If you dont have a spotter, just use dumbells on a normal bench.
When you use the "squat machine" the weight you are using is a "fake" weight. you dont have to use all your stabilizers...so the exercise becomes easier.
It is said that you can cause an injury by using the smith machine (thats what im guessing it is). Something to do with sholders operating on planes it doesnt naturaly. after time it will cause problems.
to really challenge your stabilizers try dumbells on a swiss ball....that'll kill ya.
what are stabilizers for? a friend if mine swears by the smith machine....thats all he benchs on. he came over to my house to have a work out. This was the first time he had ever done a free weight bench. He stared with a weight about 3/4 of what he uses normally to startwith...... shit man he almost killd himself almost tore his muscles.........he couldnt keep the bar from swaying back a forward almost droped the bar on me.