Confusion about positional tolerance for rectangular pins

[dulman123]

Hi I'm a new user of GD&T and recently I got confused about a partner's drawing and I would like to propose some change.
However, I got no confidence about my proposed change.
I googled a lot about positional tolerance trying to find some explanation of similar case.
But, all the posts explained positions of cylindrical holes and use surface as datum A, B, C.
I rarely see the introduction about the positional tolerance for the mid-plane of a square or rectangular hole/rectangular blocks.

I put a few question in the attachment, appreciate if anyone could help clarify and explain them.

Thanks a lot!

 

[rb1957]

IDK much about GD&T but I suspect it doesn't like mid-planes as a datum (maybe because it is hard to define, as opposed to a surface). I'd ask this of someone where you work, who may also direct you towards texts and such when you show an interest in your work. I would also not be shy in asking around "why not this ?" ... if they pick on you for showing "ignorance" then you've learnt something about the office.

 

[mfgenggear]

oP
True position for round holes or pins is insure assembly of details. if there is more than two details. then it can be matched drilled.

a rectangular hole could apply if it requires assembly.to an other details.
and the reason for rfs (regardless of feature size and MMC (maximum material condition) and more.
what to keep in mind precise very precision tolerance requires very expensive manufacturing cost. so make sure it really is required.

 

[Flying Whale]

Seems to me like the manufacturing process would most likely be on a CNC machine, in which case the result will be far more precise than the tolerance margins specified. I'd go with removing the tolerances, and just specify ISO 2768-MK in your drawing.

 

[mfgenggear]

The issue is , not knowing the required closeness or desired precise or not are all.
Will determine the tolerance of true position.

 

[drawoh]

Hi I'm a new user of GD&T and recently I got confused about a partner's drawing and I would like to propose some change.
However, I got no confidence about my proposed change.
I googled a lot about positional tolerance trying to find some explanation of similar case.
But, all the posts explained positions of cylindrical holes and use surface as datum A, B, C.
I rarely see the introduction about the positional tolerance for the mid-plane of a square or rectangular hole/rectangular blocks.

I put a few question in the attachment, appreciate if anyone could help clarify and explain them.

Thanks a lot!

Datum features B and C are centrelines, not actual features. Datums must be applied to actual features.

The rectangular slots may be used as datum features, but they are features of size. You need to think through how you will fixture to the features. If datum features B and C are applied correctly, you should call them up at MMC, and make sure the design works that way. Note how datum feature B is a slot with a width tolerance of ±0.1mm. That is not a very accurate feature for a 0.25mm positional tolerance.

Your questions...

1. The datum symbols are applied wrong. I am interpreting that they are applied to the centre lines of the slots. It actually looks more like they are applied to the centre distance between the slots. Datums must be applied to actual features. Wrong, wrong wrongitty wrong.
2. There are two issues here. I think you are agreeing with my interpretation of the 1.4mm slot being the datum feature. We could be wrong, as noted above. In any case, if the 1.4mm feature is the datum, it requires no positional tolerance. It must be perpendicular to the datum A feature. Otherwise, it is perfect by definition. If you are using a slot as a datum, it only functions in one direction. Any perpendicular datum feature is irrelevant. Your partner has correctly not applied diameter symbols to the positional tolerances.
3. The datum specification is wrong, and it is ambiguous. I have extremely limited experience with CMMs. If the parallel slot faces are datums, there must be some way of picking them up.
4. Let's look at what we think is datum B. The "pin" must pick up parallel faces, and must not pick up the ends of the slot. It cannot be round. As noted above, the datum call-ups are wrong. If the datum were applied to the 1.4mm slot width, they answer to your question would be obvious.
5. Under civilized conditions, I would apply a profile tolerance to the outlines. Your partner has called up fairly accurate widths, and has positioned them sloppily. Perhaps this is a design requirement. The call-ups are wrong. The 24mm width should be called up from A and C. The 18mm width should be called up from A and B. Again, I am not familiar with CMMs. I would measure the widths with calipres. I would measure from the datum features to the outside edges and work out the positional error. Your datum features B and C are sloppy, so you probably should account for material condition. If datums B and C are called out at MMC, you can design an inspection fixture.

Look at what we think are your secondary and tertiary datum features. Given that they are sloppy, I don't think they immobilize your part. If your datum feature are RFS, the call-up is legal, but a bitch to fixture.

 

[drawoh]

Hi I'm a new user of GD&T and recently I got confused about a partner's drawing and I would like to propose some change.
However, I got no confidence about my proposed change.
...

This question will be better answers if it posted under Drafting Standards, GD&T & Tolerance Analysis.

 

[mfgenggear]

well it helps to look at the drawing. and I specified real world concept. so after reviewing. I mean it's a simple block with two tabs. it's important to know what it'sfunction is. even a bad tolerance part a machinist would make it work. the person who would have issues is the inspector. trying to assess what the designer wants. one would have to look at how a block is squared. and how the tabs would be machined. this is a very simple part.

 

[ghostdigital]

Datum features B and C are centrelines, not actual features. Datums must be applied to actual features.

The rectangular slots may be used as datum features, but they are features of size. You need to think through how you will fixture to the features. If datum features B and C are applied correctly, you should call them up at MMC, and make sure the design works that way. Note how datum feature B is a slot with a width tolerance of ±0.1mm. That is not a very accurate feature for a 0.25mm positional tolerance.

Your questions...

1. The datum symbols are applied wrong. I am interpreting that they are applied to the centre lines of the slots. It actually looks more like they are applied to the centre distance between the slots. Datums must be applied to actual features. Wrong, wrong wrongitty wrong.
2. There are two issues here. I think you are agreeing with my interpretation of the 1.4mm slot being the datum feature. We could be wrong, as noted above. In any case, if the 1.4mm feature is the datum, it requires no positional tolerance. It must be perpendicular to the datum A feature. Otherwise, it is perfect by definition. If you are using a slot as a datum, it only functions in one direction. Any perpendicular datum feature is irrelevant. Your partner has correctly not applied diameter symbols to the positional tolerances.
3. The datum specification is wrong, and it is ambiguous. I have extremely limited experience with CMMs. If the parallel slot faces are datums, there must be some way of picking them up.
4. Let's look at what we think is datum B. The "pin" must pick up parallel faces, and must not pick up the ends of the slot. It cannot be round. As noted above, the datum call-ups are wrong. If the datum were applied to the 1.4mm slot width, they answer to your question would be obvious.
5. Under civilized conditions, I would apply a profile tolerance to the outlines. Your partner has called up fairly accurate widths, and has positioned them sloppily. Perhaps this is a design requirement. The call-ups are wrong. The 24mm width should be called up from A and C. The 18mm width should be called up from A and B. Again, I am not familiar with CMMs. I would measure the widths with calipres. I would measure from the datum features to the outside edges and work out the positional error. Your datum features B and C are sloppy, so you probably should account for material condition. If datums B and C are called out at MMC, you can design an inspection fixture.

Look at what we think are your secondary and tertiary datum features. Given that they are sloppy, I don't think they immobilize your part. If your datum feature are RFS, the call-up is legal, but a bitch to fixture.

The datum C and B are derived from the center of the two center planes derived from the features. The Datum B center planes are derived from the horizontal surfaces and the Datum B center planes are derived from the vertical surfaces features.

Both are legitimate.

 

[3DDave]

There are 3 different schemes.

The first scheme tries to derive datums from the basic dimensions; there is no reference to any surface(s) for B or C.

I have little confidence that any of the three represent a realistic interface with other components and are, instead, a CMM exercise.

For general advice, choose features/surfaces that will force the part into alignment with the mating part(s) or at least severely limit the variation in alignment as datum features. If the features available do a sloppy job of it, it is better to redesign the part rather than getting clever with datum reference symbols.

 

[mfgenggear]

Well it depends , if the tangs have to fit a female mating part. The importance is it assembles.
So I want the position of the two tangs to each other. Composite tolerance. And the outside configuration is secondary. If the tangs are made the datum as said before its legit.

 

[drawoh]

The datum C and B are derived from the center of the two center planes derived from the features. The Datum B center planes are derived from the horizontal surfaces and the Datum B center planes are derived from the vertical surfaces features.

Both are legitimate.

Maybe you can specify centre-lines in ISO. In ASME Y14.5, you apply your datum to a feature. You can apply the datum to the feature-of-size dimension. As shown on the drawing, the datum has been applied to the dimension separating the centre-lines of the two tabs. This is totally wrong.