Các dấu vết là một số dài 10mm và nó cho thấy rằng bề mặt không phải là một đường thẳng 10mm dài lý tưởng. Độ lệch so với chiều dài 10mm này từ đỉnh cao nhất tới thung lũng thấp nhất là 4,2 micron nhưng đây là một bề mặt được sản xuất bằng cách gia công chính xác. Nó không phải là chỉ có bề mặt phẳng được biến. | Dimensions symbols and tolerances 79 The trace is some 10mm long and it shows that the surface is not a 10mm long ideal straight line. The deviation over this 10mm length from the highest peak to the lowest valley is 4 2 microns yet this is a surface produced by precision machining. It is not only flat surfaces that are variable. Figure shows roundness traces from three positions along a ground hole. The traces do not indicate the diameter of the holes merely their variability. The fact that they are three concentric circles of varying Figure Trace of a flat surface showing the deviations from the ideal straightness Figure Roundness traces of a ground hole showing deviations from an ideal circle 80 Engineering drawing for manufacture diameter is due to the fact that the instrument settings are varied so that the radii can be separated. Each trace thus represents the circular trace around the ground bore and displays the out-ofroundness not the absolute diameter. Clearly each trace is far from an ideal circle showing that even a precision ground hole has some variability. The above two figures have demonstrated that a hole can never be perfectly straight or round. The same will apply to other aspects of the hole like taper and perpendicularity. The variability will be different each time a surface is produced on the same machine and also between different machines and processes. The variability will be higher with rough-machined surfaces and lower with precision-machined surfaces. The table in Figure shows the variability of some hole manufacturing processes. The data refers to processes used for producing holes 25mm in diameter. In the figure the word taper means the maximum inclination over a 40mm length. The word ovality means the difference between the maximum and minimum diameters at perpendicular positions. The word roundness means the deviation from a true circle. The words average roughness represented by Ra see Chapter 6 mean the average
