Making Sure Parts Fit and Work Together
Geometric tolerancing is an aspect of Geometric Dimensioning and Tolerancing (GD&T), a method of communicating how a part is to be produced. Typically using a series of 14 standard symbols to help guide the people who are making a part, geometric tolerancing included in an engineering drawing provides insights into positioning, cylindricity, and other characteristics that are vital to the part’s manufacturing and, ultimately, its functionality.
Balancing Functionality and Cost
When the drawing for a part is conceived, it is engineered to perfection — the ideal of how the part should be. However, reality is an imperfect world, so it is not realistic to think that every part will be 100% perfect every time. Therefore, allowable tolerances are incorporated into the engineering process to allow for acceptable imperfection to ensure that the parts will be fully functional.
That means geometric tolerancing is a formal way of communicating not just how a part is to be made, but also the allowable tolerances that are associated with the part. The tighter a tolerance is, the more difficult it will be to achieve, the greater the number of rejections, and the higher the cost. Therefore, it is always important to achieve a realistic balance by specifying an allowable tolerance that is tight enough to make the part functional but loose enough that producing the part will still be cost effective. (You can read more about what to consider before specifying a tight tolerance in our blog How Do Tolerances Really Stack Up?.
A Geometric Tolerancing Chart Can Guide You
A quick search online for GD&T will lead you to a range of resources including examples of a geometric tolerancing chart. These charts generally show the different symbols that are used, explain what they mean, and describe how to use them.
The GD&T symbols help to describe various characteristics of form, orientation, profile, runout, and location. A drawing will typically show a series of boxes with the appropriate symbol for the location of a part feature (such as its position), the shape of the feature (such as a circle for a hole), the total allowed tolerance, feature modifiers, and any required additional datum. A common modifier is the maximum material condition, which is the maximum material to be achieved, indicating the smallest hole and usually expressed by the maximum pin size allowed.
One of the most commonly used symbols in GD&T is true positioning, which is represented by a circle with a cross through it. A typical example is bolt holes in a piece of steel, where each hole needs to be positioned correctly so that the bolts align and fit properly. As you might imagine, positioning most often deals with mated parts that need to work together — as in the above example, where holes need to be placed correctly in order that bolts will match up with a corresponding steel part.
Where Geometric Tolerancing Is Used
At Metal Cutting, we cut thousands of rods, tubes, and wires every day, and while tolerances are of course involved, we are not typically producing parts that require specified positioning. However, some geometric tolerancing is necessary for complex parts that we produce on our automatic swiss-style lathe, as well as certain parts produced on our CNC lathes and mills.
In addition, day to day we often deal with an end radius that ultimately needs to fit somewhere else; for instance, we might produce a small pin that our customer will mate with a corresponding part. In these cases, we need to ensure not only that the pin is the correct diameter, but also that the end radius on the pin is not too sharp and there are no burrs that will prevent the pin from mating with a corresponding part that the customer will be using. If the tolerance of either part is off by too much, the parts won’t marry.
How Tolerances Add Up
This leads us to point out that when working with geometric tolerancing, it is important to remember that all tolerances need to be taken into consideration. For example, if you have a positional tolerance of ± 0.0010 for two parts that must fit together, the cumulative tolerancing means that each part can only be ± 0.0005. This is known as tolerance stacking, and generally we rely on our customers and their designers and engineers to think these issues through and determine the correct geometric tolerancing for their parts. However, sometimes we come across specs that just don’t add up, and at these times we need to know the right questions to ask to keep the project on track.
For example, a customer might specify a long piece of tubing that will undergo a grind, then a taper, then another grind and taper, but that has a total tolerance that is smaller than the cumulative tolerances when you add up each step of the production process. Here, we need to find out what feature is most important to the part’s functionality, so that it can have the tightest tolerance and the others features’ tolerances can be loosened to achieve the specified total tolerance. Take the example of a medical device that needs to fit and lock in place with another part. Perhaps the stepped area is most critical to it fitting together and locking in order to achieve the desired functionality? Or maybe the length of the device is more important to the fit and locking?
Bringing It All Together
It is also important to consider how a part is being made and the tools that will be used. For instance, a customer may ask us to grind a part to achieve a particular feature but be unaware that a grinding wheel is not perfectly square and will create a corner radius. Here, we would want to get more information, asking the customer if a corner radius matters to functionality — and if so, how big a radius would be acceptable and how tight the tolerance needs to be.
It is easy to see that geometric tolerancing can be critical to how parts fit and work together, which is ultimately a key to achieving the desired functionality as well as cost effectiveness. For more tips on how to create drawings and specifications that will produce the results you’re looking for, download our white paper 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.