Difference between revisions of "Drilling Sheet Metal"

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==Step Drills==
 
==Step Drills==
One way to drill sheet metal with with a special drill called a "step drill" or a "uni-bit."  These are single flute drills specifically designed for cutting sheet metal 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 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 the hand.  A drill press is recommended.  Make sure the material is well clamped a close to the drilling area as possible, preferably on either side of the place the hole is going, and that there is a place for the drill to go as it cuts through.
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One way to drill sheet metal with with a special drill called a "step drill" or a "uni-bit."  These are single flute drills specifically designed for cutting sheet metal 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 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 the hand.  A drill press is recommended.  Make sure the material is well clamped as close to the drilling area as possible, preferably on either side of the place the hole is going, and that there is a place for the drill to go as it cuts through.
  
 
[[File:Step drills.jpg]]
 
[[File:Step drills.jpg]]

Revision as of 13:40, 23 September 2011

Previous Tutorial: Keeping Bolt Holes Aligned when Drilling and Tapping

First Choice: Punch. Don't Drill

Sheet metals present a special challenge for drilling. The best way to drill in sheet metal is not to drill at all, but the 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 bolt on the punch to go through.

Whitney sheet metal punch.jpg Chassis punch.jpg

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 can, and usually does, slice into the metal, 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.

Step Drills

One way to drill sheet metal with with a special drill called a "step drill" or a "uni-bit." These are single flute drills specifically designed for cutting sheet metal 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 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 the hand. A drill press is recommended. Make sure the material is well clamped as close to the drilling area as possible, preferably on either side of the place the hole is going, and that there is a place for the drill to go as it cuts through.

Step drills.jpg

Hole Saws

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.

Hole saws.jpg

Twist drills

The closer the drill size is to the thickness of the 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, 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. 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 by clamping all of them in a sandwich with one piece of wood on the bottom, then all of your sheet metal pieces, then another 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 up to the drill chuck. It is very important to have complete control of the drilling machine and have the work tightly clamped. The sandwich approach is illustrated below in the form of a wooden drilling jig that screws together.

Sheet metal drilling sandwich.jpg

Doing this on the drill press is the best approach, but if you must use a hand drill, putting a solidly clamped depth stop on the drill will help keep the drill from pulling you and the machine into the work. A split ring drill stop is shown below.

Drill stop.jpg

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. 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