Support is one of the services I offer my clients. This particular instance required me to provide my GD&T, CMM, and quality experience to an injection-molded seat. As well, I provided my client support as a go-between with their client, an international agricultural machinery manufacturer.
To put everything into perspective, my client made a sample injection molded part from a prototype mold. The client asked me to predict how the production parts would behave once the production mold was, well, in production. It was necessary to learn the ins and outs of this part knowing I would be drilled on its behavior before even the production part came into existence. I did not have any client drawings nor any history of similar design. However, I did have a CAD model, a prototype part in the form of a seat shell, and access to a CMM.
Before we go any further, my client manufactures everything in the process. From injection mold from a block of tool steel to plastic injection molding, from mold to expansion seat foam molding, from the automation of the vinyl in the seat foam mold to snapping together. This entire process is completely automated. It’s an impressive process, to say the least.
Finding the first issues
So, onto the CMM the part goes imagining how the part would be held and referenced on the production drawings. This seat is made up of a one-piece shell that holds everything together; just imagine an open clam shell bolted to a tractor. I measured what I thought would be critical issues when the time would come time for PPAP inspections.
The major issue I found was the part wanting to close up so to speak. With the base bolted to a reference plate, I found the seat back pivoting forward on the “clam” hinge. Of course, there wasn’t a hinge, just the plastic doing its thing.
Fast-forward in time and I get the initial drawing of the part and behold! a profile tolerance of 4mm on the backrest part of the seat shell referenced to the base. Impossible! I had previously measured +10mm from nominal giving a profile value of 20 total. That would be unacceptable. We needed to control the shape of the seat back and still permit it to close up a little once in production. We needed to find a compromise.
The dimensions were linearly out of tolerance. Only out a little at the “hinge” and getting progressively worse as we measured up the seat.
I tried a composite profile tolerance to get the ball rolling. I bestfit the measurement points and they all fell into a profile of 3mm (+/- 1.5 from the nominal surface). So, I started with a profile of 20 and a secondary profile relative to the surface itself of 3. That still gave the seat back too much wiggle room since the SIP (Seat Index Point) would be affected by this large tolerance. I noticed that the seatback was in plus material up to 12mm. So how would we express tolerances of +10/0 from nominal once the molding process was optimized?
I proposed an unequally disposed tolerance profile of 12 U 10 to the constrained DRF (datum reference frame), combined with a composite profile tolerance of 4 without any constraints.
Unequal Tolerance Zones
That U… I love U! Before the 2009 ASME Y14.5 revision, we had to visually express the unequal tolerance zone with additional details on the drawing.
Now we simply add the circled-U symbol after the tolerance value to express the part of the tolerance zone we want to be away from the surface. See below.
How did it turn out in the end? Multiple parts fell into a composite profile of 5 U 5 to the DRF and 2 to itself.
What can we get out of this? The design intent is expressed within the drawing. The seat plastic’s elastic deformation permits the seat to be a little more closed than nominal, actually adding to its comfort. The second part of the composite profile relative to itself makes for a controlled visually pleasing seat shell.
GD&T lets design intent reveal itself in a less cluttered drawing.
GD&T Tips
TIP 1: ASME Y14.5 states that the value AFTER the U is the portion of the tolerance applied away from the part. Just remember that this value is applied to the plus tolerance.
TIP 2: Fusion 360 as of this post does not have the U modifier option, too bad…
TIP 3: SOLIDWORKS as of version 2019 needs a bit of persuasion to make this symbol work with the profile. Simply add <CL-U> between the desired profile tolerance and the portion of the tolerance you wish to be away from the part, see below.
