Surface Roughness of Engineering Drawing

Technical drawings use a standardized graphical language that allows subcontractors to understand designers’ intent. They contain size and position specifications, dimensional tolerances and often surface texture tolerances. The technical drawing is a contract binding the parties, that acts as a reference in case of disputes about non-compliance of the produced workpiece.

Surface texture specifications are indicated using the root symbol. This root symbol is defined in the ISO 1302 standard (latest version published in 2002). The new ISO 21920-1 series will soon replace this standard with several changes that may modify common practices.

Varieties of Surface Roughness Indicators

Definitions and presentations of arithmetic average roughness (Ra), maximum height(Ry), 10-spot average roughness (Rz), average concave-to-convex distance(Sm), the average distance between local peaks(S)and load length rate(tp)are given as parameters indicating the surface roughness of an industrial product. Surface roughness is the arithmetic average of values at randomly extracted spots on the surface of an object.

Arithmetical Average Roughness, Ra

A portion stretching over a reference length in the direction in which the average line extends is cut out from the roughness curve. This portion is presented in a new graph with the X-axis extending in the same direction as the average line and the Y-axis representing the magnitude. Ra is represented by the equation shown at right, in microns (µm).

Maximum Height, Ry

A portion stretching over a reference length in the direction in which the average line extends is cut out from the roughness curve. The gap between the peak line and the trough line is measured in the direction in which the magnitude axis extends, in microns (µm).

Ten-Spot Average Roughness, Rz

A portion stretching over a reference length in the direction in which the average line extends is cut out from the roughness curve. The average of the levels (Yp) of the highest peak to the fifth highest peak as measured from the average line and the average of the levels (Yv) of the lowest trough to the fifth-lowest trough similarly measured in the said portion is added together. Rz is this sum, in microns (µm).

Yp1,Yp2, Yp3, Yp4, Yp5 = Levels of the highest peak to the fifth highest peak in the said portion with the length (L).

Yv1, Yv2, Yv3, Yv4, Yv5 = Levels of the lowest trough to the fifth-highest trough in the said portion with the length (L).

Surface Roughness Classification

The arithmetic means roughness has been classified by ISO into 12 roughness levels, from N1 to N12. For the indication on the drawing regarding this roughness specification, it can be written directly the Ra value, or the roughness level.

Obsolete indications

The ISO 1302 standard has evolved since its first version in the 1970s, and several practices are now obsolete and should not be used anymore on modern drawings. This is why, users should be cautious with books that are quite old because they are not compliant anymore with current rules.

Here, two examples of obsolete notations, where the limit was indicated on the left of the root symbol. It was even possible to use classes (here N6). The Ra parameter was the default parameter and therefore was omitted.

Root symbol

The specified parameter should be placed under the root symbol with, at the very least, its limit value (see figure below). These parameters are defined in ISO 4287, ISO 13565 or ISO 12085. Many other indications are defined by default and can be omitted from the drawing.

Positions of Auxiliary Symbols for Surface Symbol

A surface roughness value, cut-off value or reference length, processing method, grain direction, surface undulation, etc. are indicated around the surface symbol as shown in Fig. 1 below.

Surface Symbol

a : Ra Value

b : Machine Method

c’ : Cut-off Value, Evaluation Length

c : Reference Length, Evaluation Length

d : Grain Direction

f : Parameter other than Ra (tp : Parameter/Cut-Off Level)

g : Surface Undulation (according to JIS B 0610)

Material removal

The triangle can be open or closed, or have a circle, to specify a particular constraint regarding material removal during machining.

a b c

a. Surface Symbol, b. Removal of Material by Machining is Required, c. Removal of Material is Prohibited

An open triangle does not give any constraint. A closed triangle requires material removal during machining. A circle forbids material removal.

All surfaces around

When the specification applies on all surfaces around a workpiece, the symbol just needs to have a circle at the corner between the horizontal and oblique bars. It simplifies the drawing as a single indication applies on many surfaces.

Orientation of texture lay

At the foot of the root symbol, on the right of the triangle, it is possible to specify the texture lay. Some machining methods leave a signature on the texture with oriented scratches and grooves.

The most common are parallel, perpendicular or crossed grooves, with respect to the projection plane.

Other possibilities also exist M for multidirectional, C for circular, R for radial and P for particular.

The trace left by a cutting instrument is parallel to the projection plane in the drawing. Ex. Shaped Surface

The trace left by a cutting instrument is perpendicular to the projection plane in the drawing. Ex. Shaped Surface (Side View), Circular Cut, Cylindrical Cut.

The pattern left by a cutting instrument diagonally crosses the projection plane in the drawing. Ex. Honed Surface.

The pattern left by a cutting instrument crosses in various directions or has no grain direction.

Ex. Lapped Surface, Superfinished Surface and Surface Finished with a Front Mill or End Mill.

The pattern left by a cutting instrument is virtually concentric around the centre of the plane in the drawing. Ex. Faced Surface

The pattern left by a cutting instrument is virtually radial around the centre of the plane in the drawing.

Position on a drawing

The root symbol is placed on a drawing, with the tip of the triangle in contact with the surface, including on vertical faces (it has to be rotated to the left then).

The root symbol can also be placed on a dimension lead line, as long as it stays readable. Other options are available, and in particular, simplified writing.

Surface Roughness Table in Manufacturing Process

This table is used as a reference for designers to determine the level of roughness or as a sign of workmanship in the production process. for example, that an object will be milled with a certain roughness, it must include the level of roughness, for example, N8 so that the process reaches a roughness level of 3.2 micrometres

Reference

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/08/surface-roughness.pdf

https://www.digitalsurf.com/blog/how-is-surface-texture-indicated-on-drawings/

https://guide.digitalsurf.com/en/guide-indications-iso-1302.html