![]() Without the ability to adjust the orientation of the part to minimize the amplitude of the needle, however, you are essentially evaluating parallelism based off of the table, which is sub-optimal. The more common method of using a height gage to evaluate flatness involves placing the part directly on the table (Figure 3). ![]() Using a Height Gage with the part placed directly on the surface can cause false negatives. As you can imagine, this method requires much patience and training.įigure 3. They are adjusting the orientation of the plane to come up with the smallest theoretical sandwich. The metrologist adjusts the three columns to minimize the amplitude of the needle. Then, the height gage is run across the surface while looking at the amplitude of the needle. To use the height gage correctly, the part to be measured is first placed upon 3 columns with adjustable heights. In application, one way to physically measure flatness is to use a height gage, as we can see in Figure 2. Height Gage on Columns set up to evaluate flatness It’s important to note that the two parallel planes are not necessarily aligned with anything, as we can see in the example at the bottom of Figure 1.įigure 2. Picture sandwiching the surface as tightly as possible between two parallel planes the tolerance would pass if you could get them closer together than 0.25 units. ![]() According to the standard, this means that the top surface must lie entirely between two parallel planes that are at most 0.25 units apart. At the top, we see a drawing with a flatness tolerance of 0.25 units. ![]() According to the ASME Y14.5 standard, it “specifies a tolerance zone defined by two parallel planes within which the surface must lie.”įigure 1 illustrates this concept very well. ![]() In this post, we are going to take a look at the multiple methods of evaluating flatness in GD&T and determine which is the optimal approach.įlatness is a GD&T form tolerance that is conceptually simple. ![]()
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