Tube and Pipe Bending Basics

Center-line Radius (CLR)

When referring to the radius of a die, we are referring to the center-line radius (CLR) that results from bending. CLR is the distance from the center of curvature to the centerline (axis) of the pipe. If you’re having trouble translating that last sentence into English, check out the picture above. Using the image below, you can see how the radius of a bend has a significant impact on the resulting part.

 

When choosing a die, factors that will affect the CLR you choose include the material type and grade to be bent, wall thickness, the OD, the application or design of the end product, and overall appearance required.

Achieved Center-line Radius (Achieved CLR):

This value is the radius you obtain when bending with the die. Our dies have the CLR inscribed on it. Material will spring or stretch open after it is bent; think of coiling a wire around a pencil – release the wire and it will fall off the pencil. Because of this stretching, you will find that the CLR of a bent piece of material is slightly larger than the CLR listed on the die.

Calibrated Center-line Radius (Calibrated CLR):

When using Bend-Tech Software, this value is used to specify how much stretching and compressing will occur during each bend. This value is obtained using a calibration test, by bending a test piece of material and measuring the resulting legs. The software then uses the stretch/compression data to properly calculate the amount of material in each bend as well as the proper bend locations.

Outside Diameter (OD):

The outside diameter (OD) is the distance across the extreme outside dimensions of a tube or pipe.

Inside Diameter (ID):

The inside diameter (ID) is the longest distance across the inside dimensions of a tube or pipe.

Wall Thickness:

Wall thickness is the distance between the outside diameter and the inside diameter of the pipe measured in thousandths of an inch. For pipe: Schedule determines wall thickness. It’s important to use accurate calipers when measuring this value; a tape measure or well-trained eye are not accurate enough in this case. The relationship between outside diameter and wall thickness has an important bearing on selecting a die.

Degree of Bend (DOB):

This relates strictly to the number of degrees required in a particular bend.

Springback:

The cause of springback is uneven stretching of material during a bend. The centerline material is trying to return to its original shape, yet is constrained by the unyielding material on either side. The effect is noticed when the material is taken out of the bender.

Springback must be compensated for by adding the springback factor (the number of degrees that a material springs back) to the desired degree of bend. You can easily figure out the springback factor by performing test bends. Springback is not a constant factor for all materials and can even change within materials of the same OD and wall thickness. It’s extremely important to perform test bends on each batch of material you buy.

Example:

If you want to bend a piece of tubing to 90 degrees and you determined in your test bends that the material will spring back 7 degrees, you must bend the tubing 7 degrees past 90 (97 degrees) to achieve your desired 90-degree bend when you remove the tubing from the bender.

Tube vs. Pipe:

When it comes to tube versus pipe, there’s one thing you really need to know: 1-1/2” tubing is not the same as NPS 1-1/2 pipe. For 1-1/2” tubing, the actual outside diameter (OD) is 1.500”. For NPS 1-1/2 pipe, the actual outside diameter (OD) is 1.900”. This is true for all sizes of pipe less than NPS 14. That means that if you have 1-1/2” tubing and NPS 1-1/2 pipe, you will need a different die for each size. Our benders are rated for bending Schedule 40 Pipe between NPS 1/4 and 2. The chart below demonstrates some dimensions for Schedule 40 Pipe up to NPS 14. If you’re still confused, check out the description below the chart.

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) for diameter based on inches and aschedule (Sched. or Sch.) for wall thickness. NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). Based on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below. For NPS ⅛ to 12 inches, the NPS and OD values are different. For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD. The reason for the discrepancy for NPS ⅛ to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thickness standards at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD. For a given NPS, the OD stays fixed and the wall thickness increases with schedule.

Pipe is sometimes used in structural applications like handrails, but its intended use is to transport substances which can flow (i.e. liquids, gases, (fluids), masses of small solids), so the critical dimensions are inside diameter (ID) and wall thickness. The ID determines the ability of the pipe to carry materials. ID, coupled with wall thickness, determines specifications like burst pressure. Like tubing, pipe is manufactured in different ways for different needs and applications. There are three ways to manufacture pipe.

(1) In centrifugal casting, a permanent mold is rotated continuously about its axis at high speeds as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. (2) Welded pipe is manufactured like ERW tubing and (3) seamless pipe is manufactured like seamless tubing.