I believe you may have misled your viewers on this one. When you were discussing making the curved surface a datum, you stated that the holes need to intersect the axis of the cylindrical surface. However, this is definitely not true, depending on the mating part. If the mating part is also a cylinder with a radial hole pattern, using the axis as the datum could be disastrous, resulting in a bad part that actually checks good. The reason is because as the actual size of the cylindrical surface varies, the actual radius will change (as you discussed). This means that the actual distance between the holes is required to change with the radius; i.e. the arc length between the holes will change. This effectively causes the hole pattern to shrink and grow, depending on the calculated radius of the feature. But functionally, you would need that arc length between the holes to be static. So the simulated datum needs to be at the surface itself, not at the axis. This would be exactly how a functional gage would work. The part would nest against a cylindrical surface with pins spaced at a static, basic arc length. Ergo, you do NOT want your holes to intersect the axis of the feature per se. As the actual size of the cylindrical feature shrinks and grows, you want that arc length to remain static, which will necessarily cause the axes of the holes to not intersect the axis of the cylindrical surface (except at the nominal size, of course). What would exacerbate the problem of using the axis as the datum would be that when checked with a CMM, such a small arc could potentially cause the calculated size of the cylindrical surface to vary wildly, since the software would have to ‘project’ such a large percentage of the diameter, greatly amplifying any error in the inspection. This causes the calculated radius to change wildly (relatively speaking), which, as stated above, forces the arc length between the holes’ tolerance zone to shrink and grow, which is not in any way functional. And what makes it even worse than that is that if the cylindrical surface is controlled with a profile, this would permit the size of the cylinder to truly vary considerably, even with a relatively tight profile tolerance, and the smaller the arc, the worse this gets. So your radius could vary wildly in reality, then add on top of that the error from the inspection, and you end up with a very unstable inspection. Good news is, your part would probably never check good, depending on the accuracy of the process, as well as the accuracy of the CMM. The question then would be, how do you specify the cylindrical surface itself be simulated as the datum, not the axis? Could you argue that since the cylindrical surface is not technically a feature of size (no opposing points), that forces the simulator to be at the surface, not at the axis? Again, this would be exactly how a functional gage would work.
I agree. I believe that a [BSC] modifier next to the datum reference letter D is the way to solve this issue. Similar to fig. 4-28 in ASME Y14.5-2009. This would require the datum feature simulator to be fixed in the basic radius of the profiled curved feature. Another issue with the explanation is -the curved surface is the primary datum feature for the holes' position tolerance. As a primary datum feature, it needs not to be validated with reference to A, B, C of which B and C are the secondary and tertiary datum references for the holes. Usually, the secondary and tertiary datum features are validated with reference to the primary (the tertiary also with reference to the secondary). So defining "where the datum axis is" is not part of the problem contrary to how it was presented.
@@sem7207 Great answer. In thinking about it more, it seems like on Fig 4-28, BSC helps, but there's still something missing. The hole diameter is toleranced, the hole position is toleranced, but if the part is thin, there is very little control over how normal the holes are to the contoured surface. How would you add a composite reference frame to tighten that up?
@@moynihanigan Most likely no one needs a tight control over how perpendicular the holes are to the curved surface for a part similar to the one in figure 4-28. It's a thin part, and the holes are toleranced and referenced as clearance holes; the size tolerance is not to tight, the position tolerance is specified @MMC, and pretty large datum shift allowed for the surface controlled to A[BSC], B@MMB. This indicates that the relationship of the holes with whatever goes through them is loose. I wouldn't worry about the orientation of these holes or consider a composite tolerance.
I watched lots of GD&T but I had never seen anyone show how to calculate the tolerance of position, concentric and etc... Can you make an assembly of 2 parts or more and show us how calculate the lolerance so rhey fit together perfectly?
What happens if the complicated surface is actually complicated and looks like a shape of a wave? Does it still produce a centerline that you can use as a datum?
Hi bandy32, The tolerance that is shown for this example is Profile of a Surface and the Total Tolerance is .001. Any given surface element must lie within the tolerance zone and will not be “perfect”. The larger the tolerance, the more imperfect it can be and it could start to take on the “wave” shape that you mentioned. However, this surface is merely the Datum Feature and not the Datum itself. Remember that because we would establish a Datum from this, Datums are “perfect” and just like using a planar surface as a Datum, we will be using the high points of the surface. Also, this Datum (|D|) will be “perfectly” located and oriented from the |A|B|C| DRF and because it will be a cylinder, the Datum will be an axis. Hope this helps! -Your GD&T Basics Team
how do we use a curved surface to check the true position of a hole that is not normal to the curved datum surface. Like the distance of the hole to the curved surface say the curve surfaced was 90° to the hole?
The true position of a a feature such as a hole is defined using basic dimensions back to the datums defined by the datum reference frame in the feature control frame. As long as this is accomplished, you should then be able to take the tolerance zone defined in the feature control from and center it on the true position. The measured axis of the feature must lie with in this tolerance zone. Check out our video on datum reference frames here. th-cam.com/video/D9U1YV_96kk/w-d-xo.html
Well put and understood if the curved surface has an axis. However is there a way to use the curved surface if it is not cylindrical or conical that has an axis? For example if the surface was a free-form surface?
Any surface that is mathematically defined can be a datum feature. On a side note, holes do not have to be perpendicular to the datum surface like it was mentioned here.
@@crazyingenieur3277 Understood, so if it was a completely irregular surface, then you'd need to just with "option A" here. i.e. Place datum on the flat top side and have both the irregular surface as well as the holes related to it?
@@matts9728 1. Datum features are to be selected based on the function, not based on the geometry we like. In this video, there is no information on the function of this part and mating conditions with the mating part(s). The holes shown perpendicular to the curved surface kind of hints that the bottom surface is the sitting surface hence should be selected as the datum feature (most likely primary datum feature). However, we need more information on the function and the mating part(s) to be sure. 2. Again, any feature/surface (mathematically defined, regardless of how complex it is) can be a datum feature. Refer to ASME Y14.5-2009 para 4.13 and Fig 4-28. In this figure, you will see a free-form surface selected as the primary datum feature. 3. In this video, they are talking about selecting the datum feature based on how the holes are (perpendicular, etc.), which is from a design point of view inaccurate. (see #1).
@@crazyingenieur3277 Appreciate the clarification, I completely agree that Datum features should be selected based on function. But that said, do you think there are ever situations where it's acceptable to instead datum off of easily fixtured planes for the sake of machining/inspection? I'm trying to think through it and maybe the only reason would be if you were constrained to no CNC/CMM for some reason?
@@matts9728 You can manufacture and inspect a part fastest, easiest, cheapiest way. However, if that part does not fit and functions, you will end up having something called "cheap junk".
it should be functional not easy. That looks like a wing shape there for no axis. If you are checking this on a CMM its easy if not then you need to make a master. Remember primary needs 3 points. But hell no one will inspect it and under stand it. I have seen how QA does it. They pass it if it need to be passed. Machinist will slow the feed and speed to hit the .0005 the machines are allegedly able to do or hope they can. Bottom link it will fail and use as is will be called off on most things .001 inches.
Great explanation...I believe I am starting to understand :)
I believe you may have misled your viewers on this one. When you were discussing making the curved surface a datum, you stated that the holes need to intersect the axis of the cylindrical surface. However, this is definitely not true, depending on the mating part. If the mating part is also a cylinder with a radial hole pattern, using the axis as the datum could be disastrous, resulting in a bad part that actually checks good. The reason is because as the actual size of the cylindrical surface varies, the actual radius will change (as you discussed). This means that the actual distance between the holes is required to change with the radius; i.e. the arc length between the holes will change. This effectively causes the hole pattern to shrink and grow, depending on the calculated radius of the feature. But functionally, you would need that arc length between the holes to be static. So the simulated datum needs to be at the surface itself, not at the axis. This would be exactly how a functional gage would work. The part would nest against a cylindrical surface with pins spaced at a static, basic arc length. Ergo, you do NOT want your holes to intersect the axis of the feature per se. As the actual size of the cylindrical feature shrinks and grows, you want that arc length to remain static, which will necessarily cause the axes of the holes to not intersect the axis of the cylindrical surface (except at the nominal size, of course).
What would exacerbate the problem of using the axis as the datum would be that when checked with a CMM, such a small arc could potentially cause the calculated size of the cylindrical surface to vary wildly, since the software would have to ‘project’ such a large percentage of the diameter, greatly amplifying any error in the inspection. This causes the calculated radius to change wildly (relatively speaking), which, as stated above, forces the arc length between the holes’ tolerance zone to shrink and grow, which is not in any way functional.
And what makes it even worse than that is that if the cylindrical surface is controlled with a profile, this would permit the size of the cylinder to truly vary considerably, even with a relatively tight profile tolerance, and the smaller the arc, the worse this gets. So your radius could vary wildly in reality, then add on top of that the error from the inspection, and you end up with a very unstable inspection. Good news is, your part would probably never check good, depending on the accuracy of the process, as well as the accuracy of the CMM.
The question then would be, how do you specify the cylindrical surface itself be simulated as the datum, not the axis? Could you argue that since the cylindrical surface is not technically a feature of size (no opposing points), that forces the simulator to be at the surface, not at the axis? Again, this would be exactly how a functional gage would work.
I agree.
I believe that a [BSC] modifier next to the datum reference letter D is the way to solve this issue. Similar to fig. 4-28 in ASME Y14.5-2009. This would require the datum feature simulator to be fixed in the basic radius of the profiled curved feature.
Another issue with the explanation is -the curved surface is the primary datum feature for the holes' position tolerance. As a primary datum feature, it needs not to be validated with reference to A, B, C of which B and C are the secondary and tertiary datum references for the holes. Usually, the secondary and tertiary datum features are validated with reference to the primary (the tertiary also with reference to the secondary). So defining "where the datum axis is" is not part of the problem contrary to how it was presented.
@@sem7207 Great answer. In thinking about it more, it seems like on Fig 4-28, BSC helps, but there's still something missing. The hole diameter is toleranced, the hole position is toleranced, but if the part is thin, there is very little control over how normal the holes are to the contoured surface. How would you add a composite reference frame to tighten that up?
@@moynihanigan Most likely no one needs a tight control over how perpendicular the holes are to the curved surface for a part similar to the one in figure 4-28. It's a thin part, and the holes are toleranced and referenced as clearance holes; the size tolerance is not to tight, the position tolerance is specified @MMC, and pretty large datum shift allowed for the surface controlled to A[BSC], B@MMB. This indicates that the relationship of the holes with whatever goes through them is loose. I wouldn't worry about the orientation of these holes or consider a composite tolerance.
I watched lots of GD&T but I had never seen anyone show how to calculate the tolerance of position, concentric and etc...
Can you make an assembly of 2 parts or more and show us how calculate the lolerance so rhey fit together perfectly?
What happens if the complicated surface is actually complicated and looks like a shape of a wave? Does it still produce a centerline that you can use as a datum?
Hi bandy32,
The tolerance that is shown for this example is Profile of a Surface and the Total Tolerance is .001. Any given surface element must lie within the tolerance zone and will not be “perfect”. The larger the tolerance, the more imperfect it can be and it could start to take on the “wave” shape that you mentioned. However, this surface is merely the Datum Feature and not the Datum itself. Remember that because we would establish a Datum from this, Datums are “perfect” and just like using a planar surface as a Datum, we will be using the high points of the surface. Also, this Datum (|D|) will be “perfectly” located and oriented from the |A|B|C| DRF and because it will be a cylinder, the Datum will be an axis.
Hope this helps!
-Your GD&T Basics Team
Thank you!!
how do we use a curved surface to check the true position of a hole that is not normal to the curved datum surface. Like the distance of the hole to the curved surface say the curve surfaced was 90° to the hole?
The true position of a a feature such as a hole is defined using basic dimensions back to the datums defined by the datum reference frame in the feature control frame. As long as this is accomplished, you should then be able to take the tolerance zone defined in the feature control from and center it on the true position. The measured axis of the feature must lie with in this tolerance zone. Check out our video on datum reference frames here. th-cam.com/video/D9U1YV_96kk/w-d-xo.html
Well put and understood if the curved surface has an axis. However is there a way to use the curved surface if it is not cylindrical or conical that has an axis? For example if the surface was a free-form surface?
Any surface that is mathematically defined can be a datum feature. On a side note, holes do not have to be perpendicular to the datum surface like it was mentioned here.
@@crazyingenieur3277 Understood, so if it was a completely irregular surface, then you'd need to just with "option A" here. i.e. Place datum on the flat top side and have both the irregular surface as well as the holes related to it?
@@matts9728
1. Datum features are to be selected based on the function, not based on the geometry we like. In this video, there is no information on the function of this part and mating conditions with the mating part(s). The holes shown perpendicular to the curved surface kind of hints that the bottom surface is the sitting surface hence should be selected as the datum feature (most likely primary datum feature). However, we need more information on the function and the mating part(s) to be sure.
2. Again, any feature/surface (mathematically defined, regardless of how complex it is) can be a datum feature. Refer to ASME Y14.5-2009 para 4.13 and Fig 4-28. In this figure, you will see a free-form surface selected as the primary datum feature.
3. In this video, they are talking about selecting the datum feature based on how the holes are (perpendicular, etc.), which is from a design point of view inaccurate. (see #1).
@@crazyingenieur3277 Appreciate the clarification, I completely agree that Datum features should be selected based on function.
But that said, do you think there are ever situations where it's acceptable to instead datum off of easily fixtured planes for the sake of machining/inspection?
I'm trying to think through it and maybe the only reason would be if you were constrained to no CNC/CMM for some reason?
@@matts9728
You can manufacture and inspect a part fastest, easiest, cheapiest way. However, if that part does not fit and functions, you will end up having something called "cheap junk".
it should be functional not easy. That looks like a wing shape there for no axis. If you are checking this on a CMM its easy if not then you need to make a master. Remember primary needs 3 points. But hell no one will inspect it and under stand it. I have seen how QA does it. They pass it if it need to be passed. Machinist will slow the feed and speed to hit the .0005 the machines are allegedly able to do or hope they can. Bottom link it will fail and use as is will be called off on most things .001 inches.
quite poor, you should be drilling from the front not back.. from the back you would never achieve the tolerances that are very narrow