I think that you're going about it backwards, maybe not. Way back when I first started working out, I used nothing but dumbbells, then I added cable exercises. I have always been a huge advocate of dumbbells for getting maximum results. Then, as I was lifting more and more weight, I got to the point where the dumbbells were too heavy to control. Then, I switched to mostly machines and cable exercises. Also, I would slide a bench under the squat rack to do my chest workout, using the bar with plates loaded onto it.
Split your routine in half, so that you are doing half the body parts on Monday/Thursday and the other half on Tuesday/Friday. Take Wednesday and the weekend off so your body can recover.
Everything: 3 sets of 8 reps, with the 1st set using a moderate weight, the 2nd set a heavier weight, and the 3rd set really testing your ability to get the 8 reps.
On almost every exercise, always practice keeping your knees bent a little throughout the entire exercise. It takes the strain off your back.
1) Abs: Whenever I saved my ab work until last, I often wanted to skip it because I was tired. So, I learned to do it first. Do them every day you work out. It gets them out of the way, and it warms up your body.
2) Chest: Flat presses, Incline presses, Decline presses. Form: When you are in position to start lifting the weight, there should be an imaginary line from your left elbow, through your shoulders, to your right elbow.
3) Back: Bent-over rows, using a dumbbell (one hand and one knee on a flat bench). Pull ups (Chin ups). A back machine (where you sit down and pull two bars towards you) is good too.
4) Shoulders: LIGHT-weight dumbbell raises, up and out to the side, doing both arms at once (Don't swing them in front of you. They should be coming down to the sides of your legs). LIGHT-weight dumbbell raises out and up in front of you. (Don't swing them by your sides. They should be coming down to the front of your legs).
5) Biceps: Dumbbell bicep curls.
6) Triceps: Cable exercises (the opposite movement of the biceps curl, where you are pushing the weight down and away from you.
7) Legs: Leg curls, Leg presses, calf raises.
Choose 3 of the above body parts (plus abs) to do on Monday and Thursday.
Then, do the other 3 (plus abs) on Tuesday and Friday.
Variations on the above workout routine is what I developed for myself and has worked well for me. Good luck !