face finish is not suitable for gauge aluminum bronze tube chrome plating, but is good is .0403 inch, and an 18 gauge enough for powder coating. steel tube, like Ron Guthrie If chrome plating is required, was ordering, is .049 inch. further surface finishing is reNonferrous alloys, copper quired. The material will likely base, and aluminum, use have a higher tensile strength, Brown & Sharp measureand the price will be higher. ments, and steel uses BirFor more demanding requiremingham-Wire Gage (BWG). ments, both cold and hot rolled As a result, 18 gauge BWG tubing can be drawn over a measures .049 inch. Brown & mandrel for better finish and Sharp is .0403 inch and the closer tolerances. It is often sheet steel manufacturing referred to as DOM tubing, standard for 18 gauge is .0478 drawn-over-mandrel. inch. All of which differ from The ends of these stainless steel tubes have had metal burrs Hot Rolled Seamless (HRS), wire gauge dimensions. removed after cutting. tubing drawn-over-mandrel Nobody seems to know why (DOM), and HY or High Yield welded tubing are not prowe contend with all these gauge differences. For serenity’s cesses normally required for ornamental work. sake, we accept those things we cannot change. However, HRS is done by piercing a hot billet, where heavy walls are for your own protection use decimal dimensions, not gauge, required and the steel can be a high-strength 8630 alloy with when ordering tubing. molybdenum content, which is a type of steel typically used Mr. Campbell is a senior writer for Fabricator. for automotive bushings and bearing rings. Tubing DOM will have no evidence of seams on the ID. This type of tubing is specified for hydraulic applications such as cylinders and oil lines. HY welded tubing has high strength applications, typically over 50,000 p.s.i. and is suitable for scaffolding or outside advertising supports. Tubing buyers often error in ordering the wrong specification and/or the wrong grade. On pipe there’s a 12.5 percent undersize and a 10 percent oversize tolerance on the OD. Pipe manufacturers will aim to be 8 percent undersize. The economic reason for this is that hey buy strip by the pound and sell pipe and tubing by the foot. Terry Driscoll, President of Custom Iron Inc. at Zumbrota, MN, said the only problem he knows they’ve had with square tubing was when the inside weld seam was too close to the corner radius. According to Jim Sanborn, the reason for Driscoll’s problem is the way in which the square tubing was formed. The higher quality square tubing starts with a round welded tube that is gradually rolled on all four sides to the dimension desired. This process gives clean perpendicular sides and sharp corners. The weld shadow ends up in the middle of a flat. Less expensive square tubing is made by pre-forming the strip, bending it so that two edges meet at a corner. The weld is made along the outside of that corner, which is why weld flash appears in a corner on the inside. In determining the wall thickness of square tubing you should measure the thickness of two flats and divide by two. With round tubing you have to use a ball-nosed micrometer, measure in two places, 180 degrees from each other, and divide by two. Tolerances will vary with the tube diameter, wall thicknesses, and method of manufacture. For good reasons, it’s safer to order by decimal thicknesses than by gauge. For those of us unfamiliar with tubing it’s hard to believe that there’s a difference between gauge thicknesses of steel tubing and nonferrous tubes. The wall thickness of an 18 Fill in 157 on Reader Service Card January–February 2003 n Fabricator
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