Even when you have chosen standard tube dimensions, and standard bend radii and kept to a limited number of different bend sizes, there are still bend configurations that will cause problems for tube manipulators and will often translate into higher costs for you. There is normally a way to produce most tube designs but some can be far more complicated to manufacture than you might imagine. In some cases, the only way your tube will be able to be produced is in sections and to be joined. A good weld joint, properly dressed will be all but invisible, but obviously adds to the cost, so if you can avoid it so much the better.
It would be impossible to list every tube configuration that might be problematic but a few possibilities are;
“Knots”
If there is a loop of the tube with the tube then passing through it in some way then it is highly unlikely that it can be produced on a standard CNC pipe bending machine. It is sometimes possible to produce this sort of thing on a hand-bending jig in one piece, but you will definitely face tooling costs, and even this will be limited to smaller diameter tubes that can be hand-formed. Alternatively, it may have to be made in separate pieces and joined.
Bends close to and above 180 degrees, and tight coils on stiff material.
Imagine that we want to bend a tube into the shape of something like the Greek letter α, with one “leg” of the tube crossing over the other. There are ways to get the CNC bending machine to bend the tube back on itself, and then further. The problem is that the tube will then foul itself on the carriage or other part of the superstructure of the machine. If the bend has a large enough radius and/or if the tube diameter is small enough then there will be enough flexibility to allow the tube to be lifted up and over any part of the machine and avoid fouling. Failing that it would probably be necessary to make it in sections and join.
Long bend lengths in certain configurations
Most draw bending machines are “right-handers” which means that when looking down the machine towards the bending head the bend will be to the right. So now imagine making the first bend with a length of, say, 2 meters. If the next bend requires that the tube is rotated clockwise there is a problem because the end of the 2-meter length will hit the floor. There are three possible solutions. In most cases, if the bends are started at the other end of the tube there will not be a foul condition. Alternatively, you may accept having the component made in two pieces and joined. If that is not a possibility you might need to find a tube manipulation company that has a left-hand bender!
In some, very rare, cases you might manage to design a tube that requires a bending machine that has combined left and right hand bending capability.