Understanding geometric tolerances is an essential aspect for accurate measurement that cannot be overlooked. Geometric tolerances refer to the permissible range of errors in shape, positioning, and other factors. They are crucial information on drawings to accurately convey design intent. Geometric tolerances are classified into 15 types. In this article, ACCRETECH Thailand's Sales Engineer, Mr. Hitomi and Sales Manager, Ms. TIP will explain “True Position”,classified under positional tolerance.

TIP san, have you ever seen a diagram like this?
The above drawings indicate positional degree. Can you see what kind of instructions are given?

Hmm... I've seen the symbols before, but it's a bit challenging...

Positional tolerance is a geometric tolerance often used where holes and shafts fit together. Simply put, it is a numerical representation of "how much they are misaligned from a reference point." Parts are generally set to a tolerance so that the shaft can fit into the hole even if the hole and shaft are slightly misaligned. Imagine that the diagram above shows two parts, with the female side (hole) on the left and the male side (shaft) on the right, and that these parts are used in combination. If the instructions are as shown in the diagram above, the shaft and hole will fit together without any problems if they are in the same position, but they cannot be made in exactly the same position. Some misalignment will inevitably occur. This is when we use the "positional tolerance" we discussed today. It means that as long as the positional relationship of each part is to this extent, it is okay if they are misaligned. By the way, holes are generally made larger and shafts are made smaller.

【Positional Degree】

The magnitude of deviation of a point, linear or planar form from a theoretically exact position determined in relation to the datum or other form.

Now I remember! This is an item that some of our clients have asked me about.

The most important concept in considering positional degree is "Maximum Material Virtual Condition" (MMVC). This refers to the condition where the hole is at its smallest size and the shaft is at its largest size within the tolerance zone. The role of positional degree is to establish tolerance values for fitting the hole and shaft together even if they are misaligned in this MMVC condition.

In the case of the example diagram, the clearance would be as follows:
The dimensions in red indicate the hole at its smallest size and the shaft at its largest size (MMVC). 10.1 mm (Min Hole size) - 9.9 mm (Max Shaft size) = 0.2 mm
In other words, it is acceptable for each hole to be misaligned by 0.1 mm in each direction. The position degree φ0.1 shown in the diagram represents this tolerance.

An important yet often overlooked point: Maximum Material Condition (M in a circle).

This symbol with M in a circle. Do you know what it represents?

I remember it is a symbol that had some special meaning...

This is one of the symbols used to indicate “Maximum Material Condition (Bonus Tolerance),” commonly referred to as "M in a circle," when specifying hole positions. As the name suggests, it implies loosening the range of geometric tolerances. Interestingly, even those involved in quality control often overlook this point.

I see!

The positional degree tolerance on the above diagram is indicated by the symbol "M in a cycle." Suppose the actual size of the finished hole is 10.15 mm. In this case, since it is 0.05 mm larger than the minimum hole diameter (10.1 mm), 0.05 mm will be added to the tolerance of positional degree. Bonus Tolerance = |Actual Dai – Minimum Diameter(MMC)| In other words, the tolerance is bonus applied when the actual measured hole diameter exceeds the minimum hole diameter.

This M in a circle plays an important role in expanding tolerances according to actual form, reducing the number of defects without compromising product functionality. It's a crucial point that anyone involved in QA/QC tasks should definitely be aware of!

Basic Measurement Seminars will be Held at Private Shows!

Every year at our private show, we conduct seminars in Thai on basic knowledge and concepts related to measurement. This course is ideal for beginners in measurement and also serves as a good refresher for understanding various measurement concepts. It has been well received by many participants every year.

The basic seminar is popular every year, so early reservations are recommended!

It's a course I look forward to every year!
We will announce when bookings open on our website and via social media.

ACCRETECH (THAILAND) CO., LTD.
Shota Hitomi
Senior Sales Engineer Manager
METROLOGY DIVISION Sales Engineer Section

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