GB2182591A - Method of producing a helical thread - Google Patents

Abstract

A helical thread is initially cut by machining, in the course of which a rough surface 10 is formed. In the subsequent operating step the surface is pressure-smoothed by sliding friction. The tool used for this purpose is of such dimensions that the volume of material 11 which protrudes from the desired surface is displaced into the intervening areas 12 and is smaller than the accommodating volume of the valley areas lying below the desired surface. The pressure smoothing is thereby carried out within the plastic range of the material, whereby the tools are subjected to considerably less stress than during surface compaction. Accordingly, the pressure smoothing of very small internal threads can be carried out without difficulty. In this way load-bearing surfaces of high surface pattern of which up to 80% is closed and the surface structure of which is interrupted by only a few valley areas. <IMAGE>

Description

SPECIFICATION A method of producing a screw-thread The invention relates to a method of producing a screw-thread, a screw-thread when produced by the method and a tool for use in the method.

Patent Specification DE 854291 (Geisel) discloses a method of producing a screw-thread byincbrporat- ing the production of the thread groove in a machining operation and by subsequent pressure smooth ing ofthe thread surface.

Here the screw-thread surfaces are rendered smooth by pressure polishing and are brought accuratelyto size, This publication does not disclose further details of how and with what tools the pressure polishing is carried out. Rolling tools are assumed to be used forth is purpose, as they are also used for rendering other surfaces smooth, Difficulties arise when mostly bulky tools ofthis type are used for small screw-threads. When pressure smoothing these small screw-threads surface forces must be applied which are of equal magnitude to those applied with relatively large workpieces.

Consequentlythe complicated design ofthe rolling devices often does not result in the desired surface quality. Ascrew-thread of precise dimensions and high-grade surface is especially necessaryforworm gears for converting rotary motion into linear mo tion, or vice versa. Ease of operation is required of su ch screw-th rea ds even under arduous conditions, for example under high radial loads or in a mutually tilted position. In none ofthese instances should high static friction or irregular frictional behaviour occur.

The subject ofthe invention is a method of producing a screw-thread which enables an accurate and smooth thread surface to be provided at low cost.

In one aspect the invention provides a method if producing a screw-thread comprising the steps of initially providing a screw-thread and then treating the initial screw-thread, wherein the treatment step includes pressure smoothing without substantial surface compaction.

In anotheraspectthe invention is characterised by producing a rough surface structure in the course of machining the thread with an additional treatment operation for the thread dimensions and by pressure smoothing the surface structure within the plastic range up to a surface area percentage ofthe smoothed structural peaks of between 30 and 80%, in relation to the overall surface subject to pressure treatment.

Firstly, a coarse, that is rough, surface structure is deliberately produced during for example cutting of the screw-thread in a known manner using a tap, die or the like. This surface structure among other things can be produced by the cutting edges of the cutting tool having a steep lead-in or relatively large angle in relation to the normally acute-angled shape. Another possibility is that of pre-hardening the workpiece so as to provide a slightly more brittle structure or of using a corresponding alloy material with a coarse structure. With all alternatives, afterthe initial screwthread has been cut, a surface is provided having a relativelyjagged microstructure which as a resultof the cutting action may also be of a corrugated charac ter. The structure provides raised structure peaks with adjacenttrough orvalleyareas.

Pressure smoothing within the plastic range is un dertaken in the next stage of operation. Intentional surface compaction, in its conventional interpreta tion, should be refrained from. As a maximum only those portions of material volume aretransposed from the structure peaks as can be accommodated by the valley areas. To ensure in each case that only plastics deformation of the material portions occurs without substantial structural compaction, the pres sure smoothing ofthe surface is carried out only par tially, that is even afterwards the surface still has a certain percentage of partial areas of the initial sur face structure. The pressure-smoothed surface port ions of all the structure peaks amount, when added up, to 80% for example, whereas the remaining valley areas comprise 20% of the pressure-treated overall surface.The individual surface portions thus form the load-bearing sliding surface of a thread, which is of outstanding surface quality and which is of precise dimensions because only plastic deformation ofthe material has occurred. However, if a further more intensive pressure treatment were to be carried out, then necessarily compaction ofthe material would take place when the valley areas could no longer accommodate more material upon exceeding the 100% limit. In this casethetool would have to exert substantially higher compressive forces on the surface of the screw-thread. Depending on the material of the tool, these forces increase up to a multiple of the forces which are necessaryforthe plastic deform- ation without compaction.

The described method step of "pressure smoothing" can be carried out, for example, by means ofa rolling tool which, because of the slight forces which are required for deformation with the plastic range, hasto meetonly lowstability requirements and can thus be of simple and small construction.

Preferably the screw-thread flanks are pressuresmoothed except for the transition zones leading into the root and/or crest surface. Accordingly, the trans- ition zones which in special threads, for example in a trapezoidal or acme screw-thread, do not have load-bearing function, are not pressure-smoothed.

Three separate pressure-smoothed areas are thereby created overthe complete screw-thread section, namely the two flank surfaces and the crest surface. This division improves results in the pressuresmoothing operation because, for reasons con cernedwith geometry, in the often angulartransition zones the deformation process reaches the compac tion stage earlierthan in the flatoralmostflatflank surfaces. Moreover, the application of compressive force is at least slightly reduced and is restricted to the load-bearing surfaces.

Preferably the pressure smoothing is effected by sliding friction under pressure substantially inclined to thesurface, the structure peaks being deformed partly by bending and partly by flow distortion. In contrastto a rolling tool, here the pressure smoothing is effected by a sliding member initially under pressure on the screw-thread surface. The relative movement of this sliding member on the thread sur face produces a force acting obliquely thereto, which results partly in the structure peaks being bent over in the direction of movement and partly in a flow distortion also transversely to the direction of movement. Preferably,this process takes place inthestarting area ofthis sliding member. The additional sliding area assists in improving the smoothed surface.

The results of the pressure-smoothing operation can be further improved if a supply of lubricant is provided while it is being carried out.

The described mode of operation offers,in particular, advantages in respectofthe design of the pressure-smoothing tool. Avery simple and economical tool similarto a conventional thrn6d-cutting tool can now be used. Consequently, this advantage also enables aftertreatment of very small threads to be carried-out, approximately in the range of a few mm. An internal thread with a diameter, for example, of 5 mm can thus be pressure smoothed,withoutthe relatively thin tool being exposed to any risk of damage. If the pressure smoothing were to go beyond the above-described 100% limit into the compaction stage, so-called "fretting " ofthe tool would be an inevitable consequence ofthis procedure.This method according to the invention notonly entirely obviates this danger but provides an outstanding improvement in thread quality, wherein the resulting dimensional precision and surface finish lead to reduced static friction and uniform properties over the entire thread length. Of course, both internal and external threads can be produced in accordance with this method. Atool which can be used for the pressuresmoothing stage of operation will be described ih more detail in the specific description.

In the accompanying drawings, Figure 1 shows a longitudinal section through a trapezoidal internal screw-thread in the vicinity of a thread turn, afterthethread has been initially cut; Figure 2 shows the screw-th read according to Figure 1, after pressure smoothing; Figure 3 shows an enlarged detail of a surface portion, illustrating the displacement of material during pressure smoothing; Figure4shows a cross-section through a tool for pressure smoothing an internal thread; and Figure 5shows an enlarged detail A of the initial and final treatment portions according to Figure 4.

Figure 1 illustrates an internal screw-thread groove ofatrapezoidal shape afterthethread has been initially cut. This leaves a relatively rough surface structure 10 with structure peaks 11 protruding from the surface and interposed valley areas 12. In the longitudinal section illustrated here the structure pathos- cillates about an arithmetical centre line 13.Small structural differences in the material, the condition of the cutting edge of the thread-cutting tool and its cut ting angle give the com pletely-irreguiar structure il- lustrated,the peak-to-valley height ofwhich can be as much as about 10 um and for greater clarity is shown greatly enlarged in the drawing..

Figure 2 illustrates the trapezoidal screw-thread according to Figure 1 after it has undergone pressure smoothing in a further operating stage. For this purpose a tap-like tool 14, shown in Figures 4 and 5 is used, which wili be described below. The entire screw-thread surface is smoothed, except for the transition zones 15 and 16 between the flank surfaces 17 andthe crest and rootsurfaces 18 and 19,the aforementioned tool 14 having the dimensions ofthe former centre lines 13 of the structure according to Figure 1. The structure peaks 11 are transposed by the pressure-smoothing operation, their materiai portion being displaced into the valley areas 12.This results in a smoothed load-bearing area 20 of about 80% made up of surface portions, the valley areas 12 forming the remaining surface area of about 20%.

The process of pressure smoothing at the enlarged detail of a surface portion illustrated in Figure 3 will be described in the following. This surface portion has been treated with a tool 14 according to Figure 4 and 5. The tool 14 will firstly be described so asto make the procedure easierto understand. Itcomprises a tap-like shank 21 with an external screwthread formation and two diametrically opposed axially extending cutting grooves or flutes 22. As are suIt of these grooves 22 all thethread turns are divided into two substantiallysemi-circular segments 23, each ofwhich has an initial or leading treatment portion 24 and a final ortrailing treatment portion 25.

The initial treatment portion 24 of each semi-circular segment 23 presents a trapezoidal face, the outline of which lies on the tips ofthe peaks 11 of the initial screw-thread shown in Figure 1. This applies not only to the root 19 but also to the flanks 17 and crest 18. As can be seen in Figure 4,the grooves orflutes 22 ex- tend into the shank 21. This trapezoidal outline then expands as a smooth ramp-like transition to the size of the desired screw-thread represented on the final treatment portion 25. Sothatthetransitions 15 and 16 are not pressu re- smoothed, th e edges of the thread formation of the tool 14 are relieved. Lubricant for this smoothing operation is fed along the grooves 22.

As already stated, the tool described is used forthe pressure smoothing of the initially cut screw-thread according to Figure 1. The surface structure to be treated afterthe cutting operation corresponds to the right-hand enlarged illustration of Figure 3. The surface is rough and has protruding structure peaks 11 and valley areas 12 situated between them. Upon screwing the tool 14 into the bore, the structure peaks 11 come into contact firstly with the initial tratment portion 24 ofthefirst semi-circular segment 23.

As a result of its inclination the structure peaks 11 are acted upon by a compressive force Fwhich, with re spectto the crest 18 and root 19 lies inclined between the circumferential direction and the radial direction.

This force induces a displacement of the structure peaks 11, partly by bending over and partly by flow distortion of the material into the valley areas 12, as indicated by the arrows. Upon reading the final treat- ment portion 25, the operation is concluded forthis semi-circular segment 23 and the material disposition shown on the left in Figure 3 is provided,thus resulting in the smoothed load-bearing surface 20 whose surface areas are now greater because the previously wide-open valley areas 3 have become narrower. The deformation takes place within the plastic range that is, up to the yield point of the material. Further semi-circular segments 23 follow as the tool 14 continues to be screwed into the bore, whereby the smoothing effect is optimised.

The tool 14is merelyan example. The semicircular segments 23 of the first thread turns may be disposed on a smaller diameter, whereasthefollow- ing turns lie on a steadily increasing diameter so providing a very slight but conical shape. The pressuresmoothing operation is thereby distributed over lar ger sections of the tool so reducing the forces involved.

Claims (17)

1. A method of producing a screw-thread by incorporating the production of the thread groove in a machining operation and by subsequent pressure smoothing ofthethread surface, characterised by producing a rough surface structure in the course of machining the thread with an additional treatment operationforthethread dimensions and by pressure smoothing the surface structure within the plastic range up to a surface area percentage ofthe smoothed structure peaks of between 30 and 80%, in relation to the overall surface subjected to pressure treatment.

2. A method according to Claim 1, characterised in that the flanks are pressure-smoothed except four the transition zones leading into the root and/or crest surface.

3. A method according to Claim 1 or 2, characterised in that the pressure smoothing is effected by sliding friction under pressure substantially inclined to the surface.

4. A method according to Claims 1 to 3, characterised in that the structure peaks are deformed partly by bending and partly by flow distortion.

5. A method according to Claims 1 to 4, characterised in that a supply of lubricant is provided while the pressure-smoothing operation is being carried out.

6. A device for carrying out the method according to Claims 1 to 5, characterised in that a tool corresponding to the counterpart ofthe screw-thread to be treated is provided with atleastonegrooveexten- ding substantially transversely to the thread groove and with final and initial treatment portions and in that the initial treatment portions have the dimensions of the initial machined thread groove and the final treatment portions have the desired thread dimensions, and in that a uniform transition is provided therebetween.

7. A device according to Claim 6, characterised in that the grooves are lubricating passages.

8. A method of producing a screw-thread comprising the steps of initially providing a screw-thread and then treating the initial screw-thread, wherein the treatment step includes pressure smoothing without substantial surface compaction.

9. A method as claimed in Claim 8 including pressure smoothing such that 20 to 80% of the total area subject to pressure smoothing is smoothed.

10. A method as claimed in Claim 8 or9 and not including pressure smoothing the transitions between the flanks, crests and roots of the initial screwthread.

11. Amethod as claimed in Claim 8,9 or 10 in- cluding pressure smoothing by sliding friction with a force directed at an angle to the surface being smoothed.

12. A method as claimed in Claim 8,9, or 11 including supplying lubricantduring pressure smoothing.

13. A method of producing a screw-thread sub stantially as herein described with reference to and as shown in Figures 1 to 3 ofthe accompanying drawings.

14. Ascrew-threadwhen produced bya method as claimed in any of Claims 8to 13.

15. Atool for pressure smoothing an initial screw-thread in a method of producing a screw thread as claimed in anyone of Claims 8to 13, wherein the tool has a co-operating screw-thread for mation with a leading portion corresponding to the dimensions ofthe initial screw-thread, a trailing por tion corresponding to the dimensions of the desired screw-thread, and smooth transition extending from one said portion to the other.

16. Atool as claimed in Claim 15, wherein thetool has a longitudinally extending groove forthe pas- sage of lubricant.

17. Atool substantially as herein described with reference to and as shown in Figures 4 and 5 ofthe accompanying drawings.