Toolmakers Microscope – Principle, Procedure, Parts, Applications

Toolmakers Microscope is a precision optical instrument that is used to measure and inspect the small mechanical components. It is also known as measuring microscope. It is mainly used to observe the shape, size, angle and position of small parts without touching the surface.

It consists of an optical head, rigid vertical column, heavy base and a precision measuring table. The measuring table can move in X-axis and Y-axis with the help of micrometer heads. Due to this, small movement of the object can be measured accurately.

This microscope uses optical magnification, cross-hair reticle and proper illumination for observation. The image of the object is magnified and the edge or point of the object is matched with the cross line. Then the distance or angle is measured by moving the table.

Toolmakers Microscope is used for measurement of small tools and machine parts like screw threads, milling cutters, templates, gauges and electronic circuit boards. It is useful in tool rooms, inspection laboratories and precision manufacturing works.

The development of this microscope is related with micrometry, which means measurement by microscope. Early microscopic measurement was done by Antonie van Leeuwenhoek by using fine sand grains as a measuring gauge. Later, with the development of industries, this microscope was made for accurate measurement of tools and small components.

At first, toolmakers microscope was operated manually and observation was done by human eye. Modern Toolmakers Microscope may contain digital readout, CCD camera and computer software. These parts help in quick and accurate measurement of very small dimensions.

Principle of Toolmakers Microscope

Principle of Toolmakers Microscope is based on the optical magnification and projection of the contour or surface details of a small workpiece. The object is placed on a transparent glass stage and light is allowed to pass from below or fall from above. The magnified image is then observed with the help of eyepiece and cross-hair graticule.

In shadow method, transmitted light is passed from below the stage. The opaque workpiece blocks the light and forms a sharp shadow image. This shadow image passes through the objective lens and prism system and then it is projected on the eyepiece or ground glass screen.

In reflection method, incident light is allowed to fall on the upper surface of the workpiece. The reflected light gives the surface image of the object. This method is used to observe surface details, marks and small irregularities present on the workpiece.

The measurement is done by matching the cross-hair lines with the required points of the magnified image. The workpiece is then moved by the precision X-Y micrometric stage. The distance moved by the stage gives the linear measurement such as length and width.

Angular measurement is done by using rotating stage and graduated eyepiece scale. In some accurate works like screw thread measurement, knife-edge method is also used. In this method the knife edges touch the part and the microscope measures these edges optically, so that shadow error is reduced.

Parts of a Toolmakers Microscope

1. Cast Base – The cast base is the heavy iron foundation of the microscope. It gives stability to the instrument and reduces vibration during measurement. The lower illumination unit is also fitted inside this base.
2. Leveling System – It consists of three adjustable foot screws and a box level. It is used to keep the microscope in horizontal position before taking measurement. This helps to avoid error in observation.
3. Coordinate Measuring Table – This is the stage where the workpiece is placed for measurement. It has a rotary circular table with glass plate. The table can move in X-axis and Y-axis by two measuring slides on precision ball bearings.
4. Micrometer Heads – Micrometer heads are precision screws used to move the measuring table. They measure the exact displacement of the stage in X and Y directions. In modern microscope digital encoders may be used instead of micrometer heads.
5. Vertical Column – The vertical column is fixed at the rear side of the base. It supports the optical head and carrier arm. In some microscope it can tilt the optical axis which is useful for screw thread measurement.
6. Carrier Arm – The carrier arm connects the optical head with the vertical column. It can be moved upward and downward by rack mechanism. This movement is used for proper focusing of the object.
7. Optical Head – The optical head is the main sighting microscope part. It contains the objective lenses which magnify the image of the workpiece. It helps to observe the contour and small surface details.
8. Eyepiece and Goniometric Head – The eyepiece is the upper viewing part of the microscope. It contains ocular lens and cross-hair reticle. The goniometric head has rotatable cross line, 360° protractor and vernier scale for angle measurement.
9. Illumination System – The illumination system gives light for viewing the object. Bottom transmitted light is used to form shadow profile of the workpiece. Top reflected light like oblique light, twin spot LED or ring light is used for viewing surface details.
10. Control Knobs and Locks – These are used to control and fix different parts of the microscope. Knurled knobs are used for fine rotation of the stage. Star handle screws and locks are used to hold the rotary table and carrier arm in proper position.

Construction and Working of Tool Maker’s Microscope

Construction of Tool Maker’s Microscope

1. Heavy Base – The tool maker’s microscope is constructed on a heavy and rigid cast iron base. It gives proper support to the whole instrument. It also reduces vibration during measurement and gives stable condition.
2. Leveling Arrangement – The base is provided with adjustable foot screws and spirit level. These are used to keep the microscope in exact horizontal position. Before measurement, leveling is done to reduce error.
3. Coordinate Measuring Table – The coordinate measuring table is mounted on the base. It has two slides for movement in X-axis and Y-axis direction. These slides move on precision ball bearing guideways and are controlled by micrometer heads.
4. Rotary Table – The stage has a central glass plate and circular rotary table. The rotary table is graduated up to 360°. It is used for angular setting and positioning of the workpiece.
5. Vertical Column – A strong vertical column is fixed at the rear side of the base. It supports the carrier arm and optical head. It has rack and pinion arrangement for up and down movement of optical head for focusing.
6. Optical Head – The optical head is also called sighting microscope. It contains objective lenses and eyepiece. The objective lens magnifies the image of the workpiece and the eyepiece is used for observing the image.
7. Goniometric Head – The goniometric head is provided with cross-hair reticle and graduated protractor scale. It is used for measuring angle of the object. It can rotate and align with the edge or flank of the workpiece.
8. Illumination System – The illumination system consists of two types of light arrangement. The lower projection lamp gives transmitted light from below the glass stage and forms shadow image. The upper incident light is used for observing the surface details, engraved marks, blind holes and texture of the object.

Working of Tool Maker’s Microscope

1. Placing of Workpiece – The workpiece is placed on the glass stage of the microscope. It is properly adjusted on the coordinate measuring table. Then the required light arrangement is selected according to the type of measurement.
2. Formation of Shadow Image – In this step, light is passed from below the stage. The opaque workpiece blocks the parallel light rays and a sharp shadow of the external contour is formed. This shadow image passes through the objective lens and becomes magnified.
3. Focusing of Image – The optical head is moved up and down by rack and pinion mechanism. The image is focused until the boundary of the workpiece becomes clear. The operator observes this magnified image through the eyepiece.
4. Alignment with Cross Hair – The starting edge or point of the workpiece is brought exactly on the cross-hair line of the eyepiece. This is done by moving the coordinate table slowly. The first micrometer reading is noted.
5. Linear Measurement – The micrometer head is then rotated and the stage is moved until the second edge or end point comes on the same cross-hair line. The second reading is noted. The difference between two readings gives the required linear measurement such as length, width or diameter.
6. Angular Measurement – For angular measurement, one cross-hair line is aligned with first edge of the workpiece. The angular reading is noted from the goniometer scale. Then the eyepiece graticule is rotated and aligned with second edge. The difference between two angular readings gives the angle of the feature.
7. Profile Comparison – In some works, the magnified shadow image is compared with standard profile template or engraved line pattern. This method is used for screw threads, gear teeth and complex tool profiles. The correct shape or error can be observed directly by comparison.

How to Use a Measuring Microscope

1. The specimen or target object is placed carefully on the transparent glass stage plate. The object should be kept in proper position so that it does not move during measurement.
2. The light source is switched on for viewing the object. Transmitted light is used from below when the shadow or profile of the object is required. Reflected light is used from above when the surface details of the object are observed.
3. The image is focused by rotating the adjustment knobs. The optical head is moved up or down until the image becomes sharp and clear. The observer sees the image through the eyepiece.
4. The position of the specimen is adjusted on the stage. The part to be measured is kept parallel to the X-axis or Y-axis movement of the measuring table. This helps in taking correct reading.
5. The micrometer adjustment knob is rotated slowly. The stage is moved until the cross-hair line in the eyepiece exactly touches the starting edge or point of the object. The first micrometer reading is noted.
6. The micrometer head is again rotated to move the stage. The cross-hair line is brought to the ending edge or next required point of the specimen. This position is observed carefully through the eyepiece.
7. The second micrometer reading is noted. The required measurement is obtained by taking difference between the first and second readings. This gives the length, depth, pitch or other linear dimension.
8. The measured value is written in the observation table or record sheet. In modern digital measuring microscope, the values can be directly recorded and output as CAD data.

Applications of Tool Maker’s Microscope

• Tool Maker’s Microscope is used to determine the exact length, width, depth and distance between two points of small components. It can measure the position of the parts in Cartesian and polar coordinates.
• It is used to measure different angles of tools and workpieces. Thread angle, pitch angle, rake angle and relief angle of cutting tools are measured by this microscope.
• It is used for inspection of screw threads. The thread form, pitch, major diameter, minor diameter and effective diameter of screws, thread gauges, taps and worms can be measured.
• It is used to check precision tools like milling cutters, formed cutters, punching dies, templates and cams. The shape, cutting edge and wear pattern of these tools can be observed clearly.
• It is used for surface inspection of small parts. Surface finish, surface defects, alignment of parts and hardness test indentation can be observed and measured.
• It is used in electronics for checking small components. The dimensions of Integrated Circuit (IC) package, pin pitch, solder pad alignment and track width of Printed Circuit Board (PCB) are measured.
• It is used to inspect miniature components used in watch making, automobile parts, fuel injection nozzles, medical implants, dental implants and small plastic or rubber parts.
• It is used for comparative measurement of workpiece profile. The magnified shadow image of the object is compared with standard master profile, drawn pattern or screen chart.

Advantages of Tool Maker’s Microscope

• Tool Maker’s Microscope is used for non-contact measurement of small parts. The object is measured by optical magnification, so there is no physical touching on the surface. This prevents damage and deformation of soft materials like rubber, plastic, thin metal parts and PCB tracks.
• It gives high precision and accurate measurement. Very small dimensions can be measured in micron level and in some cases sub-micron level also. So it is useful for parts having complex shape and close tolerance.
• It is a versatile measuring instrument. It can measure length, diameter, screw thread pitch, tool flank angle and different complex contours. By using accessories like rotary table, swivel centre and dividing head, cylindrical and 3D parts can also be measured.
• It works both as an observation microscope and measuring microscope. Surface finish, tool wear and material defects can be observed. At the same time, dimensional measurement of the same part can be done.
• It can measure in X-axis, Y-axis and also in Z-axis direction. Height and depth measurement can be done by focusing arrangement. This is an advantage over profile projector which mainly works in X-Y plane.
• It gives quick and direct measurement of small components. Many measurements can be taken without preparing any special image or long setting. This increases the speed of inspection and quality control work.
• It is simple to operate. The optical view, cross-hair lines and micrometer readings are easy to use. In modern type, digital display also makes the reading more easy and fast.

Limitations of Tool Maker’s Microscope

• Tool Maker’s Microscope may give operator bias during measurement. The cross-hair line is aligned manually with the edge or point of the workpiece. So the reading may change from one operator to another operator.
• It needs technical skill for proper use. Focusing, alignment and height measurement under high magnification need careful operation. If the operator has less experience then error may occur.
• It is sensitive to temperature change. Small change in temperature can expand the stage or the workpiece. This thermal expansion may cause measurement error, specially in micron level measurement.
• It requires clean and stable working condition. Dust free room, level table and vibration free support are needed for accurate measurement. If vibration is present, the image may become blurred at high magnification.
• Parallax and focusing error can occur in this microscope. If the image is not focused properly, the shadow size may change slightly. This gives wrong reading of the dimension.
• Surface condition of the workpiece affects the measurement. Dust, oil, liquid residue, uneven edge and chamfer may disturb the correct optical image. This is more important in knife-edge method and other accurate measurements.
• There is chance of damage to the objective lens. During movement of carrier arm or stage, the lens may touch the workpiece. It can damage lens coating or disturb the optical alignment of the microscope.

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