Where to placement datum feature?

[Verkstad]

Hi!
I need some good tips . I don't know where i shall place the datum features in this drawing see picture below, since i dont have three surfaces normal to each other. I am using the iso standard.

I need to put a concentric tolerance between the holes. On the big hole i need to add a tolerance on the track inside the hole for a parallell key. And a toerance for the track that goes through the hole.

If there is anything i need to clarify just ask. It would been much easier if i could add a datum features on the center line, but the iso standard wont allow it.

 

[chez311]

No offence, but you're not asking for "tips" you're asking for someone to tell you how to do your drawing. I don't mean to be rude but it seems like there needs to be more learning and base work done before asking for advice.

Cylindrical features can be utilized as datum features in both ISO and ASME. I suggest you do some reading.

Also, unless theres something I'm not seeing, I don't think you need a concentric, or coaxial control considering the holes do not share a common axis. The word I think you're looking for is position.

 

[Burunduk]

Being able to put a datum feature symbol on a centerline perhaps would be "easier" (based on how many drawing makers tend to do it anyway) but also meaningless - as a centerline is not a real feature, and you need datum features to derive the datums from.

Datum feature symbols should be associated with part features that interact with other parts in an assembly. To answer the question of where to put the datum feature symbol, one needs to know how the part functions and assembles.

My guess is that one of the holes should be a datum feature. If it is the same in ISO as in ASME, either the size dimension, the hole outline, or a feature control frame associated with the hole is where the symbol is attached.

 

[Belanger]

The primary datum feature should probably be one of the flat faces, and then you could label the next datum onto the diameter callout for one of the holes. The other hole can then be positioned to the primary and secondary datums.
That is overly simplistic -- we'd need to know more about the function, and you'd need to study more about the theory -- but it's a starting point for you.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Verkstad]

Hi!
Thank you for your answers.
I want the holes to be parallell not concentric, my mistake. The way it will be used, is that i will have two round bars through the hole. In the big hole there is a track for a parallell key

Yes i can put the datum feature in the hole. But i would get an axis not a plane and i need to add an angel on the placement of the tracks on the big hole?

A suggestion see picture below

 

[chez311]

Apologies if I came off harshly, it seemed from your initial post that no previous work had been done.

I want the holes to be parallell not concentric, my mistake. The way it will be used, is that i will have two round bars through the hole

Will anything touch the outer profile or either flat face? Or is it only constrained by (touching) these bars - ie: allowed to float along the bar axes?

Yes i can put the datum feature in the hole. But i would get an axis not a plane

This is a topic which comes up quite a bit. The actual theoretical geometry (points/axes/planes) created is not important - what is important is the degrees of freedom which your datum features constrain. This should also be driven by function - I see in your sketch you've noted the outside profile (actually only one segment - I don't think unless its attached to an all-around/between profile tolerance that we assume it applies to anything than the surface it is applied to) as datum feature A because the outer profile is already machined before the holes are. If the only thing which contacts the part is the bars through the 2x holes, then these should be your datum features, regardless of how the part is produced/processed during manufacturing.

The manufacturing process needs to be capable to produce the part to print so it satisfies the design requirements, not the other way around.

That being said, judging by the word "sheet" in your sketch and a general hunch - I assume this part is relatively thin, although you have not shown a side view or thickness. If this is the case, in order to inspect and simulate it repeatably and to prevent allowing excessive angularity error, it would be recommended that one of the flat faces is primary (A), each of the holes your secondary (B) and tertiary (C). If one of the holes is more important or perhaps has a tighter fit, it can be referenced |A|B|C| or if neither hole does it can be |A|B-C|. If they are instead referenced at MMB (or the ISO equivalent) either scheme will result in the same simulator behavior and either can be used.

 

[chez311]

Actually, I noticed you said "In the big hole there is a track for a parallell key". It is possible that the keyway could instead be your tertiary datum feature - its up to you whether its more important that the keyway be oriented to the holes, or one of the holes oriented to the keyway.

 

[Burunduk]

To add to what was said, one of the datum features for the position control on the keyway should be the large hole. It probably should be a secondary datum feature, like chez311 said one of the flat faces should be the primary if it's functional. Your key is going to be assembled in the slot on the shaft that fits in that same hole. If you want the key and keyway to fit, the keyway must be positioned relative to the hole. Also, remove the basic 1.9 - centering of the center plane of the keyway and the axis of the hole is implied.