Resource Recommendation for Constraint Design

[Jste]

I am pretty fresh to design, I've spent more time working with FEA than mechanisms and alignment. I'm working with assemblies that need to be precisely aligned and I am designing both the parts themselves and the tooling to fixture parts for machining, bonding, assembly, and match drilling.

I would appreciate any resources you may have about designing assemblies to minimize slop. I don't quite know how to word it, but a simple example is using a hole and slot instead of two oversized holes if you are attempting precise alignment without needing to specify ridiculous tolerances on the hole size and position.

I understand the basic principle that you over constrain the mating parts with 2 holes that must be concentric, but I am hoping for more in-depth explanation on how the tolerance stackup compares between 2 holes vs 1 hole and 1 slot.

I can't think of any more examples, which is the crux of the problem. I would like to be exposed to more of these concepts but I'm not sure what keywords and concepts to search for.

 

[Belanger]

Not totally sure of the question, but mainly you should think about the 6 "degrees of freedom" that need to be controlled. Presuming you have a primary plane already established, a hole is great for establishing 360 degree control, while a slot is good for stopping rotation. Having two oversized holes can also lock in all degrees of freedom, but usually that allows for more slop.
This may also get into a MMC/MMB discussion, but let's save that. Perhaps you can expand on your question or post a sketch...

 

[Jste]

Hi Belanger,

Yes that's the train of thought I was going off of. I'm cognizant of DoF especially when I'm working in CAD, I've had too many assemblies blow up on me because I over constrained them. I'm pretty well versed on GD&T, not on the level of some here but I think far more than the average entry level designer.

I'm not really thinking of a specific design, though one I am reviewing right now did start me on this train of thought. These resources are a decent example of the concepts I'm trying to learn more about:

https://www.carrlane.com/engineerin...esign-principles/locating-clamping-principles

https://www.carrlane.com/engineerin...al-workholding/locating-devices/locating-pins

I'm looking for some kind of text, discussion, whatever that gives me more ideas for intelligent part constraint. It seems like most designs rely on the basic pin/hole (or slot) fit, which seems to make sense with availability of mass produced parts. Those carrlane sources introduced me to other kinds of pins, floating pins, diamond pins. I know it's focused more on work holding but the same concepts seem useful in general assembly design as well.

I'm kind of at the point of "I don't know what I don't know". I'm just curious if anyone has come across a well constructed "beginners" guide for intelligent fitment. Or if people have more ideas outside of cylindrical features mating.

 

[drawoh]

Jste,

Try working out the design of your fabrication and inspection fixtures. This causes GD&T and datums to make sense.

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JHG