Most keys, even brass and nickel-plated key drop, will eventually wear out and break along any of a key drop cuts. But some conditions will speed up the process such as:
1. The key drop has one or more deep cuts which become the "weak point" of the key. After so much use (or just a little abuse) the key will break at these points. Having a deep cut as the first cut (next to the head of the key) can be real trouble but a deep cut in the middle of the key drop can cause big problems too;
2. The key drop is made of a light-weight material not suitable for everyday use. Keys cut on aluminum blanks are a prime example. They will also break at a deep-cut "weak point" only sooner. Identified by their light weight and either grey in color or sometimes bright colors such as red, green, blue or gold.
Usually before a key breaks, one or more tiny cracks will appear by the cut about to give way. You may want to check your key drop right now - at least the ones that you use and depend on every day. If you see a tiny crack being formed by one of the cuts or if the key is bent or misshapen in any way, go to your nearest locksmith or key shop and get another one or two made. And just to be safe, don't carelessly toss the old key away. A new key could be made from the pieces and used to access your property.
BROKEN KEY REMOVAL TOOLS
We locksmiths have a variety of tools for removing broken keys from locks. One of the more common tools I use is made of springy steel that is 5" long, 5/16" wide at the handle and .022" thick. There is a small tapered barb at the end of this tool that will dig or "bite" into the broken key piece so the piece can be pulled out.
It is not absolutely necessary to invest in a broken key extractor tool. As long as the tool you use is made of sturdy metal, is narrow enough to enter the keyway, and has a barb to grab on to the broken key drop you will likely be successful. A scroll saw or jig saw blade should do the trick.
PROCEDURE #1 - Partial Key Broken Off In Lock
When a deep cut gets too weak somewhere in the middle of a key, you end up with the front part of the key in the lock and the remainder in your hand. The good news is the lock is probably in the proper position for key removal (more on that later).
FIRST - unless the lock has been in good working order (with the key going in and out smoothly) prior to the key breaking off, I suggest spraying a lubricant such as WD-40 or Tri-Flow in the lock's keyway first. You want the broken key piece to slip out as easy as possible and a dry or gummy lock will make the job harder.
SECOND - insert the removal tool into the lock's keyway so that the barb (or teeth) can contact the cuts of the key piece to be removed. You should be able to tell how far to insert the tool based on how much of the key you still have in your hand.
THIRD - let the barb "bite" into one of the cuts of the broken key piece and give the tool a sharp tug towards you. The key piece should come flying out.