You can cut hardened metal bolts that a Hacksaw can’t with a Dremel using their disks.
To cut and shorten a bolt with a Dremel, first thread a nut onto the bolt past the point where the cut will be made. The nut helps protect the threads and can straighten them after cutting. Mark the cut line with a marker. Clamp the bolt firmly in a vise at the end side of the threads you are discarding. The nut should be beyond that point because you will use the side of the vice clamps as the guide to cut a straight line and the nut protects the good threads.
Install the metal cutting disk and wear safety glasses. Hold the cutting wheel square to the bolt. Let the disk do the cutting; do not force it. Work slowly around the bolt until the unwanted section drops off.
When cutting, ensure the clamped bolt’s marked cut line extends beyond the vise jaws. Hold the Dremel with both hands and keep the cutting disk as close to 90° as possible. Don’t push hard because the disks are brittle and excessive pressure can easily crack them.
For the filing step, first flatten any rough high spots on the cut end with a small flat file or grinding stone to clear and clean the threads. Finally, unscrew the nut you installed before cutting. As the nut is unscrewed, it helps reform and clean the threads so the bolt will start into a nut or threaded hole more easily.