GD & T Characteristics - B-1 Parallelism

 

B-l: Parallelism : 

It is a desired characteristic between two surfaces (Opposite faces Of a fixture plate or guideway surfaces of a bench vice); one surface and one axis (lead screw axis and bed of a lathe or Axis Of spindle and table surface of a milling machine); two axes (T Slots of milling machine table or two guide pillars of a draw die). It is important due to the level of accuracy expected during the motion of the concerned parts or fitment of the concerned parts. 

At any Of such cases, parallelism as a desired geometric characteristic shall be indicated by means of a symbol consisting of two parallel lines with 606 slant. with their height equal to 1.5 times the height of any alphabet or numerical figure at the feature control frame; and spacing of around half of the height of any alphabet. To indicate the permissible deviation at parallelism for any feature; some opposite datum feature shall be essential. 

Fig.1 Parallelism

Accordingly, a typical feature control frame for parallelism shall appear as shown. The permissible deviation for the parallelism is always smaller than the permissible size variation. In the illustrative case, the upper surface B is expected to be parallel to the datum feature (surface) A within 0.2 mm. But neither the datum feature "A" nor the upper surface "B' t can be expected to be absolutely plain and flat. Hence, the datum feature plane "A" shall be defined either by a datum simulator (like surface plate below the surface "A") or a simulated datum at "A". 

Fig.2 Parallelism

Similarly, since the surface "B" under discussion also cannot be absolutely plane or flat, its possible orientation is to be first. This can be done by identifying the local ups and downs due to the waviness or warpage of the surface and aligning them at possible orientation. Now the deviation at parallelism can be detected.

At the illustration "AA" and "BB" are the possible orientations of the upper face 'B', even considering the waviness and warpage Of the surface 'B'. Now the condition for parallelism States that the level difference between the uppermost and lowermost locations within the surface 0B' must be within the specified tolerance. However, these levels must within the specified size tolerance. 

Fig.3 Parallelism

An alternate exists where the surface B is supplied with a flat surface simulator from the upper side which can be a small size thin flat plate. Such arrangement shall result in four extreme levels within the component width and the length; as l, 2, 3, and 4. Now the deviation within l. 2, 3 and 4 shall be the actual deviation which shall be comparable to the permissible deviation mentioned at the component drawing. Since the upper face simulator is tangential to some limited high points of the surface 'B' , this concerned plane is called the Tangent plane. It must be understood that the "Possible orientation plane" (discussed earlier) and the 'Tangent plane" need not be the same. The possible orientation is arrived by averaging the ups and downs but the tangent plane is concerned regarding the "ups" only. 

When this alternate method is proposed to be used, it is indicated on the drawing by a 'Tangent plane symbol" at the feature control frame. This symbol 's the letter T in a circle, put immediately after the amount of prescribed tolerance at the second chamber of the feature control frame. Such a symbol at the drawing shall suggest that the confirmation of the parallelism shall be done as per the suggested procedure. 

Fig.4 Parallelism

The tangent plane identification method is more suitable to those desired parallel surfaces involving relative motion. The method involving possible orientation is more suitable to those desired parallel surfaces involving static fitments.

When any axis some long O.D. or long bore is required to be parallel to some datum surface, the allocation of the symbol and construction of the feature control frame follows the same method. Here, the deviation at parallelism can be due to some tilt or misorientation of the cylindrical feature. The amount of permissible deviation in the second chamber of the feature control frame shall mean that, within the available length of the cylindrical feature (150 mm), the permissible deviation at the parallelism of the axis with respect to surface "A" is 0.2 mm max. The permissible deviation at the parallelism can be larger than the tolerance at the size but must be smaller than the tolerance at the position (if any).

Fig.5 Parallelism

Such parallelism by means Of slip gauges inserted at within the 150 mm length and the difference to be noted: or by moving a dial indicator (to be rested on datum A and aligned at the 4086 outer diameters) from one end of the 150 mm length to the other end, In fact, such methods are plenty in number and differently suitable to the comments under discussion. 

Fig.6 Parallelism

Sometimes, one axis of an outer diameter is required to be parallel to Some other axis. However, here the case cannot be similar to the previous cases. In case Of parallelism with respect to a datum surface, the deviation at parallelism can be only in one direction i. e., perpendicular to the datum surface. However, in the case of the parallelism with respect to an axis, the expected deviation at parallelism can be in any possible direction. Therefore, a zone descriptor symbol becomes essential to be mentioned here at the second chamber, Herc, the zone descriptor symbol is "φ" indicating a circular zone of diameter 0.2 mm.

Fig.7 Parallelism

Accordingly, the feature control frame shall mean that the axis of 30 mm pin can be allowed to have a deviation at its orientation (not position) but to be contained in an imaginary cylinder of 0.2 mm diameter and 50 mm height. At the figure for interpretation. "PP" and "QQ" are the deviated orientations resulting in error at parallelism: but they are to be contained within a 0.2 diameter imaginary cylinder.

Measurement of such is because of any possible direction for 'PP' or 'QQ'. Here the qualification or disqualification of the Component can be done by a special purpose qualifying gauge, Here, the special purpose gauge can consist of cylindrical sleeves a fixed center distance 100 mm; one of them to have a bore to correspond to the size 40f7 and the other one to have its bore to correspond to Size MN.

If compared to each other at the same time; then the amount of the parallelism tolerance can be larger than the tolerance at the size but must be smaller than the tolerance at the position. 

Similarly. if two bores (or internal dimensions) are expected to be parallel to each other, then the corresponding feature control frame can be seen and explained in a similar manner.

Fig.8 Parallelism

The feature control frame for the parallelism of 30H8 bore with respect to 50H8 shall mean that the axis of the 30H8 bore is allowed to orient in any direction but must lie within an imaginary cylinder of 0.25 mm diameter and 40 mm height. Such deviation at the parallelism can be checked by means 01 a special purpose qualifying gauge which shall have two pins at a center distance 150 mm, One pin shall have its OD. corresponding to 50H8 but the pin shall have its OD corresponding to the size MN. Such a gauge is expected to completely fit into the component.

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