[This article was originally published in the NOMMA Fabricator]
How old were you when you learned that a “2 by 4″ isn’t a piece of lumber that measures 2 inches by 4 inches? Have you ever been told that 1-1/8” pipe doesn’t exist? Using the correct terminology when ordering material (or fittings, tools, or other items that must be used with these materials) can save a lot of time, headaches, and money!
Many products have a name that for convenience only approximates the material’s size. These are sometimes referred to as nominal dimensions. Webster’s describes nominal as “in name only.” In other words, you can’t trust the “name” dimensions in actual measurements or calculations. Differences and difficulties in correctly describing a tube and pipe are common in the metalworking industry.
Pipe is a commonly used material in the ornamental iron industry. Pipe and tubing are not the same materials! Pipe was originally used for the movement of water, and therefore the ID (inside diameter) was the critical dimension. The nominal dimension for pipe is the ID. So, 1½ inch pipe is NOT 1½ inch outside diameter, but instead is nominally, (approximately) 1½ inch inside diameter. Pipe is typically manufactured to looser tolerances and less expensive to purchase.
The wall thickness of pipe is designated by various “schedules,” most commonly Sched. 5, 10, and 40. The exact wall thickness of anyone schedule changes with the pipe size. A 1 inch Sched. 40 pipe has a .133 inch wall, but a 2 inch Sched. 40 pipe has a wall thickness of .154 inches.
Tubing, on the other hand, is typically produced to tighter tolerances and designed for consistent mechanical and structural properties. To further complicate matters, tubing can be manufactured to pipe size dimensions, and some companies market pipe to the fence industry as “fence tubing!” The thickness of a tube’s wall is normally described as a gauge. A specific gauge is consistent regardless of tube OD (outside diameter).
|Name||Outter Dim.||Wall Thickness|
|1 ½” x schedule 5 pipe||1.90″||.065″|
|1 ½” x schedule 40 pipe||1.90″||.145″|
|2” x schedule 10 pipe||2.375″||.065″|
|2” x schedule 40 pipe||2.375″||.154″|
|17/8″ OD x 18 gauge tube||1.88″||.049″|
|2” OD x 16 gauge tube||2.00″||.065″|
Let’s look at a typical example…
John needs to buy a pipe notching tool. When John calls and asks for a “2 inch pipe notcher,” there are actually many different sizes that could possibly fit this description, as shown above.
This is only a partial list! You can see that all of the materials listed are about 2 inches, but depending on the tolerance and precision of the tool or mating part, describing all of them as 2-inch pipe is bound to cause problems.
So, given all the possible tube and pipe sizes and all the various names used to describe them, how is the average person supposed to keep it all straight? Whether you think your material is tube or pipe, when describing it, remember these key points:
- Is a dimension OD (outside diameter) or ID (inside diameter)?
- Is the dimension precise, (taken with a micrometer or calipers)?
- Is the dimension approximate, (taken with a tape measure, eyeballed, etc.)?
- What is the wall thickness/schedule/gauge?
If John had instead described his material as “a pipe just under (indicating that the dimension is approximate) 2 inch OD (outside diameter) by about 1/16-inch wall,” then he would have narrowed down the possibilities. Most likely, he is using 1-½ inch by Sched. 5 pipe.
You probably work with the same few sizes over and over. Look and see what your material supplier calls your material. You may wish to obtain and make a permanent file of “material spec sheets” for each size of pipe or tube you commonly use. This document should show precise dimensions, tolerance range, manufacturing method, coatings, or treatments, etc. These documents can be valuable and even serve as a sales tool. Your customers who scrutinize every detail may want to see why the same chain link fence job can cost so much more, based on the quality of the material ordered.
You probably have reference charts in your office for fractions to decimals, inches to millimeters, etc. If tube and pipe are a routine material for your business, post a chart showing pipe sizes by OD, various schedules, and gauges for tubing. Make sure the employees who do purchasing understand these distinctions and are comfortable in correctly describing the various materials.