How to Read Screw Thread Callout?

1. Read the numbers. They will look something like one of these:
   • #4-40 x .5
   • 1/4-20 x 5/8
   • M3-0.50 x 10
2. 2
   

   The major diameter for the threaded portion of the screw.
   Interpret the first number. The first number gives the major, or largest, diameter.
   • In Unified threads (measured in inches) there are numbered diameters #0 through #10, with 0 the smallest and 10 the largest. (Diameters #12 and #14 may also be found, but are usually on older equipment and needed for repairs or restorations. #14 is close to, but not exactly the same as, 1/4-inch.) The major diameter in Unified threads = 0.060" + 0.013" (numbered diameter). So, #2 has a major diameter of 0.086". The odd numbers exist, but the even numbers are in far more common use. For screws larger than a #10, the diameters are listed in fractional inches. For instance, a 1/4-20 screw has a 1/4-inch major diameter.
   • For metric threads, e.g. M3.5, the number following the M is the major diameter of the external thread in millimeters.
3. 3
   

   The distance between adjacent threads, or thread pitch.
   Interpret the second number. It has to do with the distance between adjacent threads. It may be given as the number of threads per unit length, or it may be given as the distance between threads (also called the thread pitch).
   • For Unified threads, the number given is threads per inch. For instance, a 1/4-20 screw has 20 threads per inch.
   • For metric threads, the thread pitch is given in millimeters per thread. Thus, an M2 x 0.4 screw has threads every 0.4mm. Although most metric fasteners have two or more standard pitches (fine & coarse threads), the pitch is often omitted from a thread callout; it is always helpful to carry a sample with you to the hardware store.
     • Two major metric "industrial standards": DIN Deutsches Institut für Normung (German) and the JIS Japanese Industrial Standards. Although these standards are closely related and often identical, there will be cases where say a JIS M8 bolt may not have the same pitch as a DIN M8 bolt. The American National Standard (ANSI) Metric thread is more commonly used in the USA.
4. 4
   

   The length of most screws is measured from the bottom of the head.
   Read the length, which is generally given after the x. The length of most screws is measured from the bottom of the head, as shown. Note, however, that a flathead screw, designed to sit flush in a countersunk material, is measured to the top of the head.
   • For unified threads, the length is given in inches. A 1/4-20 x 3/4 screw is .75 inches long. The length may be given in fractional inches or the decimal equivalent.
   • For metric threads, the length is given in millimeters.
5. 5
   Understand some other nomenclature that sometimes goes with screw threads.
   • 

     Nuts have internal threads.
     Thread classes refer to fit: how loosely or tightly the screw fits in the nut. The most common thread classes are 2A or 2B. A indicates an external thread, such as on a screw or bolt. B indicates an internal thread, such as on a nut. The 2 (or, far less commonly, 1 or 3) describes the tightness of the fit.
   • You may see the abbreviations UNC and UNF. These stand for unified coarse and unified fine, respectively, and they refer to standard series of thread pitch. Each series assigns a pitch to diameter. For instance, a #10 UNC screw has 24 threads per inch, whereas a #10 UNF screw has 32 threads per inch. If a thread is specified by its series, look for the pitch in a table.
   • Minor diameter is the smallest diameter of the thread, the innermost diameter. Major diameter is the largest diameter of the thread, or the outermost diameter. The diameter given is typically the nominal major diameter of an external, or male, thread.
   • This guide is for machine screws. Other threads, such as wood screws, follow slightly different guidelines. Other thread series, such as the PG series and British Whitworth also exist, but they are relatively rare.
   • Screw thread callouts are not printed on the fastener. If you have an unknown fastener, a thread gauge or screw checker can be a big help in determining its size. If no such instrument is available, try screwing your fastener into a known mating thread. Stop immediately if you feel undue resistance to avoid stripping threads.
     • Markings may appear on the fastener to indicate the grade or material, particularly if the fastener is special (such as if it is an aircraft grade).
   • A good rule of thumb with machine screws is that a minimum of three full threads should engage the mating thread. If there are not at least three threads engaged in a thin material, use a nut or other reinforcement.
   • Consult a size chart for more information.
   • To read aloud, say these callouts as follows:
     • #4-40 x .5 -- Say "Four-forty by point five" or "Number four-forty by a half."
     • 1/4-20 x 5/8 -- Say "Quarter-twenty by five eighths."
     • M3x.5 x 10 -- Say "Em three by point five by ten."
   • Screws typically come in certain round-numbered lengths, so a 1/4 inch screw may be far easier to find than a 5/32 inch screw.
   • 

     These screws are the same.
     One way to check whether two screws are the same is to set them side by side facing opposite directions. If their threads mesh, they have the same thread pitch. This is also a quick way to check length.
   • The majority of machine screws are right-handed threads, meaning that the screw will turn clockwise to insert and counterclockwise to remove. Remember, "right, tight; left loose". One common exception is the thread holding the left pedal on a bicycle to the crank arm.

   Warnings

   • When specifying a fastener, make sure that the fastener is adequate for the job and compatible with the materials and environment.
   • Tapered pipe threads follow entirely different rules.
   • Metric standards are often very close to English standards and can often be confused. Sometimes context can be helpful (e.g. if the hardware is from a non-American car, it's probably metric).

   
   
   
   

How to read Vernier Caliper and Micrometer?

How to use and read a vernier caliper？

An ordinary vernier caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.
The basic steps are as follows:

1. Preparation to take the measurement, loosen the locking screw and move the slider to check if the vernier scale works properly. Before measuring, do make sure the caliper reads 0 when fully closed. If the reading is not 0, adjust the caliper’s jaws until you get a 0 reading. If you can’t adjust the caliper, you will have to remember to add to subtract the correct offset from your final reading. Clean the measuring surfaces of both vernier caliper and the object, then you can take the measurement.
2. Close the jaws lightly on the item which you want to measure. If you are measuring something round, be sure the axis of the part is perpendicular to the caliper. Namely, make sure you are measuring the full diameter. An ordinary caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.
3. How to read the measured value:

1), Read the centimeter mark on the fixed scale to the left of the 0-mark on the vernier scale. (10mm on the fixed caliper)

2). Find the millimeter mark on the fixed scale that is just to the left of the 0-mark on the vernier scale. (6mm on the fixed caliper)

Shop By Products

• + Digital Calipers
• + Micrometers
• + Depth Gauges
• + Height Gauges
• + Digital Indicators
• + Dial Indicators
• + Thickness Gages
• + Dial Caliper Gages
• + Dial Calipers
• + Vernier Calipers
• + Special Calipers
• + Others

Home > Education > Vernier Calipers Information

How to use and read a vernier caliper？

An ordinary vernier caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.
The basic steps are as follows:

1. Preparation to take the measurement, loosen the locking screw and move the slider to check if the vernier scale works properly. Before measuring, do make sure the caliper reads 0 when fully closed. If the reading is not 0, adjust the caliper’s jaws until you get a 0 reading. If you can’t adjust the caliper, you will have to remember to add to subtract the correct offset from your final reading. Clean the measuring surfaces of both vernier caliper and the object, then you can take the measurement.
2. Close the jaws lightly on the item which you want to measure. If you are measuring something round, be sure the axis of the part is perpendicular to the caliper. Namely, make sure you are measuring the full diameter. An ordinary caliper has jaws you can place around an object, and on the other side jaws made to fit inside an object. These secondary jaws are for measuring the inside diameter of an object. Also, a stiff bar extends from the caliper as you open it that can be used to measure depth.
3. How to read the measured value:

1), Read the centimeter mark on the fixed scale to the left of the 0-mark on the vernier scale. (10mm on the fixed caliper)

2). Find the millimeter mark on the fixed scale that is just to the left of the 0-mark on the vernier scale. (6mm on the fixed caliper)


3). Look along the ten marks on the vernier scale and the millimeter marks on the adjacent fixed scale, until you find the two that most nearly line up. (0.25mm on the vernier scale)

4). To get the correct reading, simply add this found digit to your previous reading. (10mm + 6mm + 0.25mm= 16.25 mm)

4). To get the correct reading, simply add this found digit to your previous reading. (10mm + 6mm + 0.25mm= 16.25 mm)


4.Maintenance
Clean the surface of the vernier caliper with dry and clean cloth (or soaked with cleaning oil) and stock in a dry environment if it stands idle for a long time.
Buy Quality Vernier Calipers at Tresnainstrument.com Now!


Reading the Metric Micrometer

To read the metric micrometer, first read the number of whole millimeters on the bottom row of the sleeve of the micrometer (Figure 3). These are the whole millimeters. If there is an additional line uncovered on the top row, this is equal to one-half a millimeter (Figure 3). So 0.5 mm would need to be added to the measurement. Then add the thimble reading to whole and half millimeter sleeve readings. Refer to figures 3,4, and 5 and try to read the measurements before looking at the answer in the figure explanation.

Figure 3  Metric micrometer reading equals 23.15 millimeters.

23 whole divisions are uncovered = 23 millimeters. 0 millimeters divisions are uncovered = 0.0 millimeters 15  0.01 millimeter divisions line up on the thimble = 0.15 millimeters.

Figure 4a  Metric micrometer reading equals 18.60 millimeters.

Figure 4a  Metric micrometer reading equals 18.60 millimeters.

Reading the Vernier Metric Micrometer

The vernier metric micrometer has the ability to measure to two thousandths of a millimeter (0.002-mm). 0.002 mm is equivalent to approximately 0.00008 of an inch.
The vernier is graduated in increments of 0.002 mm starting and ending with 0 (Figure 5).
If either 0 on the vernier graduation scale lines up with the thimble reading, no additional thousandths of a millimeter are added to the reading.


Figure 5  The two-thousandths vernier scale line that lines up with the graduated line on the thimble is added to the reading. In this case 0.006 mm must be added.

The reading of the vernier metric micrometer is accomplished by adding the whole millimeter, the half-millimeter, and the hundredths of a millimeter just as before. To this reading is added the number of two-thousandths of a millimeter, which is read off of the vernier scale (Figure 5).

Figure 6  Metric micrometer reading equals 15.584 millimeters.

15 - whole divisions are uncovered = 15 millimeters. 1 - 0.50 millimeters division is uncovered = 0.5 millimeters 8 – 0.01 millimeter divisions line up on the thimble = 0.08 millimeters. 4 – vernier divisions line up on the vernier scale = 0.004