It’s Round, It’s Straight, It’s 3D!
As part of our continuing exploration of the Geometric Dimensioning and Tolerancing (GD&T) standards that are used to communicate how parts should be manufactured, this week we are taking a look at cylindricity, another characteristic of geometric tolerancing that may be called out on an engineering drawing.
What Is Cylindricity and How Is It Measured?
In the most basic terms, cylindricity is an indicator of roundness along the full length of a part. That means unlike concentricity, which compares roundness at two different points or compares an outside diameter and an inside diameter (OD and ID), cylindricity is a three-dimensional tolerance that requires you to consider straightness along the part’s axis as well as the part’s roundness. It is a characteristic that may be indicated in specifications for shafts, pins, and other parts that need to be both round and straight along their axis.
A part’s cylindricity tolerance must be less than its diameter tolerance. The tolerance zone — that is, the area into which all points along the entire length of the cylinder would have to fall — is two concentric cylinders. If a part is not both sufficiently round and sufficiently straight along its axis, it would not fit into that zone and, therefore, it would not qualify as cylindrical. To measure cylindricity, a part would be rotated around the spindle of a precision measuring device while a probe is used to record the surface variation along the length of the part. The results are graphed and then checked against the allowable tolerance zone.
Working with Rods and Tubes
At Metal Cutting Corporation, we specialize in working with small metal rods and tubes — typically, taking parts that have the same diameter throughout and cutting them to specified lengths. Therefore, the cylindricity tolerance we can hold is subject to whatever tolerance the rod or tube manufacturer can hold. However, our capabilities also include centerless grinding, which can be used to improve cylindrical characteristic. Therefore, it may be called out on some machined parts, where we perform additional work that allows us to hold tighter tolerances for overall diameter and cylindricity than what the rod or tube manufacturer can hold.
That being said, the reality is we don’t often get callouts for cylindricity at Metal Cutting; our customers are far more concerned with tight tolerance diameters and characteristics such as parallelism, angles, and burr-free, perpendicular end cuts. Additionally, if you hold a tight diameter tolerance along the entire length of a rod or tube, chances are pretty good that it will also be suitably cylindrical. Remember, though, that in addition to measuring a rod or tube diameter at different points along its entire length, it is important to look at the part’s straightness — that is, how the part rotates on its axis at different points — if you want to be sure it is cylindrical. If the part is not sufficiently cylindrical, it will not rotate on its axis.
For the Best Results, Call It Out!
A true cylindrical part will be round and straight within the specified tolerance along its entire length. As with other GD&T features, providing Metal Cutting with detailed drawings and complete specifications up front, including callouts for cylindricity and other important characteristics, is crucial to ensuring the best results for your small metal part needs. In addition to helping us deliver a quality product, these drawings and specifications play a vital role in helping to control costs, reduce waste, and maintain timely and efficient production schedules.
For more tips on how to maximize the accuracy of your project quotes and manage your total parts cost, 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.