Compare "Coarse / Fine" Thread

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Pitch of Coarse thread and Fine thread comparison table by Nominal Diameter
Nominal Diameter M1 M1.2 M1.4 M1.6 M1.8 M2 M2.5 M3
 M3.5
 M4
Coarse thread 0.25 0.25 0.3 0.35 0.35 0.4 0.45 0.5 0.6 0.7
Fine thread 0.2 0.2 0.2 0.2 0.2 0.25 0.35 0.35 0.35 0.5

 

Use Coarse Threads

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The set screw is usually a coarse pitch of thread. All JIS standard set screws have a coarse pitch of thread.

The coarse thread has a greater engagement between the male and female threads than the fine thread, so the thread strength is greater.

In other words, even if the threads are heavily loaded and repeatedly tightened, it is less likely that the male and female threads will wear out.

 

Use Fine Threads

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  1. Set screw with extremely short overall length.

    To make the short overall length set screw, it is being need with two threads on that set screw.
    If you need a more shorter overall length set screw, it is better to use fine thread instead of coarse thread. (Sample photo of Fine thread M2xP0.25xL1.0)

  2. Set screw used as an adjustment screw.

    In the case of a set screw used as an adjustment screw, it is unlikely that a large load will be applied to the thread, so use a fine pitch. Use a fine pitch, it is aimed at improving the adjustment accuracy.
    (Reference: M2 screw is coarse pitch = 0.4 mm, fine pitch = 0.25 mm.)
    (Sample photo of Fine thread M2xP0.25xL4.0)

    If you need higher screw accuracy for fine pitch, please contact us. At our company, we can manufacture by dividing into normal grade pitch accuracy and precision grade pitch accuracy.
    In the case of normal grade, the single pitch accuracy is within 0.010 mm, and the cumulative pitch accuracy is within 0.020 mm.
    In the case of precision grade, the single pitch accuracy is within 0.005 mm, and the cumulative pitch accuracy is within 0.010 mm.

    However, since the fine pitch is a made-to-order product, the cost is high and the delivery time is long. Therefore, considering the low cost and short delivery time, it would be a good idea to use the oval point set screw in DELscrew standard as the adjusting screw.

  3. Fine thread screw are harder to loosen than coarse thread screw.

    The fine thread has a smaller lead angle than the coarse thread. A small lead angle means that it will not loosen easily due to some vibration.

    It is expected that it will be possible to prevent the screw from loosening after the adjustment is completed by the time the screw is locked.

  4. Manufacture of fine thread requires higher processing accuracy than coarse thread.

    Finer thread require higher screw accuracy because the meshing height of male and female thread is smaller than that of coarse thread. Also, when making female threads for fine thread, the dimensional accuracy of the prepared hole diameter is higher than that for coarse thread.

    The specific screw accuracy differs between coarse and fine threads, depending on the "Tolerance Quality" and the nominal diameter of the screw, and there are a wide variety of combinations, so they are omitted here.

 

Tolerance Quality

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By the way, in the JIS standard of "Tolerance Quality",
"Allowable limit dimensions and tolerances for metric coarse threads" B0209-1997 ,
"Allowable limit dimensions and tolerances for metric fine screws" B0211-1997 and
Tolerance Quality : Fine, Medium, and Coarse : there are three types.

This division is called by the new ISO, and in the old JIS, it was called 1st class, 2nd class, 3rd class.
JIS has the following description as "correspondence between screw tolerance quality and class".

 Classification  Application example
Fine  Precision screws with particularly little backlash.
Medium  General-purpose screws used for machines, tools, structures, etc.
Coarse  Screws used in environments where dirt and scratches are likely to occur, such as building construction work and installation of equipment, or screws that have difficulty in threading, such as threading on hot rolling rods and threading on long blind holes.