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Back
to basics: taps
We’ve
been discussing tapping and some of the challenges in understanding
threads, selecting drill sizes and different types of taps.
One particularly challenging tap situation is in tapping a
blind hole.
Unlike
a through-hole, a blind hole does not go completely through the
part.
The chips made in drilling the hole and cutting the tap, are
pushed to the bottom of the hole.
This can cause much frustration in broken drills and taps.
The
standard tap design has a straight flute with a spiral point.
These taps are designed for through hole applications and do
not promote chip clearance.
It is common to use this type of tap in producing a blind
hole because, like most people, we use the tooling that’s
available in the shop.
When
the part has a minimum amount of clearance behind the drilled hole,
the chips are forced into the bottom of the hole, promoting 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.
Try using a turbo tap as a hand tap – you’ll soon see
what I mean.
Another
good alternative is a roll or form tap.
Recall from last month’s issue, roll taps form the thread
through material displacement, not by cutting the material.
The
pre-tap drilled hole needs to be larger than for a standard tap.
We’ve placed a drill chart for roll / form taps on our web (www.rose-training.com)
site for reference.
When
using a roll tap you may also experience a challenge obtaining the
correct size tolerance for both the thread class and the post-tap
minor diameter size.
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When
we use roll taps we normally cover the bases and make sure that we
have a variety of H sizes taps available. As this process relies on
metal displacement we cannot forecast the sizes that will be
obtained by a roll tap in different types of material. For example
we may need to use a H5 size tap to produce a class 2B size thread.
To
obtain the correct minor diameter we can change the size of the
drill.
A smaller drill can produce a smaller minor diameter but it
may also add tapping force due to the greater amount of metal
displacement.
The smaller size drill results in greater metal displacement
by the tap.
Roll
taps are well liked it our shop because we do not have pesky chips
at the bottom of the hole that can cause tap breakage.
These taps also eliminate the challenge of removing chips
prior to shipping the part.
Whether
we are using a cut tap or a roll tap we are always dealing with attaining
the full thread depth as specified on the part print.
Blind
holes might be less trouble if design engineers were aware of these
issues.
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.
We
have all seen prints where the specifications require a 1.00”
drill depth and a 7/8” full thread depth. Sometimes this type of
specifications may be unavoidable but whenever possible we hope
design engineers allow enough room between the bottom of the hole
and the depth of the thread when available.
Anything
that makes tapping easier must help everyone.
Typo
notice: Did
you catch the typo in last month’s column?
The chamfer length of a bottoming tap should be 0.050 –
0.100, not 0.050 – 0.010.
As in real estate it’s location, location, location of the
decimal point!
Thanks! |
