Hi, 

If you use Robotiq vacuum grippers (either EPick or AirPick), it is possible to use "Grip Check" function to differentiate part detected or not detected. This way you can execute code accordingly.

If you control yourself a pneumatic system, you can use Robotiq "Force Copilot" to detect the part. It can be done while inserting in the CNC or before. Simply use a "find surface" with a maximum distance where you can expect to get a contact. Use low force detection to be gentle on the grip and on the part you test the presence. In other ways: You can add a contact sensor to detect the part. You can track picking and placing with variable. You can have a "Machine B " input targeting if you drop to machine b or if it is down and you drop else where before re-picking. 

In brief:
use Robotiq vacuum grippers and grip check 
or
manage the workflow with variable at vacuum activation and vacuum deactivation and an IF statement 
or 
(If the robot control the vacuum with a Digital Output) manage the workflow using the output as the flag of "part presence" or "part absence" and an IF statement. 

As the master of the robot you decide what he will do. The "IF" and the "Switch-Case" statements are the way you can, in context, use some code and/or skip some other code. 
In this specific case, you want to skip placing in the Machine B and you want to add an "else" to the "if" to place the part onto a temporary deposit point. 

Using Force Copilot "Find surface" feature to protect the robot from falling in protective stop. You make the end of the place travel with it to detect if it gets in contact with a part before the target distance. 

I believe you now have plenty of solutions to try. Use the one that match your needs. 

Regards, 

David Gouffé
Integration Coach
Coach en intégration