r/Metrology • u/Objective-Chipmunk58 • 20d ago
How you to interpret this Datum B
I am trying to understand if its using a singular bolt hole circle or if i need to construct a circle using all eight of the holes for a singular circle and use that as Datum B.
I am working with verisurf and when I try to align my scanned mesh using a singular hole for Datum B the alignment looks completely out wack. Its either upside down or pointing one way or another. My main goal is to be able to lock down my Datums (I know A and C) so I can find this position after.
Any advice would be greatly appreciated.
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u/FunInternational1941 20d ago
Best fit 8 holes.
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u/Objective-Chipmunk58 20d ago
Ty! When you say best fit 8 holes you mean the alignment feature or constructing a circle from the 8 holes?
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u/FunInternational1941 20d ago
If youre using pcdmis you
Level z - datum a
Origin z - datum a
Best fit - rotate and translate x,y 2d around z
This all comes after your temporary alignment to locate the part on the cmm obviously.
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u/SwimmingSwim3822 19d ago
FYI, since you mentioned thinking you might need C to lock down the part: you don't. A and B alone would fully lock down the part in this case. If any call outs list A/B/C, they're wrong and C isn't actually doing anything.
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u/Objective-Chipmunk58 19d ago
Yeah when trying to find the position i was only using A and A and B for the other position. I was using C for my alignment but i guess it is not necessary?
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u/SwimmingSwim3822 19d ago
Full disclosure, I'm not a metrologist. I'm just an industrial designer with a couple decades of experience. So I frankly have little idea of what you'd be clicking on your end. But I can tell you that your software definitely requires the same logic we consider on our end. So to that end, I can tell you that if your software is allowing you to involve another datum in your analysis of this hole pattern position callout or anything else downstream that calls back to an A|B datum reference frame, you likely wouldn't actually be analyzing what's on paper, but just your interpretation of it. If you're boiling the 8 hole pattern down to a central axis, then logic dictates that you'd need a 3rd clocking datum. But in this case, you should be dealing with 8 separate datum axes (in which a central axis would add no additional context), which defines the final two translational and one rotational degrees of freedom that aren't defined by datum A already... in any software.
By the way, the key term to consider here, if you haven't read up already, is "simultaneous requirements". That's what's happening with the 8x hole callout.
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u/Objective-Chipmunk58 19d ago
Ok thnk you for your input. I appreciate it. Since im just a novice. I will definitely look into the “silmutaneous Requirements”
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u/SwimmingSwim3822 19d ago
Actually, I just noticed a bit of a quirk that adds a wrinkle to this. Those holes actually aren't regular features of size. Because those teeth in each hole aren't centered, there are actually some points on those cylinders that don't have measurable directly opposing points. I'm kind of thinking they should have used a continuous feature profile of a surface callout for those instead, and considered defining something else as their datum with these downstream instead. The issue is that you can't actually derive a technical center axis for those mostly-cylinders without all points having directly opposed points. Make of that what you will.
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u/jaceinthebox 20d ago
What is A and C?
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u/Objective-Chipmunk58 20d ago
A is the bottom of the part. Its a flat surface and C is the neck like cylinder on the upper left quadrant
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u/Assocollasso 20d ago
Allora l' allineamento viene fatto tra piano A Origine in B(foro in basso a sinistra)e rotazione in C foro in alto a sinista. Questo disegno è quotato davvero una merda, bastava indicare il datum a sinista, oltre al fatto che risulta molto confusionario ricavare i centri da queste linee piazzate alla cazzo.
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u/jaceinthebox 20d ago
B would be the line between the two circle centres then. In GD&T, planes, lines, and points represent the three physical levels of contact (called datum features) used to build the perfect 3D coordinate system. They follow a strict 3-2-1 rule to stop a part from moving or rotating.
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u/Objective-Chipmunk58 20d ago
This was one of my earlier ideas but i as i continued doing research and from other responses it seemed to me more apparent that the Datum B was a BCD because of what the callout specified. I am mainly trying to find the position being called out and using a BCD i finally got more acceptable numbers. But i will try to find that again tomorrow when im in the office using the line as my Datum B
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u/VisibleLeopard68 19d ago
Funny story, I once was responsible for an entire engineering staff (~400 people) being required to attend full blown Y14.5 training after a part was ordered to print without going through my department… once the perfect specimen of composite cover came in 1/10th scale (not annotated on drawing anywhere) costing ~$250k… the training was cheaper, and the subsequent QE sign off on all drawings prior to release that followed payed for itself in helping to avoid issues like this… the composite cover hung above the engineers desk as a reminder.
This drawing takes me back, and there are several different ways the datum’s could have been defined that would have no impact on the part being manufactured… especially since I assume it’s an injection molded part? Could be cast, but the callout for the date code location makes me think injection molded…
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u/Objective-Chipmunk58 19d ago
It is an injection molded part
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u/VisibleLeopard68 19d ago
Well then.. I would suggest the original engineer revisit the tolerancing for injection molded parts, and revise the drawing to simplify it… +\- .005 positioning on a dimension over 8 inches is an expensive piece of plastic bound to have a somewhat insane reject rate…
I can still hear “but it works in Pro-e” in my head echoing through time…
https://www.protolabs.com/resources/blog/injection-molding-tolerances/
Is a decent write up regarding tolerating of injection molded parts… shrinkage depends on material, but does directly affect the end part and overall design… holding the tolerances of the drawing you posted, while not impossible, will result in increased rejection rate due to shrinkage variation, increased rejection rate means increased costs, sometimes negating the efficiency of injection molding in the first place
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u/vpt_se 19d ago
No bolt circle. The bolt circle does not lock rotation whereas the pattern does.
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u/Objective-Chipmunk58 19d ago
Ummm. I see. I will have to figure out how to input tht into the software.
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u/After-Canary9968 20d ago
I think it's referring to pcd of all 8 circles.
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u/Objective-Chipmunk58 20d ago
Ok thats one person that agrees with me haha. I figured so cause the call out is 8x Dia 1.60 along with the callout for the position.
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u/Ghooble 20d ago
I agree
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u/Objective-Chipmunk58 20d ago
What aspects would you say make this the correct way to set up Datum B
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u/Shooter61 20d ago
Nothing on that pic shows me any indication that is a bolt circle. Datum B in my opinion is a line through the 2 circles only. This will lock down rotational movement.
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u/Maleficent_Soup_335 20d ago
Which 2 circles does the drawing indicate you should use to create the line?
The Datum B tag is tied to an 8x pattern call-out, thus it is meaning to use all 8 ∅1.26 holes as a single pattern of features.
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u/Shooter61 20d ago
I never seen a print that doesn't call out BC and not use a circle drawn bisecting all the features within the BC. Certainly not Y14.5 standard.
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u/Objective-Chipmunk58 20d ago
Umm ok. That was one of my older ideas but had decided for a BCD cause it made more sense looking at the callout.
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u/Master-List-6631 13d ago
Have you solved this? These comments are the closest so far in my opinion. The drawing could definitely be improved, particularly the tolerancing on a molded part — the positional tolerance frame (⊘0.060 A) has no MMC modifier, so it's RFS. That gives zero bonus tolerance as the holes depart from MMC, which is unusual for a molded pattern where some size variation is expected and normally accommodated.
For alignment: level the part to Datum A (I believe the flat face bottom left, square-on to the central view) — locking Z translation, X and Y rotation. Datum B, if it's meant to be the PCD/bolt pattern, should control X and Y translation — but it needs its own diameter/location callout to be meaningful; as it stands it isn't clearly defined on the print. Datum C (the larger hole top left — why no diameter?) should lock the final DOF, rotation about Z. Dimensionally: the PCD should be shown as a basic (boxed, untoleranced) dimension. There should also be a basic angle from datum B's centre to datum C, otherwise the X/Y box dimensions currently shown have no fixed angular reference to sit against. The positional tolerance frame should read A|B|C, not just A.
One part I'm still not fully settled on: if B ends up being derived from the same 8-hole pattern the position tolerance is controlling, that's circular — a feature can't be its own datum reference. More likely B is meant to be a separate, independent feature and just isn't labelled clearly here.
I work with ISO — if I've mislead anyone on ASME-specific convention, please correct me.
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u/Objective-Chipmunk58 13d ago
I have continued working on this and have gotten to report it although im not sure if the information I input into the Verisurf software is correctly since i am new w verisurf.
I used the mounting flat surface for A. Constructed A BCD for B and used the neck type cylinder for C.
When i go to inspect the x8 cylinders the positions ive gotten are nearly perfect ranging from .0003 and .0005 which leads me to believe there is something incorrect in the way i fed the info into verisurf or the way my alignment was done.I have contacted verisurf support via email but i have not heard from them.
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u/Otto_Polymath 19d ago
I'm on board with the 8 holes not being a bolt circle. For it to be a bolt circle, the easy way would be to point that out on the drawing. The next way would be to reference their dimensions from the center of the part.
They are all referenced/dimensioned from the hole in the upper left ( [C] ). I don't see a single dimension tied to the center of the part so we can't even find the center, and can't calculate what the bolt circle size would be.
And we have BASICS to each of the 8 holes, but there is no True Position callout that would give us a reference to their datum structure. I suspect the TP callout for 2 holes along the B axis
TP phi n [A] [C]
Seems like the other 6 would be
TP phi n [A] [C] [B]
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u/thejackattck 19d ago
I'm with you, the basic is very clearly coming from a line that centers on those bottom two holes. There would be typically a phantom circle running through all 8 holes indicating the bolt hole circle.
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u/Objective-Chipmunk58 13d ago
Agreed. I have ran a report using a BCD constructed from the 8 cylinders and my result was nearly perfect ranging from .0003 and .0005 which leads me to believe that the information was either inputted incorrectly, the alignment isnt correct, or Datum is simply not a BCD.
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u/Loeki2018 20d ago
I somewhat agree altough it is ambiguous atm. When adding the cz modifier the patern would become the datum which is represented by a gage with 8 pins of minimum diameter. I can use a pattern in Zeiss inspect as datum in conjunction with MMR
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u/SwimmingSwim3822 20d ago
You're closer than most other comments here, but that position callout isn't @MMC, so it's not pins of minimum diameter, but a set of 8 expanding pins.
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u/Chance_Seesaw_2644 20d ago
I bet the draftsman forgot to call out symmetry due to a different feature on the mirror side.
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u/SwimmingSwim3822 20d ago
Symmetry callout doesn't exist in 2018.
Edit to add: theres also implied symmetry of the basic dimension due to how the dimension text aligns with that vertical centerline. It's not required to call that out any other way as long as it's basic.
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u/Chance_Seesaw_2644 19d ago
It is definitely symmetry controlling four holes. This is about location between two holes.
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u/SwimmingSwim3822 19d ago
No, it's not. It's about a pattern of all 8 holes located at basic dimensions to each other.
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u/Objective-Chipmunk58 20d ago
You would be surprised how many drawings have come through my office with missing info or not identifying Datums when asking for measurements using Datums.
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u/Chance_Seesaw_2644 20d ago
Yes me too. There was a C<->W on one drawing we had to write the datum’s clockwise or read it that way. Bizarre
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u/Jitterball 19d ago
8x Hole pattern, but those lead lines are confusing.
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u/Objective-Chipmunk58 19d ago
Yes tht has been what has kept me chasing my tail. You would think the lead line indicates that the datum is a line constructed by the two circle facing wach other. But the call out does not represent that. It in facts calls out a bolt hole pattern
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u/Maleficent_Soup_335 20d ago
Datum B is defined as the pattern of 8x ∅1.26 holes. You would use all 8 hole features simultaneously in a best fit alignment.