What is the difference between roughness and flatness

In mathematical terms, this is the integral of the absolute value of the roughness profile, divided by the profile length. Because of this averaging, the Ra value is lower than the actual height of the roughness variations. To calculate Rz , the roughness profile is divided into five equal lengths. The height difference between the highest and lowest point in each of the five sections is known as the total roughness, or Rt , for that section.

Rz is the average of the five Rt values. The calculated Rz value is approximately the height of the most severe roughness variations. Since Ra represents average values, and Rz is based on maximum values, Rz is almost always greater than Ra. The difference between the two parameters will depend on the uniformity of the roughness profile.

If one value is known, it is possible to estimate a maximum for the other, but this approximation should not be used for critical applications.

As a rough rule, if only Rz is known, Ra can be approximated by dividing by a factor of 7. If Ra is known, the value of Rz for the same surface can be up to 20 times higher and a little more difficult to approximate. The unit used in the United States for roughness measurement is micro-inches. The corresponding international SI unit is micrometers, or microns for short.

Just as one meter is The basic surface finish symbol is a check mark with the point resting on the surface to be specified. Variations of this symbol provide additional instructions as described in the table below. Numbers near the basic surface finish symbol are used to provide different surface finish parameters. The location of the number in relation to the symbol determines which parameter is indicated.

The letters in the figure below show the proper location for each parameter according to the ASME Y Where a represents the average roughness value Ra , and b represents the production method, coating, note, or other additional information. The letter c provides the roughness sampling length in millimeters or inches, while d gives the direction of the surface lay.

The value of e indicates a minimum material removal requirement in millimeters. Finally, if an alternate surface finish parameter is provided, the parameter symbol and value are provided in location f ie: Rz 0.

The lay direction is parallel to the drawing plane of view where the symbol appears. Surface finish is highly dependent on the process used to manufacture the part, and very smooth surface finishes usually require additional processing such as grinding or polishing.

In a pipe it is the roughness of the pipe over the diameter of the pipe. Log in. Industrial Engineering. The Difference Between. Study now. See Answer. Best Answer. Study guides. Q: What is the difference between roughness and flatness? Write your answer Related questions. What is monochromatic checklite?

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What are the two factors that affect the friction force between two? How can you increase the amount of friction between your shoes and the ground? How do you use roughness in a sentence? Related Projects. I suppose my question is really about why would I ask the machining to make me a flat surface or a finished surface when the flat surface would do for both flatness and surface finsih? Except in extreme cases flatness and roughness are on a different scale of surface deviation.

Additionally, roughness is typically specified as some kind of average value, while flatness is more of a worst case measurement. In your example you probably need to specify both flatness and roughness.

There may be reasons you want a flat surface with some roughness. An example is a sliding surface such as a way of a machine. A scraped surface holds lubricant to improve sliding. A scraped surface can be extremely flat. This same situation occurs on clyindrical surfaces. Some roughness will hold lubricants. The norm was to be flat within. That tolerance was relatively easy to produce when you used a lapping machine.

The finish was extremely smooth but was not measured. The tightest flatness spec I have seen was a charge pump body which gave a flatness of.

We measured the parts with an optical flat and mono-chromatic light. Aat one time we used thousands of metering pumps for polymer that carried some very tight specs on flatness of the end plates and gears. We would lap our spinnerets flat to less than a quarter light band over an 8" span.