Thread Snap Gages
Thread snap gages are also
called a thread calipers. Thread snap gages are only
used for checking external threaded parts.
Thread snap gages are available in either a Knife Edge
Style or a Roller Style. The front set of gage members
are a complete GO gage. The components should pass through
the GO rollers under their own weight or with the very
slightest of pressure. Under no circumstances should
any force be used to assist entry. No element of a screw
which passes through the GO members can be too large.
The rear gage members constitute a NOGO effective diameter
gage, the threads being cleared away at the crest and
root. Standard thread forms such as UN Series and Metric
Series are readily available. Non-standard forms such
as Trapezoidal, Buttress, Acme, Whitworth, and others
are also available and of course are priced accordingly.
Multi-start threads may also be gauged using thread
snap gages. There are some lead-diameter combination
The Thread Snap Gage is ideal for gauging components
produced in high volume in all types of material. They
check parts faster than GO/NOGO ring gages. Thread snap
gages have a GO and a NOGO portion similar to thread
ring gages. Thread snap gages are used just like plain
snap gages. To use thread snap gages, apply the part
to the gage by sliding it into the gage from the side
of the part thread. The increased speed of inspection
comes from this sliding action which replaces the screwing
action of thread ring gages. The part is passed through
the GO segments. Assuming that it does in fact pass
the segments the first part of the part inspection is
completed. The part is then attempted to be passed through
the NOGO segments. At this point the hope is for the
part not to pass through the segments. If it does pass
through the segments the part has failed the inspection
Thread snap gages do not wear out as fast as thread
ring gages. Thread ring gages wear rather quickly because
of the contact friction of the gage against the part
through the entire helical path of the thread gage application,
and then again through its removal. Snap gages will
wear less than ring gages because the contact with the
part being measured is momentary and only at two points
on the part. Knife edge thread snap gages will wear
faster than roller style thread snap gages. The knife
edge contact is a dragging action where the roller style
is a rolling contact which has low interactive friction.
Regardless, both styles of snap gages are much longer
wearing than ring gages.
Thread snap gages especially lend themselves to gauging
components manufactured from soft alloys such as stainless
steel, aluminum, bronze and brass where the problem
of adhesion or galling of the alloy to the thread ring
gauge can be experienced when using thread ring gages.
This is again because of the minimal contact that the
gages have with the part being measured. Also, the threads
on the gage members of thread snap gages are annular
and of corrected form so that there is no interference
caused by the helix angle of the thread being gauged.
The gauge frames are typically made of cast iron and
are sometimes coated in an epoxy resin coat, which gives
the gage a high level of protection from accidental
damage which may occur if the gage should be dropped,
or of any other type of accidental impact.
LEAP TALL BUILDINGS IN A SINGLE BOUND [Back]
They can be used to detect: out-of-roundness; malformed
major diameter; radius at root oversize; pitch error;
To identify an out-of-round condition, pass the part
through the gage twice. On the second pass, present
the part rotated 90° from the position of the first
presentation. If the part is out-of-round to a point
of being out-of-tolerance an oversize or undersize condition
will be found.
To identify various other errors use the snap gage
as a form of optical comparator. This can be done by
placing a reflective material, like a piece of ordinary
white paper, between the GO and NOGO rollers. Then with
the aid of a light source, and the work piece between
the GO members, defects in the piece should be detectable.
See the samples below for the type of errors detectable
and an example of what they may look like.