gd t is a language of symbols used to communicate design intent now i've identified four major concepts that i'm going to cover in this video that's going to help you with your gd t studies the concepts are one datums two feature control frames three material condition modifiers and four basic dimensions if you know what these are you can identify them and you can apply them to a drawing you're gonna have a pretty good handle on how the system works let's get started right away with datums now the definition of a datum is that it is a theoretical point line or plane now we can't touch or measure something that's theoretical so we identify features on the part to serve as datums and simulate them with datum feature simulators which are just inspection equipment whether it's a surface plate to simulate a plane or a gauge pin to simulate a cylinder we're just going to get as close as we can to that theoretical perfect datum now the other way to understand datums which is very very important that datums are places to measure from let me give you some examples of what i mean on the board i've got two uh drawings on the board let's talk about this one first we've got a stepped block with a leader directed note that indicates parallel to five thousandths now this is definitely not allowed anymore but if you look at drawings that are you know 50 80 years old you might see something like this this is what's known as an implied datum it's up to whoever is interpreting the drawing to decide what this surface is supposed to be parallel to now you might say duh it's this large surface down here you'd probably be correct but somebody else could say hey i know how this part functions it actually fits upside down that surface is supposed to be parallel to that surface right so what datums do is specify exactly which surface something is supposed to be oriented to or located from let's go into our next example we've got a block with a hole in it now we've got a center line going through the block and then one dimension covering the height this indicates that that hole is centered in the middle of the block okay so that center line and then that dimension indicates this we don't need another dimension for the location of that hole but one person could say let's measure from this surface make it one inch plus or minus ten thousandths another person might say one inch plus or minus ten thousandths from that surface another person might say well why don't we take the block and put it in a vise and then find the center right and simulate that center line and then get the location from there now none of them are really correct right plus or minus dimensioning really doesn't have rules for how to do this stuff you're leaving the interpretation up to whoever has the drawing who's making the part who's inspecting the part what datums do we can identify this is a datum and clearly state that the center of this part is a center plane datum and then the hole is located from the center of the part it's all about being explicit about the drawing requirements now we'll talk more about datums as we go through this talk right now but this is good enough for now datums are places to measure from next up let's talk about feature control frames now this example here i've just written it out in a note and that was the way to do it older drafting manuals would just spell things out hey make this cylinder concentric to this cylinder make the total indicator read out this everything was written in notes the problem with notes is that it leaves room for interpretation think about if you ever took an english class and you're reading an essay your teacher expects you to interpret it but they don't they know every student isn't going to interpret it the exact same way so notes should be as short as possible that reduces the room for interpretation but if we can use symbols instead we further reduce the room for interpretation so to make this into a proper g d t call out we're going to use the feature control frame right so feature control frame is a box in this case it's leader directed the very first box will have the geometric characteristic symbol the next one will have the tolerance after the tolerance will be the datums in this case with this particular tolerance we only need one datum we're going to write our datum feature symbol down here connect it to the bottom surface so now it's explicit what the drawing requirements are okay so next up let's talk about feature control frames now i've replaced the note on this drawing with the feature control frame that accomplishes the same thing the first symbol is the geometric characteristic symbol then their tolerance then our datum references in this case we're just saying that this surface is parallel to five thousandths to datum a which is this bottom surface identified with the datum feature symbol now let's move on to a large a couple more examples of feature control frames so feature control frames differ depending on how they're applied what geometric characteristic is implying and how many datums there are now there's more videos i've made about this specifically the different kinds of geometric characteristic symbols but let's just talk about a few first is form tolerances like flatness typically don't or never have a datum reference because form tolerances only apply to individual features there's never a need to relate them to anything else so in this case a flatness tolerance is directed with a leader to a surface it's just saying that surface is flat to within 10 thousands next up we've got a profile which is very tricky profile can do a lot of different things it can function just like flatness or it can locate surfaces in this case the datum reference to datum a it's doing the job of flatness parallelism and locating a surface if we have a basic dimension and then another form tolerance straightness applied to the surface of a cylinder this controls the line elements around the cylinder very simple feature control frame just the symbol and then a tolerance okay let's talk about something a little bit more complicated we'll get into position which generally have the most complicated feature control frames so allow us to talk about all of the elements of the feature control frame so here is about as complicated as a feature control frame can get but it's not too bad so we've got a block with a hole in it we've got basic dimensions locating the center of the whole feature we've got three datums so a b and c on perpendicular surfaces of the part we're leader directed to a size dimension now position is almost always applied with the directly tolerance dimension that applies to a feature of size feature of size real quick is something you can measure with opposing points so this width of the block we can measure with opposing points this hole we can measure with opposing points okay so a quick definition of feature of size so our feature control frame like i mentioned the first block is the geometric characteristic symbol the next block is the tolerance and associated adjectives to the tolerance so in this case with position we have a diameter symbol here to indicate that the tolerance zone is a cylinder so you can imagine there's a cylinder centered where these basic dimensions terminate of 30 thousandths that's where the axis of this hole must lie next we have a material condition modifier in this case it is mmc this just means that as this hole gets larger more locational or positional tolerance is allowed and i'll i'll put a card up i made a whole other video about it we'll just gloss over it for now next now this is important are datum references most of the time they will be written a b c because it's a popular choice for designers it makes it easy to read drawings because people are used to seeing it does not have to be abc it's in the order that the part is set up for inspection so datum a should be a mating surface the largest and most important mating surface normally for a block like this it would be one of the largest planes right that gives you good repeatability for inspection doesn't have to be the second one is our secondary datum typically the second largest surface but it can be other kinds of features as well it could be a feature of size like a slot or a cylinder so we could switch this [Music] it could just as easily be acb in this case we'd set up to a b and then c okay so they are in order and it does matter that's known as primary secondary and tertiary some people know this as the three two one rule so three points for the primary two points for the secondary one point for the tertiary the idea is to completely immobilize the part and all six degrees of freedom when you're inspecting it right it's stated on the drawing exactly how it's supposed to be set up for inspection again this reduces errors in interpretation if everybody is trained up on how to apply and interpret gd t so now let's move on to material condition modifiers this seems to throw people for a loop but it's a simple system when you look at it we've got three material condition modifiers to worry about mmc with its little m with a circle or written mmc or spelled out as maximum material condition next is lmc l with a circle or lmc spelled out it is least material condition and then i put the symbol here it's an s with a circle currently the symbol is not used and it has not been used since the asme y145 1982 but some people still you know are comfortable with this symbol and notes and things so it's spelled out as rfs we've talked a little bit in previous videos about the concept of mmc it simply states that external features at their maximum material condition are at their largest size so if you have a pin that's plus or minus you know 40 thousandths the mmc is 1.04 the largest pin it's the opposite for internal features so if you have a hole at mmc it's the smallest the hole is allowed to be within the limits of size it's a great way to compare whether parts will fit and how much allowance is between them another way to think of it is that the mmc is the heaviest part so the largest pin is the heaviest pin the smallest hole creates a heavier part than having a larger hole now lmc is the opposite so the lmc of a pin is the smallest it's allowed to be the lmc of a hole is the largest it's allowed to be we generally split these up into internal and external features so the pin would be our external feature whether it's a pin a block or anything else it's external internal could be a slot or a hole or a groove or any number of different kinds of features rfs does not have a symbol because it is the default requirement for feature control frames applied to features of size so what's a feature of size i talked about it a little bit before let's go a little bit more depth a feature of size is something with opposable points that can be measured let me give you some examples you have a block the width dimension that is a feature of size if you have a pin with the diameter dimension that's also a feature of size it can be measured with two points same thing with a slot or a groove that would be a feature of size because you can measure it with two opposing points so think about a caliper the inside measuring part of a caliper what is not a feature of size is something like this radius right you can't get two points on that radius now you could make an argument that okay if the center line is the radius is on the same plane as the top of this part you could the problem is you got a tolerance on that radius so if the radius comes in small then you can't get two points it comes in large maybe you can but you wouldn't want to call that a feature of size next something like a chamfer not a feature of size because you can't measure out into air with two opposing points same thing with this open cut right here that would not be a feature of size so it's important to understand what is and isn't a feature of size to know what material condition it can apply at so things that are not features of size cannot have material conditions you can never have more or less tolerance than stated for something that isn't a feature of size now let's talk a little bit about what these symbols do in practice so i've got a pin with a size dimension and a straightness tolerance applied to the derived median line this just means real quick that the center of that pin must lie within a cylinder that is 20 thousandths in diameter and that's all we'll worry about for now so there's no mmc or lmc symbol but it is a feature of size in that case the regardless of feature size the one where we don't use the symbol applies it is the default requirement this is known as rule number two in the asme standard so all that means that no matter what size this pen comes in at there's always 20 thousands of geometric tolerance so pretty straightforward right it's exactly what you think it would be now let's try the mmc modifier so if this tolerance applies at mmc it's best read as that applies only at mmc 20 000 only at mmc so let's write that right here when the pin comes in at 1.02 the mmc of this size 20 thousands of tolerance is allowed but if the pin comes in uh toward its lmc so it comes in smaller more geometric tolerance is allowed equal to the difference the mmc to the actual size all that means if it comes in ten thousand smaller you get ten thousands more geometric tolerance simple enough right same thing we just keep going down the list if the pin comes in as lmc you're allowed 60 thousands of geometric tolerance that's also equal to the total size tolerance forty thousands right so we just add forty thousands to the twenty thousands to see the total amount of geometric tolerance that may result if the pen comes in toward its lmc okay so let's try the lmc modifier instead of the mmc modifier so now we read this so we have twenty thousandths of geometric tolerance only at lmc so we'll reflect that right here if the pin comes in toward its mmc you get more tolerance equal to the difference so if the pin comes in at 0.99 you get 30 thousandths 1.0 you get 40. and so on lmc is just the reverse of mmc in this situation okay so rfs is default for geometric tolerances applied to features of size mmc and lmc can be used to allow more tolerance depending on the actual size the part comes in at okay let's move to another concept which might be a little advanced for a video like this but let's go ahead and tackle it let's talk about datum feature material boundaries these are known as mmb lmb and rmb which sound a little like you know mmc lmc and rfs they're the same thing but they're for the datums so let me get this drawn up on the board so i've got a drawing on the board here we've got a donut shape with two dimensions applied to it the outside diameter is identified as a datum with a size tolerance two inches plus or minus 20 thousandths the mmc of that feature is 2.02 the largest it can be we've got an internal diameter with a size tolerance and a position tolerance so we're positioning the smaller hole to the outer diameter the feature control frame position diameter tolerance zone 0 and mmc this just means that if the hole comes in at its smallest size it has zero positional tolerance but if the hole comes in any bigger you get more positional tolerance just kind of like the chart we were doing before datum a is our primary datum so we've got to establish three points of contact with datum a here but datum b applies at maximum material condition or mmb maximum material boundary what this is is an optional inspection technique you don't have to do this you could inspect it with a closing shape just like a rfs inspection but you could also measure it with a fixed gauge so what this says is what this is saying is that you can check it at the mmc or virtual condition of of this feature so the mmc is 2.02 you can make a gauge for this part and check it that way right you're saying the inspection here is fixed so what would the gauge look like the gauge would be a ring with a pin in the middle the ring to simulate datum b at its mmc so it would be set to 2.02 inches the maximum material boundary the pin would be set to the mmc of this feature so one inch plus or minus 20 thousandths we would simulate it at 0.98 so if the part fits in that gauge we've established that it has not violated this boundary okay now all we'd have to do is check and make sure the hole wasn't too big so these aren't always used but they are a technique to save you know money and inspection if you can set your dementia or your tolerances like this you can automate inspection to a pretty serious degree right you could have a robot drop this washer or this donut into a gauge and if it fits it's good if it doesn't fit it's bad it go in a different bucket okay so that is a brief introduction to material condition modifiers next we'll move to basic dimensions which is mercifully simple and it'll be the last thing i talk about in this video so basic dimensions are theoretically exact dimension values applied to size or location of features that are controlled with a feature control frame it's very important that basic dimensions are associated with feature control frame so in this case i've just given the left right basic dimensions but these basic dimensions show you where the tolerance zone should be for this position tolerance another example are things like profile that control the location of surfaces with basic dimensions now since basic dimensions are theoretically perfect they don't count in a tolerant stack up so they can be written differently than plus or minus dimensions so if you did a plus or minus dimension like this you could have a lot of stack up with basic dimensions as long as they connect together you can interpret them you can interpret the distance from the centers of the holes as one inch okay now at inspection you really shouldn't indicate the tolerance or the actual dimension for basic dimensions okay so all of these dimensions here are associated with either this position or this profile that's where the tolerance for the basic dimensions is and the last thing about basic dimensions is that they must come from datums that's where you're measuring from the basic dimensions have to lead back to datums now they can loop so datum c is here we can go two inches and then back and then locate this but they've got to be connected okay so that's it for this video i just wanted to go over these four major concepts of gd t i hope you got something out of it if you enjoyed the video please like and subscribe leave a comment down below and stay tuned for more videos like this coming soon you