Languages for engineering communication - 2.

In similar to the laws regulating scientific reporting engineering communication also developed international laws for the technical drawing. These laws form the base for interchangeability חלפיות של חלק במכונות

2.1 Interchangeability

Interchangeability החליפיות של חלק allows using all over the world the similar parts of machines which are produced in various countries.

There are two possible ways to ensure interchangeability :

The first is to produce the part individually and with absolute accuracy. This wave is complex and uneconomic.

The second way is to divide the parts, into a number of groups with the typical or standard dimension which is equals to the arithmetical mean for this group.

To reduce the number of the standard parts of the machines, אוסף של חלקים סטנדרטים all the dimensions in machining design are rounded off to the nearest whole millimeter. These rounded dimensions are known as the nominal size מידה יסמל .

2.2 Types of the dimensions מדיםיסוגי מ

The rounding of the dimensions נוהל של עיגול introduces two types of dimensions: a) the real dimensions, known as the actual size מידה פועלבb) rounded dimensions are known as the nominal size מידה סמלית.

The surfaces פניו של הגוף also have various types of dimensions. When contacting each to other their dimensions called mating dimensions. All other dimensions are called free dimensions.

The mating surfaces usually classified as external or internal and their dimensions are shown in Fig. 1.

Fig. 1 Mating parts: 1- enveloping surface hole(הקדח)

2- enveloped surface (shaft) (הגל).

Internal surfaces of different shapes can be referred to as holes (הקדח). Their mating external surfaces can be referred to as shafts הגל. The typical example is a keyway and its key.

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2.3 Tolerance סיבולת .

The international laws for technical drawing are based on system of fits and tolerances ת וסיבולתיואפיצו . We can say that without the system of fits and tolerances interchangeability of the parts in machinerywould be impossible. The tolerances סיבולת postulates the arithmetical mean diameter for all dimensions within each group of the parts.

Each production has its accuracy (דיוק של היצור) which in turn is characterized by limits of sizes reached in production (גבולות של המידות) . Tolerances are based on measuring the actual size of the part. The maximum limit Ab of a hole, or Bb of a shaft is the maximum permissible actual size in production.

The minimum limit Am of a hole, or Bm of a shaft is the minimum permissible actual size in production.Tolerance is the difference between maximum and minimum limits and denoted in Fig. 2 by the Greek letter

with a sub index:

hole ((הקדח and (shaft)( הגל)

Since the tolerances are based on the actual size of the part this is characteristics of the production. The tolerances stipulates the permissible error in manufacturing. The value of tolerance shows if the part can serve as duplicate for original or not. The large tolerance makes easier manufacturing while prevents the part from to be interchangeable. The part is interchangeable if the actual size of the part is between the Limits of sizes.

Fig.2 Tolerance of a dimension

2.4 Deviation סטייה

If tolerance is based on actual size, the deviation is based on nominal size מידה יסמל .The upper maximum deviation (סטיה עליונה מכסימלית) for a hole is the difference between maximum limits

and the nominal size מידה יסמל , d of the association

The lower maximum deviation for a hole is the difference between minimum limits and the nominal size d of

the association: .

The upper maximum deviation for a shaft, is the difference between their maximum limits and the nominal size

d of the association: .

The lower maximum deviation, for a shaft, is the difference between minimum limits and the nominal size d of

the association:

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Fig.4 Maximum deviations for a hole

Fig.5 Maximum deviations for a shaft

When one of the limits is equal to the nominal size, the maximal deviation is equal to zero. (0). Relation between the deviation and the tolerance is rather simple:

סטיה מכסימלית עליונה - סטיה מכסימלית תונהח ת =סיבולת

Notifications of the tolerance

A common diagram of limits and tolerance zones in Fig. 6 composed of :1. The line corresponding to the nominal size is called the base line ציר מוצא האפס 2. Lines representing the upper elements of holes or shafts and located above the base line correspond to positive deviations ( יה חיובית ט ס ) ; 3. Lines representing the upper elements of holes or shafts and located below the base line correspond to negative deviations ( ( יה שלילית ט ס

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Fig.6

4. The upper deviation coincides with the upper boundary of the tolerance zone. The lower deviation coincides with the lower boundary of the tolerance zone.

Common notations

1. Tolerances are סיבולת)) expressed in millimeters as the maximum deviations סטיה מכסימלית from the nominal size מידה יסמל . 2. The upper deviation סטיה עליונה is arranged above the lower deviation (סטיה תכתונה) . 3. The figures (מספרים) showing deviations are smaller than these for the nominal size.4. Deviations equal to zero are omitted. 5. The deviations of the hole are usually located above while the deviations of the shaft are below. 6. The numbers of the parts can be also indicated for clarity.

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Example 1

:סיבולות דו-כיווניות

סיבולות חד-כיווניות:

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Conclusions:

1. Tolerance based on actual size and is characteristics of production.2. Deviation based on nominal size and is regulated by the standard for this part.3. Tolerance and deviation both affect the interchangeability of the part.

In order to check where or not the part is interchangeable, one should measure the actual size. The part is interchangeable if the actual size is within the so-called tolerance zone (between the upper and lower deviations.

סיבולת המידה נקבעת משיקולים של:

עלות

דרישות מכניות

עלות היא נגזרת של שיטת ייצור כמות ומהירות הייצור.

.דרישות מכניות נובעות מהתאמה לחלקים אחרים

Fits 2.5האפיצות סוג המגע בין שני חלקים מכנים :אפיצות

The quality of the contact between the parts is specified by the type of the fit, It is difference between the diameters of hole and shaft before association.

Clearance ( אפיצת( חופש - hole is larger than shaft and there is freedom to relative move. Interference (אפיצת לחץ) - hole is smaller than shaft and fixed relative to each.

ISO SYSTEM OF FITS AND TOLERANCES

A tolerance system (הסיבולת) may be subdivided:1) According to the basis, hole (הקדח) or shaft ) (הגל 2) According to the size of the tolerances, which specify the grades of manufacturing accuracy (איכות של היצור)3) According to the size of the clearances or interferences, types of fit(האפיצות) .

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Fig.10

The hole and the shaft base systems

In the hole basis system the hole minimum limits are constant and not depends on fits when the grade of

accuracy and nominal size are fixed. As shown in Fig.10 in order to change the fit (האפיצות) one should vary the .(הגל) of shaft(גבולות)In the shaft (הגל) basis system the shaft maximum limits are constant for all fits and to change the fit one should vary the hole limits (גבולות של הקדך).

The hole basis system is more economical since fixed diameter tools reproduce basic size rather well. Hole based tolerance is marked by large English latter H. A single type of the tolerance for hole is usually of . This means that lower limit is equal to nominal dimension while upper limit is usually larger then nominal dimension.

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Tolerances for shaft are usually marked by small English letters from “a” to “zc”. The shafts produced with tolerances from “a” to “g” are usually enter the hole with clearance .אפיצות מרווח The shafts produced with tolerance from “p” to zc, are usually larger then hole and result in אפיצות מדחק. Shaft based system is marked by small English latter h. Upper limit is equal to nominal dimension while lower limit is usually smaller then nominal dimension רשון הביואזור של הסיבולת חוא תמיד לכ .Tolerances for holes are usually marked by large English letters from “A” to “ZC” (Fig. 13).

Basic notation of the tolerance and fits is composed of:1. 2; מידה יסמל) נומינלית) . maximum deviations ( סטיה מכסימלית) or tolerance (סיבולת); 3. fits (אפיצות); 4. grades of accuracy to which the given fit is to be manufactured (דרגה סבולת).

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Small difference exists between the machine drawings and the designer’s drawings .

On the machine drawings (סיבולת) indicated directly after the nominal size in millimeters. On the designer’s drawings (סיבולת) indicated by symbols only and thereafter technician convert this notation to millimeters.

Examples.

40f7 is a typical example of the designer’s drawings notation (there are no tolerance numbers). It is the shaft (small f);Its nominal diameter is 40 mm and f7 grade of accuracy ensures the clearance fit (see graph 12)

All rest examples are typical for סרטוטי ייצור .

30h6 is again shaft (small h), nominal diameter 30mm which is equal to upper maximal diameter. Lowest diameter is 30-0.013 mm and h6 degree of accuracy ensures the clearance fit which is rather close to boundary of clearance כמעט גבול של אפיצות חופש (see fig. 12).

30F7 is the hole (large F) with nominal diameter 30mm, unidirected upper and lower deviations and H7 degree of accuracy of the fit.

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Tolerances of shape and positionISO standard stipulates notation of the shape and fits on the drawings.

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Examples

To convert format from designer to machine drawing one should specify the values of tolerances and fits using special tables.

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2. Questions to exam.

1. What is the interchangeability of the parts in machinery and how to reach this requirement?

2. Why all the dimensions used in machining design are rounded off עיגול to the nearest whole millimeter.

3. Explain the difference between:

a) the actual size מידה בעליל and the nominal size

מידה יסמל ;

b) mating dimensions and free dimensions;

c) external and internal surfaces.

4. Explain a standard characteristics of the permissible error in manufacturing?

5. When one can say that the part is interchangeable?

6. When the maximal deviation is equal to zero?

7. Explain difference between positive and negative deviations.

8. What kind of association performances is characterized by the fit?

9. Explain difference between the hole and shaft systems.

What kind of the system is more effective economically?

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