Mandrel for expanding internallyfinned tubes

April 19, 1949. _A. _w. _Hallberg 2,467,668

MANDREL FOR EXPANDING INTERNALLY-FINNED TUBES

Filed Oct. 30. 1947

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Attorneys

Patented Apr. 19,1949

2,467,668

UNITED STATES PATENT OFFICE

2,467,668

MANDREL FOR EXPANDING INTERNALLY FINNED TUBES

August W. Hallberg, Waterbury, Conn., assignor to Chase Brass &amp; Copper Co. Incorporated, Waterbury, Conn., a corporation

Application October 30, 1947, Serial No. 783,076

Claims.

The present Invention relates in general to the manufacture of finned heat-exchange tubes and more especially to a superior method and means for improving the physical characteristics of an internally-finned tube produced by hot extrusion techniques.

In general, internally-finned tubes produced by hot extrusion are quite soft and have very little rigidity, as a consequence of which the tubes, including their internal fins, are easily bent and rendered unsatisfactory for practical purposes. It is desirable, therefore, to impart an appreciable degree of rigidity to the internally-finned tubes and to straighten the fins so as to provide a product acceptable to the trade. Moreover, as extruded, the transverse dimensions of the Aimed tube may vary slightly throughout the length of the tube and from tube to tube and, hence, it is desirable to subject the tube to a sizing operation to produce accurately-dimensioned tubes.

An object of the present invention is to provide a superior method of imparting rigidity to hotextruded internally-finned tubes.

A further object of the invention is to impart rigidity to a hot-extruded internally-finned tube by a cold-working operation wherein the tube is expanded radially in successive stages.

A still further object of the invention is to cold work an internally-finned tube by passing a mandrel therethrough having a stepped enlargement and longitudinal fin-receiving grooves whereby the tube is expanded radially and the internal fins simultaneously straightened in one pass of the tool.

A still further object of the invention is to provide an improved mandrel for expanding an internally-finned tube while simultaneously straightening the internal fins thereof.

A still further object of the invention is to provide an improved mandrel for cold working an internally-finned tube wherein a stepped enlargement at one end of the mandrel expands the tube in progressive stages.

With the above and other objects in view, as will appear to those skilled in the art from the present disclosure, this invention includes all features in the said disclosure which are novel over the prior art.

In the accompanying drawings, in which cer- 50

Cl. 153—80.5) tain modes of carrying out the present invention are shown for illustrative purposes:

Fig. 1 is a perspective view of a finned tube formed by a hot-extrusion process and charac₅ terized by both internal and external fins;

Fig. 2 is a side elevation in section of the tube on line 2—2 of Fig. 1 and including a portion of the improved mandrel for cold working the tube;

Fig. 3 is an end elevation of the tube and man10 drel on section line 3—3 of Fig. 2; and

Fig. 4 is a perspective view of the improved mandrel for cold working the finned tube.

Referring to the drawings, the type of internally-finned tube to which the improved method 15 of this invention is applied is indicated in Fig.

and comprises a cylindrical body-portion 10 of any desired length having internal fins II which project inwardly radially from the inner wall of the body i 0 of the tube and extend longi2o tudinally thereof. In the present embodiment, each fin is substantially rectangular in cross section and there are eight fins in number, but it will be appreciated that these specific features do not limit the application of the invention.

Moreover, the tube shown in Fig. 1 has a plurality of longitudinal external fins 12 which extend outwardly radially from the outer surface of the tube, the number of external fins being twice the number of internal fins I ί and every other ex30 ternal fin being in substantial alignment radially with one of the internal fins. The number of external fins may vary, of course, from the number shown and, in fact, the invention is equally applicable to an internally-finned tube having no 35 external fins.

The finned tube is generally formed of a ductile metal such as copper or a copper-base alloy and by a hot-extrusion process such as described more fully in my copending application, Serial No.

713,160 filed November 29, 1946, and issued as Patent No. 2,461,640 on Feb. 15,1949.

In practicing the improved method of this invention, a length of the hot-extruded internallyfinned tube is held securely on a draw-bench, 45 whereupon a mandrel or other suitable tool is drawn through the tube from one end thereof to the other to expand the tube radially, the radial expansion of the tube being effected in successive stages, each of progressively greater diameter and in one pass of the tool, whereby the tube

2,467,6 is cold worked to such a degree as to have the rigidity required by the trade. The degree of cold working is measured, in general, by the amount of radial expansion of the tube and the characteristic feature of this invention is the 5 discovery that the radial expansion of the tube must be effected in two or more successive stages in order to control the size of the tube. Hence, the ultimate radial expansion necessary for building the required rigidity into the tube is accom- 10 plished in successive stages, the initial stage effecting a relatively slight increase in the diameter of the tube and the next succeeding stages effecting progressively greater Increases in the diameter of the tube, the ultimate increase in diam- ₁₅eter of the tube for cold working the tube sufficiently to produce the rigidity required being substantially 10% of its original diameter.

Simultaneously with the cold working of the tube, the body 10 of the tube and its internal ₂₀fins I ί are straightened. To the latter end, the mandrel or expanding tool is provided with longitudinal recesses or grooves spaced circumferentially around the tool at distances corresponding to the circumferential spacing of the internal fins 25 of the tube. Thus, as the tool is forced into one end of the finned tube, the internal fins are adapted to engage in the longitudinal recesses or grooves of the tool, whereby movement of the tool and, in particular, its grooves relative to the ₃₀tube, effectively straightens its fins.

As shown especially well in Figs. 2 and 4, the tool 13 comprises a substantially-cylindrical bodyportion 14 having a drawbar 15 fixedly secured to one end thereof in alignment with its longi- ₃₅tudinal axis for pulling the tool through the finned tube by means well known in the art.

The body 14 of the tool is substantially cylindrical and for superior performance is a few thousandths larger than the internal diameter of 40 the tube 10. Although the body 14 of the tool will thus expand the tube radially slightly, the cold working accomplished thereby is insufficient to impart any appreciable rigidity to the tube, the slightly greater diameter of the body-portion 14 of the tool constituting, primarily, means for straightening the finned tube as the tool is pulled therethrough from one end to the other.

In this connection, it has been discovered that the over-all length of the body of the tool must be a multiple of its diameter for effectively straightening the tube.

Formed integrally or in any other suitable manner at one end of the body-portion 14 of the tool is an enlarged cylindrical head, hereinafter referred to as a stepped annular enlargement, which is adapted to cold work the inner wall of the tube to build the required rigidity into the tube In one pass of the tool therethrough. This stepped annular enlargement of the tool is indicated generally at 16 and comprises two successive concentric annular flanges or steps Π and IB of progressively increased transverse dimensions, that is to say, the diameter of the inner flange or step 17 is several thousandths greater than the diameter of the body-portion 14 of the tool, while the diameter of the outer flange or step 18 is equally again as great as the diameter of the inner step. The outer step 18 is blended to the adjacent section of the inner step 17 by a radius 19, the inner 70 step 17 being blended, in turn, to an adjacent section of the body-portion 14 of the tool by a radius 20.

The specific dimensions of the tool and, in particular, the transverse dimensions of its stepped 75 enlargement are In large measure determined by the size of the tube being cold worked and the degree of cold working necessary to accomplish the required rigidity. For the purpose of illustration, however, the diameter of the body of a tool for cold working an internally-finned tube of substantially three-quarter inch outside diameter and one-sixteenth inch wall thickness is from about 0.005 inch to about 0.010 inch greater than the internal diameter of the tube, the diameter of the inner annular flange of the stepped enlargement being about 0.031 inch greater than the diameter of the body 10 and the diameter of the outer annular flange being about 0.031 inch greater than the diameter of the inner annular flange. The longitudinal dimension of the stepped enlargement 16 is preferably a fraction of the over-all length of the tool and in the embodiment shown is substantially 10% thereof.

As pointed out above, the tool also embodies means for receiving the internal fins of the tube and for straightening the fins during the coldworking operation on the tube. To this end, a plurality of longitudinal recesses or grooves 2i are formed in the tool, each recess extending throughout the combined lengths of the bodyportion 14 and enlarged head 16 of the tool and each groove being spaced apart circumferentially from its neighboring grooves a distance corresponding to the circumferential spacing of the internal fins 11 of the tube, whereby the fins are adapted to engage therein. The body of the tool is thus divided into a plurality of longitudinal sectors equally spaced circumferentially about the longitudinal axis of the tool. Moreover, each groove 21 corresponds in cross section substantially to the cross-sectional dimensions of the internal fins 11 of the tube, although the depth of each groove is slightly greater than the transverse dimensions of its respective internal fin 11, as shown clearly in Fig. 3. Consequently, as the grooves are moved longitudinally relative to the tube, the internal fins of the tube will be straightened.

By practicing the above described method by means of the particular tool shown and described herein, internally-finned tubes are adapted to be transformed from their relatively soft and unstraightened condition as extruded to a per⁵⁰ fectly straight tube which has been cold worked by radial expansion to acquire a relatively-high degree of rigidity. Moreover, during the coldworking operation performed by the tool 13, the tube 10 has been sized to a predetermined di□5 ameter to within very close tolerances while the internal fins 11 of the tube have been straightened.

The invention may be carried out in other specific ways than those herein set forth with00 out departing from the spirit and essential characteristics of the invention, and the present embodiments are, therefore, to be considered In all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.