Artigo

Understanding composite positional tolerances in GD&T

Fig1 Hole Pattern

By Onat Ekinci

Introduction

Composite tolerances in GD&T define multiple levels of positional control for patterns of features. Given their multi-layered complexity, they may look very challenging at first sight. The goal of this article is to present different variations of composite tolerances and discuss their differences. The difference between composite positional tolerances and single-segmented positional tolerances will also be discussed.

Composite tolerances are used when we have relatively looser location requirements but tighter orientation tolerances. The full definition of composite tolerances can be found in section 10.5 of the ASME Y14.5-2018 standard[1].

A simple example would be a set of holes (pattern) used to affix a name plate. The relative position of the holes is important (tighter tolerance), in order to match the same hole on the plate itself, but the absolute position of the entire pattern on the part may be less critical (looser tolerances) as long as the orientation is good.

In this blog post, we shall present the following variations of positional tolerances:

  1. Composite tolerance with only primary datum in the lower segment
  2. Composite tolerance with primary and secondary datums in lower segment
  3. Two Single-Segmented Position Tolerancing

Terminology:

In the ASME Y14.5 standard, a feature control frame is a rectangle divided into compartments containing the geometric characteristic symbol, followed by the tolerance value and datum references. Composite position tolerances and two single-segment position tolerances follow the same logic with one major difference:

  1. In composite tolerances, the geometric characteristic symbol contains a single entry (Figure 1)
  2. In two single-segment feature control frames, there are two separate geometric characteristic symbols (Figure 2)

Composite feature control frame

Figure 1. Composite feature control frame.

Two single-segment feature control frame.
Figure 2. Two single-segment feature control frame.

Composite tolerance with only primary datum in the lower segment

Hole pattern

Figure 3. Hole pattern with only primary datum repeated in the lower segment of composite position tolerance (created in BuildIT Metrology Software).

In Figure 3, we observe a composite tolerance created using BuildIT Metrology Software. The algorithms implemented in the software evaluate the measured features according to specifications of ASME Y14.5, explained in this post.
Hole pattern located by composite tolerancing
Figure 4. Hole pattern located by composite tolerancing (Primary datum only in lower segment). This figure represents one of the possible displacements of the pattern. Note that the yellow and blue circles represent the tolerance zones with 0.5 mm and 0.1 mm diameters, respectively.

Figure 4 shows the tolerance zones and their functions for composite tolerances with only primary datum in the lower segment:


Yellow Tolerance Zones: Pattern locating tolerance zone framework (PLTZF).

Blue Tolerance Zones: Feature relating tolerance zone framework (FRTZF).

The PLTZF constrains the location and orientation relative to datum ABC.

The FRTZF:

  1. Constrains the location and orientation of individual hole features within the pattern
  2. Constrains the orientation only relative to datum A

The axis of the holes must reside within the tolerance cylinders of both the PLTZF and FRTZF.

Animation showing hole pattern located by composite tolerancing
Figure 5. Animation showing hole pattern located by composite tolerancing (Primary datum only in the lower segment). Any of the displacements in this animation represents a possible configuration for composite tolerancing with only primary datum in the lower segment. Note that the yellow and blue circles represent the tolerance zones with 0.5 mm and 0.1 mm diameters, respectively.

In the animation in Figure 5, the yellow tolerance zone pattern designates the PLTZF and the blue tolerance zone pattern designates the FRTZF.

Again, the important difference between the upper and lower segment is that in the lower segment, the tolerance zones are only constrained in orientation with respect to datum A. The lower segment does not locate. Thus, the blue tolerance zones (the FRTZF)

nly in rotation (they must stay perpendicular to A), but they are free to translate and free to rotate with respect to datum B or C and stay inside the larger tolerance zones.

Note that in certain positions, portions of the smaller blue tolerance zones can fall beyond the limits of the larger yellow tolerance zones. However, these portions of the smaller tolerance zones are not usable because the axes of the features must not violate the limits of larger tolerance zones (ASME Y14.5-2018, 10.5.1.1).


Composite tolerance with primary and secondary datums in lower segment:

ole pattern with primary and secondary datums repeated
Figure 6. Hole pattern with primary and secondary datums repeated in the lower segment of composite position tolerance (created in BuildIT Metrology Software).

In composite tolerances with primary and secondary datums repeated in lower segment, one more degree of freedom is constrained. In Figure 6, we can see an example of a composite tolerance with primary and secondary datums in the lower segment created using BuildIT Metrology Software.
Hole pattern located by composite tolerancing
Figure 7. Hole pattern located by composite tolerancing (primary and secondary datums in lower segment). This figure represents one of the possible displacements of the pattern. Note that the yellow and blue circles represent the tolerance zones with 0.5 mm and 0.1 mm diameters, respectively.

Figure 7 shows the tolerance zones and their functions for composite tolerances with primary and secondary datums in the lower segment:

Yellow Tolerance Zones: Pattern Locating Tolerance Zone Framework (PLTZF).

The PLTZF constrains the location and orientation relative to datum ABC.

Blue Tolerance Zones: Feature Relating Tolerance Zone Framework (FRTZF).

The FRTZF:

  1. Constrains the location and orientation of individual hole features within the pattern
  2. Constrains the orientation only relative to datum A and datum B.

Animation showing hole pattern located by composite tolerancing

Figure 8. Animation showing hole pattern located by composite tolerancing with primary and secondary datums in the lower segment. Any of the displacements in this animation represent a possible configuration for the composite tolerancing with primary and secondary datums in the lower segment.

The important difference between the upper and lower segment is that in the lower segment the tolerance zones are only constrained in orientation with respect to datums A and datum B. The lower segment does not locate with respect to datums A and B. Thus, the FRTZF can move up and down and right and left (Figure 8).


Two single-segmented position tolerancing:

 Two single-segmented position tolerancing
Figure 9. Two single-segmented position tolerancing (created using BuildIT Metrology software).

Two single segmented position tolerances are not composite tolerances. An example of this can be seen on Figure 9. Both segments control the location and the orientation defined relative to their datum reference frames.

Hole pattern located by two single-segmented position tolerancing.

Figure 10. Hole pattern located by two single-segmented position tolerancing.

Figure 10 shows the tolerance zones and their functions for two single-segmented position tolerancing:


Yellow Tolerance Zones: Represent the upper segment.

The upper segment constrains the location and orientation relative to datum ABC.

Blue Tolerance Zones: Represent the lower segment.

The lower segment constrains the location and orientation relative to datum A and datum B.

As shown in Figure 10, the blue tolerance zones are now also located by datum B (as opposed to composite tolerances which only constrained orientation). They are only free to translate left and right, as datum C allows.
Animation showing hole pattern located by two single-segmented position tolerancing
Figure 11. Animation showing hole pattern located by two single-segmented position tolerancing.

As shown in the animation on Figure 11, now the lower segment also constrains the location, in addition to the orientation. Thus, the blue zones are also constrained in moving up and down (the location constraint imposed by datum B). They can only move right and left, since the datum C is not constrained in location.


Conclusion

Composite position tolerancing is an advanced conceptual tool for fine tuning the required orientation in parts with hole patterns. It provides the ability to adjust location and orientation requirements on these complex parts.

The upper segment in the control frame specifies both location and orientation, thus establishing translational and rotational constraints, whereas the lower segment specifies orientation only, establishing only the rotational constraint. The lower segment imposes tighter tolerances for orientation than the upper segment, thus allowing a fine tuning in rotational adjustment.

References:

1. ASME Y 14.5-2018, Dimensioning and Tolerancing. New York: American Society of Mechanical Engineers.

Artigo
Fabricação
Inspeção e controle de qualidade
Aeroespacial
Automotivo
Educação
Energia e recursos naturais
Equipamentos pesados
Prestadores de serviços de medição
Metalurgia, usinagem e montagem
Borracha e plásticos
Construção naval
Software — Controle de qualidade e metrologia
BuildIT Metrology
Inspeção e controle da qualidade (BP)
Conhecimento
Melhores Práticas
Indústria e Desenvolvimento

Assine nosso boletim informativo para manter-se informado.

Conteúdo relacionado

Filtros

Filtros

비디오

Pratt Miller가 3D 기술을 활용해 차량 개발을 개선한 방법

Pratt Miller가 FARO Leap ST 핸드헬드 스캐너를 어떻게 차량 조립의 정밀한 설계, 테스트, 반복 작업을 위한 최첨단 도구로 활용하고 있는지 알아보세요.
인포그래픽

Blink 시작하기

이 인포그래픽을 통해 Blink가 어떻게 사진을 찍는 것처럼 쉽게 현실 데이터를 캡처하고 공유할 수 있는지 알아보세요.
비디오

Blink 이미징 레이저 스캐너 개봉하기

Blink는 간단한 워크플로, 자동화된 인사이트, 시각적 현실 캡처 방식을 위해 설계된 소형 이미징 레이저 스캐너입니다. 언박싱 영상을 시청하여 포함된 구성품을 확인해 보세요!
비디오

Blink by FARO®: 간편한 레이저 스캐닝

누구나 현실을 캡처할 수 있게 도와주는 Blink 이미징 레이저 스캐너에 대해 알아보세요. 가이드 워크플로와 포인트 클라우드 자동화를 통해 누구나 데이터를 스캔하고 공유할 수 있습니다.
비디오

Blink로 시작이 쉬워 집니다

FARO® Technologies의 Blink 이미징 레이저 스캐너를 사용하여 스캔을 설정하고 시작하는 방법을 알아보세요. 간단한 워크플로를 따라 정확한 현실 데이터를 쉽게 캡처하고, 보고, 공유할 수 있습니다.
비디오

Blink 이미징 레이저 스캐너: 기술 제품

Blink 이미징 레이저 스캐너의 활용 사례를 확인해 보세요! 이 이미징 레이저 스캐너는 3D 포인트 클라우드과 360° 사진을 손쉽게 캡처합니다. 스캔, 프로세싱 및 공유가 얼마나 쉬워지는지 직접 확인해 보세요.
브로셔

FARO® Technologies의 Blink

자동화된 워크플로, 뛰어난 비주얼, 원터치 인사이트를 통해 누구나 리얼리티 캡쳐가 가능합니다.
ARTICLE

Understanding Handheld 3D Scanning and Smart Manufacturing

The smart factory of today and current iterations of handheld scanning are inseparable companion tools. Learn how the FARO Leap ST is helping lead the way.