Drilling Sheet Metal
First Choice: Punch. Don't Drill
Because they are much thinner than the cutting parts of all but the smallest drills, sheet metals present a special challenge for drilling. The best way to drill in sheet metal is not to drill at all, but to use a punch. A sheet metal punch is basically a round shear. It pushes the part you want removed into a sharp edged hole of the same size and shape, cleanly shearing the hole. A couple of examples are shown below, a Whitney punch and a chassis punch. With a chassis punch, you need to start with a smaller hole for the screw on the punch to go through, but this does not have to be a clean or accurate hole as long as you properly position the punch for the final hole.
In addition to punches, there is another kind of miniature shear called a nibbling tool, or nibbler. This can be used to enlarge a hole or change its shape by cutting out little rectangles of metal, one for each squeeze of the tool. It does not make nice smooth round holes, but is very handy for odd shaped holes and slots. You start in an existing hole (3/8 to 1/2 inch) and nibble out the edges. There are manual nibblers and power versions. The second picture shows the waste rectangles from cutting a rectangular notch in a piece of sheet metal.
Since it is not always practical to have all of the punches you need, we will talk here about drilling with a regular twist drill, which is tricky, and can be dangerous. The most dangerous part of drilling in sheet metal is that it is thin and the drill's cutting edge can, and usually does, slice all the way through the metal on contact, transferring most of the torque to the work. Sheet metal must be very strongly clamped to keep this torque from turning the work into a spinning knife, which can inflict serious damage to your body.
One way to drill sheet metal is with with a special drill called a "step drill" or a "uni-bit." These are single flute drills specifically designed for cutting thin materials by starting with a small hole and progressively enlarging it to your final desired size. This kind of drill does not pull the drill into the metal, but relies on you to push into the metal to the desired depth/diameter. For each step, the drill slices out a little strip instead of trying to cut the whole thing at once. A step drill should be run at a slow to moderate spindle speed or it will chatter. If you are using a hand held machine, having a side handle reduces the tendency for the machine to twist out of your hand. A drill press is recommended. Make sure the material is well clamped as close to the drilling area as possible, preferably on at least two sides of the place the hole is going, and that there is a place for the drill to go as it cuts through. Clamping with a piece of plywood with a hole in it is a good way to clamp all the way around the drilling site so that each step enters the material evenly.
You can cut sheet metal with a hole saw. It is best to heavily clamp the metal to a piece of wood, or better yet, between two pieces of wood, and then cut through the whole stack. With thinner metal, there is a risk of tearing if the material is not very well supported, because the sheet metal is a lot thinner than the depth of the saw teeth, which will grab and try to spin the work when they go partially through.
The closer the drill size is to being less than the thickness of the sheet metal, the easier it is to drill with a twist drill. Drilling a tiny hole in sheet metal is pretty much the same as drilling in any material. But if you are drilling a 1/4 inch hole or a 5/8 inch hole in a thin piece of sheet metal, usually significantly less than 1/16 inch thick, you are really subverting the way the drill works, because the first bite the drill takes goes all the way through the material. Once this happens, the drill stops cutting, and just screws its way through the work, grabbing it and yanking it up toward the chuck. Your job is to keep this from happening so that the lands on the outside edges of the drill can slice through the rest of the metal, which is not a job they were designed to do. How to you get around this? You have to make the sheet metal part of a thicker stack of material so that when the drill tries to pull the metal up, it gets stopped by whatever is on top of it. A very effective way of doing this is to tightly clamp the sheet metal between two pieces of scrap wood. If you have multiple pieces to drill, you can save time and increase safety and quality by clamping all of them in a sandwich with one flat piece of wood on the bottom, then all of your sheet metal pieces, then another flat piece of wood. Even with this arrangement, the drill will cut all the way through the sheet metal at the beginning and try to pull the whole sandwich into the drill chuck. It is very important to have complete control of the drilling machine, have the work tightly clamped, and be prepared for the drill to try to self feed. Many times you will have to hold the drill press feed lever up as the drill pulls downward. The sandwich approach is illustrated below in the form of a wooden drilling jig that screws together.
Doing this on the drill press is the best approach, but if you must use a hand held drill, putting a solidly clamped depth stop on the drill, just far enough from the point to allow the cut to complete, will help keep the drill from pulling you and the machine into the work. The split ring type drill stop shown below is an excellent choice for this. Another drill stop technique that works for odd size drills or when a drill stop is not handy is to use a piece of tubing or dowel cut to the appropriate length and pulled all to way up to the chuck, also shown below.
Keep Your Fingers
One of the most common injuries related to drilling is lacerations from out of control sheet metal. Sheet metal is thin and relatively flexible, so it can rip free of clamps and wrap around a drill bit, taking body parts with it, with or without gloves (I.E. gloves are not a great help in this case.) This is more likely to happen when drilling small pieces, so use overkill when clamping and use the full sandwich approach.Previous Tutorial: Keeping Bolt Holes Aligned when Drilling and Tapping