How to control the Size and Position of 2 Very Odd Openings

[Tarator]

Hi all,

I am trying to find a way to control the size and position of 2 very odd openings of a molded plastic part. The opening looks like a perfect circle on one end, and a slot/ellipse on the other due to the draft and odd angles. This part mates with another plastic part which has 2 cylinderical holes. 2 self tapping screws are used to asseble both parts, so it is a fixed fastener assembly. The openings on my part function to clear the screws. Ideally, I would like to be able to use MMC.

Can I use a Surface Profile (ALL AROUND) for the size and a Position (BOUNDARY) for the position of these openings?
Or do I have to use Surface Profile callouts as in the attachment?

Thank you all for your feedback.

 

[pmarc]

Yes, you can use Position at MMC (BOUNDARY) to locate the openings and Profile of Surface (ALL AROUND). Just make sure that true profile (contour) of the openings is fully and correctly defined with the use of basic dimensions. The boundary generated by Position at MMC callout will have the same odd shape as true profile, but will be offset from the true profile by half of profile of surface tolerance value plus half of positional tolerance value.

 

[Tarator]

Thank you for the quick resposnse!

It is very hard to fully define the opening with basic dimensions on the drawing due to the nature of the geometry of the opening. I do have a general note that the math data has to be referred to.

The reason why I was hesitating to use Profile+Position BOUNDAY was that in all the books (incl. the standard), the openings are perfectly extruded normal to the view plane.

I attached the STEP file, maybe that will help to picture the openings better.

Thanks again.

 

[pmarc]

1. I do not have access to CAD at the moment, so I can only imagine the geometry we are talking about.

2. The approach with a general note that the math data has to be referred to is convenient, reasonable and commonly used option. As long as the note says something like: "Unless otherwise specified, CAD model defines basic geometry...", I think we can consider the true profile of the openings fully defined.

3. I said it not that long ago in one of discussions on this forum: "Do not expect to find each and every case in the standard or in any other publication". Y14.5 defines rules, but if we want to deal with designs that are more than just simple rectangular-block-with-round-holes geometries, we are often forced to go with our thinking way beyond the box labeled "Figures shown in Y14.5". And as long as the thinking is not in conflict with the rules, I see nothing wrong with this.

By the way, which version of Y14.5 are you following? Per 2009 edition this opening geometry would fall under "irregular feature of size type B" definition, as defined in para. 1.3.32.2(b).

 

[Tarator]

I use Y14.5M-1994.

The opening is very irregular... even the draft angle is not constant due to the odd angle between the Datum A plane and the face where the opening is.

I prefer using Profile+Position BOUNDARY so I can use MMC, and they can use a functional gage to check.

 

[axym]

Tarator,

I agree that the combination of Profile for size/form control and Position for orientation/location control would work with this feature.

Keep in mind that the functional gage will only check the Position tolerance, and the Profile tolerance will have to be checked with some other method.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Tarator,

Keep also in mind that even if you stayed with two profile FCFs, you would also be able to use "functional" fixed in size gage. It would verify inner boundary of profile tolerance zone defined by upper FCF.

Granted, with 2 profiles approach you would have one boundary more to verify comparing to profile/positon approach, but that does not mean that profile tolerance can't be verified with hard, fixed in size, gage at all.

Additionally another gage can be used to verify inner boundary of profile tolerance that controls size/form of the openings, regardless of tolerancing scheme chosen.

 

[Tarator]

Thank you all for your input. I really appreciate it.