Irregular closed feature 4

[HC1AutoEng]

Received a drawing for a flexible plastic part stating GDT on irregular closed features.

I have no experience with this callout.

Could someone explain it to me. I have attached a picture of the callout.

 

[drawoh]

HC1AutoEng,

If datums A, B and C are orthogonal, I have problems with the positional tolerance. It ought to have a diameter symbol in front of the 2. I am not sure about the word "BOUNDARY", either.

If I assume that the 2 is really Ø2, the irregular form should be located inside a 2mm circle centred on the nominal position. The irregular form outlines must be within of their correct position, allowing for positional error. The error can be increased at LMC of the irregular features.

You could print the maximum and minimum outlines of the irregular features on clear film. Sit the film on the face such that the feature outline is contained within the piece of film. You must be able to do this. Each feature on the film must be sitting within 1mm of its nominal position. Otherwise, you reject the part.

JHG

 

[KENAT]

Boundary is in ASME Y14.5M for slots & the like to support use of MMC/LMC, see 1.3.1 & 1.3.2 & 6.5.5.1 & figure 6-19.

Figure 6-19 has a similar callout, no dia symbol for the position, however it is 2 separate FCF but attatched to the same leader, perhaps because the all around can't apply to the position.

Take a look at 6.5.5.1 & fig 6-19 that explain it.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[TheTick]

I thought position was only for features of size (holes and slots), not for irregular features.

I think a composite profile would be more appropriate.

 

[dingy2]

Boundary is used on a non cylindrical feature of size such as a rectangular hole, etc. where the boundary is the virtual condition size, shape, location and orientation of this feature. It must be confirmed with a checking pin though. Unfortunately, most Designers do not use the term "BOUDARY" when applying positional tolerances to slots but it is a better call out.

I do not see a problem with reference datums A,B, & C - primary, secondary and tertiary. Datum A is for perpendicularity to the mounting surface while datum B & C will give the location of the centre of the irregular shape hole.

The leader line with a circle around the elbow shown with the profile of a surface simply means "all around".

I don't see anything wrong with the call out.



Dave D.

http://www.qmsi.ca

 

[PaulJackson]

I don't see anything wrong with it either. This type of callout can be used to control any feature's odd or simple virtual condition boundary of position relative to the DRF.

paul

 

[dingy2]

Paul:

I cannot believe that you are agreeing with me???

There is just one exception I have though to your statement. The term "BOUNDARY" is used on noncylindrical features of size rather than any feature as you stated. A round feature of size at MMC really has the virtual condtion boundary superceding the centres as we have discussed here many times before.

See -

6.5.5.1 Boundary Control for Noncylindrical Feature - page 168

Dave D.

http://www.qmsi.ca

 

[TheTick]

Datum A is for perpendicularity to the mounting surface while datum B & C will give the location of the centre of the irregular shape hole.

How is this center determined?

Can someone with a standard give a ruling on the "legality" of using position for an irregular shape? The references I have seem to indicate it is not proper. It just doesn't sit right with me.

 

[dingy2]

I quoted above the section in the ASME Y14.5M-94 standard that covers irregular shaped features of size using "BOUNDARY". The centres should be shown with basic dimensions from datums B & C on the drawing but one does not confirm centre location but the virtual condition boundary about the true position.




Dave D.

http://www.qmsi.ca

 

[drawoh]

TheTick,

I thought position was only for features of size (holes and slots), not for irregular features.

The profile tolerance of the irregular feature is very much more accurate that the positional tolerance. The specification makes sense to me, although it is worth looking at the composite profile tolerance in Figure 6-25 in ASME Y14.5M-1994.

If I were inspecting, I would require each feature of the irregular hole to be located within the positional tolerance.

JHG

 

[dingy2]

If one uses a profile of a surface for an irregular shaped hole, one would use a coordinate measuring machine to sweep the surfaces of the hole. This is really costly to perform and it probably wouldn't be done on a regular basis.

If one used a positional tolerance at MMC on an irregular shaped hole using BOUNDARY, it could be checked with a checking pin and fixture simulating the virtual condition boundary, location and orientation. It could be checked on a regular basis (in the Control Plan) and once the fixture is made, the cost on the shop floor would be minimal.

Dave D.

http://www.qmsi.ca

 

[KENAT]

Tick, use of profile/positIon like this is perfectly legal, both dingy & I have given the reference in the standard.

The only thing I see on the post that is slightly different from the example in the standard is the arrangement of the FCF, see link for what the standard has.

Very simplistically, the position is controlling the location of the irregular feature on the part, the profile is locating the size/shape of the irregular feature.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[PaulJackson]

All features of size with tolerances specified at MMC have a virtual condition boundary whether stated so additionally beneath the feature control frame or not. Paragraph 5.2(b) explains this. Paragraph 5.10.1 explains ways to interpret the position tolerance of Noncircular Features at MMC ... "In terms of"...(a),(b),(c), and NOTE. The words in (c)... "To invoke this concept, the term BOUNDARY is placed beneath the feature control frames" does not supercede what is previously stated in paragraph 5.2(b), it is however confusing, because it might lead someone to believe that... without it there wouldn't be a boundary without it.

In paragraph 5.3.2.1 Explanation of positional tolerances at MMC the arguements "In terms of..." are preceded by the statement "A positional tolerance at MMC may be explained in either of the following ways"... where in 5.10.1 the word "may" is missing... Go figure! Both paragraphs attempt to accomodate how one evaluates the control but they do not or cannot contradict paragraph 5.2.


Dave, You said,

There is just one exception I have though to your statement. The term "BOUNDARY" is used on noncylindrical features of size rather than any feature as you stated. A round feature of size at MMC really has the virtual condtion boundary superceding the centres as we have discussed here many times before.

Maybe I missed the

as we have discussed here many times before

but paragraph 5.2(b) pertains to all positional tolerances with material condition modifiers. Therefore the surface "or boundary" with MMC or LMC tolerances always takes prescedance over centers no matter what type of feature it is.



HCIautoEng, As stated earlier by Kenat and reiterated by Dave... 6.5.5.1 & fig 6-19 explain your situation.

Kenat, I don't have a problem with one leader line, an ALL AROUND symbol, and a position control attached to a profile control. I don't think that it is improper since it doesn't modify the position control... only the profile control.

 

[KENAT]

Paul, I was just flagging it as different, with a hypothesis of why the example in the standard shows the way it is. I'm definitely not stating it's definitely wrong.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[HC1AutoEng]

Thanks for all of your replies. My customer has put this callout in the general notes section of the drawing (the leader line does not point to anything in particular) and is using this to cover any feature on the part they did not give a specific tolerance to.

 

[MechNorth]

Dave, you nailed it. I've only seen the hybrid callout of position@MMC with a profile control a couple times, and a composite profile would be a lot easier for everyone to understand. I don't agree with calling them irregular features of size though; FOS have toleranced sizes and directly opposed points. The term irregular enclosed features is better. Here's some applicable references for review:

http://www.tec-ease.com/tips/september-00.htm

http://www.tec-ease.com/tips/october-00.htm

http://www.tec-ease.com/tips/november-00.htm

To the comment above that position only controls center planes and center axes of features of size, the previously posted Y14.5 references establish that the position control at MMC, when applied to an irregular feature whose size is controled by a surface profile, provides the limitation of boundary location rather than center axis/center plane.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com