How big (or small) is the OD and ID of the tube I need to bend? Generally speaking, the larger the diameter, the easier the tube is to bend. However, the thinner the wall, the more likely the tube wall is to collapse while being bent.
How steep of a angle do I need to bend? Obviously the greater the angle of bend, the more difficulty you will have in maintaining the wall integrity. We recently did a job for a customer who needed a 135° bend (in essence a V shape.) In this job the issue was not how to maintain the tubes ID, but how much could we minimize and control the collapse of the tube walls during the bending process.
How long can the radius be? The compression / tension forces on the tube walls are exponentially reduced as the radius increases. So your success rate will significantly increase as the radius increases.
Here is an example of the inter-relationship of all of this. Even though stainless steel hypodermic tubing is a full hard tube, when you get down into the very small ODs like 0.013”- .032” the tubing does not have enough tensile strength to remain at the radius they are formed (they spring back). So you wind up having to bend them at a much smaller radius than your finished radius, thereby compounding your problems.