Describing a bent metal tube sounds simple enough. It’s grey/silver. It’s metal. It’s tube shaped. It has a curve. But you could take a completely different metal tube, and all of those descriptions would apply to it as well.
To order the correct metal tubing, you need to get specific.
We’ve talked to countless manufacturers who have struggled to describe what they need. Often times, they really don’t know where to start. They simply know what they need the tubing for. Thankfully, you don’t have to be a tube bending expert to order some metal tubing.
We’ve helped people of all knowledge levels through the tube ordering process, and today, we’d like to do the same for you. Here are a few factors to know when describing metal tubing to a supplier, customer, or your own team.
The Right Materials
It starts with knowing what the tube needs to be made out of. Most metal tubes are going to be made out of steel, copper, or aluminum. Each material comes with its own pros and cons making it more ideal for certain situations than others.
Even if you know the general type of material, it may not be enough. Steel tube can be 1008, 1010, 1020 and even HSLA steel. These designations describe the chemistry of the steel. In the case of 1008 steel, it is 99.92% iron and 0.08% carbon while 1020 steel is 99.8% iron and 0.20% carbon.
HSLA describes high–strength, low–alloy material that uses other elements in addition to carbon to build strength into the steel and is usually followed by a number describing the expected yield strength. HSLA 50 would refer to steel with 50,000 LBS minimum yield. A knowledgeable tube supplier can help identify the optimum material depending on application and required volume.
Some materials are more commonly available commercially and in small quantities.
Type of Tube
As the old fabricator saying goes, there’s more than one way to form a tube. Steel tubing can be extruded or made from flat steel ribbon coil formed into a tube. It can be made of hot rolled steel, cold drawn steel, or worked into DOM (drawn over mandrel).
Cold Drawn Seamless (CDS) is normally made from SAE 1018, having precise tolerances and good surface finish, while Hot Finished Seamless (HFS) normally made from SAE 1026, having less critical tolerances and a scaly finish. It’s not as strong as CDS, and it’s usually an expensive option for specific applications
Drawn Over Mandrel (DOM) tube is an electric welded tube that is drawn over a mandrel and through dies to make as perfect of a tube as possible. It is then normalized to remove all stress and to homogenize the steel and the weld. This is used when precision OD, ID, or roundness are critical.
Electric Resistance Welded (ERW) is the most common tube type for most applications. A strip of steel of the correct gauge to produce the required wall thickness is folded or formed up into a tube via a series of form rollers. A welder joins the two edges of the ribbon as they meet to form the tube OD. ERW tube can be either hot rolled steel or cold finished steel.
The cold finished steel has been rolled while cold to develop stronger properties and a smoother appearance, although with today’s technology, it can often be difficult to tell them apart. The tube weld seam should be noted as welded, flash controlled, flash rolled, etc. Most commercial tube is flash controlled if under 1” and Flash removed or Rolled if over 1” OD.
Shape of the Tube
This is a pretty easy one to describe, regardless of knowledge level. Is the tube round? Square? Oval?
Round tube in the USA is commonly sold to standard fractional sizes like ½”, 1”, 1.25”, etc. With a minimum order of about 10,000 FT, however, any tube OD may be custom ordered.
Square tube and rectangular tube are produced as round tubes and then rolled to the square or rectangular form. Location of the weld seam and radius of the corners must be recognized. There is little choice when buying commercially standard tubing, but there are many options when ordering mill direct.
Oval tubes as a standard commercial shape are few, but they can be ordered if quantity allow for mill direct orders.
If you’d like to learn more about the shape of tubing, make sure to check out this post here.
Diameter of the Tube
The diameter of a tube is, of course, the distance from one side of the tube to the other. The specifics of how its measured can vary between tube types. ERW Tubing is specified by Tube OD and Wall thickness while DOM tubing is specified by tube OD and Tube ID with the allowable tolerances called out.
Wall thickness is important to ensuring your tube can handle the stress of the job its being used for. This measurement is calculated by taking the outside diameter minus the inside diameter and dividing this number by two. This is the gauge of steel used to form the tube. In the US, these are in multiples of 1/64”. For tubes under 2” OD, the most common thicknesses are 0.028”, 0.035”, 0.049”, 0.062”, 0.075”, 0.092”, 0.120” wall and 0.125”.
Degree of Bend
This is to what degree the tube is bent. To find this measurement, you can lay a protractor on the part to determine the angle.
The Centerline Radius (CLR) refers to how tight the bend is. It is the distance from the inside center point of the curvature to the middle (the centerline axis) of the tube/pipe. Empty bending (where no mandrel is used inside the tube to prevent flattening) can be done on most tubing if the CLR is equal to 1½ times the tube OD or larger and up to 10% flattening is allowed for the outside of the bend. Tighter bends, as well as higher quality bends are made using a mandrel to control the tube flattening during the bending process, maintaining a CLR equal to the tube diameter.
Number of Bends
This is a simple yet overlooked characteristic. How many bends does the part need? One? Two? Five?
Plane of Bends
Is the part rotated out of square in a cross-sectional view between bends? If the part will lay flat on the ground, it is likely a single–plane part. If not, then it is a multi-plane part.
Defining the Bends
There are many options to define the bends. When the tube is bent, it is programmed as the feed length to the bend, then the degree of bend followed by the feed to the next bend, and finally, the rotation of the tube to set the angle between the bends and the degree of bend.
Making Sure You Get the Right Tubing for Your Needs
We realize we may have lost some of you during a few parts of this process. Don’t worry. The professionals at Ever-Roll have the experience and industry know-how to guide you through the entire ordering process, ensuring you get the tubing best suited for your needs.
For tube fabrication, contact Ever-Roll today!