Last week, we looked at two examples of 3D printed objects with very different tolerances and allowances – a small tree frog and a train console. Since we already covered why you should use GD&T when 3D printing, I thought we might dive a little deeper into the “what” and “how.” Let’s return to our previous examples for a moment.
The frog
The frog arm is built (via AM) with a honeycomb internal structure (a common CAD model modification that maintains rigid structure, but reduces material usage).
Let’s consider the frog’s measurements:
- The wall thickness of the frog arms measure between 0.030 and 0.050 inch.
- The arm outer diameter measure between 0.150 and 0.200 inch
- The arm cross section is oblong and not consistent in size throughout the length of the arm.
If those thin frog arms were allowed to vary +/- 0.050 inch, then it would certainly break just lifting it off the build plate. Worse yet, if +/- 0.050 inch is the precision of the manufacturing machine, then the frog’s arm could not be built given the “as designed” wall thickness of 0.030 to 0.050 inch.
The console
In contrast, let’s consider the train console:
- Varying the overall profile tolerance within +/- 0.050 inch will be just as stout for its intended purpose to hold switches and levers, while allowing manufacturing flexibility, which ultimately means lower price options.
- The cutouts in the train console where the switches are mounted are an example of design features that require tighter tolerances than +/- 0.050 inch. The mating part (light-up switch) is “snap-fit” into place, driving tighter and more specific tolerance methods to be specified for the switch cutout.
Both the profile tolerance feature control frame, and the switch cutout geometry, with limit tolerances identified, are examples of GD&T. These profile and feature of size “callouts” convey design and manufacturing intent to downstream users.
Wall thickness & variation
Because the design requirements of the train console don’t include sustaining the weight of a truck, if the overall wall thickness and overall profile geometry are allowed to vary 0.050 inch, then the design intent is met for the “as built” part. However, if I did need this part to carry “truck driving on-top” type loads, then the 0.050 inch variation in wall thicknesses may not be adequate.
As an example, the overall wall thickness cross-section for this complex shape is modeled to nominal at 0.200 inch. If the overall profile manufacturing allowance is 0.050 inch profile tolerance, this means an acceptable measurable dimension of the “as built” surfaces may be nominal minus 0.025 inch, and nominal plus 0.025 inch. This results in a wall thickness that varies between 0.175 and 0.225 inch.
Clearly bigger is better, but the profile tolerance as called out in this example, is both bilateral and symmetric, directing that the allowable “as built” measured surface may also be 0.175 inch thickness. Depending on the structural and dynamic load cases for this part, 0.175 inch wall thickness may not be sufficient . Therefore, given increased strength requirements on this product, I might disallow a variance of 0.050 inch profile tolerance and make it tighter – say 0.010 inch profile tolerance.
The key take-away
The key take-away from 0.050 inch versus a 0.010 inch profile tolerance is that a 0.050 inch profile tolerance allows for a larger variety of manufacturing methods, and likely less expensive methods. Tighter tolerances generally mean that the part will be more expensive to manufacture, although not always. Again, knowing the manufacturing capability of your product and properly communicating the allowance mathematically is the key principle of GD&T.
Grabbing my new digital, dual-unit calipers, I am happy to report that even though this part was printed using a fairly inexpensive FDM method, it yields dimensions to within 0.010 - 0.020 inch of the surface and greatly exceeds my 0.050 profile requirement.
I do recognize that this is not a “complete” inspection method of this part, but bear in mind that I have identified the key feature (switch cutout feature) using GD&T methods and communicated that the remainder of the part is acceptable within a .050 inch profile tolerance.
If one were to use a reasonable interpretation of this product definition set, the resulting shape probably does not require detailed inspection methods such as 3D scanning the part or inspect it with a 3D CMM (Coordinate-Measuring Machine). However, it is necessary to point out that if a 3D scan or 3D CMM is completed, then the “as designed” nominal geometry can be automatically compared (via software and without human calculations) to the manufacturing allowances. The software can then produce a report stating “as built” versus “as designed” compliance.
This process is complete and unambiguous verification of the “as built” against the “as designed” part, which ultimately drives ROI by improving the product and saving time. The process still provides accurate results and dramatically increases the amount of data we can pass on about the product.
Tolerances and cost
Since most 3D printers build precision is much tighter than 0.050 inch, I can make an adjustment to the train console geometry with no added cost. By reducing the overall profile tolerance from 0.050 inch to 0.020 inch, then I increase the quality of the product without increasing cost. This is not the case for all hardware product and manufacturing machine complements, so it is critical that the CAD designer and the product documenter (and many times a manufacturing engineer) match the requested tolerance with the manufacturing method.
Using GD&T in a 3D context is more intuitive than it is in a 2D drawing. - Gary Leblanc (Senior Aerospace Propulsion Engineer)
GD&T plus 3D printing capability is a super-power with limitless capability. I suggest we use it, but with every super-power come responsibility. How will you use your super-power?
