WO1994004295A1

WO1994004295A1 - Method of manufacturing helical pitch components

Info

Publication number: WO1994004295A1
Application number: PCT/BE1993/000045
Authority: WO
Inventors: Jean Deroulou; Claude Van Dorpe
Original assignee: Forges De Courcelles-Centre S.A.
Priority date: 1992-08-25
Filing date: 1993-06-29
Publication date: 1994-03-03
Also published as: BE1007097A5; AU4301993A

Abstract

A method is disclosed for manufacturing helical pitch components by direct forging. It applies essentially to the manufacture of relatively large helical components having a deep screw groove. Straight sections (1) having ribs (2) radially located about an axis are cut into suitable lengths (4) and are twisted while hot.

Description

Method for producing parts with helical pitch.

The invention relates to a method of making parts with helical pitch by forging. The method according to the invention applies essentially to the production of relatively large helical parts with deep teeth and in particular to semi-finished blanks for screw compressors. The invention also relates to the profiles necessary for carrying out the method.

There are proven methods and equipment for the production at an accelerated rate of parts with helical pitch in screws and bolts; all, however, relate to parts of reduced diameter and work cold and in shallow grooves.

Large helical parts for power machines, extruders, compressor screws are generally made by removing material. Rotating or milling cylindrical bars or cylindrical blanks already having pins. It is also possible to assemble (by welding or brazing) bent parts. We admit a high labor cost and also a significant loss of raw material per unit produced due to the high cost of the tools used. A substantial gain could be made if it were possible to produce large helical parts with an advanced degree of semi-finishing. This would indeed reduce both the machining time and the waste of raw material.

We know, in the field of woodworking, tap bits made by twisting. This technique is however limited to parts of small diameter which do not, moreover, require very high dimensional precision.

US-A-3608400 thus describes a method for the formation of drill bits by twisting a rod comprising initially straight ribs. This process is normally carried out cold but the part can optionally be heated during or prior to torsion ^* . We start from parts of relatively small diameter. The teeth thus formed are not deep teeth. After torsion, the section of the part is not noticeably changed, and, moreover, no demand is made for formed parts with high dimensional accuracy, the drilling diameter being determined by inserts, fixed on the point of the wick. US-A-2174814 describes a training method for helical gears of short length (of the order of 2.5 cm) cut from a ribbed bar of some 3 m after twisting thereof; the bar is kept under longitudinal tension during torsion. An appropriately shaped wrench is provided so that an operator can manually correct the degree of torsion of the bar during operation.

Patent US-360941 describes a method of manufacturing by hot twist extensions of mine drills from a straight rib axis.

The parts with helical pitch obtained by the various methods described in these documents do not require extreme precision and the deformation of the section is always considered to be a negligible factor.

The object of the invention is the development of a forging process making it possible to obtain, in a reproducible manner, parts with helical screw thread of relatively large dimensions and in particular parts with deep teeth.

Another object of the invention is the economical production of blanks of semi-finished helical parts of various sections.

Another object is to obtain helical parts having reduced manufacturing tolerances with respect to the dimensions of the finished part.

The invention also aims to reduce the waste of raw materials (expensive or high quality metals).

The object of the invention is a method of producing a helical metal part with regular pitch. This process includes the following operations:

- manufacture of a starting profile having ribs arranged radially around an axis,

- heating of this profile so as to obtain a uniform temperature, - fixing of this profile on a device capable of applying a torque to it around its axis,

- torsion of the profile, so that the ribs take a helical shape, the section of the profile being such that one obtains after the deformation due to torsion, a section corresponding to that desired for a determined helical part,

- torsion stopped when the helical shape reaches a determined pitch corresponding to the desired section, - cooling of the helical part obtained.

The starting profile is preferably obtained by cutting a profile of great length.

The starting profile is advantageously obtained by forging or by stamping. According to an advantageous alternative embodiment, this starting profile is obtained by forming ribs over part of the length of a precut bar, the ends of this precut bar retaining their original section, intended to be fixed by means of fixing to the device capable of applying a torque to the precut bar.

According to an advantageous embodiment, the method according to the invention further comprises the following operation: - machining of the ends of the starting profile so as to form support and drive means at these ends. This process also preferably includes the following operation: finishing by machining the helical part obtained.

Advantageously, the starting profile is made of steel and it is heated uniformly to semi-hot temperature, the semi-hot domain being conventionally located for steel between the blue stain and the passage to the austenic domain Ac3.

According to another way of proceeding, the steel is preferably heated to a temperature between 600 and 1100 ° C.

Note that if you want to twist an aluminum part or one of its alloys, the temperature will be between * 300 and 500 ° C. For the torsion, according to the same process, of a brass bar, one will preferably operate between 600 and 850 ° C.

The profiles can be heated for example in an annealing oven.

The invention also relates to a compressor screw produced by one of the methods as described above.

The invention also relates to a profile for carrying out one of the above processes, which comprises ribs extending radially around an axis, the section of this profile being such that after heating and deformation under torsion , a section corresponding to the section of a determined helical part is obtained.

An advantage of the process according to the invention is that, when the manufacturing parameters have been perfected, complex helical parts can be produced in very large series, therefore inexpensively, while maintaining a high degree of precision.

Another advantage is that the power of the apparatus necessary for the actual shaping is very reduced. According to a first example, the power developed during the torsion of a 10 cm piece of diameter and a length equal to at least 2 times its diameter is less than or of the order of 1 kW.

According to another example, the shaping by torsion of a piece 14 cm in diameter and 0.6 m long requires a power of less than 6 kW.

Another advantage lies in the simplicity of the tools used.

Another advantage of the process is that the torsional shaping applies not only to steel of different grades, but also to other metals or forging alloys.

In addition, for certain materials, it is possible to work at relatively low temperature, which limits the problems of metal oxidation. Other features and advantages of the invention will emerge from the description of embodiments below, reference being made to the appended drawings, in which: FIG. 1 is a perspective view with interruption of a section segment with a straight groove serving as a starting point for the method according to the invention; Fig. 2 is a perspective view of a starting profile before twisting; Fig. 3 is a perspective view of the starting profile after torsion; Figs. 4 (a, b, c, d) are superimpositions of the profile of a part seen in cross section in the initial state and at different stages of torsion; Fig. 5 shows, in cross section, the deformation of another type of profile after torsion; Figs. 6 to 9 are graphs showing the relative evolution of different dimensional parameters during torsion; Fig. 10 is a superimposed view of the section of a section obtained by calculation and of this same section after torsion; Fig. 11 shows, side view with interruption, an Arched screw made using the method according to the invention. Fig. 12 is a view of a starting profile obtained by forging-hammering from a precut bar.

Fig. 1 shows a rib section 1 as used as a starting base in the method of producing a helical part according to the invention. It has the appearance of a spur gear of infinite width. The section of the ribs of this spur gear does not seem, at this stage, anything functional. In fact, it is calculated so that one obtains after a rigorously controlled deformation, in torsion, of the starting rib, a shape approaching as close as possible to that of the helical toothing which one wishes to obtain . A rib section 1 as shown can be produced according to different methods, and in particular by forging, by hammering with several hammers, by stamping, by extrusion, by rolling or any other shaping process. The profile shown in Fig. 1 has four ribs 2 arranged around a "heart" 3.

Depending on the shape that one wishes to obtain, and according to the equipment available, this section 1 can have any number of ribs, and even an even or odd number depending on the eventuality.

Fig. 2 shows, in perspective, a starting profile (pre-draft) 4 cut from a section of profile as shown in FIG. 1. For a reason which will appear later, the pre-draft 4 is longer than the future finished part, and in a determined proportion with it.

The pre-blank comprises at each of its ends, pins 5 by means of which it is possible both to support it and to apply it a torque.

Fig. 12 shows another form of pre-cover, obtained by forging and in particular by hammering-forging of a precut bar. As shown in the figure, the ribs 2 have been preformed over the entire length of the initial bar, with the exception ^* of its ends 5a, which have retained their initial section, square in this case. A pre-draft thus prepared offers an improved grip for applying the torque since it offers a larger diameter, and it does not require prior machining of the pin 5 as in the pre-draft shown in FIG. 2. Note that, in order to save material, it is also possible to braze pins 5 of adequate section on a pre-cut section.

This pre-draft is then heated, in the case of a steel, to a "semi-hot" temperature, or between 600 and 1100 ° C. depending on the working conditions.

It is important for the reproducibility of the process according to the invention that a uniform temperature is obtained over the entire length and at all points of the pre-blank 4. This can be obtained by using for example an annealing oven, which does not '' does not exclude other heating means, such as induction furnace, etc.

The pre-draft 4 is then mounted between two axial supports by means of which it is possible to apply a torque to it, without significant axial stress. The pre-blank is subjected to a slow twist via the axial supports 5. Given the temperature to which it has been brought, the part deforms under the effect of the twist and the ribs 2, initially straight, adopt a helical configuration.

It will be noted that the presence of the axial supports 5 has nothing decisive for the method and that the torsional torque can perfectly be applied via plates or by hooking on the ends of the pre-blank. Under the conditions imposed, it can be seen that the deformation of the section takes place in a practically uniform manner, both at the ends and in the middle of the part. We therefore obtains (Fig. 3) a perfectly regular propeller.

The operation also brings about certain unexpected, even a priori paradoxical transformations on the part 4: the latter has indeed contracted in the axial direction, the outside diameter of the teeth 6 has narrowed, the "heart" 3 on the contrary has extended, finally the profile of the initial ribs 2 has generally changed.

The inventors endeavored to control, by tests and projections, the phenomena governing the deformation of the section, so as to be able not only to obtain a regular and rigorous pitch, but also to determine by calculation the section of an initial rib approaching as much as possible, after twisting, of the desired helical profile. In this way, the finishing of the part can be reduced to a few finishing passes with a lathe or a milling machine.

Figs. 4a, 4b, 4c, 4d show in superposition, the initial section 7 of the pre-blank 4 and the same deformed section 8, 9, 10 after rotation of 180, 270 and 360 ° respectively. These figures make it possible to visually appreciate the progressive effect of widening the profile and shortening the ribs 2.

The same phenomenon is highlighted in FIG. 5 for a pre-blank with another initial section 11 after 360 ° twisting (dotted profile 12). The greater the torsion, the more rounded the sides 13 appear, which demonstrates that the widening of a tooth is not the simple result of the progressive inclination of a rib relative to the axis of the part. By even slightly modifying the section, it is therefore possible to modify the deformations generated by torsion accordingly. It can also be seen, if cuts are made at different points on the axis, that the sections obtained are perfectly superimposed. This shows that the process is well mastered. Indeed, we had noted, during previous tests, an inversion of the profile on either side of the section center of the room. This was obviously incompatible with the aims of the invention. The development of the process having been continued in spite of these unfavorable indications, the phenomenon has been eliminated by respecting more homogeneous temperatures.

On the basis of these premises, the evolution of the different variables (section, terminal dimensions, etc.) has been determined experimentally and by finite difference calculation as a function of parameters directly measurable on samples (initial length, diameter, twist angle, nature of the material, temperature ...).

It can easily be demonstrated that, in the case of an initial cylindrical shape, with a given angular torsion, the mean equivalent deformation is proportional to the diameter and inversely proportional to the length. The calculation also shows that this axiom remains true for a piece of arbitrary section. The shortening of the part being (a) function of the deformation and also of the temperature, one effectively determines from experimental values the evolution of the configuration of a pre-draft during torsion and, starting from there, one establishes the section of a profile giving, after controlled torsion, a helical part of determined section. According to one of the initial hypotheses, the part obtained after twisting is a blank which must in any case still be machined to reach its final stage; there is therefore a latitude in the precision of the contour of the order of 1 to 2 mm. We note, in practice, that we can stay below this threshold, but it goes without saying that we can reserve more latitude and plan a greater allowance. In any case, the quantity of metal to be removed remains far less than the quantity of waste that is obtained during conventional machining of comparable parts.

We also observe that the extreme edges of the pre-blank, through which the torque twist is applied, oddly enough, do not participate in the twist (this phenomenon affects a "slice" of metal of about 15 mm).

Figs. 6, 7 and 8 show the trend of the various variables in the case of a part as shown in FIG. 4.

The graph in Fig. 6 gives the variation in diameter (in%) as a function of the shortening measured (in%) with regard to the core of the sample (curve 14) and for its periphery (curve 15).

The graph in Fig. 7 shows the relationship between the contraction rate of a given type of steel (low carbon content) and its average equivalent deformation, and this at different test temperatures, ie at 400 ° C (curve 16) at 700 ° C (curve 17) and at 900 ° C (curve 18).

The graph in Fig. 8 relates the rate of tooth enlargement (for a part of the shape of that shown in Fig. 4) as a function of the radius for different values of the contraction, ie a value of (-6.72%) (curve 19), of (-12.12%) (curve 20) and of (-13.92%) (curve 21).

The graph in Fig. 9 establishes the tooth enlargement rate (in%) as a function of the contraction of the part (in%) at different radii (measured with respect to the axis of the part).

Fig. 10 shown, superimposed, the section

22 of a straight rib 1, established by calculation, the section

23 of the tooth obtained after torsion of the part, and the final section (24) of the helical part after machining. The sections being determined from experimental data, they can be usefully adjusted by carrying out, as in other operations for shaping metal parts, a "volume balance" before and after twisting to compensate for possible material deficits .

We have also reported in FIG. 10 the envelope curve E, which represents the profile of a blank cylindrical to obtain a part having the same helical profile as shown in curve 24 by removal of material, that is to say by a conventional machining process. By way of comparison, it will be noted that the part along curve E has a mass of 28 kg, the blank along curve 23 a mass of 19 kg. The mass of the finished part (identical in both cases) is 17 kg. These figures eloquently illustrate the economic aspect of this method. One of the peculiarities of the shaping process according to the invention, which clearly distinguishes it from any known torsion shaping process, resides in the control of the deformation, which is calculated and carried out so as to obtain a shape section. predetermined. This section is determined, while the part is being twisted, indirectly, by the measurement of the pitch obtained, and not by the angle of torsion imposed on the part.

The stopping of the torsion phrase is therefore subject to the measurement of obtaining a determined step.

This measurement can be carried out visually, with an appropriate form comparator, or automatically, by laser measurement coupled to the control of the torsion machine.

Fig. 11 is an interrupted view of an Archimedes screw made according to the method of the invention. The cost price of such a part, normally made by assembly, becomes particularly advantageous if the method according to the invention is used, which can open up outlets for it in certain industrial assemblies, such as for raising products. liquids or viscous for example.

Claims

1.- Method for producing a helical metal part with regular pitch, characterized in that it comprises the following operations:

- manufacture of a starting profile (4) having ribs (2) arranged radially around an axis,

- heating of this profile so as to obtain a uniform temperature, - fixing of this profile (4) on a device capable of applying to it a torsional torque around its axis,

- torsion of the profile (4), so that the ribs (2) take a helical shape, the section of the profile (4) being such that after the deformation due to torsion, a section corresponding to that desired for a specific helical part,

- torsion stops when the helical shape reaches the desired section,

- cooling of the helical part obtained.

2.- production method according to claim 1, characterized in that the starting profile (4) is obtained by cutting a profile of great length (1).

3.- Method according to any one of claims 1 and 2, characterized in that the starting profile (4) is obtained by forging.

4.- Method according to any one of claims 1 and 2, characterized in that the starting profile (4) is obtained by stamping.

5.- Production method according to any one of claims 3 and 4, characterized in that the starting profile (4) is obtained by forming ribs over a part of the length of a precut bar, the ends (5a ) of this precut bar retaining their original section, this original section being able to secure the precut bar with the device intended to apply the torque.

6.- Method according to any one of the preceding claims, characterized in that it also comprises the following operation:

- Machining the ends of the starting profile so as to form support and drive means (5) at these ends.

7.- Method according to any one of the preceding claims, characterized in that it also comprises the following operation: finishing by machining of the helical part obtained.

8.- Method according to any one of the preceding claims, characterized in that the starting profile (4) is made of steel and in that it is heated uniformly to mid-hot temperature.

9.- Method according to any one of claims 1 to 7, characterized in that the starting profile (4) is steel and in that it is heated to a temperature between 600 and 1100 °.

10.- Method according to any one of the preceding claims, characterized in that the heating of the profiles (4) is carried out in an annealing furnace.

11.- Compressor screw, characterized in that it is produced by a process according to any one of the preceding claims.

12. Profile for carrying out the method according to any one of claims 1 to 10, characterized in that it comprises ribs (2) arranged radially around an axis, the section of this profile being such that after heating and deformation under torsion, a section corresponding to the section of a determined helical part is obtained.

13. Profile according to claim 12, characterized in that it is obtained from a precut bar having an original angular section, the ribs (2) being formed over the entire length of said bar, with the exception of axial ends of said bar precut which keep the original section of said bar.