What does M in fasteners business mean? Why we couldn`t call Tapping/Drilling screw as M?


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    When we are talking about fasteners sizes, we might have heard about Mxxx such as M10x30, M16x100 or some sort of this that beginning with Mxxx a lot. Then why we can call Hexagon Bolt or other machine screw as M but for drilling and tapping screw, we couldn’t mention M as their sizes. Today I will explain about this issue.

    “M” refers to fasteners thread form in Metric system which we known as JIS, DIN ,and ISO fasteners. Thread angle between each will be 60°for Metric system. For example M10x1.5P “10” refers to Nominal size of fasteners in millimeter (mm) and 1.5P is refers to pitch distance between each thread, 1.5P for M10 is a coarse thread which is normal thread for this size but 1.25P or less, will be fine thread.

Before we fasten screw or bolt into the hole, we must drill at hole using drilling bit to make initial hole into the desired surface then using hand tap and machine tap to make a thread into drilled hole as below illustration.

1.       Drill an initial hole with drill bit


2.       Use hand tap or machine tap to make threaded hole

3.       Fasten desired fastener


    For example, if we would like to fasten M10x1.5Px50 Hexagon bolt (with general situation without concerning any application condition), we have to drill the hole at 8.5mm within tolerance of  8.38mm (-) tolerance and 8.68mm (+)Tolerance

Then we will use hand tap size M10x1.5p to fasten to that pre-tapped hole to make thread into the hole. After cleaned the hole, we can fasten this M10x1.5Px50 Hexagon bolt.

According to above process, we need at least 2 process to fasten Metric system fasteners.

    The point is, when we use a hand tap to pierce through the hole, it will expand straight hole into systematic threaded hole which tolerance and pitch are designated as the standard. To make it ready for bolt or screw to be fasten drilling screw doesn’t need pre-tapped hole because of their piercing function.

Tapping screw, do need some pilot hole at the object with smaller drill bit to the screw size to make it more precise and easier to pierce through the object. Most of people may drill it directly from the first, this might cause misdirection of the tapping screw installation.

Thus, the hole which be pierced with those piercing screw can be done in one process, in the other word, just fasten them to the metal or wood!

The screw will pierce the material and make itself as a thread and fasten to the desired surface or material.

    Anyway, thread system is not referred to Metric system and doesn’t require pre-tapped threaded hole, so we can’t use M before their sizes. They have their own specific diameter that will pierce through the surface Ex. A-Tapping Pan Head Screw 6x30 or Hex Drilling screw 6x19. Which are not refer to Metric thread system.

I will talk in detail about tapping screw and drilling screw in upcoming blog.

If there are any issue you would like us to write, please feel free to share your though with us!


Written by

Mr.Nuttapol Sirilert

International Sales Dept.

Hanshin Neji Co.,Ltd.