I love this website but some redditors go a little too far on the anti-HR rhetoric. Yes, it is true that HR is not here to work for you, and there are some very shitty HR departments out there. But, for the most part, what they are there to do is to try and work up a disciplinary plan that appeases as many people as possible. Their goal is to keep the issue under wraps and from blowing up any further.
Some people take that to mean that if you don't threaten blowing the situation up by means of getting lawyers, police, or the local news involved, HR will not take your situation seriously. A few things to keep in mind: Doing these things isn't always going to stun HR into 'submission'. In fact, depending on the circumstances, they may expect that you've retained a lawyer or filed a police report, and declaring that fact up front may not impact the process at all. It will just out you as trying to “win” the HR interaction.
For another thing, remember how HR views you as either an asset or liability? Well, which do you think your workplace is going to view you as if you develop the reputation of being the guy who threatens lawyer and police involvement every time something happens to you at work? You'll be viewed as a liability and shoot yourself in the foot.
If you really want to do things in a way that will maximize your chances of having the other party be seen as a liability in need of reprimanding by HR, then your best bet is to write down what happened as quickly and accurately as possible. Document what happened in any charting system you have (if you work in a hospital, you may already know to do this). Get statements from your manager(s) and other witnesses of the event. Pull CCTV if the incident was possibly filmed. Print out every company policy that even remotely deals with employee code of conduct and highlight portions you strongly believe were violated within the interaction.
HR will appreciate solid proof of infractions and consistent feedback a whole holy hell of a lot more than threats of litigation and “exposure.” Yelling “I'm getting a lawyer!” at the HR meeting isn't the MacGuffin some Redditors want you to think it is.
Also, obligatory: This advice may vary. If your problem with a coworker involved serious violations of conduct, or the breaking of state/federal law, this advice of avoiding the threat of lawyers and police involvement may genuinely not apply to you.