Does Flatness Matter in Small Parts Production?
This week, we take a quick look at surface flatness, another aspect of the Geometric Dimensioning and Tolerancing (GD&T) standards use to communicate how a part should be manufactured. Along with positioning, cylindricity, and other geometric tolerancing characteristics, flatness tolerance may be indicated on an engineering drawing to help ensure the metal parts that are produced will have both the correct form and the desired functionality.
What Is Surface Flatness?
Flatness is a measure of a surface’s form compared against itself, indicating that all the points along the surface lie in the same plane. Unlike parallelism, which compares the orientation of opposite surfaces, or perpendicularity, which compares the orientation of adjoining surfaces, surface flatness does not consider any other data or features.
Flatness is symbolized in GD&T by a parallelogram. The flatness tolerance zone is the area between two parallel planes, into which the surface in question would need to fit in order to qualify as “flat.” The flatness tolerance would be the allowable distance between those parallel planes. Looking at it another way, the tolerance zone is the highest and lowest (±) acceptable points across the plane of the surface that is being measured.
Flatness allows you to limit waviness or variation in a surface without tightening its dimensional tolerance. The characteristic of flatness is useful in helping to ensure even wear across a surface or where two surface must meet flush and form a tight seal, as with two surfaces bolted together with a gasket in between.
Flatness Takes a Back Seat
At Metal Cutting, we produce thousands of rods, tubes, and wires cut to length every day. The reality in our world is we don’t often work with flatness tolerance for the surfaces we cut, due to their very small sizes; instead, our customers typically are more concerned with parallelism, angles, and the perpendicularity of the end cut rather than surface flatness — which in honesty, would be virtually impossible to measure on parts such as tiny tubes with a diameter of 0.040″ (1.016 mm). Additionally, our customers are looking for end cuts that are burr-free, especially with parts that need to mate with other parts.
However, customers do on occasion ask us to help them achieve a specified flatness tolerance on parts that have a larger surface area or that have been machined, as well as pre-made parts that customers supply to us for additional finishing processes. In these instances, we make good use of our lapping equipment to attain smooth, flat surfaces that meet customer specifications.
How Is Flatness Measured?
Flatness is notoriously hard to measure. For one thing, there really is no such thing as “perfectly flat.” Second, the flatness of a part can’t be measured by placing it on a table and running a height gauge over the part, because the table would then be part of the data and you would be measuring parallelism, not flatness. Instead, you need to make sure the points along the surface of the part are within the tolerance zone, comparing the surface to two imaginary parallel planes.
Coordinate measuring machines (CCMs) allow for a type of 3D measurement, creating virtual planes to which your part’s surface profile can be compared. Therefore, a CCM can be used to measure points across the length and width of the part to check that the entire surface is within the flatness tolerance.
Here at Metal Cutting, we typically fixture the part so that the surface to be measured is level; we confirm this by finding three opposite points on the part surface that are the same height. We then put our height gage on various other points across the part to note any differences and check that the entire part is within specifications for flatness. We can also use our OGP SmartScope system to measure multiple random points along the surface plane of a part to make sure the highest and lowest points are within the allowable tolerance range.
GD&T for Accurate Specs
For the complete range of GD&T features, from parallelism, perpendicularity, and angularity to circularity, straightness, and flatness, the goal at Metal Cutting is always to keep production cost-effective while delivering the very best quality for our customers’ products. By providing us with detailed and complete specifications up front — including clear and carefully thought-out tolerances for surface flatness and other important characteristics — customers play a truly vital part in helping us manage both costs and quality.
For tips on how to maximize the accuracy of your project quotes, manage your total parts cost, and achieve the quality metal parts you need, download our free guide, How to Fine-Tune Your Quote Request to Your Maximum Advantage: Frequently Asked Questions in Small Parts Sourcing.
This week’s blogger, Barbara Osborne, is the Quality Assurance Manager at Metal Cutting Corporation.