Servo - May 2008

FIGURE 3. This is a very good example of appropriately using a tapped hole. Notice the top and bottom plates both screwinto the uprights. This allows for quick assembly and disassembly.

diameter bit, I would use 250 rpm. When working with aluminum, simply double the speed that you would use with steel.

This is only a rule of thumb, so if a speed isn’t working, play around and see if something else works better for your situation. The softer the material you are drilling, the faster you can run the drill bit. If you can’t tell what rpm your drill is spinning at, then just remember that the smaller the drill bit, the faster you want to go, and vice versa. (Again, remember to always where safety glasses!)

Tapping

in it (the same size as you are trying to drill) will serve as a guide in keeping the drill straight up and down. To build this guide, find a small flat piece of aluminum or steel about 1/2” thick and drill a perpendicular hole using the methods we just talked about. Now you can slide the guide over the drill bit so that it spins freely and doesn’t bind. Next, line the drill bit up with your dimple and then slide the guide down flush with the material. Now you can drill the hole, using the guide to keep the drill straight up and down.

Drilling a hole properly involves turning the drill bit at the appropriate speed. You may have a drill press with markings that show what rpm it is spinning at for different settings. If so, then the rule of thumb is that for a 1/2” diameter bit in steel, spin at 500 revolutions per minute. For different drill sizes, simply apply the same proportion such that when the diameter in inches and the speed are multiplied together, the result is 250. For example, for a 1/4” diameter bit I would spin at 1,000 rpm or for a 1”

Tapping is the process of forming or cutting threads using a tap in a predrilled hole (of the appropriate size) so that you can screw into it using common machine screws or bolts. Usually, this is done in metals and plastics. Tapping is very useful when you want to connect two things so that they can be assembled and disassembled repeatedly. It is the equivalent of drilling through two pieces of material and using a bolt with a nut, but easier because you don’t have to have access to the backside to hold the nut in place. You can also use this when a bolt and nut aren’t possible, such as connecting two frame members at a right angle as shown in Figure 3.

For every size screw, there is a matching tap and drill bit. Screw sizes are designated using two numbers: the diameter and the number of threads per inch. For example,

a 1/4-20 screw has a 1/4 inch major diameter (the diameter of the outermost part of the threads) and 20 threads per inch along the length of the screw. Also, for each major diameter there is typically a fine and coarse thread size, for example, 1/4-20 and 1/4-28. I recommend coarse threads for most applications because it is less likely the screw will pull out from excessive force. There is also a metric screw system, but it doesn’t work with any other screw types.

A tap looks like the combination of a drill and a screw. However, do not attempt to drill a hole with a tap — it will break! There are two basic types of taps: spiral point and bottoming. The spiral point tap is used for through-holes, or holes that go all the way through the material. They are stronger and easier to use, but they push away all of the chips that they cut down into the hole. So, if you try to use one for a blind hole (a hole with a bottom), it will fill the bottom with chips and you will not be able to tap the full depth of the hole. Don’t try to force the tap to push the collected chips down because you will risk