Mechanical comparator
These are of 2 Types
- Dial Indicator
- Read type mechanical comparator
- Mechanical comparator employs mechanical means for magnifying small deviations.
- The method of magnifying small movement of the indicator in all mechanical comparators are effected by means of levers, gear trains or a combination of these elements.
- Mechanical comparators are available having magnifications from 300 to 5000 to 1.
- These are mostly used for inspection of small parts machined to close limits.
1.Dial Indicator
- A dial indicator or dial gauge is used as a mechanical comparator.
- The essential parts of the instrument are like a small clock with a plunger projecting at the bottom as shown in fig.
- Very slight upward movement on the plunger moves it upward and the movement is indicated by the dial pointer.
- The dial is graduated into 100 divisions.
- A full revolution of the pointer about this scale corresponds to 1mm travel of the plunger.
- Thus, a turn of the pointer b one scale division represents a plunger travel of 0.01mm.
Experimental setup
- The whole setup consists of worktable, dial indicator and vertical post.
- The dial indicator is fitted to vertical post by on adjusting screw as shown in fig.
- The vertical post is fitted on the work table; the top surface of the worktable is finely finished.
- The dial gauge can be adjusted vertically and locked in position by a screw.
Procedure
Let us assume that the required height of the
component is 25mm.
Initially this height is built up with slip gauges. The
slip gauge blocks are placed under the stem of the dial gauge.
The pointer in
the dial gauge is adjusted to zero.
Now he slip gauges are removed.
The component to be checked is introduced
under the stem of the dial gauge.
If there is any deviation in the height of
the component, it will be indicated by the pointer.
Mechanism
The stem has rack teeth. A set of gears
engage with the rack.
The pointer is connected to a small pinion.
The small
pinion is independently hinged. I.e. it is not connected to the stern.
The
vertical movement of the stem is transmitted to the pointer through a set of
gears.
A spring gives a constant downward pressure to the stem.
2. Read type mechanical comparator
In this type of comparator, the linear
movement of the plunger is specified by means of read mechanism.
The mechanism of this type is illustrated in
fig. A spring-loaded pointer is pivoted.
Initially, the comparator is set with
the help of a known dimension eg. Set of slip gauges as shown in fig.
Then the
indicator reading is adjusted to zero.
When the part to be measured is kept
under the pointer, then the comparator displays the deviation of this dimension
either in ± or— side of the set dimension.
Advantages
It is usually robust, compact and easy to
handle.
There is no external supply such as
electricity, air required.
It has very simple mechanism and is cheaper
when compared to other types.
It is suitable for ordinary workshop and also
easily portable.
Disadvantages
Accuracy of the comparator mainly depends on
the accuracy of the rack and pinion arrangement. Any slackness will reduce
accuracy.
It has more moving parts and hence friction
is more and accuracy is less.
The range of the instrument is limited since
pointer is moving over a fixed scale.