Servo - May 2008

breaking the tap.

A bottoming tap is used for — you guessed it — getting to the bottom of blind holes. This type requires more care when using than a spiral point tap because it draws all the chips up out of the hole. So, you need to worry about chip build-up which can cause binding and tap breakage. I recommend spiral flute bottoming taps.

You will also need a tap handle, which come in two styles. The t-handle looks like a “T” as its name suggests, and is good for smaller tap sizes such as a 1/4” and below. Figure 4 shows a t-handle tap being used to bottom tap aluminum. There is also the straight handle tap wrench, which is often larger and consists of a single straight handle. This is good for larger taps where more leverage is required. The third option for tapping is using a hand tapper as shown in Figure 5. This allows you to easily keep the tap straight when tapping. Now that we know the tools necessary to tap, we can go over the actual tapping procedure!

6) When you are sure your threads are fully formed, unscrew the tap from the hole. Be careful because this is the stage when tap breaks are easy. The tap is weakest going backwards and if a few chips get in the way, it will snap the tap off in the hole. If you encounter any resistance when backing out, simply go back down a turn or so and try to come back up.

7) Now you can clean the hole out by blowing into it with compressed air; just be sure to wear safety glasses.

1) Obtain the correct size and type drill, tap, and tap handle. Drill the hole to the appropriate depth, ensuring that it stays perpendicular to the material. Next, add some oil to the tap and hole; WD- 40 works well but there are chemicals specifically meant for tapping (such as TapMagic mentioned previously). If you are tapping plastic, water will work fine.

2) Start tapping the hole by setting the tap into the hole and slowly turning the tap handle. Be careful to keep the tap perpendicular to the material and in line with the hole. This is critical because if this is not straight, it will cause your tap to break as you get deeper into the hole.

3) As you begin to cut threads, you will feel a little
resistance. The amount of resistance varies for every tap
size and material type, so judging how much is too much
comes with experience. However, if you feel like it is
surprisingly difficult to turn, stop immediately and back the
tap out of the hole to check things out. You may
not be in straight, the hole may be too small, or
you may not have enough cutting oil.

As a beginner, you should only be tapping by hand. Power tapping can be done using a reversible cordless drill but unless you want to risk breaking taps off in the material, stick to hand tapping.

They say there are two kinds of people in this world: those that have broken a tap and those that will. This is because even with the proper techniques, right tools, and extensive experience, it is very easy to break a tap! So, what do you do when you break your first tap? One option is to use a dremel tool and small rotary cut-off disk to grind a slot into the broken tap. Then use a flat head screwdriver to try and unscrew it. You can also use a center punch and hammer to get the tap to rotate out, however, this is easier said than done. Tap removers do exist but only work well under perfect conditions and tend to be expensive. I bought one a few years ago and have yet to use it successfully. There are also chemical dissolvers you can buy that will, over days, etch away enough of the steel tap body allowing you can crack it out. (Obviously they only work on aluminum, plastic and other non-ferrous metals.)

The final and most effective solution is to simply move on and drill a hole next to it to try again. Just remember to grind down any sharp edges from the broken tap!

Welding

In smaller robots, welding usually does not have much

4) Assuming that everything is going smoothly, you now want to turn the tap to start cutting threads in increments. For every two turns, you must go backwards at least one quarter turn. This will break up the chips and prevent binding.

5) If you are tapping a through-hole, then make sure to tap past the edge of the hole, because the end of the tap doesn’t cut a full size thread. If you are tapping a blind hole, stop when you feel the tap hit the bottom of the hole; you can tell by a dramatic increase in resistance.