Using a step drill bit is ideal when you are working on lexan, plexiglass, and fiberglass because it will not break or damage them like a regular twist drill will. With these materials, it will almost always be much easier to opt for a step drill over a reamer. The same holds true for sheet metal. Not only is sheet metal very thin, which gives you ample control over the size of the hole that you create; but it is also relatively thin.
There are some limitations to what a step drill bit can do though. The first thing to keep in mind is the depth that you will be working with. These types of bits get progressively larger the further into the surface they go. If you are creating a very deep hole, then there is a chance that the bit will make the top portion larger than it should be. In general, you will need to use a reamer if the material is more than ¼ inch thick, although some people argue that anything beyond an eighth of an inch thick is too much for this type of drill bit. Keep in mind that the thicker the material, the more lubricant you will need to use.
As you can see, there are a variety of situations in which a step drill bit can completely replace your reamers. However, when looking at the big picture this isn't the case. There are still plenty of scenarios in which using a reamer will be more effective. Most notably, when the hole that you are creating is going to be fairly deep or the material that you are working with is too thick. The final conclusion is that it is ideal to have both in your toolbox.
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