US 3856906 A
Apparatus is provided to perform the following work steps in sequence. First, one end of a plastic tube is heated while under circumferential compression. The compression deforms the heated end inwardly and when the apparatus exerting the compressive force is removed, the heated end is inserted in, and gripped by segmented dies having pipe threads on their gripping surfaces. An expansion tool is inserted into the heated end of the tube and an elastomeric annulus which is part of the tool is compressed axially. Said axial compression displaces the annulus radially against the inner surface of the tube which, in turn, is expanded radially and deformed into the thread form of the dies. Next the axial pressure on the annulus is released, the annulus recedes radially and the expansion tool is retracted from the tube end. A cooling probe is inserted from the opposite end to hold the threaded tube in place as the segmented dies are separated. The cooling probe will freeze the heated end of the tube such that the thread form will be permanently fixed in the exterior tube surface.
Description (OCR text may contain errors)
nited States Patent [191 Edgar Dec. 24, 1974 METHOD FOR FORMING THREADS O PLASTIC NIPPLES 1 James S. Edgar, 1306 S. Columbus Airport No. 3, Columbus, Ohio 43207 22 Filed: Mar. 12,1973
21 Appl. No.: 340,462
Primary Examiner-Richard R. Kucia Attorney, Agent, or FirmFay & Sharpe  ABSTRACT Apparatus is provided to perform the following work steps in sequence. First, one end of a plastic tube is heated while under circumferential compression. The compression deforms the heated end inwardly and when the apparatus exerting the compressive force is removed, the heated end is inserted in, and gripped by segmented dies having pipe threads on their gripping surfaces. An expansion tool is inserted into the heated end of the tube and an elastomeric annulus which is part of the tool is compressed axially. Said axial compression displaces the annulus radially against the inner surface of the tube which, in turn, is expanded radially and deformed into the thread form of the dies. Next the axial pressure on the annulus is released, the annulus recedes radially and the expansion tool is retracted from the tube end. A cooling probe is inserted from the opposite end to hold the threaded tube in placeas the segmented dies are separated. The cooling probe will freeze the heated end of the tube such that the thread form will be permanently fixed in the exterior tube surface.
4 Claims, 8 Drawing Figures l l l l l l PATENIEDUEBWW 3.856.906
sum 3 BF 'IIIIIIIIIIIIIIIIIIIIA ll iillllllllllllllm METHOD FOR FORMING THREADS ON PLASTIC NIPPLES BACKGROUND OF THE INVENTION Metallic tubes are traditionally used in plumbing but, more recently plastic tubing has become more in demand particularly PVC plastic tubing and other less expensive tubing of thermoplastic polymers. The reasons for the desirability of plastic tubing is its light weight and low corrosion potential. The detrimental factors include the difficulty ofjoining and separating the tube lengths.
One often used mechanism for joining is bonding by chemical reaction where the end of the tube and an associated sleeve are painted with some reacting chemical. When the tube is inserted into the sleeve, the chemicals react to bond the two together. Unfortunately, getting them apart again requires cutting because the bond is permanent. Additionally, if for some reason the chemical reaction does not work to completely seal the joint, leakage occurs and severing is required to replace the sleeve. There is no way to tighten the frozen joint nor any economically practical way to put an additional seal at the joint.
More recently, threaded plastic tubing has been used in the same way as threaded metallic tubing and this solves the problem to a great extent. Thus, there is a critical need for threaded nipples of standard lengths such as 3 inches, 6 inches, 1 foot, etc., at various standard diameters. Unfortunately, there is substantial difficulty in threading plastic nipples, and that is the reason for the aforementioned need.
The inexpensive thermoplastic polymers economically practical for use in plastic tubing are not nearly as strong in tension and compression as equivalent copper or steel tubing. As a consequence, when nipples are to be threaded, they require gripping to prevent rotation when the threading die cuts the end, internal support to prevent either the collapse of the tube or inadequate depth on the cut threads, or some compensation for the problem. It goes without saying that this becomes a very difficult problem when one considers trying to thread nipples a quarter inch in diameter and in close or short lengths.
This invention solves the problems by approaching them with an entirely new and different theory which avoids the gripping and collapse problems inherent in cutting threads in a conventional manner.
BRIEF DESCRIPTION OF THE INVENTION The apparatus performing the method of this invention includes a reamer which, initially cuts a frustoconical surface inside the end of the tube to be threaded. The reason for this will be explained subsequently. Then, a clamping force is circumferentially applied on the exterior surface of the tube and together they are heated to a temperature which will soften the plastic but, will not be so high as to melt it.
After the tube is heated to an adequate temperature, it is removed from the heating means and the clamping force removed. The heated end of the tube is then inserted into and gripped by segmented dies having a pipe thread pattern on its interior gripping surface.
With the tube locked in place, a force or expansion tool is inserted into the heated end of the tube which, when actuated, will force the tube end radially outward to completely fill the thread form of the die. When the force tool is removed and the tube is cooled by a probe projecting from the opposite end of the tube, the segmented dies are retracted and the threaded nipple adequately cooled and placed in storage or elsewhere for subsequent use or for threading the other end if the need arises.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view ofa single operator, table model of the apparatus of this invention;
FIG. 2 is a fragmentary sectional view showing a tube end being reamed prior to its heating;
FIG. 3 is a fragmentary sectional view of a heated tube with a circumferential clamp around the heat end;
FIG. 4 is a fragmentary sectional view of the heated end ofa plastic tube clamped in threading forming dies;
FIG. 5 is a fragmentary sectional view similar to FIG. 4 but, with an expanding tool inserted into the heated end of the tube and the tube being shown expanded radially into firm contact with the thread form pattern;
FIG. 6 is a fragmentary view of the threaded nipple of FIG. 5 with the dies retracted and the nipple being removed by a cooling probe;
FIG. 7 is a perspective view of the preferred gripping clamp of this invention; and
FIG. 8 is a fragmentary sectional view of the force or exxpanding tool of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring particularly to FIG. 1, most of the apparatus to accomplish this invention is illustrated mounted on a table top 10. It includes a heater 12, illustrated with three nipples l4 projecting therefrom. Split or segmented dies 16 are illustrated as being mounted on a frame 18 and actuated by hydraulic means 20. Hydraulic fluid will obviously be delivered from a reservoir under pressure to means'20; the piping. control mechanism, etc. are not illustrated as any skilled mechanic could design an adequate system and they are not a part of the invention. As will be clear to those having ordinary skill in the art, the frame 18 and hydraulic means 20 are but one of a plurality of equivalent means for accomplishing the result desirable which, will be explained in more detail subsequently.
A force tool 22 is aligned with the dies 16 in guide channel 24 and a cooling probe 26 is in a similar guide channel 28 on the opposite side from the force tool.
For convenience, the invention will be described hereinafter by the preferred process used in making the threaded nipple. First, let it be said that the invention being described is concerned with forming pipe threads on plastic nipples. American Standard pipe threads diverge from the axis of a pipe at an angle of approxiamtely l 47 or an included angle of 334. With metallic nipples, the excess metal is cut away from a cylindrical periphery by thread dies which rotate relative to the tube. As a general rule, such a procedure puts too much pressure on the conventional plastic tube and will often cause collapse or tearing. This invention overcomes that complex problem and one of the first things done in furtherance of this objective is illustrated in FIG. 2; that is, a reaming tool 30 initially reams the inside end surface of the tube which is to be threaded. It is preferred that the reaming tool cut a frustoconical surface which will diverge toward the open end of the tube at an angle not substantially greater than 3 /2. The
reason for this is that the usual amount of material removed from the exterior surface of a metallic tube by cutting dies is to be removed in this casefrom the interior surface by the reamer. In sequence, this will be followed by a radial contraction of the end to be threaded and subsequently, by a radial expansion.
The intention of this combination of three steps, namely, reaming, contracting and expanding is to have a relatively uniform interior tube diameter from end to end. As will be explained subsequently, the radial expansion of the tube which forms the threads could result in a series of undulations in the fluid flow path with every joint in the pipe. Obviously, this is undesirable because, it would create undesirable turbulence within the fluid itself.
As a consequence of this desire for a smooth interior, the reamer is first used to cut the stated amount from the inside surface of the tube 14 as illustrated in FIG. 2. Next a modified split hose-type clamp 32 is contracted around the exterior surface of the reamed end with the resulting inward radial pressure and deformation illustrated in FIG. 3.
The clamp itself is more clearly illustrated in FIG. 7. Because it is well known to those in the automobile repair and plumbing trades no detailed explanation of the clamp appears necessary, except to say that a spring steel rod 34 combines with the split metallic sleeve 36 to provide a semi-controlled compression tool in the obvious way.
Referring to FIG. 1, it will be seen that each nipple 14 has one end inserted in the heater 12. The inserted end has a clamp 32 mounted thereon as illustrated in FIG. 3. The clamp 32 serves two purposes. The compression feature has been discussed earlier. However, it also serves one other very useful and important purpose. That is, it shields the end of the tube from the radiant heat of the heating element within the heater 12. In addition because the sleeve 36 is metallic, the heat is relatively uniformly distributed around the periphery of the tube, metal being a good heat conductor. As is well known, plastic per se is not a good conductor of heat and hot spots might result without the metallic band at the periphery. Radiant electric heating is not a critical feature of the invention but is preferred.
After the appropriate temperature is generated on the tube it can be removed for subsequent work. The temperature desirable for subsequent work will vary from one material to another. For example, a temperature of about 300 F. is satisfactory for polyvinyl chloride and about 320 F. is suitable for polypropylene. Exact parameters can be tailored for the particular materials and machines in use but, the principals set out remain the same.
After the tube is taken from the heater and the clamp removed, the heated end of the tube is inserted into the split die 16 and the hydraulic means 20 is actuated to grip the hot end. It will of course. be recognized that the FIG. I showing of two split dies is merely illustrative of the concept. The split dies copuld be of three or four or five segments or any other number that was found convenient. In addition. the illustration of the die working on only one end of the tube in FIG. 4 can clearly be modified such that the dies can operate on both ends of the tube simultaneously. There is no apparent need to illustrate this series of possible modifications, as the principals are not changed.
Taking the nipple from the heated environment of the heater l2 and inserting it into dies 16 could cause the nipple 14 to cool and harden too soon because, the metallic dies 16 would seve as a heat sink. For this reason, heating elements 38 are preferably incoporated into the dies to keep them at a temperature of 275300 F. and thus, they will draw off very little heat from the tube itself.
One end of the force tool 22 is inserted into the tube 14 as illustrated in FIG. 5. The portion illustrated includes a rod 40 coaxially mounted within a hollow rod 42. A deformable resilient, relatively incompressable annulus 44 circumscribes the rod 40 and is sandwiched between two radially extending flanges 46 and 48. The flange 48 is merely a washer held in place by a nut 50 threaded onto the end of rod 40. v
In operation, the tube 14 is clamped into the dies I6 and the force tool 24 slides forward on the channel 24 until the elastomeric annulus 44 slides into the end of tube 14 and flange 46 abutts the tube end or the die face. Handle 52, is pushed downward, thereby retracting rod 40 relative to hollow rod 42 which, deforms the resilient annulus 44 axially. Because the annulus is relatively incompressable, it expands radially as it contracts axially; this forces it into contact with the interior surface of the nipple and in turn forces the interior surface of the nipple radially outward into the thread pattern 54 where it is plastically deformed into the desirable shape.
After adequate force has been transmitted and the threads properly formed, the handle 52 is raised allowing the annulus 44 to contract radially and the force tool is retracted from the end of the nipple. At that point, the cooling probe 26 is inserted into the tube from its opposite end. Many means of cooling the probe are within the contemplation of this invention but, no specific cooling mechanism or tubing arrangement has been illustrated, as such is obvious. In any case, as illustrated in FIG. 6, when the cooling probe is inserted and it sufficiently cools the tube to freeze the threads 56, the dies 16 may be retracted and the probe 26 will hold the nipple in place where it may be removed for storoage or whatever need may arise.
While the inventive concept illustrated and described with great particularity in this specification has been limited to forming pipe threads on the end of a plastic tube, it will be recognized by those having ordinary skill in the art, similar processes and modified equipment may be used to form flanges, cupped or belled ends, grooves, notches, splines, decorations, etc., on the ends of plastic tubes. Certain of these shapes would require modifications of the annulus 44 and certainly these fall well within the skill of the art and are within the contemplation of this invention. Additionally, the clamp 32 could be eliminated with adequate safeguards on heat distribution and some apparatus for performing the heated end of the nipple prior to its insertion into dies 16.
As will be obvious to those having ordinary skill in the art. certain additional modifications may be made in the method and apparatus described without departing from the spirit of the invention. It is not the inventors intention to be bound by the language used in describing the invention nor the single illustrated embodiment. Rather he intends to be bound by only the appended claims.
1. A process of forming threads on the end of a cylindrical plastic-tube comprising:
a. removing material from the inside surface of the tube at the end to be threaded, said removal comprising cutting the material away to form a frusto conical interior surface diverging toward the open end of the tube at an angle not substantially greater than 3 /2,
b. heating the end of the tube to be threaded and deforming it radially inward to reform the interior surface of the tube to roughly a cylindrical surface while reforming the exterior surface at the end of the tube to a frusto conical surface converging toward the end of the tube,
c. gripping the heated and deformed end of the tube with segmented dies, said dies having a thread form pattern on their inner surface in contact with said heated tube end,
d. applying radial pressure from within the tube against its interior surface to plastically deform said tube radially against the dies with force sufficient to cause plastic flow of the heated end into the thread form pattern of said dies and e. cooling the tube by application of cooling fluid to quickly freeze the thread form pattern on the end of the tube and prevent damage to the thread due to the soft nature of the hot plastic.
2. The process of claim 1 including placing a circumferentially extending clamp on the tube end prior to its insertion into said dies to compress the heated end radially inwardly and removing the clamp prior to placing the tube within said dies 3. The process of claim 2 wherein the expanding step is accomplished by inserting annulus of resilient, substantially, incompressable material into the heated end of said tube and deforming the annulus axially, thereby forcing it to expand radially against the interior surface of said tube.
4. The process of claim 3 wherein the segmented dies are heated continually to maintain their temperature near that of the heated tube to prevent premature freezing of the heated end and improper deformation. l l l