Vernier Height Gauge (Metrology)

2.30.
[Fig. 2.101] This is also a sort of vernier caliper, equipped with a special base block and other
attachments which make the instrument suitable for height measurements. Along with the sliding
jaw assembly, arrangement is provided to carry a removable clamp. The upper and lower surfaces
of the measuring jaws are parallel to the base, so that it can be used for measurements over or
under a surface. The vernier height gauge is mainly used
in the inspection of parts and layout work. With a scribing
attachment in place of measuring jaw, this can be used to
scribe lines at certain distance above surface. However dial
indicators can also be attached in the clamp and many
useful measurements made as it exactly gives the indica-
tion when the dial tip is just touching the surface. For all
these measurements, use of surface plates as datum sur-
face is very essential.
2.30.1.

Specification of Vernier Height

Gauges. For specifying the vernier height gauge, one has
to specify clearly the range of measurement, the type of
scales desired, and any particular requirements in regard
to the type of vernier desired. Generally, all the parts of
the height gauges are made of good quality steel, or stain-
less steel also in certain cases. At the time of fabrication,
the blanks for all height gauges are suitably heat-treated
so as to remove any stresses resulting from possible cold
working. The underside of the base and the faces of the
measuring jaw and scriber are hardened to about 700 HV
(58 RHC) for good quality steel and up to 650 HV (56 RHC)
for stainless steel. In selecting a suitable high carbon
stainless steel to give a hardness of not less than 650 HV,
a balance should be drawn between hardening qualities
and resistance to staining, because a steel which gives a
high hardness number is likely to have low acid resisting
qualities. The various parts and the nomenclature of the
height gauge are illustrated in Fig. 2.102.
Vernier Height Gauge.
Fig. 2.101. Vernier Height Gauge.
Nomenclature for Vernier Height Gauge.
Fig. 2.102. Nomenclature for Vernier Height Gauge.
2.30.2.


Description of parts and their requirements

Base. It is made to ample proportions to ensure rigidity and stability of the gauge. The
underside of the base is relieved leaving a surface round the outside edge of at least 7 mm width
and an air gap is provided across the surface to connect the relieved part with the outside. The base
is ground and lapped to an accuracy of 0.005 mm as measured over the total span of the surface
considered. The departure allowed from the true flatness is only in the nature of concavity, as
measured over the total span of the surface concerned.
Beam. Its section is chosen such that it ensures rigidity during the use. The rigidity should
be such that an upward force of 1 kg applied at the tip of the scriber when the measuring jaw is in
its highest position in a plane containing the measuring jaw, shall not cause a change in the
parallelism of the jaw with the base of more than 0.005 mm/10 mm of projection of the working
surface of the scriber. The guiding edge of the beam should be perfectly flat within the tolerances
of 0.02, 0.04, 0.06, 0.08 mm for measuring range of 250, 500, 750, 1000 mm respectively. The faces
of the beam should also be flat within the tolerances of 0.04, 0.06, 0.10, 0.12 mm for vernier
measuring heights of 250, 500, 750,1000 mm respectively. These faces should also be square to the
base within 0.04 mm/100 mm.
Measuring Jaw and Scriber. The clear projection of the measuring jaw from the edge of the
beam should be at least equal to the projection of the base from the beam. For all positions of the
slider, the upper and the lower gauging surfaces of the measuring jaw should be flat and parallel
to the base to within 0.008 mm. The actual depth of the measuring jaw should agree with its nominal
depth to within 0.013 mm. The nominal depth of the measuring jaw is always clearly marked on it.
The measuring faces of the scriber should be flat and parallel to within 0.005 mm. The scriber
should preferably be of the same nominal depth as the measuring jaws so that it can be reversed
and should confirm to this depth to within 0.013 mm. The actual depth of the scriber is also clearly
marked on it.
The projection of the scriber beyond the jaw should be at least 25 mm. Vernier height gauges
may also have an offset scriber and the scale on the beam so positioned that when the scriber is
coplanar with the base, the vernier is at zero position.
Graduations. The following requirements should be fulfilled in respect of graduations on
scales. All graduations on the scale and the vernier should be clearly engraved and the thickness
of graduation both on scale and vernier should be identical and should be in between 0.05 mm and
0.1 mm. The visible length of the shortest graduations should be about 2 to 3 times the width of the
interval between the adjacent lines. The perpendicular distance between the graduations on scale
and the graduations on vernier should in no case be more than 0.01 mm.
In most of the cases the graduations on the beam give the reading of heights from the
underside of the base to the top of the measuring jaw, but in some spherical types of height gauges
using offset scribers, the scriber can be positioned co-planar with the underside of the base and in
that position the vernier reads zero. Generally provision is also made for adjustments of the vernier
on the slider in order to reset the zero position if wear or any other cause makes this necessary.
For easy reading, it is recommended that the surfaces of the beam and vernier should have
dull finish and the graduation lines blackened in. Sometimes a magnifying lens is also provided to
facilitate taking the readings.
Slider. It has good sliding fit along the full working length of the beam. A suitable fitting is
incorporated to give a fine adjustment of the slider and suitable clamp provided so that the slider
could be effectively clamped to the beam after the final adjustment has been made.
2.30.3.

Precautions.

The height gauges are generally kept in their cases when not in use.
Every care should be taken, particularly in case of long height gauges, to avoid its heating by warmth
from the hands. The springing of the measuring jaw should be always avoided.
2.30.4.


Tests for accuracy.

For testing the accuracy of the vernier height gauge, the
following conditions shall be kept in mind :
(i) The standard reference temperature of 20°C shall be maintained in the room where the
test is going to be conducted.
(ii) The various tests shall be carried out by resting the height gauge on a precision surface
plate.
(iii) The test measurements shall be made at a distance of 25 mm from the end of the jaw.
(iv) The standard gauges shall be used for testing purposes.
(v) The slider shall be firmly clamped.
(vi) The errors in the reading at any portion within the measuring range of height gauge
should not exceed ± 0.02 mm for measuring range of height gauge of 250 mm, ± 0.04 mm for above
250 mm and upto 750 mm, and ± 0.05 mm for range of more than 750 mm.
The most important tests on vernier height gauge are straightness and squareness and the
parallelism of the working faces of the beam, measuring jaw and scriber ; and the accuracy of the
scale readings. However, the various recommended tests are described below :
(a) Hardness of working faces. It is assessed by any of the conventional methods. However,
the scratch test is most commonly used. This test is performed by a hand scriber consisting of a
small round Swiss file, the end of which is ground to a smooth hemisphere of about 0.5 mm radius,
and is mounted on a wooden handle. The scriber is first drawn on a standard test piece of hardness
700 HV and then on the face under test with the same force. The resulting marks are compared
and estimate of hardness of tested surface made.
(b) Flatness of base. This test is performed by blueing a known flat surface and then rubbing
it against the surface to be tested. Afterwards the visual test which is sensitive to well within 0.003
mm may be employed. In this test a ‘knife-edge’ straight-edge is placed against the surface to be
tested against a source of light and the colour of the light as visible against the illuminated
background is noted which gives the measure of the flatness.
(c) Rigidity of beam. The change in parallelism between the surface of the scriber and the
base, when a load of 1 kgf is applied to the tip of the scriber with the measuring jaw in its highest
position, in a plane containing the measuring jaw, may be checked by a suitably mounted indicator
and a precision square block as shown is Fig. 2.103 (a).
Accuracy Tests.
Fig. 2.103. Accuracy Tests.
(d) Flatness and squareness of beam. The test for the flatness and squareness of the beam is
carried out with the aid of precision square block (whose tolerances of the squareness are known)
and a precision surface plate. The square block is placed on the surface plate and height gauge also.
The dial indicator is made to rest on the square block and its reading against the guiding edge of
the beam is noted in several positions and thus the flatness and squareness* assessed. [Refer
Fig. 2.103 (b)]
(e) Flatness of the working surface of the measuring jaw and scriber. This is checked either
by applying a ‘knife-edge’ straight edge or by the optical flat if the finish of the surface is very good.
(f) Parallelism of the measuring jaw and scriber with the base. This test is carried out in a
similar manner as (e). The indicator is suitably mounted on the stand and indicator base directly
made to rest either on surface plate or on the precision parallel block as the case may be and readings
noted both on the measuring jaw and scriber and the parallelism assessed.
(g) Thickness of graduations. It is checked with a microscope fitted with an eye-piece graticule
scale engraved at 0.03 mm interval when a graduation line thickness may be read directly against
the graticule scale.
(h) Accuracy. The accuracy of reading of vernier height gauge is checked by making setting
on end-bar and slip gauge combinations of known size. The combination and the gauge are made
to stand side by side on a precision surface plate and the scriber is brought in position for the test
and the reading on the scale is noted down. The size of the standard combination used shouW be
chosen so as to cover a number of points over the range of the instrument and at the same tinMto
provide a check on the accuracy of the vernier scale.
(i) The perpendicular distance from the graduated face of the beam to the graduated bevelled
face of the vernier is checked with a 0.1 mm feeler gauge.
2.30.5. Master Dial Indicator Vernier Caliper. It combines the best features of the
Vernier Caliper and the Dial Comparator Gauge in one instrument and greatly reduces the problem
of operator’s sense of feel. This is ideally suited for both external and internal measurements.

Next post:

Previous post: