|
Back
to basics: taps
Once
again we left you thinking about threads and taps.
Last month we ask for the difference between a cut tap and a
roll tap. Well, just like the name says, a cut tap cuts the material
to form the thread. A
roll tap forms the thread through metal displacement; it pushes the
material into the thread shape.
Both
taps create an ID thread, but a roll tap is a stronger tool and has
the advantage of not producing chips.
No
matter which type of tap you select, the ease of use depends a lot
on your pre-machining planning.
When you plan ahead and select the most appropriate drill
size first, you give yourself a head start to successful tapping.
Look
around your shop – do you see a drill and tap wall chart?
Most companies have these charts that advise a drill size to
be used when tapping a specific thread. The drill sizes recommended
on these charts are a starting point -- as with most things we may
need to investigate further to obtain the optimum drill size.
Why
does the drill size matter? Well if you drill the hole too small you
can expect to break taps. If
the hole is too big your thread form will be incorrect.
If the wall charts are just starting recommendations how can
you find the best drill size?
Recall
from our past discussions there are three diameters on a thread: the
major, minor and the pitch diameter.
Look up the thread size in your Machinist Handbook.
Each of these diameters has a tolerance range.
For an internal thread, the minor diameter represents the
largest drilled hole allowable inside the part.
Use the tolerance of the minor diameter to determine the best
pre-tap drill size. Don’t
be surprised if this drill size is not the same as that listed on
the wall chart.
Selecting
a more accurate drill size in the beginning makes the tapping
process much easier. Be aware that as you increase the size on the
drilled hole you must be sure to gauge the resulting thread to
ensure the minor diameter is within tolerance.
The
larger the initial hole, the less material will be removed by the
tap resulting in a reduction in potential tap breakage.
Another
aspect of pre-machining planning is selecting the style of tap
chamfer. In tooling
catalogs, these styles are called bottoming, plug and taper style
taps. Longer chamfers
(taper and plug taps) help to distribute the cutting force along the
length of the tap. The
shorter chamfer means there is greater force as the tap enters the
drilled hole.
|
The
length of each individual chamfer is determined by the thread size.
Each tap chamfer style length is based on the pitch of the
thread being made with the tap.
Chamfer
lengths
*bottoming tap chamfer length = 1-2 times pitch
length
*plug tap chamfer length = 3-5 times pitch length
*taper tap chamfer length = 7-10 times pitch length
Using a 1/4-20 UNC tap as an example, the pitch
length of this thread is 1/20 = 0.050”.
For each type of tap the chamfer lengths are:
Bottoming tap: 0.050
– 0.010
Plug tap: 0.150
– 0.250
Taper tap: 0.350
– 0.500
Taper taps are rarely my first choice.
I tend to select a plug tap, depending on the clearance
allowance specified on the print.
Bottoming taps are good choices when there is minimum
clearance at the bottom of the hole.
A blind hole with insufficient clearance can be
frustrating. Think about
the problems caused by this situation and possible solutions.
We’ll meet back here next month to continue the topic.
As an aside, it seems that some design engineers
don’t give enough consideration to the blind hole clearance.
When you have a blind hole with minimum clearance between the
drilled hole and the tapped hole, it’s not just the machinists who
suffer. Tap breakage due
to insufficient clearance is more then just frustrating to the
machinist, it increases production costs in terms of material,
tooling and time.
A basic tap has a straight flute with a spiral
point. This type of tool
forces the chips into the drilled hole.
Again, with minimum clearance, forcing the chips into the
bottom of the hole can promote tap breakage.
An alternative is a turbo tap.
This tool uses a high spiral flute to force the chips out of
the hole. In my
experience a turbo tap requires more force to feed forward but it
does evacuate the chips.
When
chips are a problem, a good alternative is a roll tap.
|
