Hey man, I’ve been watching your UA-cam videos for free. You are the reason I learn this stuff. With my first proper salary, I’m gonna buy your course just to thank you. These lectures are banger. Thanks boss
9:28 - To help you decide where the C-Datum should be, you need to decide what you want to measure on your gauge. Earlier on, besides worrying about constraining the part and how it gets assembled (two very important details), also think about how the part will be measured on the gauge and what you want to measure. Choosing the pin hole is a good idea, it allows you to more easily measure th latching surface of the bracket.
Both that hole and angled face are rotationally important. So, I would agree that it's up to the designer to decide how they want to collect the inspection data. Align to the hole and check the face or align to the face and check the hole. Either way inspection can do it.
On Seat Latch Bracket Application (9:50), if the hole (3.0 - 3.2) is the datum feature C, then the DRF should have been | A | B | C > |, where `>` represents the translation modifier. Unfortunately, this detail has been missed. Another alternative DRF would be (maybe more realistic): | A Ⓜ | B Ⓜ | C Ⓜ |
I do like your recommendation of the translation modifier for C. The mating part does self-center in there. However, I would not recommend material boundary modifiers in any of these parts in the video. Those modifiers allow datum feature shift and cause major stack-up issues. I only recommend modifiers for datum features if the part is sloppy and adjustable.
Seems like the recommendation in this video is to always constrain 6 degrees of freedom. But what if we constrain less and then rely on simultaneous requirements to take care of mutual location and orientation between those features considered as followers? For example in the seat latch bracket case, the hole used as datum feature C is just where the handle for operating that mechanism mounts. I wouldn't really say the handle is imposing a constraint on this part in assembly. So I would think of using only A and B, and then the profile tolerances on other important features such as the face where the square holes are and a position tolerance on that hole, all referencing A and B in this order, would form a simultaneous requitement that would mutually control them. That way only 5 degrees of freedom would be constrained and 2 datum references used, but still everything would be fully defined. Are there any cons to doing it that way?
Good question. If you leave a degree of freedom "free" then the the rest of the features must be "best fit" in that final DOF because of the simultaneous requirements. It can be complicated to best fit it all. Best fitting and optimizing is not fully standardized. Its good for pass/fail but can be hard to get consistent measurement data. If there is a clear "leader feature" that does the rotational work, then I say "lock it down"., as its much more repeatable. Of course there are situations where best fitting can be functional, but generally, constraining all six DOF is the way to go.
Thank you very much for the answer, the info you give here is priceless! I have a follow-up question after you said that best-fitting is problematic: isn't best fitting required often when inspecting gd&t ? For example, the recommended practice for tightening the "in-pattern" relationship between a group of holes and loosening their locational control to the datum reference frame is a composite position. I guess best-fitting the group of holes in unlocked degrees of freedom is what is needed for measurements for the lower segments of the composite tolerance, isn't it? Are those measurements also unrepeatable? Thanks in advance!!!
Quick mention...the base part is missing B datum in the secondary prequalifier callout. Terrific vid btw..my favorite for detaling out a functional alignment scheme, and how to bring up VC and boundary conditions (fastener size).
Thanks for the compliment! You may have missed the C pin has a position in the left/right direction of .005 to AB. The perp of .001 is only refining it in one direction to datum A.
In the sheet metal part example, when you use an all surfaces profile tolerance to datum a b and c with unspecified dimensions basic in addition to a plus/minus thickness tolerance, which side of the surface is controlled position and orientation by the profile tolerance since there is now a thickness tolerance involved?
Yes, good comment. It should be made clear which side of sheet metal is profiled and which side is stock thickness. This is easy in model based definition (digital association). On 2D drawings, I like to point to all the faces directly that have the profile or size tolerance. Use the UOS profile for the cut edges. I think the sheet metal example (seat latch bracket) in my video is clear, right?
Do we have any solutions to avoid the secondary and tertiary datums created by threaded holes when it has to (bolted with threaded holes and against a flat surface which is datum A )? It's reaaly hard to get a constant testing value when threaded holes marked as datum B & C on the drawing?
Threaded holes are an option for the datum feature, and I would select these if that's the way the part functions. They can be hard to align to in inspection though. Slightly tapered (expandable) thread gages can "self center" to eliminate slop. If the threads are large enough, CMM can do a helical trace to get a repeatable thread axis. Another option in inspection is to measure from the minor diameter instead and keep track of the measurement uncertainty (guard-band the tolerance).
The origin of the DRF is where the datum A axis and datum B plane intersect. Datum feature C only constrains the final rotation around the origin. The DRF is established when all 6 degrees of freedom are constrained.
In that assembly, one of the pins fits to a hole, the other mounts to a slot. The pin-to-hole is the more important interface and self-centers the two parts. The pin only stops rotation in the mating slot. This is a classic B then C datum reference frame.
5:45 Datum C used constrain purpose but All other holes are created based on datum B hole. Reference taken from Datum B hole. Then why mention all other dimensions? Datum A and B is enough na?
Datum features A and B establish a plane and axis. This is not enough. Rotation around the axis is not constrained yet. You don't know North, South, East, West until you've rotationally aligned the part. Datum feature C is needed for the final rotational constraint. ABC are a full 3-plane datum reference frame.
You could go with a partial datum feature for that lid, yes. Hole and slot contacting two pins is usually set up as B then C. The alignment hole does more work than the slot. I don't agree that they are equal.
An explicit specification „geometric feature functions“ between geometric design and GD&T is missing in the world of advanced mechanical engineering. There are only three well known functions in the nature of mechanical parts: Fit, touch and non contact. These functions apeare in natur at the level of an assembly - not in a single part drawing. A single part specification needs the simulation of two contacts: Datums for fit and touch! And a synthese of the natur-function-tolerance. Let‘s realise a software to handle feature functions: Created with constrains in a 3D-assembly and stored into a PLM-system (like we still handle the structure information which ends in a BOM). The step to a GD&T will not longer be the job of an designer. Based on this new specification a GD&T is created in a predefined workflow with additional input from manufacturing issues. Revolution not Evolution! Mechanical engineering should be pushed to the next level. Unfortunately there will be everytime a difference between the function in the natur and the GD&T-modell. We should talk about: How can we control this problem? (Should we reduce tolerance?)
Good comments. I wonder if software could help give better recommendations to designers for functional datum features based on how they align in the assembly. There will be options though for some parts. A part may mate to many other parts. A designer must prioritize one fit vs another to determine the proper DRF. Also, another alignment besides fit and touch is done with fixturing. This happens in inseparable assemblies (weldments, match drilling). Certain features are fixtured (with either hard or soft tooling) before being bonded together. This fixturing sets the datum reference frame for the individual components (not necessarily the contacting features.)
@@GeoTolPro „Inseparable assemblies": Every time we talk about a technical drawing, we should clarify what the purpose of this specification is. This is not easy, because it means that we have to fit this specification into the product development process, which is different in every company. For your example first we need a functional based GD&T of the finished inseparable assembly (which we want to purchise…) ready for the next assembly step. So we are talking again about fit, touch and non contact. I'm afraid it's the same game every time. Second, we need a verification specification based on the first GD&T. This should take into account the measurement uncertainty by reducing the tolerances. This document should be a part of the contract with the supplier (not the first functional GD&T!). Third, a manufacturing specification is required. GD&T is not suitable for this because manufacturing processes cannot directly control geometric quality. Ultimately, the manufacturing process must be controlled by measurement.
Yes, the mechanical engineering world is waiting for a concept of how a basic datum system with GD&T should be built. It is always very helpful to start with the function itself. Which function is the most well-known in engineering practice today? It is bolting: firstly, a surface contact and secondly, the fit of through holes or threads.
I disagree, A should be on the opposite side, A doesn't exist until machined, sure you want to use functional feature if it makes sense, holes as datums ya, when it makes sense,
Datum features are selected based on how the part functions (how it mounts in the assembly). It doesn't matter the sequence of operations from manufacturing. When the part is all finished, what are the functional requirements?
I agree. A lot in this system is what machinists and inspectors would call natural and common sense. Geometric tolerancing is just giving us the language to communicate that "common sense" in a more explicit way.
Hey man, I’ve been watching your UA-cam videos for free. You are the reason I learn this stuff. With my first proper salary, I’m gonna buy your course just to thank you. These lectures are banger. Thanks boss
Hi,
This is the first time I discovered your channel.
Thanks a lot for your valuable work.
Do all holes have to be located using one of the datums or can there be basic dimensions locating hole to hole distances?
Thanks, many videos on gd&t was there teaches theory, thing that set you apart is realtime examples. Great work
Nice explanation.. Expecting more videos on selecting datums
9:28 - To help you decide where the C-Datum should be, you need to decide what you want to measure on your gauge. Earlier on, besides worrying about constraining the part and how it gets assembled (two very important details), also think about how the part will be measured on the gauge and what you want to measure. Choosing the pin hole is a good idea, it allows you to more easily measure th latching surface of the bracket.
Both that hole and angled face are rotationally important. So, I would agree that it's up to the designer to decide how they want to collect the inspection data. Align to the hole and check the face or align to the face and check the hole. Either way inspection can do it.
Excellent video, i understand the complex parts very easily ❤
On Seat Latch Bracket Application (9:50), if the hole (3.0 - 3.2) is the datum feature C, then the DRF should have been | A | B | C > |, where `>` represents the translation modifier. Unfortunately, this detail has been missed.
Another alternative DRF would be (maybe more realistic): | A Ⓜ | B Ⓜ | C Ⓜ |
I do like your recommendation of the translation modifier for C. The mating part does self-center in there.
However, I would not recommend material boundary modifiers in any of these parts in the video. Those modifiers allow datum feature shift and cause major stack-up issues. I only recommend modifiers for datum features if the part is sloppy and adjustable.
Seems like the recommendation in this video is to always constrain 6 degrees of
freedom. But what if we constrain less and then rely on simultaneous requirements to take care of mutual location and orientation between those features considered as followers? For example in the seat latch bracket case, the hole used as datum feature C is just where the handle for operating that mechanism mounts. I wouldn't really say the handle is imposing a constraint on this part in assembly. So I would think of using only A and B, and then the profile tolerances on other important features such as the face where the square holes are and a position tolerance on that hole, all referencing A and B in this order, would form a simultaneous requitement that would mutually control them. That way only 5 degrees of freedom would be constrained and 2 datum references used, but still everything would be fully defined. Are there any cons to doing it that way?
Good question. If you leave a degree of freedom "free" then the the rest of the features must be "best fit" in that final DOF because of the simultaneous requirements. It can be complicated to best fit it all. Best fitting and optimizing is not fully standardized. Its good for pass/fail but can be hard to get consistent measurement data. If there is a clear "leader feature" that does the rotational work, then I say "lock it down"., as its much more repeatable. Of course there are situations where best fitting can be functional, but generally, constraining all six DOF is the way to go.
Thank you very much for the answer, the info you give here is priceless! I have a follow-up question after you said that best-fitting is problematic: isn't best fitting required often when inspecting gd&t ? For example, the recommended practice for tightening the "in-pattern" relationship between a group of holes and loosening their locational control to the datum reference frame is a composite position. I guess best-fitting the group of holes in unlocked degrees of freedom is what is needed for measurements for the lower segments of the composite tolerance, isn't it? Are those measurements also unrepeatable? Thanks in advance!!!
Nice video mate!
Excellent video! Much appreciated!
this is great information thank you for that
5:30 Do the center plane of slot C must go thru circle B?
Yes, ASME Y14.5 requires the true geometric counterpart for C to be located (centered in this case) to datums A and B.
Quick mention...the base part is missing B datum in the secondary prequalifier callout. Terrific vid btw..my favorite for detaling out a functional alignment scheme, and how to bring up VC and boundary conditions (fastener size).
Thanks for the compliment! You may have missed the C pin has a position in the left/right direction of .005 to AB. The perp of .001 is only refining it in one direction to datum A.
In the sheet metal part example, when you use an all surfaces profile tolerance to datum a b and c with unspecified dimensions basic in addition to a plus/minus thickness tolerance, which side of the surface is controlled position and orientation by the profile tolerance since there is now a thickness tolerance involved?
Yes, good comment. It should be made clear which side of sheet metal is profiled and which side is stock thickness. This is easy in model based definition (digital association). On 2D drawings, I like to point to all the faces directly that have the profile or size tolerance. Use the UOS profile for the cut edges. I think the sheet metal example (seat latch bracket) in my video is clear, right?
Very well explained 👍
Do we have any solutions to avoid the secondary and tertiary datums created by threaded holes when it has to (bolted with threaded holes and against a flat surface which is datum A )? It's reaaly hard to get a constant testing value when threaded holes marked as datum B & C on the drawing?
Threaded holes are an option for the datum feature, and I would select these if that's the way the part functions. They can be hard to align to in inspection though. Slightly tapered (expandable) thread gages can "self center" to eliminate slop. If the threads are large enough, CMM can do a helical trace to get a repeatable thread axis. Another option in inspection is to measure from the minor diameter instead and keep track of the measurement uncertainty (guard-band the tolerance).
another good one...
really good lecture!
can you explain me where 0,0 from on that seat bracket?..how 3- DRF planes will come perpendicular to each other
?
The origin of the DRF is where the datum A axis and datum B plane intersect. Datum feature C only constrains the final rotation around the origin. The DRF is established when all 6 degrees of freedom are constrained.
Shouldn’t the one with 2 pins have B and C called out as a single datum feature by calling out the position of the two pins wrt datum A??
In that assembly, one of the pins fits to a hole, the other mounts to a slot. The pin-to-hole is the more important interface and self-centers the two parts. The pin only stops rotation in the mating slot. This is a classic B then C datum reference frame.
5:45 Datum C used constrain purpose but All other holes are created based on datum B hole. Reference taken from Datum B hole. Then why mention all other dimensions? Datum A and B is enough na?
Datum features A and B establish a plane and axis. This is not enough. Rotation around the axis is not constrained yet. You don't know North, South, East, West until you've rotationally aligned the part. Datum feature C is needed for the final rotational constraint. ABC are a full 3-plane datum reference frame.
@@GeoTolPro Would it be also correct to rotationally align the part by using the symmetry plane of the part as datum feature C?
Datum feature C must be a real and physical feature. A symmetry plane is not real and cannot be a datum feature.
Example 1: B and C are equal. Contact at C touches not at the complete feature.
Example 2: Touch at A is not against compleate feature. B and C are equal. C is a fit between different types of features!
You could go with a partial datum feature for that lid, yes. Hole and slot contacting two pins is usually set up as B then C. The alignment hole does more work than the slot. I don't agree that they are equal.
An explicit specification „geometric feature functions“ between geometric design and GD&T is missing in the world of advanced mechanical engineering.
There are only three well known functions in the nature of mechanical parts: Fit, touch and non contact. These functions apeare in natur at the level of an assembly - not in a single part drawing.
A single part specification needs the simulation of two contacts: Datums for fit and touch! And a synthese of the natur-function-tolerance.
Let‘s realise a software to handle feature functions: Created with constrains in a 3D-assembly and stored into a PLM-system (like we still handle the structure information which ends in a BOM). The step to a GD&T will not longer be the job of an designer. Based on this new specification a GD&T is created in a predefined workflow with additional input from manufacturing issues.
Revolution not Evolution!
Mechanical engineering should be pushed to the next level.
Unfortunately there will be everytime a difference between the function in the natur and the GD&T-modell. We should talk about: How can we control this problem? (Should we reduce tolerance?)
Good comments. I wonder if software could help give better recommendations to designers for functional datum features based on how they align in the assembly. There will be options though for some parts. A part may mate to many other parts. A designer must prioritize one fit vs another to determine the proper DRF.
Also, another alignment besides fit and touch is done with fixturing. This happens in inseparable assemblies (weldments, match drilling). Certain features are fixtured (with either hard or soft tooling) before being bonded together. This fixturing sets the datum reference frame for the individual components (not necessarily the contacting features.)
@@GeoTolPro „Inseparable assemblies": Every time we talk about a technical drawing, we should clarify what the purpose of this specification is. This is not easy, because it means that we have to fit this specification into the product development process, which is different in every company. For your example first we need a functional based GD&T of the finished inseparable assembly (which we want to purchise…) ready for the next assembly step. So we are talking again about fit, touch and non contact. I'm afraid it's the same game every time. Second, we need a verification specification based on the first GD&T. This should take into account the measurement uncertainty by reducing the tolerances. This document should be a part of the contract with the supplier (not the first functional GD&T!). Third, a manufacturing specification is required. GD&T is not suitable for this because manufacturing processes cannot directly control geometric quality. Ultimately, the manufacturing process must be controlled by measurement.
Yes, the mechanical engineering world is waiting for a concept of how a basic datum system with GD&T should be built. It is always very helpful to start with the function itself. Which function is the most well-known in engineering practice today? It is bolting: firstly, a surface contact and secondly, the fit of through holes or threads.
I disagree, A should be on the opposite side, A doesn't exist until machined, sure you want to use functional feature if it makes sense, holes as datums ya, when it makes sense,
Datum features are selected based on how the part functions (how it mounts in the assembly). It doesn't matter the sequence of operations from manufacturing. When the part is all finished, what are the functional requirements?
Experienced machinist naturally do this only different is designers should really consider this to make life easer for shop floor people
I agree. A lot in this system is what machinists and inspectors would call natural and common sense. Geometric tolerancing is just giving us the language to communicate that "common sense" in a more explicit way.