Designing parts to permit manufacture from tubing, or tubing in conjunction with other material forms, can provide dramatic cost reductions. At the same time, quality and function of parts can be improved.
It is no secret that today's designers are under greater challenge and pressure than ever before. Competitive pressures for cost reductions and pressures of Federal and State safety regulations, and the manufacturer's desire for product reliability have imposed design conditions to degrees not previously encountered.
More and more designers are turning to miniature metal tubing, since tubular parts can provide structural integrity superior to that achievable with solid cylindrical and most shaped sectional elements. There is virtually no fabrication operation that cannot be performed on small diameter metal tubing while maintaining rigid tolerances. In addition, tubular parts can simultaneously provide functional utility for transmission of liquids, gases, wiring and other functions.
Because of many design factors, including today's emphasis on miniaturization, the use of such tubing in component manufacturing has been increasing at a rapid rate. Most tube mills recognize that there is very limited background information for design engineers on tubular fabrication and therefore are willing to work closely with designers or manufacturers on alloy selection and the best method of producing a particular part.
The benefits of tubing define the areas where the designer should consider the use of tubular parts. Generally speaking, tubular components should be seriously considered where:
- Difficult miniature shapes are required
- Close tolerances are essential
- Reduced size or weight is desirable
- Maximum electrical and mechanical properties are required
- Smooth, burr-free surface finishes are needed
- Reducing materials waste would significantly reduce costs
- Alternate production methods require expensive machining or finishing operations
- A specific alloy is essential because of the particular application.
Guidelines, however, are just that--"guidelines." They can point the way to investigating the use of small tubing. It is up to tube mill technologists to maximize these benefits for the manufacturer. Today's advanced tube mill has not only the capacity to produce fine tubing, but also to produce sophisticated tubular parts and assemblies, utilizing other components such as wire, plastic inserts, and so forth.
The secret is to know what can be done with small tubing, and then work to prevent over-design, which is unnecessary and often costly. For example, if a light flare will substitute for a chamfer, dollars will be saved in processing because the flaring operation is not as complicated. 0r, to take another example, if an OD tolerance of .002" to .003" will suffice rather than a tolerance of .005" to .001", machine speeds can be increased and unit costs reduced.
Before moving into the area of fabricating operations, it is well to stress that small tubing not only has obvious material saving advantages, but also has great strength and stability, exceeding those of solid cylinders by a significant amount.
The differences can be exemplified via the laws of physics: a tubular element in which OD equals do, ID equals di, w equals wall; and a solid cylindrical element or rod of diameter dr. One can set up mathematical equations to compute bending and torsional stresses under any practical load condition for the values of do and di such that the cross-section of the tube is identical to that of the rod, or (do2-di2) = dr2.
Thus, when using a tube, increasing the diameter decreases the stress load. For example, increasing the tube diameter to twice the rod diameter decreases stress by about 70%. These facts can be exploited by recomputing the tubular element cross-section necessary to achieve a predetermined level of design torsional stress. One can then repeat the calculations and derive the tubular cross-section necessary to maintain the same bending or torsional stress in terms of tube OD to rod diameter, and determine the resulting material savings.
It should be emphasized that the large diameter tube is easier to process, hence there is a potential for reduced costs. These facts are significant, yet indicate only part of the tube advantage story; another may be in the making of the part. Depending on just what type of part or alloy used, it might be drilled, machined, and cut-off on a lathe. Or, it might be better to use an automatic screw machine for drilling, turning and cutting. In this latter case the time required to mechanically cut the material--and also the scrap material--must always be considered.
Therefore, some of the basic design considerations where tubing has a natural advantage are:
- structural integrity
- functional utility
- material savings
- machining time
There are basic tube fabrication operations which lend themselves to high speed, automatic techniques. These will be considered first, then followed by a description of those techniques requiring more skilled detailed processing.
This is the first and perhaps the primary technique, and from the standpoint of accuracy and production rate, cutting has great value in manufacturing methods. Depending on dimensions and material to be cut, there are several employable methods which affect both quality and cost.
Note that there are similarities in end result, and also note the differences in production rate and deburring requirements. Of the seven methods described for cutting, only three yield essentially burr-free ends. If any of the other four methods are used, an additional operation is necessary to remove the burr, which adds to the cost. In the case of very fine tubing, where the ID is .250" or less, removing ID burrs becomes more difficult. The cut tube, independent of cutting method, serves as the basis of all further forming operations.
This term describes an angular expansion, usually of both ID and OD, the purpose being to provide a lead-in to the ID Flares assist in automatic assembly and/or for sealing. Flared tubes can be made as small as .025" OD. They are generally formed with a flaring tool which has a radius shape as opposed to an angular shape, and the flare diameter is normally 1.3 to 1.4 times the tubing diameter.
Differentiating from a normal flare, which has a radiused contour, is a set flare which has a specific conical configuration, the purpose being for definite sealing--the cone fitting into a corresponding matching part.
Flanged tubes are produced using forming tools which generally produce a 90 degree flange, creating a relatively accurate OD, depending on wall thickness. There will always be at least a .005" radius under the flange. Since the flange diameter is formed, not trimmed, flange diameter tolerance must be at least plus or minus .003". Depending on alloy and OD size, flanged tubes can be produced with wall thicknesses from .005" (sometimes less) to a maximum of about .020".
Tubes can be supplied with one or both ends rounded or tapered to close the end(s) down to approximately 50% of the OD. Although the preference is to have a full spherical radius, smaller radii can be supplied, depending on the wall thickness of the tube. The inside surface is free-formed, not supported, on production equipment so control over ID corner radii is limited. Round end tubes can be produced from tubing .030" OD and up.
Tubes can be flattened at any point along the length, but are normally flattened at the end of the tube. The flattening may either be on the center of the tube or offset. In addition, a flattened tube may be trimmed or pierced, for use in applications such as connectors. All flattening is done generally without restriction of material, and the flat width and thickness is relative to the tubing size selected. The flattened ends are free formed and therefore require trimming for certain applications.
Following are examples of difficult and involved fabrications of tubing, and tubing used in conjunction with other components. Many more possibilities exist, but these should stimulate the imagination as to what can be accomplished in the realm of small tubing.
In practically all cases, the parts presented can be formed automatically--the rate of formation being dependent on specific geometries and alloy used.
More complex, yet not expensive, tubular part cross sections are generally developed in the tube-drawing process by controlling the tools. Close tolerances can be achieved, but the degree of tolerance will also determine the cost.
There are effective ranges for coiling and bending. In general, a tube can be bent satisfactorily around a radius 2.5 times the OD of the tube without internal support. For tighter bends, a special mandrel is used to avoid kinking and collapse. This raises costs, but makes it possible to bend tubing around a radius as small as the OD of the tubing. A good rule to follow is to avoid compound bends wherever possible.
A tubular part may involve a multiplicity of secondary operations to achieve the desired result. Note that bulging, tapering, flattening, grooving or detent, and end-coining can all be involved.
Bulging, tapering, and closing the end of a tubular part are demonstrated in the previous examples. The closure and its shape can be varied depending upon the application. Some closures can be hermetically sealed.
A modern tube mill is well-equipped to handle assemblies made from tubing. Utilizing its specialized forming machinery, the mill can cut and form tubular parts to make them an integral part of simple and even complex sub-assemblies.
Summarizing the advantages to be derived from using tubing for parts formation, and the basic operations that can be applied:
- Parts can be produced at relatively high production speeds, with excellent uniformity, because OD, ID and concentricity are implicit in the tubular product, and accuracy is essentially unaffected by cutting and forming techniques developed and used. Because of this uniformity, parts can be hopper-fed into special equipment for additional automatic or semi-automatic operations.
- Tubing can be manufactured in various geometries, such as triangular, rectangular and oval cross-sectional shapes. Parts can subsequently be produced for a virtually unlimited number of applications. For example, telescoping tubes with square shapes might provide a means of registration between movable parts.
- Tubular parts can be cut to length and have holes pierced through one or both walls automatically. They can be flared, flanged, radiused, swaged, bulged, corseted, dimpled, bent and given a host of other secondary operations. For example, slotting and crimping operations can be automatically performed in the end of a tube, before heat treating, to create a connecting device.
With the technological advances that have been made in tubular drawing and fabricating in today's tube mill, the potential for tubular components has expanded. Tube mills can produce both simple and sophisticated tubular components to close tolerances. The designer's imagination is perhaps the only limitation in tubular fabrication.
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