The depth micrometer, as the name implies, was designed to measure the depth of holes, grooves, spaces, keyways, etc. There are versions with analog reading and also, some more modern, with digital reading.
This measuring instrument is composed of a hardened, ground and polished base, combined with a micrometric head. The rods are introduced through a hole in the micrometer spindle and are placed in the correct position using a knurled nut.
The micrometric spindle is rectified, with high precision, and has a course that is usually 25mm or 1 “, The rods are provided with differences of 25mm (or 1”) each. Each rod emerges from the base and advances according to the rotation of the drum.
How to Use a Depth Micrometer?
The reading is obtained exactly in the same way as in an external micrometer, except that, in this case, the cylinder has the graduation in the opposite direction.
Before using a depth micrometer, make sure that the base, the tip of the rod and the piece to be measured are clean, and that the rod is perfectly placed in the micrometer head.
When measuring, firmly hold the base of the micrometer against the base of the object to be measured, as shown in the video below. Turn the drum until the rod touches the bottom, activate the lock and remove the micrometer to read.
With frequent use, may arise, to compensate for them, certain models have the option of making adjustments from a nut located at the top of the rod. Whenever you make any modifications, then check the calibration using a pattern block.
What are the Pattern Blocks that we can use to modify?
The standard blocks, coves or standard gauges, longitudinal pattern blocks (BPL) or Johansson blocks – in honor of its inventor – are solid pieces in the form of a parallelepiped, in which two of their parallel faces (or measurement faces ) present an excellent specular polishing that ensures exceptional parallelism and flatness, being able to materialize a certain length with high precision.
Generally, they are presented by sets of a variable number of pieces and thanks to the fine finish of their measurement faces can be adhered to each other by a simple manual slip, combined in the amount necessary to have any existing nominal value within its field of use, with steps of up to 0.5 microns.
From these characteristics, it follows that the pattern blocks are the most accurate materialized length devices that exist. Since they appeared in the market, at the beginning of the 20th century, and up to the present, their design and construction have continuously evolved and today they are subject to compliance with the international standard ISO 3650.
That is why the requirements that the master blocks must comply with our rigorous and based on their aptitude to be calibration instruments. These requirements are:
- Geometric and dimensional accuracy: they must meet the requirements of length, parallelism and flatness.
- Adhesion capacity to other standard blocks: determined by its surface finish.
- Dimensional stability through time, that is, they must not “age.”
- The coefficient of thermal expansion close to common metals: this minimizes measurement errors against temperature variations
- Resistance to wear and corrosion.
Materials that make up the pattern blocks
The standard blocks are usually made of steel, but they are also presented in other materials of greater hardness and strength, such as hard metal and ceramics, so the use of pieces of one or another material will depend on the budget and the application.
The average hardness of the steel used in standard blocks is 64 HRc (Rockwell scale) and it presents great precision and dimensional stability, as well as a low coefficient of thermal expansion. However, the pieces require a meticulous cleaning after use and storage conditions must be taken care of, to protect them from moisture and corrosion.
The blocks of hard metal, usually tungsten carbide or chromium carbide, have twice the average hardness concerning steel and therefore can offer a solid adhesion and great resistance to wear.
So far, the pattern blocks of ceramic are more resistant to wear and corrosion and have the best adhesion properties and stability. They are pieces of zirconium oxide with a special treatment to achieve their exceptional characteristics, which reach an average hardness of 130 HRC. Also, the advantage they have over metal blocks is that no type of magnetic impurities is adhered, for example, iron filings or steel shavings, which would introduce errors in the measurements and damage the piece.
Precision grades and uses
Even within each class of materials with which they are constructed, the standard blocks are available in different qualities or degrees of precision (in numbers or, formerly, in letters), each grade properly classified by ISO 3650 and subject to the tolerances stipulated by it. Once again, the use of this or that degree of precision depends on the application.
Presentation and coupling of pattern blocks
As already noted, standard blocks are usually supplied in sets presented in wooden cases, although they can also be purchased per unit. While there are many, the most common names are those of 56 and 112 pieces, which allow 1-mm and 0.5-micron steps, respectively. The nominal length of the smallest block of the 56-piece set is 1 mm and that of the largest block is 200 mm. In the set of 112 pieces, the nominal length of the smallest block is 0.5 microns and that of the largest is 100 mm.
Often the situation arises in which a value that does not respond to any of the individual blocks must be materialized, that is, it is more likely that we have to materialize, for example, 12,028 mm than 10 mm exact. This situation is solved by the process known as coupling, combination or assembly of standard blocks.
By the fine finish that their measurement faces have, the standard blocks can be adhered by these faces to form a coupling able to reach the measurement that we wish to materialize. However, it is not a question of grouping blocks randomly, but of following careful rules to achieve the necessary precision and protecting the blocks from any damage.
1) Group the master blocks following the sequence I-IV indicated by the figure, that is: cross them at right angles (I), turn them gently in the indicated direction (II), slide one on top of the other in the indicated direction to eliminate the film of air between both (III) until achieving a perfect adherence (IV).
2) Unless it is essential, the use of thinner standard blocks should be avoided, as they are very fragile and can easily deteriorate.
3) Always try to use the smallest possible number of standard blocks (no more than five in total) so that the sum of the small errors of the block remains with a negligible value (<0.001 mm), although condition two must be respected. The more complete the pattern block game, the greater the chance of using the least number of blocks.
4) Begin by choosing the blocks that determine the smallest values, for example, of the thousandths, then those of the hundredths, the tenths, etc.
5) For your protection, the thinnest standard blocks in the group should be placed in the middle and the larger blocks at the ends.
6) It will be necessary to achieve a certain balance of values between the thinnest and the largest standard blocks.
For example, suppose that we want to materialize the measurement that we exemplified previously, that is, 12,028 mm and we have a set of standard blocks of 112 pieces.
Starting with the figure that determines the smallest values, we could choose the 0.008 mm block. However, condition two stated above advises us not to use such thin blocks. Therefore, we will choose the piece of 1,008 mm. Then we require the measurement of 0.02 mm, but again, to satisfy condition 2, we choose the block of 1.02 mm. Finally, we added the 10 mm block to complete the total of (1,008 + 1,02 + 10) mm = 12,028 mm.
Care of the pattern blocks
Like any piece of precision, extraordinarily delicate and of high cost, the blocks require extreme care, both in handling and storage, to avoid damage and loss of their properties. For this, we must take into account the following:
- The work environment should be at 20ºC or at a temperature as close as possible to that value since it is to which the standard blocks are calibrated. It must be an environment protected from damp, dusty or corrosive atmospheres, as well as from sunlight, thermal radiations, magnetic or electric fields.
- For the care of the master blocks, always work on soft surfaces (rubber, suede, etc.) and use gloves or tweezers, avoiding using unprotected hands, which could be dirty or damp. Before use, the blocks should be carefully cleaned with appropriate solvents to remove the lubricant that protects them. They must be handled without touching their measuring faces or taking several at the same time in the hand, nor letting them remain in hand for a long time so that they do not get hot.
- If couplings are built, they should never be forced to fit into the housing to be measured and should be removed as soon as possible to prevent the blocks from sticking permanently.
- After use, the master blocks should be cleaned again, lubricated and stored in their case.
Observing this care, the useful life of the standard blocks is very long. It is common for manufacturers to also market assembly elements, accessories, gloves and cleaning kits together with the sets of standard blocks.
Although the rectangular section pattern blocks are the most common, some firms also commercialize block blocks of square section, and both types are available in metric and inch sizes, with a wide assortment of games that respond to the needs More demanding.