Thanks guys.
The battery has been recharged, so I don't think it is the problem, but I will check the cables.
When the engine stops cranking and the starter makes a grinding noise, I notice that sometimes the starter gears are extended and have not meshed properly with the flywheel gears. When I loosen off the starter the gears retract again. This makes me think that the alignment could be an issue
Thanks for that link Denny, it was very helpful. How can I extend the starter gear to measure the space between the two sets of teeth?
Once again, I really appreciate your guys advice and patience.