How to Apply GD&T Position Tolerance to a Circular Hole Pattern

[@tjvanderloop1686]

This was well done, it is vital in the "Industry that GD&T" is understood. You did a great job! Thank You. T J (Tom) Vanderloop, Author, Technology Instructor & Manufacturing Consultant, AWS & SME Professional

[@StraightToThePointEngineering]

Hello Mr. Vanderloop! I appreciate the kind words of acknowledgement and encouragement! This is one of my older videos and so I am continuing to improve the quality as I create more. I appreciate your viewership and I will look for you in the comments sections in the future. Stay safe and healthy!

[@flewdefur]

Ive watched several of your GD&T videos and found them all very helpful and clear. Thanks!

[@solonglife8130]

The basic dimension of 2.000 and 3.000 only show the position of the center of PCD. How to know the positioning dimension of the 8 small holes from the datum ? Is it we need to make some simple calculation (cos45 or sin 45) manually to get the distance of the small holes to datum B and C? Then with this number of distance, we measure by CMM and get the deviation of the small holes from datum. Thanks.

[@ginotacobell3098]

It looks simple but then how you inspect those 8x holes?

[@freerangequark]

Does the Y14.5 standard allow the holes in a composite controlled pattern to be of different size? If so, how would that be annotated? Thanks!

[@CharlesBecket]

Greetings, sir. Re-entering the quality field after a 30-year absence. Trying to get back up to speed. You say to forget the traditional +/- system of stating a tolerance but you use .281 +.007 / -.002. How does this interact or have an effect on the .014 in the box? Does the .014 refer to the amount of distance each hole can be off with respect to the centerline of all the holes? Thank you.

[@kingbaldy100]

Hey Charles, I'm pretty sure STTP is no longer active so here is an explanation for you! The .281 +.007 / -.002 is the diameter for each of the 8x thru holes that are in a circular pattern for this part. The boxed dimensions 2.000, 3.000, 1.500 and the general note of "BC EQL SP" would allow the operator to find the coordinates for each the thru holes, relative to the datum's defined for this part. The .014 positional tolerance would then be applied to each of the thru holes, with respect to each of their absolute coordinates which were defined by the boxed dimensions and note above. The diameter of the thru holes are independent of the location of the thru holes.

[@CharlesBecket]

@@kingbaldy100 Thank you very much for the explanation. Wishing you well.

[@kalyanb278]

Sir if the part is circular shaped and of dia. 100mm , then 4 holes on PCD dia. 60 mm . Tolerance for the hole position is +/- 0.15 then how to show it symbolically, which will be the datum reference.

[@vj-jx2db]

Why you have taken a hole diameter 0.014 in the box but actual diameter written over the box is different?? Plz ans me

[@yonatanshenhav1208]

simple and to the point - a great video

[@InfinitiQX70]

How to pick XYZ tolerance from GD&T position of .056M|A|B|C?

[@armandmatossian2081]

Does the positional tolerance apply to the center location of the circular hole pattern? Or does it apply to the individual 8 holes in the pattern?

[@StraightToThePointEngineering]

Good Morning Armand Matossian! Thanks for the awesome question! The short answer: when you apply a GD&T positional tolerance to a circular hole pattern, it does NOT establish any sort of tolerance zone at the center of the hole pattern. Instead, applying a positional tolerance to ANY type of hole pattern means you are establishing an individual tolerance zone at each individual hole's location. The dimensioning scheme you provide such as a "bolt circle" diameter simply provides a "path" for the machinist and inspector to follow to get to each hole's individual location. The long answer and some information. How do you know what the hole orientations and locations are? Each hole's orientation and location and its related tolerance zone is established by the datums you list in the Feature Control Frame along with the basic dimensions you place on the face of the drawing. FYI, here's a better way to look at GD&T tolerances in general. It helps if you understand that when you use GD&T to control a part feature, you are NOT directly controlling that feature. You are instead establishing the orientation and location of a tolerance zone that the feature then must lie within. So for example. When you apply a positional tolerance to a pattern of holes. The datums you list along with the basic dimensions do NOT directly control the part's physical holes. I mean, how can a piece of paper with text and symbols control a part feature? Instead the machining equipment, setup, tooling, and programming directly control the placement of the physical holes. The GD&T (including the datums and basic dimensions you list) you specify technically only provides instructions to the quality inspector on how to: 1. Establish the inspection equipment surfaces which will simulate the datums you listed; 2. How to orient/locate the cylindrical tolerance zones for each hole relative to the datums being simulating with the inspection equipment surfaces. Now, after creating the inspection setup, they will begin inspection by placing the actual physical part into that inspection setup. Then they will probe the actual part holes and determine the actual physical orientation and location of those hole's axes. Finally, they will compare the actual physical hole axes to the cylindrical tolerance zones they established earlier. If each individual hole's axis lies within its related cylindrical tolerance zone, then the part "passes" inspection. If one or more hole's axis falls outside its related cylindrical tolerance zone, then the part "fails" and it must either be re-worked or scrapped. So to recap. GD&T does NOT directly control part features (essentially it does NOT control the outcome of the machining process). Instead it is instructions for the inspector on how to establish the inspection equipment and tolerance zones to then determine if the actual physical part features lie within those tolerance zones. Now, an experienced machinist will pay attention to the GD&T and will perform machining processes that will ensure the part features pass inspection based on the requirements established by the GD&T you specify. I hope this information answers your question as well as helps you understand how GD&T actually works and how it is consumed by the machinist and inspectors. Have a wonderful day and stay safe and healthy!

[@armandmatossian2081]

@@StraightToThePointEngineering Awesome, thank you!!

[@StraightToThePointEngineering]

@@armandmatossian2081 You got it! I know I've edited the response several times, however, I want it to be a well thought out and well written response for both you as well as anyone else reading the comments section who are trying to get a better understanding of GD&T. Again, thanks for watching and for participating in the comments section! I hope this helped!

[@williamlee7371]

With just one positional tolerance on that dimension that applies to all individual 8 holes in relation to the Datums. ASME and BS allows you to further specify how these bolt holes can be positioned by using composite tolerancing which is useful if the position of the actual pattern to the datum is different to the position of the holes to each other.

[@najibjama3510]

How does one come up with the position tolerance itself? I know there is limits and fits for holes and shafts are there charts one can use to come up with the position tolerance itself thanks.

[@StraightToThePointEngineering]

Hi there psv head! That is a great question but it is a rather complex answer that of course is "it depends". I can't really answer that in just a comment and so at a certain point I will attempt to create a useful video covering tolerance selection. It essentially comes down to designing, selecting or calculating an allowable tolerance "budget" for your whole design, and then assigning a portion of that "budget" to each tolerance in the stack up of parts. It can be a bit of a "chicken or the egg" when it comes to selecting vs. calculating what a tolerance should be but the best method is to start somewhere that you know or can apply an achievable common-sense tolerance and then move onwards and upwards from there. A great position tolerance that I fall back on a lot is .014. This is because a .014 position tolerance is equal to the traditional ±.005 location tolerance that we are all used to seeing and applying to our drawings before learning GD&T. A ±.005 tolerance is good starting point because it is a fairly precise but easily-repeatable and low-cost tolerance that most machinist can achieve with most machines and setups. So start with .014 and then as you become a more practiced designer you can begin to calculate and determine the best tolerances for your designs. I hope this helps and feel free to comment back with more questions and additional clarification. Have a great day!

[@najibjama3510]

@@StraightToThePointEngineering Hey thanks I really appreciate your reply. This topic is one of those things that really never gets answerd in technical trainings for mechanical designers, so naturally most of us develop our own ways of going about it. I have always wanted to see how experienced designers and engineers get it done. I pretty much use the method you described along with cheat sheets I compiled of what worked and what didn't from previous projects I have done, and I feel like there is probably a better way of going about it than what I'm doing. Again thanks I look forward to your video Posts I really enjoy your channel its very informative.

[@StraightToThePointEngineering]

Hey psv head, thanks for the reply! Since it sounds like you have a bit more experience I will dive in just a bit deeper on this. From a true technical perspective, tolerances should be ultimately derived from your engineering requirements that govern the function of your assemblies. As a matter of fact, application of GD&T such as datum selection and tolerance selection cannot be correctly applied unless you are designing from the higher-level assembly and working your way down. For example let's look at the part from this video you are commenting on. You cannot really specify correct functional datums unless you know what surfaces of this part interact with the surfaces of the other parts in the higher-level assembly. You cannot determine the correct minimum and maximum size for the holes unless you first determine their functional purpose and then determine what the positional tolerance you are going to apply to them is. You cannot determine what positional tolerance to apply to the holes unless you know how much the plate is allowed to move around in the higher-level assembly. You cannot know how much the plate is allowed to move around in the higher-level assembly unless you know what your overall tolerance budget is at the higher level assembly. In order to determine what your overall tolerance budget is at the higher-level assembly, you must refer to your engineering requirements. Or in other words, is there some engineering or scientific reason/purpose to the assembly that is going to guide the mechanical design and limit the total amount of tolerance stack up within the assembly. So as you can see, when you try to look at just the part and its features with blinders on, and you do not look at the higher-level assembly, there is not really a whole lot you can accomplish. Instead, you need to start with the higher-level assembly, particularly the engineering requirements that are governing its function, and then work your way downwards. Let's imagine a scenario for this plate from the video. Imagine that a photo-sensor mounts to the top of the plate. That photo-sensor is activated by a light-emitting trigger device that is mounted above in some mechanical relation to the plate. Let's also say that the manufacturer insists that the beam emitted by the light-emitting trigger cannot be more than .050 out of axial alignment with the sensor. Boom! Now you have your engineering requirement! From this you know that your overall tolerance budget (or tolerance stackup) between all of the parts that relate the sensor to the trigger cannot be more than .050 combined. Now you as the designer need to go in and begin applying tolerances using your best mechanical, manufacturing, and inspection judgment. Say there are ten stack up locations that create the path between the sensor and the trigger. Yes, you could simply divide .050 by ten which leaves .005 as your allowable tolerance at each point. However, there may be features that are harder to manufacture and so you may choose to give them a bit more (say .010) and then reduce other feature tolerances to be able to stay within .050 budget. Now, if your design or particular part features do not really have a related engineering requirement and so the tolerance selection is up to you, then it is important that you as the designer do two things: 1). ensure that the size and tolerances you choose are such that you can assemble your parts interference-free and 2). you choose tolerances that are manufacturable, are relatively low cost, generate low-scrap rates, and allow for relatively low manufacturing lead times. If you do this you will make your employer money and you'll make the big bucks! I hope this helps and feel free to respond with comments and questions!

[@GTPhan]

Do we have to convert the diametric position tolerance into Cartesian coordinates (X,Y) to know the actual position of the hole pattern center point , given the basic dimension relative to datum B, C, and its positional toleance? The circular hole pattern center is located around .014" dia of its true location.

[@StraightToThePointEngineering]

Hello G. T Phan! Thank you for the great question! Let me attempt to provide a helpful answer. In GD&T we are really specifying the orientation and location of tolerance zones relative to a set of datums. Then the actual physical part is placed into the inspection setup and the part features of interest are then probed/inspected to see if those features fall into those tolerance zones we specified. In this particular example we are establishing the orientation and location of 8 individual tolerance zones. When the part is inspected, each axis of each of the 8 individual holes must fully reside in their respective tolerance zone to "pass" inspection. Let me be clear, we do NOT establish a tolerance zone to locate the bolt circle itself. It is not a "feature" that we want/need to control. The bolt circle is simply there to help establish the location of the 8 individual tolerance zones. So in this example the center of the bolt circle is located exactly 3.000 inches horizontally from Datum B and then exactly 2.000 inches vertically from Datum C. The bolt circle is then given a size of exactly 1.500 inches. Then 8 tolerance zones are exactly placed Equally Spaced around that bolt circle. With 8 tolerance zones, the math will make each tolerance zone exactly 45 degrees apart (360 / 8 = 45). It is apparent from the drawing that the upper most tolerance zone is located exactly along the vertical centerline of the bolt circle and so this conveys the exact starting point for the pattern (GD&T allows you to make these kinds of inferences). Additionally, it is important to note that the orientation of each of the 8 tolerance zones is exactly 90 degrees (perpendicular) to the specified Datum A (another allowed inference made from the drawing). Now once the machinist has manufactured this part including drilling the holes, this part will then be sent to the inspector. The inspector will establish the exact orientation and location of all 8 tolerance zones using the interpretation I just described. They will then place the part into their inspection setup. Finally, they will probe the orientation and location of each hole's axis and determine if each axis resides in their respective tolerance zone. If the answer is "yes" for each of the 8 holes, then those holes "pass" inspection. If any of the holes do not, then the entire part "fails" and must be either re-worked or scrapped. I hope this helps you understand how tolerance zones are established to control part features. I hope this clarifies that the bolt circle is not a part feature and so we do not establish a tolerance zone to control its orientation and location. Instead, the bolt circle is simply a tool we use on the face of the drawing to help convey the orientation and location of the tolerance zones that the axis of the holes then must reside inside of. Thank you again for the great question! If anyone reads this long-winded reply, I'm sure they will appreciate the fact that you asked. If you have any comments or need further clarification, feel free to reply! Have a wonderful weekend!

[@northeastpsychos]

In an example like this, do all 8 holes share the tolerance? For instance if each hole measured .002” from true position that would be a total of .016” and out of spec from the required .014”. Or does each individual tolerance zone get .014” to work with?

[@jasondean37]

2 years late...lol. But each one gets the tolerance

[@mohammadrezahamidi4818]

Whet is the meaning of M in circle?

[@newsfit21]

GD&T is so confusing i wish you would make many more videos you are fantastic with explaining

[@donaldkelley7692]

What, if any, does the size of the basic bolt circle size variation influence the positional tolerance.

[@StraightToThePointEngineering]

Hello Donald! You will notice that the bolt circle size dimension is basic (has a box around it) just like the 2.000 and 3.000 linear dimensions. This means that it is a theoretically perfect number that you would enter into your inspection software to "perfectly" locate the tolerance zones for each hole as part of your inspection setup. Remember, inspection equipment is relatively precise and calibrated and so we use "perfect" basic dimensions to first locate our tolerance zones that then each feature specified must then be located within. Now, once you establish the "perfect" location of the tolerance zones using all of those basic dimensions, then you would inspect each hole feature to make sure each hole's axis fell within their respective, basically-located, tolerance zone. FYI, you need all three dimensions including the bolt circle size along with the statement "EQL SP" to fully define each hole's location. To finish this long-winded answer, no, the bolt circle has no size variation because it is a basic dimension and so as a result the bolt circle size does not influence the positional tolerance itself.

[@JoshuaJones-te1ef]

I have seen another method for applying GD&T to PCD like patterns. I often see it where Datum A is based off the dimension of the centre hole or point of the PCD.

[@idasmith8644]

[Nephew of Ida] Excellent. To erase the +-0.1 and transform to base dimensions is genial. Thanks for the video.

[@StraightToThePointEngineering]

Hello there Ida Smith! I can't take the credit for the ideas unfortunately ;) Thank you for watching and for leaving the positive comment!

[@miao1616]

fabulous work

[@spec24]

How do you control the tolerance (spacing) of the holes along the BC? EQL SP doesn't cut it.

[@edwardalexander2569]

Yes it does. The machinst knows that a circle is 360 degrees and that there are 8 holes. So the theortical exact location of the holes will be on the bolt circle diameter of 1.5" and at angles of 0, 45,90,135,180,225,270 and 315 degrees. so for the hole located at 45 degrees it exact theoritcal location from Datum B is 3" + (Diameter/2)*sin(45) = 3.530" and from datum A will be 2.530", so as long as the center of the hole is no further than 0.014'/2 (= 0.007') in any direction from that exact location it's within tolerance. This is actually much clearer than specifying and angle with a tolerance, do you take it as a stacking tolerance where you measure between each hole, in which case the tolerance would need to be extremly tight or do you take it as absolute tolerance where each hole can only vary by the tolerance from its true position?

[@devilslilhelper]

Straight to the fuckin' point, no waffle, NICE ONE

[@StraightToThePointEngineering]

Hi devilslilhelper! Thanks for the positive, no-bullshit review here in the comments section! It's appreciated man! Stay safe and healthy!

[@ElltoRToisedelPaPito]

Awesome information…Thank you.

[@SandeepKumar-ur4ig]

Nice platform to know about gd&t

[@joellasseigne4652]

Is there a reason for not applying a control on datum A? EI flatness?

[@joellasseigne4652]

Now that I look again. You did not control any of the datums. Perhaps this should be renamed to Pick 4 method? 2-Control your datums etc.

[@joellasseigne4652]

Or just control datums in item 3

[@StraightToThePointEngineering]

Hi again Joel! One of my professors once told me that "the art of teaching is knowing what to leave out." In this case, I did not want to include controlling the form and orientation of the datums as part of the tutorial as I felt it would get too in-depth and would confuse beginners. Looking back combined with your keen observation, I should have at least pointed out that an experienced designer would also control the quality of the datum features with form and orientation controls such as flatness, parallelism, and perpendicularity. Thanks for the constructive comment and I will look at where to begin incorporating your suggested concept in future videos! Have a good evening and stay safe and healthy!

[@kulathuranivijay1272]

Sir in position tolerance for hole with reference to circular or cylinderical Dia datum with respect to the axis how we can going to specify the tolance.. please explain

[@StraightToThePointEngineering]

Hello Vijay, there are multiple scenarios that could be derived from you question and so I am not exactly sure what you need clarified. To start though, look at Figures 7-2, 7-5, and 7-18 in ASME Y14-5-2009. These figures can provide some reference guidance on how to define hole(s) which need to be located relative to a cylindrical datum feature. If you have a specific example you would like me to look at feel free to email me at [e-mail] and I will communicate with you as well as look at creating a video to explain further. Thanks for watching and commenting and have a great rest of your day!

[@danialraja]

Great video but I still feel as tho I’m a little rusty. Could you make a video in the future about your calculations to getting that position tolerance with other values and dimensions and tolerances? That would really help me.

[@nickritter1992]

what does the M with the circle around it in the GD&T too. mean?

[@StraightToThePointEngineering]

Hi again Nick, this is what we call the "Maximum Material Condition (MMC) Modifier". I will try to explain this as simply as possible. Specifying MMC allows you to have a variable Positional tolerance that can expand or contract as the size of the feature it's controlling moves away from or back towards its MMC size. If you do not specify MMC or LMC, then the Positional tolerance size is just a single locked value no matter what size the actual feature turns out to be during inspection. So in this example, when the hole is at its MMC diameter of .279 the .014 tolerance applies (for an internal feature like a hole it is when its at its smallest diameter). As the hole moves away from it's MMC size towards its LMC size of .288, the amount that the hole diameter gets bigger is actually allowed to be added to the Positional tolerance (this is commonly known as "Bonus" tolerance). So say the hole is inspected and its actual size is .286. This means the actual hole diameter is bigger than its MMC size of .279 by .007. So you can now add that .007 to .014 for a total allowed Positional tolerance of .021. This type of GD&T is heavily used when the feature you are controlling is a clearance feature for something else to fit inside. In this example the holes are actually clearance holes that bolts are going to fit through. As a hole diameter gets bigger, that hole's position can shifts sideways even more and the bolt will still clear through it because of the fact that the hole is now actually bigger. I hope this helps. I will be shooting a video specifically on MMC here soon and will be posting it. Let me know if you have more questions or need additional clarity. Thanks for watching!

[@Hallowed1907]

Sir can you add an example of hole as a datum plane ?

[@StraightToThePointEngineering]

Hey Amit, that's a great idea! I will work on a video that describes how to use a hole as a Datum to locate other feature from. Thanks for watching and commenting and be on the lookout for more videos. I will be posting on a much more regular basis due to all the positive feedback I am getting from everybody including yourself. Have a great rest of your Sunday!

[@sleino55]

@@StraightToThePointEngineering Hi there! Is there already a video on this? I am really struggeling with positioning screw holes around a hole for a rod.

[@dsh1001]

I have the a similar question. It is not a bolt hole circle but is a circle of holes with position related to a bore. There is no set X/Y per se since the holes need to be equally spaced but do not need to relate to any specific X/Y axis. I am recommending a concentricity callout for this instead but they are adamant that it has to be a position tolerance.

[@dreadmuthu]

That's a great explanation btw how did u decide that dia 0.014 in that tolerance box. If u can explain that calculation then it would be better

[@dreadmuthu]

FlatusOhlfahrt thanks. Can you make a video on those formula of all other gd&t with an example

[@StraightToThePointEngineering]

Hi DREAD, FlatusOhlfarhrt did a good job of explaining the math but here is the reasoning. We are all used to seeing a +/-.005 tolerance in most of our drawings. This is a tolerance zone that has a square shape and the total size of this square tolerance zone is .010 x .010. With GD&T positional tolerance you are specifying a tolerance zone that has a circular shape instead. A lot of the time we would like to specify a GD&T circular tolerance size that is similar in scale to the +/-.005 square tolerance zone because we know most machine tools can hit that familiar tolerance. To find a comparable circular tolerance zone size we can draw a circle whose arc lines are drawn tangent to each corner of the .010 x .010 square. I suggest you draw this scenario in SOLIDWORKS or some CAD software as a sketch and then measured the diameter of the circle and you would notice that it's diameter is very, very close to .014. So to answer your question, in any situation where I would like to specify a GD&T circular tolerance zone and use a size that mimics the familiar +/-.005, then I will specify a circular tolerance zone diameter of .014. FYI, trigonometry can also be used to calculate .014 and that is what FlatusOhlfahrt used in his explanation of how the mathematics works.

[@danialraja]

@@StraightToThePointEngineering do you have the link to ttheir video

[@airlanggak5391]

thanks so much man!

[@rrk0681]

how to locate a circular pattern of curved slot using GD&T

[@VinothKumar-qo7ry]

Great sir

[@SandeepKumar-ur4ig]

Keep it up sir

[@StraightToThePointEngineering]

Hey there Sandeep, thanks for the positive words of encouragement! Let me know if there's a topic you'd like/need covered and have a great day!

[@marktrotter6930]

BC and EQ SP do not exist in GD&T

[@Kenjanalkaline]

More example sir

[@StraightToThePointEngineering]

Hi there! I am back and just posted another video. Also, moving forward I will be posting videos on a more regular basis due to all of the positive feedback from viewers saying that the content is helpful. Thanks for watching and commenting! It is very appreciated! Enjoy the rest of your Sunday!

[@pgabli10]

You are not telling the machinist you are telling the inspector. Most drawing are not machining drawing they are inspection