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
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
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!
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.
FIGURE 5. A hand tapper is used so that the tap always
remains perfectly upright and in line with the hole.
SERVO 05.2008 39