In an ideal world, manufacturers could make parts perfectly all the time. Every customer would have an unlimited budget for materials and labor, and every machine would perform flawlessly under all conditions, repeatedly producing precise replicas of a part made exactly to specifications.
In the real world of complex CNC machining, however, slight variations in machined parts are the norm. That’s why tolerances exist. A part may not be “perfect,” but it does hewclosely enough to those exact specs that it functions as intended.
Functionality, of course, is the ultimate goal. And it might surprise you to learn that tighter tolerances aren’t always the best path to getting functional parts. That’s why our team encourages engineers to think critically about their tight tolerance specifications. Calling out unnecessarily tight tolerances can lead to a slew of problems, including scrapped parts, increased costs, and—yes—even final parts that don’t function well in an assembly.
A better alternative, in many cases, is to use the GD&T symbol true position, which enables a precision machine shop to determine the tolerance range that will ensure superior functionality.
Why Is True Position Valuable in Precision Machining?
The most commonly used GD&T symbol, true position indicates the optimal position for a hole that will create the tightest possible tolerance in an assembly. Identifying this point in a design allows us to calculate a 3D radius of tolerance for any hole.
When we see the true position symbol, we know that cutting a hole anywhere in the zone between the true position and the edge of the radius will yield a tolerance that ultimately maintains the integrity of the assembly.
Why does this zone of tolerance surrounding the true position make such a big difference in precision machining results?
1. Greater flexibility
Traditional methods of notating hole measurements place the desired tolerance at one point along a 2D X-Y axis. When engineers use GD&T to notate true position, on the other hand, the resulting zone in which holes can be cut extends to all three dimensions: X, Y, and Z.
We calculate the radius by multiplying the minimum allowed tolerance by 2.8, with the resulting measurement applicable to the X, Y, and Z axes. In short, true position indicates many locations where a hole can be cut, providing machinists with a much wider range of options.
2. Cost and time savings
When tolerance requirements are unnecessarily rigid, precision machine shops may have to scrap more parts, which uses valuable machining time and results in higher machining costs. In many cases, those parts would still function properly with looser tolerances.
Because true position is design-specific, it allows us to determine the acceptable tolerance for each unique part. When we go through true position exercises with our customers, we sometimes find that the tightest tolerances are unnecessary for many parts and can even be detrimental to the final assembly.
For a simple example, think about the grate on a heating vent, which generally has room to move around a bit in each direction. That additional “wiggle” room provides the tolerance needed to ensure that the entire vent can be properly mounted—each hole is within tolerance, and together, all holes properly align so that the vent is functional.
3. Minimal risk from unknown variables
When engineers don’t use true position, they forfeit a wealth of valuable information that can save their parts when unexpected obstacles arise during precision machining.
Say we receive a part with a hole that has been cut at a slight angle from the location marked on the design. If we have the true position of that hole, we can see that the imperfect hole still lies within the tolerance zone. We can then proceed with the next step of the assembly. Without this information, we might instead determine that there is no choice but to scrap the part.
True position also helps avoid tolerance stackup, a problem that can occur when mating two parts together, each with their own tolerances, without clearly understanding the cumulative effect on the final assembly.
If you use true position, we can use the tolerance radius to ensure your parts mate together perfectly. True position gives us more information to make these kinds of critical decisions than any other metric.
How Reata Engineering Handles Tight Tolerance Requests
Customers often request tight tolerances because they assume that tight tolerances = superior results. Our experience tells us that this assumption isn’t always accurate.
When customers specify tight tolerances for certain features, especially holes, we take it upon ourselves to explain to them that there may be an opportunity to loosen their tolerances and save money without negatively affecting the outcome of the part.
If a customer is firm about their tolerance requirements, as is sometimes understandably the case, we may provide a “best effort” quote indicating that we can’t promise a successful result with the restrictions they’ve specified. Our team excels at complex CNC machining, but we never make a promise we’re not sure we can keep.
Fortunately, using true position callouts is one simple way to avoid unnecessarily tight tolerances while ensuring that parts function as intended.
No matter the requirements for your project, Reata Engineering is here to get you the parts you need. Request a quote today to work with us!