Many people who aren't strong enough to do bodyweight pull-ups resort to using some type of pull-up machine that assists them by pushing up on their feet or knees. Sorry, but those machines won't make you better at regular pull-ups.
For one thing, most of them don't go high enough to allow the chin to clear the bar, or where the bar should be. Second, they don't work on the capacity to move your body in space since the body is fixed (no tridimensional movement). Pulldowns don't help much either.
Besides using bands to assist you, there is one machine that can help you build your pull-up strength, if you use it right:
Most people rely subconsciously on momentum and body swing to get the weight down in a lat pulldown machine. This is why when they try to transfer to pull-ups they can't do it. You need to get stronger muscles, not stronger at taking the weight from point A to point B regardless of how it gets there.
The Hammer Strength machine (and others like it) is more fixed, which reduces the use of lower back swinging. Adding bands compensates for any momentum that can be created by increasing the difficulty of the movement as you go along. This will also strengthen the finish of the pull, which is normally the weakest point in a chin-up or pull-up.
Do these rows with minimal momentum. Don't try to accelerate. Feel the muscles doing the work every inch of every rep.
Perform sets of 8-10 for a few weeks, then heavier sets of 6-8 when your mind-muscle connection gets better.