Pipe and tubing are formed by taking strips of steel from a coil and progressively rounding up the edges until a tube is formed. Then a welding process welds the edges of the strip together. Depending on several factors, speed of the mill welding equipment, skill of operators, etc, this seam protrudes to varying degrees inside the tube. Incidentally, square tubing starts out the same way, then after the welding process, the sides of the round are progressively rolled in to form a square or rectangle. Keep in mind that the tube mill process runs a continuous strip of coil that is cut to length after it is welded and all this happens at around 300' per minute.
Mechanical tubing, at first glance appears to be a thick walled pipe. On closer inspection, you will notice it has no seam. This is because mechanical tubing is formed very differently. When looking at mechanical tubing specs you will find a much closer tolerance in inside and outside diameters, and no weld seam. In fact, you would be hard pressed to state whether a shorter piece of mechanical tubing was tubing or a part turned and drilled in a lathe. This makes it an ideal receiver for bushings and bearings, if you can find a suitable size. Because of the additional processing, and the fact that more pipe and tubing is sold, mechanical tubing is much more expensive to buy and more difficult to locate off the shelf.
