How to interpret Baxter forces

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I want to keep moving the baxter mounted with a 2 parallel jaw gripper into a flat table and stop motion when i sense that force exerted on the baxter gripper due to table contact is increasing beyond certain limit. For this purpose, I want to get the force exerted by Baxter along gripper axis when its gripper is thrust into a flat table. Logically there should be a force in z-axis.

So I use the command:

rostopic echo /robot/limb/right/endpoint_state

Part of the output it gives me is:

wrench: 
  force: 
    x: 0.000755130895413
    y: -0.922724783421
    z: 0.56328082085
  torque: 
    x: 1.53568863869
    y: -0.00801796652377
    z: 0.366737604141

It gives me different force values arbitrarily and sometimes close to zero even when there is tight contact between gripper and table. Even when Baxter end-effector gripper is in mid-air then also it gives me a non-zero force in z-axis. I cannot understand how to get the force along the gripper axis using ROS in baxter in a way that makes sense, eg. when baxter is in mid-air force should be zero at the end-effector, right? Is there no such force sensing capability in baxter?