How Many Datums are Too Many Datums?

[Glass_Half_Full]

Hi everyone,

Its great all the knowledgable people on this forum who are happy to help others. Hopefully one day I will be in a position where I can do the same.

I'm a bit stumped trying to tolerance a drawing I am working on, in my last job I thought my GD&T was passable but either the new field has thrown me or I have forgotten something significant.

I am drawing up the base plate for a machine I am designing and it has a few major interfaces with different parts of the machine. Two key interfaces are the rails for the y-axis and the side plates that form the structure of the z-axis. I do not come from a machine tool background, and the machine I am designing represents a significant step up in precision for my company, so I don't know the established way of doing this in machine tool design.

I have attached a sketch of a comparable setup of base plate with side plates and linear guide rails and an illustrative orthographic hand drawing showing the approach I am taking at the moment. Apologies but I cannot upload the actual drawing or images of the actual CAD, so I hope these are clear enough. A key detail that might not be obvious from my sketching is that the rearmost side-plate hole on either side is a dowel hole that is used as the tertiary datum for the lengthwise position of holes on that side.

But to summarise, it seems to make sense to me, functionally, that the mounting holes for a side plate should be positioned relative to the locating edge for that side plate, and the mounting holes for a rail should be positioned relative to the locating edge for that rail. This is the approach I would take with +/- dimensioning where I would dimension from each locating edge. Further, since the locating edges divide the top surface into four, I feel it makes sense for the holes to be positioned perpendicularly to the surface they sit on, and that should be the primary datum for that hole.

The upshot is that I have 11 datums on my actual real drawing, and this just seem nuts. I have probably but 5 datums on the occasional drawing before, where say it was mostly cube shaped but had a significant bore for instance. But eleven seems excessive. However it feels wrong to me to use the locating edges in pairs to create a mid-plane datum and position the holes relative to that, as then if the distance between locating edge increases (i.e. the size of the datum feature increases) the tolerance zone of the holes will not move to accommodate that and the holes will be closer to the locating edge (and misaligned with the part they interface with) than intended, and I will have to tighten the tolerance to account for this possibility.

At the same time, I really don't want to quadruple the workload of the inspector if this is going to make them do more, time consuming set ups. I have to admit my knowledge of inspection processes is lacking. If they can easily move measuring tools to check different datum reference frames then that wouldn't be so bad, if they have to move the plate into a different position in the fixturing that would be awful.

Really appreciate any insight any of you could shed on this.

Thanks.

 

[Burunduk]

Glass Half Full,
To provide different position tolerance values in different directions you can use this method per ISO 1101-2017:

Or this former practice that is also still allowed in ISO 1101-2017:

 

[3DDave]

Glass_Half_Full,

If you really want to open up the tolerance in one direction, use a vertical (in the given view orientation) slot instead of a hole.

 

[Glass_Half_Full]

Thanks 3DDave, the mating part is actually a flat surface, so kind of like a slot, which is why I could afford to open up the vertical tolerance for the pin hole.

 

[Glass_Half_Full]

Thanks Burunduk, funny I can't find that method in my BS 8888:2020, even although it does discuss situation feature modifiers. Maybe I will have to get my hands on ISO 1101:2017.

 

[greenimi]

That's why I am asking if in ISO the rules are the same?
The OP clarified that his drawings are per ISO GPS (not ASME), hence per my knowledge (but I might be wrong here) the rules are different (at least per today's rules and not per tomorrorw ISO new revisions).
Should we use diamond pins or should be use cylindircal pins?
Otherwise stated: is CURRENT ISO GPS rules in alignment with ASME Y14.5-1994 or the current ISO rules are in agreement with ASME Y14.5-2009 or 2018?
I am leaning to say the current ISO rules are per 1994 hence a diamond pins could be used/ should be used. Or per pmarc's suggestions those diamond pins are unnecessary distractions anyway.
Hence my questions..............for clarifications only and learning the differences between the systems.

pmarc,

Sorry to bother you directly, but I remember not long ago we talked here on the forum about my inquiry. I cannot remember what was the outcome, nor I can find the thread/ discussion. Must be my old age.
Thank you very much for your help in indicating (if you know) the thread or answer my question once again here on this discussion, if you don’t mind.

 

[Kedu]

Coming to the rescue...or at least I hope.

"I would say there would be no difference in ISO between both scenarios. In both cases the hole and slot would be fully filled by the associated feature, and the reason being - as you pointed out - that ISO does not enforce the associated features to be perfectly located relative to each other, only oriented.

I hope the members of ISO committees will chime in here. From what I've heard, the new version of ISO 5459 will change the default relationship between the associated features to perfect orientation and location."


pmarc,
I copied from your LI group. Am I on the right track here?
Please let us know.

 

[pmarc]

Come on, greenimi. I find it hard to believe that you are not able to find the outcome. You who's able to mine through the forum like noone else and need just one word I or others said to start a conversation ;-)

Kedu,
I am assuming greenimi is looking for confirmation that the rules for the relationship between tertiary (sometimes secondary) datum feature simulator and the higher order of precedence simulators are different in ISO than in Y14.5 (2009 or 2018). The answer is they are, but the quote you brought up comes from a purely-ISO discussion and so I don't think it fits in here.

 

[greenimi]

Come on, greenimi. I find it hard to believe that you are not able to find the outcome. You who's able to mine through the forum like noone else and need just one word I or others said to start a conversation winky smile

pmarc,

Well, well, sometimes I remember only few important words which, in my brain, I label the entire thread/ discussion. And fortunately (or unfortunately) those relevant words come from you…not sure why ?
But ISO versus ASME is one of my top curiosities…..I want to learn more about differences between the systems.
Thank you Kedu for the posting. I lately did not have time to spend on Linkedin.
Maybe because it is called linkedin and not linkeiso (I like to know ISO GPS system and less German DIN specific national standard)

 

[3DDave]

greenimi,

Perhaps creating a new thread with links to all the research you have done so far and an analysis of what you have discovered is in order.

I'd prefer to avoid LinkedIn as that is a marketing harvesting site masquerading as a jobs/discussion board; a feeder directly to Microsoft, its owner.

 

[greenimi]

Perhaps creating a new thread with links to all the research you have done so far and an analysis of what you have discovered is in order.

I have not done any research nor any analysis and I am not intending to do such.
Was just a question for pmarc (because I knew he is going to give me a straight answer--and THANK YOU pmarc) to understand the concept and principles pertaining to my side question.
That's all.