Recommanded tolerance for datum features

[supergee]

Hello all,

As some of you might know, I am teaching GD&T at the college level. I was wondering if there was a rule of thumb about the value to give for datum tolerances. I got a tip about that a few years ago from someone in the aerospace industry. The rule of thumb was to make the datum 10 times more precise than the least tolerance referencing that datum.

This would mean a flatness of .0005" for a datum when a hole position tolerance is .005". That seems to make perfect sense in the aerospace business, but .0005" is very precise for many companies.

While I am in favor of teaching the best practices of the industry, I want to teach the students not only the best but also what is reasonable after doing a cost-benefit analysis. For instance, 10x more precise if people's lives might be at risk, such as critical components with significant impact in case of failure (e.g., aircraft engines), and 5x more precise for noncritical components with low impact (e.g., low-cost washing machines).

Is there some standard or reference on this subject you might know?

Gee

 

[mfgenggear]

OP
The standard is experience.
It all depends on the product and application.
Eg ground shafts, gears, critical complex details, sheet metal details, rough castings forgings,

 

[supergee]

Thanks for your input, mfgenggear.

As a teacher, my job is to take existing experiences and summarize them so that students don't start from scratch. My predecessors used to teach GD&T symbols and their meanings.

I try to teach them the effect of GD&T on practical cases: I don't want them to just read GD&T, I want them to design with GD&T.

That is why I need real-life scenarios. I actually give them an oral exam, where they need to explain WHY they chose the tolerances and datums on their homework. Ever since, most of my students have been able to explain when and why they should use certain tolerances and even the values for location. The datum value is a recurring question, which is why I ask here.

Gee

 

[3DDave]

There is a fundamental flaw that is not discussed in the ASME Y14.5 standard.

The standard makes a promise that nothing is allowed to cross the boundary of the datum; that the datum is absolute and that the True Geometric Counterpart" or "Datum Feature Simulator" is a suitable stand-in, themselves taken as sufficiently perfect.

The flaw is that that the datum is held to be representative of the limitation imposed by the mating part, but the reality is the mating part is irregular.

This irregularity isn't mentioned.

A simple stack calculation based on the AME of mated parts may give the same as the AME of the mated parts, but will often be less.

In ASME Y14.5: 2+2 <= 4

It is also possible for the mating of irregularities to allow unaccounted for rotations.

This can cause an additional datum shift that isn't otherwise accounted for.

Making the datum features excessively precise is one way to deal with the problem. The other is to make the extent of the datum features so small that the effect is minimized.

There is also another factor - QC wants their job to be easier and throughput to be higher. Not dealing with irregularities does that. If the form of the feature is nearly perfect they don't need to spend time looking for high and low spots; they might be able to choose a single CMM hit to represent an entire surface. The conflict with those who make the parts is obvious.

Just keep in mind that setting a part on the surface plate will contact high spots. Those spots aren't necessarily what the mating part will contact.

 

[mfgenggear]

3d Dave very good.

OP
The answer is complex. I will try to give examples.
Keep ot simple, give tolerances of ÷/- 010 and +/- .005. In general. True position .010 and .005 when possible.
My specialty is gears. Gear shafts.
On shafts manufactureers like between centers, and datum at each end. You can give .0005 runout and .0005 diameter tolerance. Yet give. 0005 tolerance and run out on other ground diameters to the datuma. And it is done often.

As a manfacture they tighten those tolerances due to stack up of tolerances as machining . This is also done often.
Place the AANSIY14 procedures. But tolerance to perform the following.
Given enough tolerance so the manufacture
Can machine at the most cost efficiency.
Yet maintain the requirements and integrity
Of the detail or assembly.
It's a fine line.
I recommend when possible send out drawing for concurrent engineering and manufacturing review.

 

[Ryan6338]

I agree 100% with all the replies. There's really no general rule that can be applied here. The drawing should be designed to guarantee fitment and function with the widest tolerance possible. The idea of increasing a tolerance just because something is safety critical doesn't really make sense. When it comes to tolerances, the goal is always to maximize tolerance zones while still meeting all functional requirements. Sometimes this results in a tight tolerance, and sometimes it doesn't. On the contrary to your example, it's often much more expensive to design things with wide tolerance zones.

Things like aircraft parts may be given a tighter tolerance just because the increased costs of high-tolerances is negligible compared to other factors of the design and the quantity isn't high enough to justify significant time spent on cost-reduction by widening tolerances as much as possible. Products that need to be low-cost may require significantly more design effort to reduce their cost as much as possible. They end up getting a wide tolerance at the cost of significant engineering effort ensuring everything will work using the cheapest possible manufacturing methods.