GD&T Rule #1 and Free State 1

[JoelDonn]

Does "rule number one/ the envelope rule" apply to injection moled parts as they could be considered a "free state" exception?

 

[weavedreamer]

That might depend on what material is being injected into the mold. Aluminum has substantially different 'free state' properties, than say, silly string.

 

[drawoh]

JoelDonn,

Rule #1 applies when your part is contstrained by your datums.

ASME Y14.5M-1994, Section 6.8 has a bunch of suggestions for managing free state variation. How you deal with this depends entirely on your application. On sheet metal parts, I usually place a note stating that Datum[ ]A applies when the part is clamped to a flat surface.

How flexible are your injection moulded parts? The whole point of free state is that your parts are deforming due to weight and flexibility. If your parts are warping permanently when they are demoulded, then there is no free state. The parts delivered to you must be within your original tolerances, somehow.

JHG

 

[axym]

The use of injection molding (or any other production process) does not relieve Rule #1 requirements.

If the part is to be treated as non-rigid, the drawing should include a restraint note that describes the exact clamping conditions that the tolerances apply in. Drawoh's note describing clamping the part to a flat surface is an example of this. I would go a couple of steps further and describe the clamping locations, forces, and sequence as well.


Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[demmagod]

Joeldonn,
If the parts you mold are made from Plastic then no the Envelope Principle doesn't apply, Plastic parts are non rigid, you should add a free state symbol or note stating that parts are to be measured at a free state.

 

[KENAT]

demmagod, may just be your wording but rule one does apply - regardless of material/process, it just may not be appropriate to invoke.

KENAT, probably the least qualified checker you'll ever meet...

 

[demmagod]

KENAT,
You are correct, I should have clarified myself,
What I meant is that because most plastic parts have some form issues and maybe subject to free state variation in the unrestrained condition that joeldonn should look at adding the Free state symbol or a note that parts should be measured at a free state or restrained, joeldonn could solve this problem by invoking a form tolerance.

 

[Bizantine]

I must agree with demmagod's first post. Rule #1 does not apply to "parts subject to free state variation in the unrestrained condition. [2.7.1.3]" You still have to ask yourself if your part is subject to free-state variation. Some (probably most) injection-molded parts are rigid enough that I don't think this exception would apply. The free-state modifier means that the stated tolerance _does_ apply in the free-state comdition, without the modifier, Rule #1 does not apply.