effects on the performance of a form tap or the shape of the formed threads.

FIG. 3shows one example of a grinding tool that can produce a form tap having no lead error. As shown, grinding tool200(for example a grinding wheel202) includes a first and second ribs210and220extending radially from its peripheral surface. The grinding tool200also defines a second axis of rotation204. In use, the grinding tool200is mounted on a suitable thread grinding machine, such as thread grinding machines manufactured by Jones & Lamson, Matrix, Drake or Normac (not shown). The first and second ribs210,220engage the form tap100to form the threads110. Details relating to the configuration of the first rib210and second rib220are explained below with reference toFIGS. 4 and 5.

FIG. 4is a partial sectional view of the grinding tool200taken at section A-A showing the first rib210and the second rib220separated by a trough240. As shown, the first rib210comprises a first rib leading face212and a first rib trailing face214that meet to define a first rib peak216. The first rib210may also be described as the larger rib, the trailing rib or the finishing-grinding rib.

In some examples, the first rib leading face212and the first rib trailing face214may intersect to form a sharp edge at a theoretical first rib peak217, illustrated using dashed lines inFIG. 4. However, the first rib210preferably comprises a peak216having a first peak radius r1, instead of a sharp edge. Optionally, the first peak radius r1may be made as large as possible without creating an unsuitable shape of the root radius117(or threads110formed by the first rib210) because increasing the first peak radius r1may decrease wear on the first rib210and extend the life of the grinding tool200.

The first rib210also defines i) a first outer diameter OD1, which is the distance between the second axis of rotation204and the first rib peak216, and ii) a minor or clearance diameter MD, which is the distance between the second axis of rotation204and the grinding tool shoulder206. The difference between the first outer diameter OD1and the minor diameter MD is at least equal to the thread height H doubled.

The grinding tool200also comprises a second rib220. The second rib220may also be described as the smaller rib, the leading rib or the chamfer-grinding rib. The design, size and shape of the second rib220are related to, but are not equal to the dimensions of the first rib210. To help illustrate this relation between the first and second ribs210,220a phantom second rib226is shown using dashed lines inFIG. 4The phantom second rib226represents a second rib that is identical to the first rib210and is spaced apart from the first rib210by a thread pitch120. The phantom second rib226is only included for illustrative purposes and to help clarify the relation between the first rib210and the second rib220.

As shown inFIG. 4, the second rib220defines a second rib leading face222and a second rib trailing face224that intersect to define a second rib peak221. As described above in relation to the first rib peak216, the second rib peak221may intersect to form a sharp edge229or it may comprise a second peak radius r2, as shown inFIG. 4. The second rib leading face222is parallel to, but offset from the first rib leading face212while the second rib trailing face224coincides with the face of the phantom second rib226as shown.

The second rib220also defines a second outer diameter OD2, that is the distance between the second axis of rotation204and the second rib peak221. As shown, the second outer diameter OD2is less than the first outer diameter OD1. That is, the first rib210extends further from the grinding tool shoulder206than the second rib220. Each rib210,220also defines a rib width W1, W2respectively. The first rib width W1is greater than the second rib width W2.

As exemplified inFIGS. 4 and 5the second peak radius r2is smaller than the first peak radius r1. Optionally, the second peak radius r2may be made as small as possible. In some examples, the second peak221may be initially formed as a sharp edge which then wears to form a second peak radius r2over time during use. Based on the performance of the grinding tool200and the required tolerances of the threads110formed using the grinding tool200, there may be a range of second peak221profiles and second peak radius r2sizes that are acceptable. In such situations, a grinding tool200having a second peak radius r2that is as small as possible (or begins as a sharp edge) may allow that grinding tool200to be used for a longer period before the second peak radius r2exceeds a pre-determined maximum size and the grinding tool200has to be refurbished or dressed.

The trough240is defined by the depression or valley contained between the first and second ribs210,220. The trough240includes the leading edge212of the first rib210, the trailing edge224of the second rib220and the tap OD grinding surface242at the bottom, or base of the trough240. The height of the trough240relative to the first rib210(i.e. the distance between the grinding surface242and the first rib peak217) is equal to the thread height H of the threads110on the finishing portion of the form tap100. Accordingly, when the grinding tool200is in use, the grinding surface242is in contact with and shapes the crests112of the threads110of the finished form tap100. As exemplified inFIGS. 4 and 5, the grinding surface242may be generally planar in order to form a crest flat114on each thread110, but it is understood that in other examples of the grinding tool200the grinding surface242may comprise an arcuate shape having a radius to form a desired crest radius113on each thread110of the form tap100. The distance between the second rib peak221and the grinding surface may be referred to as a second rib height and the distance between the first rib peak216and the grinding surface may be referred to as a first rib height.

The first and second ribs210,220are separated by a peak spacing distance230that is measured from the centre of the first rib210to the centre of the second rib220. The spacing between the first and second ribs210,220is based on the desired thread pitch120of the threads110on the completed form tap100that is to be manufactured using the grinding tool200. The peak spacing distance230is not equal to the thread pitch120or an even multiple thereof.

Optionally, the grinding tool200may be constructed so that the first rib210and second rib220are spaced to engage adjacent threads110on the form tap100. In such a configuration the peak spacing distance230is less than the pitch120.

FIG. 5is a schematic representation of the sectional view ofFIG. 4showing the geometric relationship between the first rib210and the second rib220that determines the location of the second rib220. For the purposes of calculating the geometric relationship between the first and second ribs210,220, measurements and calculations may be based on the locations of the theoretical first and second rib peaks217,229. The theoretical first and second rib peaks217,229represent the location of the first and second ribs if both ribs were formed as sharp edges.

As shown inFIG. 5, the location of the second rib220relative to the first rib210is (for example the straight-line length L) is based on the desired pitch120and chamfer angle αcof the form tap100calculated to give the required peak spacing distance to provide the lead error correction.

Because the configuration of the first and second ribs210,220and the grinding surface242may be based on a desired combination of thread profile, pitch120, root design length L and chamfer angle αc, a particular grinding tool200may be useful for creating a particular combination of these characteristics. For example, a grinding tool200configured to create a form tap having pitch 1.5 mm and a chamfer angle of 5 degrees may not be suitable for creating a form tap having a pitch 1.5 and a chamfer angle of 10 degrees. In some instances, a separate grinding tool200may be created for each desired form tap configuration/combination. However, in each example of the grinding tool200used, the resulting form tap100will have a constant crest-to-crest spacing as a result of the lead error correction/lead error adjustment described above.

The grinding tool200may be formed from a suitable material known in the art, including vitrified, ceramic and borazon. Also, the profile of the grinding tool200(i.e. the shape of the first and second ribs210,220) may be created using any known process including CNC dressing, roll dressing and crush forming. Preferably, the grinding tool200may be formed and shaped using a diamond dressing roll.

In the examples described, the grinding tool200has been shown as being a single tool, for example single grinding wheel having integrally formed first and second ribs, however; it is understood that the first and second ribs may not be integrally formed with the grinding tool200. For example, the first and second ribs could be formed on a separate band, ring or collar that is connected around the periphery of a grinding wheel or other, non-consumable grinding tool body. Optionally, the first and second ribs may be formed on separate bands or rings, each of which is secured around the perimeter or periphery of the grinding tool200. In yet another example, the grinding tool may comprise two separate grinding wheels spaced apart at an appropriate axial distance from each other along the second axis of rotation; each wheel comprising one of the ribs.

To manufacture the form tap100having no lead error (i.e. having a constant crest-to-crest spacing) using the grinding tool200as described above, an operator may install the grinding tool200on a grinding machine (not shown) so that it rotates about its second axis of rotation204. A form tap blank (which will be formed into form tap100) is then positioned in the thread-grinding machine. It is understood that the blank is loaded and secured in the machine in a known manner so that the blank rotates about the first axis of rotation104. The first axis of rotation104is parallel to, and offset from the second axis of rotation204. Once both the blank and grinding tool200have been mounted in the thread-grinding machine they are rotated about their respective axes of rotation.

With both the blank and the grinding tool200rotating, the second axis of rotation204is moved toward the first axis of rotation104so that the ribs210,220of the grinding tool200engage (or interfere with) the surface of the blank of the form tap100.FIG. 6is a partial section representation of the grinding tool200ribs210,220engaging the form tap100. As both the form tap100and grinding tool200are rotated about their respective axes of rotation, the grinding tool200is advanced along the length of the form tap100(from the chamfer portion150to the finishing portion140) at an axial feed rate, in generally the machine direction as shown inFIGS. 4-6.

In this application, the terms “machine direction” and “grinding direction” describe the relative motion between the grinding tool200and the form tap100. It is understood that the necessary relative motion may be achieved by holding the form tap100in place and moving the grinding tool200from right to left as shown inFIG. 6, by holding the grinding tool200in place and moving the form tap100from left to right as shown inFIG. 6, or by a combination thereof. It is also understood that the machine direction is intended to describe the general direction of movement between the grinding tool200and the form tap100and that when the grinding tool200is forming the chamfer portion150of the form tap100the grinding tool200may move at an angle relative to the first axis of rotation104. Preferably, the angle is the chamfer angle of the form tap being manufactured.

As exemplified inFIG. 6, when the grinding tool200is used to create a form tap100, the different portions of the grinding tool200, i.e. the first and second ribs210,220and the trough240may perform different grinding or forming functions. Specifically, the second rib220and trough240perform a majority of the shaping and grinding on the chamfer portion150of the form tap100, (shown as position1) whereas the trough240and the first rib210cooperate to shape the threads110on the finishing portion140of the form tap100(shown as position2). The operation of the grinding tool200in use is explained in further detail below.

When grinding tool200is used to form the chamfer portion150(position1) of the form tap100, the initial grinding of each thread110in the chamfer portion150is performed by the leading face222, the second rib220and the second rib peak221. For example, when the grinding tool200first contacts the form tap100, the leading face222of the second rib220grinds and forms a corresponding face on a first thread110and the second rib peak221grinds the root116of the first thread110. As the grinding tool200is advanced along the length of the form tap100(from right to left inFIG. 6as shown using different line types to represent different positions of the grinding tool200), the leading face222of the second rib220engages the next, adjacent thread (i.e. a second thread) on the chamfer portion150and simultaneously the trough240will engage and shape both faces and the crest and first thread. As shown, when the trough240is positioned about a thread110, the second rib trailing face224, the grinding surface242and the first rib leading face212cooperate to grind both faces and the crest112of the thread. When the trough240is engaged with a given thread, the leading edge212and second rib peak221of the second rib220may engage the next, adjacent thread.

To form the chamfer portion150, the grinding tool200is advance along the length of the chamfer portion150at an angle relative to the first axis of rotation104, preferably the chamfer angle. As a result of this angular displacement away from the first axis of rotation104, as the grinding tool200advances the first rib peak216does not contact the roots116of the threads110in the chamfer portion150that were formed by the second rib220. Therefore, as shown inFIG. 6the roots116of the threads110in the chamfer portion150of the form tap100are shaped by the second rib peak221and have a root radius that is substantially equal to the second peak radius r2. As described above, the shape of the roots116in the chamfer portion150is not critical because the chamfer roots116do not form part of the functional or useful portion of the form tap100(i.e. the thread crests112and the roots116of the finishing portion140). The shapes of these roots116are not adjusted or modified by the first rib210.

When the grinding tool200reaches the transition thread148the motion of the grinding tool200relative to the form tap100changes such that distance between the first axis of rotation104and the second axis of rotation204remains constant whereby the grinding tool200moves along the form tap100but does not move further away from the first axis of rotation104. As a result of this change in relative movement, when the grinding tool moves along the finishing portion140of the form tap, the first rib210will follow the same path as the second rib220. This may be advantageous because it enables the first rib210to complete threads110along the finishing portion140that have been partially formed by the second rib220.

Specifically, as has been described above in detail, the first rib210is shaped to correspond to the desired finished thread110profile and the second rib220is narrower and shorter than the first rib210and has a second peak radius221that is smaller than the desired root radius of the finished form tap thread110. Therefore when the second rib220grinds a thread110on the finished portion140an amount of residual material “X” (shown as the hatched region on the left side ofFIG. 6) is left behind between the leading face222of the second rib220and the desired finished thread shape, shown in dashed lines.

During a subsequent revolution, as the grinding tool200is advanced along the flat, finishing portion140, the first rib210will correct the finished thread shape by removing the residual material “X” and correcting the root radius using the leading face of the first rib212and the first rib peak216respectively. A schematic illustration of the position of the grinding tool's200movement from a first position (Short dashed lines) to a second position (long dashed lines) is shown inFIG. 6.

The grinding tool200is also shown in a third position or transition position (solid lines) in which the trough240is shaping the transition thread148and the leading face222of the second rib220would be partially forming the adjacent thread110a.When the grinding tool200advances toward the second position (shown in long dashed lines) the leading face of the first rib210corrects and completes the shape of thread110aby removing the residual material and shaping root116a.

As shown, throughout the grinding process the crests112are shaped by the grinding surface of the trough240. As the grinding tool200moves along the form tap100, the trough is always “in lead” or on pitch which allows the crests112along the entire length of the form tap100to be free from lead error. In contrast, when the grinding tool200transitions from the chamfer portion150, in which all the roots116are shaped by the second rib peak221, to the finishing portion140, in which the roots116are corrected by the first rib peak216, a lead error A is created between the roots of the chamfer portion150and the roots of the finishing portion140due to the spacing between the first and second ribs210,220. However, as described above, because the roots of the chamfer portion do not form part of the functional portion the lead error A may not affect the performance of the finished form tap100. This preservation of the crest-to-crest spacing at the expense of the root-to-root spacing enables the form tap100to have no lead error (as defined above) and may be an advantage of the grinding tool200.

In the example shown (inFIG. 1), the form tap100includes three (3) threads110in the chamfer portion150; however, it is understood that the chamfer portion150of a form tap may include a greater or fewer number of threads110in the chamfer portion150, based on the user requirements, including pitch, thread height, and desired chamfer portion length.

In some examples, it may be possible for both the finishing portion140and the chamfer portion150of the form tap100to be formed using the grinding tool200in a single-pass process. That is, a blank is shaped into a form tap100by forming the threads110of both the finishing portion140and the chamfer portion150in a single, continuous operation by advancing the grinding tool200along the length of the blank, from the chamfer portion150to the finishing portion140.

In other examples, the grinding tool200may be used to form the threads of a form tap in a multi-stage or multi-pass process. In such an example, during each successive pass the grinding tool200may remove progressively more material from the form tap blank. During each pass the grinding tool200may be moved along the entire length of a form tap blank, from chamfer portion150to finishing portion140, to rough-in (or partially form) the threads of the form tap100. Then, once the first pass has been completed, the grinding tool200may be repositioned closer to the first axis of rotation104of the form tap (generally in advance of the transition thread) and advanced along the length of the form tap for a second, rough-in pass. During the second pass, the ribs of the grinding tool may follow and further shape the partially-formed threads from the first rough-in pass. Generally, the final pass of a multi-pass process is referred to as the finishing pass in which the threads110are formed into their final shape. An advantage of the present grinding tool200may be that the same grinding tool200may be used throughout a continuous, multi-pass process to form both the chamfer portion150and the finishing portion140of a form tap100without requiring a tool change or adjustment.

Preferably, the multi-pass process includes four passes, comprising three rough-in passes and one finishing pass.

The multi-pass process may be advantageous because it may i) reduce the wear on the grinding tool, and ii) reduce the heat generated during the thread forming process thereby reducing the cooling and lubrication requirements of the manufacturing process.

The present invention has been described here by way of example only. Various modification and variations may be made to these exemplary embodiments without departing from the spirit and scope of the invention, which is limited only by the appended claims.