Screw Gauge
Screw gauge is a device
which can measure very small lengths (or thickness) up to one – hundredth part
of a millimeter. It can be used to measure diameter of a wire, thickness of a
thin metal sheet ,etc.
Description:
A Screw Gauge is shown here and its main parts are
described below.
C-Frame:
It’s a c shaped frame as identified in the picture, is
a rigid part that has both holding points for a job or object to be measured.
Its size depends on micrometer measuring range so size of c frame increases as
range expands to bigger.
Its main function is to provide basic structure of a
micrometer in which stationary anvil located at one end and moveable spindle
slides inward or outward trough other end of c frame.
Zero adjust screw:
It’s a screw located back end of anvil shown in
figure. As name shows it is to correct or adjust the zero error of micrometer
if there is some error found before the measurement during test.
Anvil:
As discussed above it is a small stationary
cylindrical part of micrometer located in far end of c-frame and acts as one
holding point for measuring objects. So we can say it’s a one of rigid
measuring and holding point of micrometer.
Spindle:
A cylindrical long part which is mounted through all
other parts sleeve, lock nut and thimble. It is moveable part and has a
connection with ratchet as we rotate the ratchet clockwise or counter clockwise
the spindle slides out or inward to adjust it with compare to measuring object
size.
Anvil Face and Spindle Face:
Faces of both anvil and spindle which are opposite to
each other are the measuring points of micrometer and hold the measuring object
collectively.
Lock nut:
As we know the mechanism of micrometer based on precision ground
threads of spindle so the lock nut works as stationary nut for this mechanism,
so rotation of this mechanism into lock nut controls the spindle movement.
Sleeve:
It’s a barrel type cylindrical part which mounted on
spindle and is main scale of micrometer because main scale is engraved on the
sleeve . Thimble rotates around that sleeve and spindle. Its main function is
indication of reading in millimeter in case of imperial micrometer.
Thimble:
Thimble is also mounted on spindle and a scale is
engraved around it perimeter of thimble. Scale of thimble is to show the
measurement value in fraction.
Ratchet:
It’s a knurled thumb gripe to rotate the spindle into
desired direction for measuring process, provided with ratchet action to avoid
over tightening of micrometer across the measuring object and also ensures
equal pressure force of each measurement.
Principle of the screw guage:
The screw gauge works on the principle
of screw. When a screw is rotated in a nut, it exhibits both linear and
rotational motions. When a screw is moved in a fixed nut , the linear distance
travelled by the screw on the main scale when the circular is given one
complete rotation is called Pitch of the screw.
Least count of the screw guage:
Least count
(L.C) = Pitch of the screw / Total number of divisions on the circular scale
Generally , the pitch of the screw guage is 0.5 mm and it has 50 divisions on
its circular scale . Hence , L.C. = 0.5 mm / 50 = 0.01 mm or 0.001 cm
Determination of the diameter 'd' of a
wire:
Place the wire between
the stud and spindle end as indicated in the diagram. Rotate the thimble until
the wire is firmly held between the stud and the spindle.
The rachet is provided
to avoid excessive pressure on the wire. It prevents the spindle from further
movement - squashing the wire!.
To take a reading first
look at the main scale and note the main scale reading (M.S.R).
Note the division of
the circular scale which coincides with the base line of the main scale.
This circular scale
division (C.S.D) when multiplied by the least count(L.C) ,gives the circular
scale reading(C.S.R) Add the Main scale reading (M.S.R) and the circular scale
reading (C.S.R).This gives the observed reading.
Hence, Observed Reading = Main scale reading +
Circular scale reading
Observed Reading = Main scale
reading +( Circular scale Divison*Least Count)
Depth Micrometer
Depth micrometers are
excellent for determining the depth of small holes, extruded cuts, or odd
geometries unable to be measured by inside micrometers or outside micrometers.
These micrometers only come in the 1" variety in the Gauss Johnson shop.
Here is a picture of the anatomy of a depth micrometer:
How to
Operate:
Depth micrometers have a base that is placed flat against a
surface and the thimble is twisted to extend the rod. The rod contacts the
bottom of the hole or slot and tends to lift the base. When this occurs the
user is to retract the rod until the base is returned to its flat position and
the user will twist the cap until it clicks. Common practice is to move or
slide the depth micrometer along the flat surface to ensure there is drag along
the bottom of the hole or slot. Once this occurs the user will engage the lock
nut and remove the depth micrometer for measurement reading.
How to
Read:
Depth micrometers are read in the same fashion as the outside
micrometer. However, the numbers are placed in the opposite order as those
displayed on an outside micrometer. For example the following depth micrometer
reads .3765": The numbers read the same, but are placed in opposing order.
This can be disorienting to those familiar with outside micrometers. Don't
over-think the measurement, it is read the same as an outside micrometer.
Thread Micrometer
This is just like an ordinary micrometer with the difference
that it is equipped with a special anvil and spindle. The anvil has an internal vee which
fits over the thread. The anvil in this case is not fixed but is free to rotate. Thus vee of the
anvil can accommodate itself to any rake range of thread.
The spindle on the other hand has a ground conical shape. When the conical spindle is brought into contact with the vee of anvil, micrometer reads zero. Different set of anvils are provided for different types of threads and the contact points of the micrometer are so designed that some allowance for thread clearance is always made.
The spindle on the other hand has a ground conical shape. When the conical spindle is brought into contact with the vee of anvil, micrometer reads zero. Different set of anvils are provided for different types of threads and the contact points of the micrometer are so designed that some allowance for thread clearance is always made.
Thread micrometer is used for the measurement of the pitch
diameter but the accuracy is influenced by the helix angle of the thread.
It is used for accurate measurement of
pitch diameter of screw threads.
The micrometer has a pointed spindle and a double V-anvil, both correctly shaped to contact the screw thread of the work being gauged. It directly reads in terms of pitch diameter as the zero reading of the micrometer corresponds to the closed position of anvil and spindle when both are in perfect match with each other.
The micrometer has a pointed spindle and a double V-anvil, both correctly shaped to contact the screw thread of the work being gauged. It directly reads in terms of pitch diameter as the zero reading of the micrometer corresponds to the closed position of anvil and spindle when both are in perfect match with each other.
The angle of the V-anvil and the conical point at the end of
the spindle corresponds to the included angle of the profile of the thread. The
V-anvil is allowed to swivel in the micrometer frame so that it can
accommodate itself to the helix angle of the thread. The extreme point of cone
is rounded so that it will not bear on the root diameter at the bottom
of the thread, and similarly clearance
is provided at the bottom of the groove in the V-anvil so that it will not bear
on the thread crest.
The spindle point of such a micrometer can be applied to the thread of any pitch provided the form or included angle is always the same. The V-anvil is however limited in its capacity; a number of different blocks being required to cover a full range.
The spindle point of such a micrometer can be applied to the thread of any pitch provided the form or included angle is always the same. The V-anvil is however limited in its capacity; a number of different blocks being required to cover a full range.
Stick Micrometer
Stick micrometers are
designed for the measurement of longer internal lengths. These comprise of the following parts :
(a) A 150 mm or 300 mm micrometer unit fitted with a micrometer of 25 mm range and having rounded terminal faces.
(b) A series of extension rods which together with the micrometer unit, permit of a continuous range of measurements upto the maximum length required.
(a) A 150 mm or 300 mm micrometer unit fitted with a micrometer of 25 mm range and having rounded terminal faces.
(b) A series of extension rods which together with the micrometer unit, permit of a continuous range of measurements upto the maximum length required.
How the stick micrometer will look like is shown Fig. Its
accuracy is of the order of ± 0.005 mm throughout the range. It is essential that
the radius of curvature of the terminal faces of the micrometer unit be
slightly less than one-half the smallest measuring range of the micrometer
unit, and abutment faces should have minimum radial width of 2 mm and properly
lapped ; these should be parallel to each other and normal to the axis
of the micrometer unit to within 0.005 mm across their diameters.
It may be noted that screwed joints are used for joining the
end piece, extension rod and the measuring unit. The screw units generally have threads
of 0.5 mm pitch. The extension rod is generally hollow and has a minimum external diameter of 14 mm. In the design
of extension rod shown in Fig., it
will be noted that the contact faces come together by screwing and the
pressure between the contact faces depends upon
the heavy or light tightening of the screw. In order to overcome this
difficulty, the extension rods of type shown in Fig., are generally employed.
This type consists of length standards which are mounted in
protective bushing and when these extension rods are combined, the contact faces of the
length standards come together under constant pressure and thus the
measurement is not affected by heavy tightening of the screw. The accuracy
of this instrument is of the order of ± 0.005
mm, i.e. the micrometer unit with terminal piece in position should record correctly throughout its range
to within ± 0.005 mm. In this type of micrometer, there should be sufficient play between the external and
internal threads of the joint to
permit the abutment faces of the various parts of the micrometer to butt
together solidly.