November 21, 2002
Bending tube and pipe has become increasingly popular throughout the metal fabrication industry. Bending inherently saves money over other fabrication methods, such as the cutting and welding of standard fittings or the use of cast elbows. However, fabricators can reduce costs further by following simple procedures while designing or ordering bends.
Use common sizes, wall thicknesses, and grades of material. Tube and pipe charts available from any tube and pipe supplier show all sizes that can be made. Many of those sizes are available only at a premium because they are so seldom produced. If you are not aware of the sizes and grades commonly produced, check with a tube or pipe supply house to see if the product you have in mind is normally available.
Examples of tube and pipe sizes found on charts that cost more because of their rare usage include 4-1/2 inch pipe (measures 5 inches outside diameter [OD]), 11-inch-OD tubing, 9-inch pipe, or 7- by 4-inch rectangular tubing. The use of odd-sized materials can double your material cost and easily double bending cost because your supplier may have to build or modify tooling to perform the odd job.
To bend tube and pipe, almost all bending machines require more material than the usable arc in the finished bend. These bending tangents, or the straight piece of the tube on each end of the bent arc (see Figure 1), are required by the bending machine to apply leverage to the material during bending.
These required tangents vary but range from two to four times the tube and pipe's OD on each end. Let these required bending tangents work for you and not against you.
If the bent part will be attached to more straight material in service, design the bend so that these bending tangents remain on the bend and are not removed by the bender. This reduces labor at the bender and the amount of straight material needed at the next fabrication step.
This also reduces the cost to attach the bend, in terms of labor and welding, to the rest of the fabrication because this extended end of the bending tangent is inherently less distorted by the bending pressures and will mate with tube and pipe of the original size with little or no roundup or rework.
Another reason to consider tangents when designing bends regards the standard tube and pipe lengths commonly available. Before designing bends, determine the common lengths available that will work for your project, then design with these lengths in mind to avoid higher than necessary material costs.
To get the maximum bent arc from the available lengths, subtract the required bending tangents from the available length to determine the maximum economical bent arc per piece. Otherwise, additional material must be welded to the material to be bent to allow for these required bending tangents. This additional material cost will be minimal compared to the welding stub cost. Experience has shown that the wise use of bending tangents generally can save from 10 percent to 50 percent on your next bending job.
Today's modern bending techniques allow almost any tube or pipe to be formed to almost any desired bending radius. However, designing bends to some radii will cost more than to others. Some radii require custom-built dies or expensive machinery and labor to achieve the bend. Many bending applications have some flexibility in the radius they can use.
When possible, use a common bending radius and adjust the tangents on each end of the bend to yield the overall required dimensions. Small, tight radius bends should use the largest radius possible. Typically, the bigger the radius, the easier the bend is to make without serious deformation, such as ovality or wrinkles in the material.
If the radius can be big enough, design it in increments of feet, i.e., 12-inch radius, 24-inch radius, 36-inch radius. If your project does not allow such a big radius, design the radius as a multiple of the nominal outside diameter of your material, such as 3R, 5R, or 10R, where R stands for the radius factor.
In other words, if you are designing bends in 2-inch pipe (2.375-inch OD), a 3R bend would be 2-inch (pipe) X 3 (radius factor) = 6-inch radius; a 5R would produce a 10-inch radius; and a 10R would produce a 20-inch radius.
Designing bend radii around the common dies available through most bending operations will also reduce the cost of your bending project.
When designing bends with seamless tube or pipe, consider using material with a longitudinal weld seam or welded material over seamless material. Welded material generally costs less than seamless material.
Because of higher-quality welding and nondestructive examination techniques used in the tube mill industry today, many designs can use the less expensive welded tube and pipe in places that previously required seamless material. Again, experience has shown that generally this substitution can save from 25 percent to 40 percent on material cost.
Know the proper terms and needed information to communicate your bending needs to your bender. Lack of good communication can be costly when ordering any custom-produced product, especially tube and pipe bends. See the Sidebar for a list of common bending terms.
A complete description of a tube or pipe bending job includes:
The end preparation specified on bends, whether the ends have a specified tangent or a specified condition such as a beveled end or a square cut end, can determine a large part of the cost of the bend. If the bends will be cut by the next fabricator, ask what is the least expensive end preparation designation. This prevents you from paying for an end preparation that won't be used.
If tangents are required, specify a minimum tangent as opposed to an exact tangent. Requesting an end preparation combined with a requirement that no straight tangent be on the finished bend, known as zero tangents, is the most expensive end preparation. This requires cutting the bend after bending, rounding the ends because they have some ovality caused by the bending pressures, then applying the end preparation.
Avoid minimum charges on small orders. Many bending operations have standard minimum charges, such as a setup fee for each bending machine. If you order only one bend, your unit cost for the bend can be three times or more than your unit cost if you had ordered 10 bends or more. So consider grouping orders of different bends, or if the bend is a repeat need, consider increasing the order quantity to reduce the unit cost.
Good suppliers will offer cost-saving ideas. However, sometimes you have to start that conversation. This applies especially to custom-produced products like tube and pipe bends, because minor changes in the bend can yield cost reductions.
Ask your bender what changes in your bend would reduce cost. In many cases, you can avoid costs that are not adding value to the bend from your perspective.
Consider having the bender arrange the shipment of the bent product from the bender's plant to its next destination. Shipping tube and pipe bends that have irregular shapes can have its difficulties. The bender's experience in shipping such items can help reduce shipping costs and save you time.
In a global manufacturing environment, fabricators must constantly look for ways to reduce cost, avoid waste, and improve effectiveness. They can achieve this if they make informed decisions. Using the information presented here and following these steps, they can reduce the cost of designing and ordering bends in their tube and pipe.
TPJ - The Tube & Pipe Journal® became the first magazine dedicated to serving the metal tube and pipe industry in 1990. Today, it remains the only North American publication devoted to this industry and it has become the most trusted source of information for tube and pipe professionals. Subscriptions are free to qualified tube and pipe professionals in North America.