[Ch...] Posted March 12, 2019 Share Posted March 12, 2019 Imagine this scenario: Two circles are created on a cylinder. Those two circles are then recalled into a 3D line and also recall feature points into a cylinder. So now I have one 3D line and one cylinder both created from the same points. Now each of those are intersected with the same plane. Challenge: Who can best articulate the difference in position results you get between the cylinder and the 3d line intersections. I can understand the difference inside my head, but I am having trouble finding the right words to use when explaining to people who are not CMM people. Can anyone do a better job is explain this than I can? I'm getting ready to have a argument over a few microns worth a lot of money and with people who don't understand GD&T or CMM's. I'm not looking forward to this. Also, can anyone add how machine resolution will impact these measurements. Lets say the tolerance is +-.005mm and using a Accura II with Vast XT gold. Link to comment Share on other sites More sharing options...
[Te...] Posted March 12, 2019 Share Posted March 12, 2019 The axis lines are created differently. Recall uses the center points of the Circles ONLY. Recall Feature Points creates the axis (line) from the data set (ALL points) do not forget to apply filter and outlier when you recall feature points into a cylinder (or any feature for that matter). Link to comment Share on other sites More sharing options...
[Cl...] Posted March 12, 2019 Share Posted March 12, 2019 For what its worth, I've had better luck using the 3d line option. More stable/repeatable results. Link to comment Share on other sites More sharing options...
[Cl...] Posted March 12, 2019 Share Posted March 12, 2019 The center line of the cylinder is created from hundreds, maybe thousands of recalled point centers creating more of a skewed centerline compared to just a centerline created from those same points, but combined into just two complete circles. This (in my mind) should be much more stable. Link to comment Share on other sites More sharing options...
[Ch...] Posted March 12, 2019 Author Share Posted March 12, 2019 I have tons of data showing that 3D line is more stable; However, which is more representative of how the part will function? Link to comment Share on other sites More sharing options...
[Me...] Posted March 12, 2019 Share Posted March 12, 2019 Please sign in to view this quote. This is exactly what I remember from training as well. Link to comment Share on other sites More sharing options...
[Me...] Posted March 12, 2019 Share Posted March 12, 2019 Please sign in to view this quote. If you're looking for function, wouldn't you want to use Outer Tangential and constrain to a plane? Link to comment Share on other sites More sharing options...
[Ch...] Posted March 12, 2019 Author Share Posted March 12, 2019 Shawn, I see your point that that is what I would typically do. I am using Outer Tangential here. My problem is that there is no plane that is precise enough to use. It is a set of centers which consist of just arbors that slide inside of a bore. The only functional datum on the whole gage is an straight line axis. Which is why I need to choose my words carefully when i'm explaining this to people. Understanding something and being able to articulate to other people are two totally different beast. I was once told that you have to know 10x's more about what you are talking about than what you actually say if you want to actually sound intelligent. Link to comment Share on other sites More sharing options...
[Br...] Posted March 12, 2019 Share Posted March 12, 2019 Please sign in to view this quote. I think the Cylinder with outer-tangential is closer to an actual functional feature. The 3D line connects the dots on two 2D circles. Those circles due to their 2D nature, are constrained to the plane they are nominaly on. Look at your features, and look at the spacial angles for the circles. The actuals should be exactly the same as the nominals. The Cylinder however, disregards this and creates a fit based on the collection of points at the same time. Tell them to imagine an expanding or contracting cylinder(depending on if the feature is an OD or ID), that expands/contracts until it contacts the highest points. Where it stops defines the axis. There will be a very slight difference between the two. I would guess that the difference between the two methods would be more pronounced depending on the form error. The cylinder with outer-tangential is going to be closer to representing the "Actual mating envelope" described in Y14.5. Link to comment Share on other sites More sharing options...
[Me...] Posted March 12, 2019 Share Posted March 12, 2019 Advanced training states you should take a minimum of 5 - 7 circles in a cylinder if it is that critical. Use your cookbook. That combined with outer tangential and proper filtering and outlier elimination should put you right on the money. Also it will greatly increase your repetability. Link to comment Share on other sites More sharing options...
[Ma...] Posted March 13, 2019 Share Posted March 13, 2019 The cookbook states (pages 26-27) for true position that you should use MICI or MICY (Maximum Inscribed Circle or Maximum Inscribed Cylinder) for both the feature and cylindrical datums. If the datums are planes then use Outer Tangential on the planes. Page 16, the page that defines machined bores, states that if the bore depth is 1-3 x the diameter then use 3 circle paths. If the bore depth is greater than 3x the diameter then use 5 circle paths. Your filtering and outlier, and measuring speed depends on the bore size. If you don't have the cookbook you should purchase it. Mark Link to comment Share on other sites More sharing options...
[Br...] Posted March 13, 2019 Share Posted March 13, 2019 Please sign in to view this quote. I know it says that, but does calypso not do the same exact algorithm for Outer Tangential and MaxInscribed\MinCircumscribed? I've tested this on a few occasions and get the same exact number to the micron between the two options. Link to comment Share on other sites More sharing options...
[Ma...] Posted March 13, 2019 Share Posted March 13, 2019 I've not done that test so good to know Mark Link to comment Share on other sites More sharing options...
[Al...] Posted March 13, 2019 Share Posted March 13, 2019 Please sign in to view this quote. Zeiss teaches that Outer Tangential is exactly the same As Max Inscribed and Min Circumscribed, for circles and cylinders. Link to comment Share on other sites More sharing options...
[Me...] Posted March 13, 2019 Share Posted March 13, 2019 Wouldn't it be... Outer Tangential = Min Circumscribed Inner Tangential = Max Inscribed ? Link to comment Share on other sites More sharing options...
[An...] Posted March 14, 2019 Share Posted March 14, 2019 Please sign in to view this quote. The tilted MI Cylinder always yields an ellipse that has to be fitted into the point cloud. The 3D Line always relates to the center of the MI Circle. (not MI Ellipse) See attached.Contribution_14_03_2019.pdf Link to comment Share on other sites More sharing options...
[Er...] Posted March 14, 2019 Share Posted March 14, 2019 That could be kind of missleading Andreas. Mathematically it is a cylinder expanding. The epiliptical form you are talking about is always present on a cynindrical shape, depending in the angle of view. Even if the touch points that are determing the size occurs in different planes, There is no eliptical calculation going on. Its still Mathematically a cylinder since it has a perfect shape. Link to comment Share on other sites More sharing options...
[Aa...] Posted March 14, 2019 Share Posted March 14, 2019 For holes: Max Inscribed = Outer Tangential Min Circumscribed = Inner Tangential For pins/bosses: Max Inscribed = Inner Tangential Min Circumscribed = Outer Tangential Inner/Outer has to do with whether the simulator approaches from inside or outside the material. Brett is spot on when he points out the topic of form error. You will always have some amount of orientation instability to any tangential fit 3D object (cylinders, widths) primarily due to taper. This is because the fitting algorithms are required to expand/contract the simulators to their maximum/minimum size. So picture a data set for a hole that is tapered or waisted. The simulator is going to contact at the narrowest axial position. And imagine that it was slightly out-of-round. A larger simulator size could be achieved by slightly tilting the simulator. It will still contact 3 points at that one axial position, but it will also contact one point at another position. Y14.5 talks about optimizing unstable datums, and references Y14.5.1 for mathematical methods and limitations of said optimization. Conceptually, this optimization sounds really nice, but to put it into practice, there is only ever one potential datum feature simulator that is the largest/smallest. Optimization would mean having to pick one that is different from that, and Y14.5.1 would have to prescribe the criteria for how close in size to the largest/smallest potential a "candidate datum" would have to be (similar to how it lays down limitations in how close to the edges of a planar datum feature that "rocks" the contact points with the simulator are allowed to be). But the Y14.5.1 subcommittee didn't address orientation instability of datum features of size. Link to comment Share on other sites More sharing options...
[An...] Posted March 14, 2019 Share Posted March 14, 2019 Please sign in to view this quote. Correct! Just wanted to demonstrate by a very simple scetch,what the difference makes - by means of a decreasing ellipse. Diameter b smaller than MIC. Link to comment Share on other sites More sharing options...
[Pe...] Posted March 14, 2019 Share Posted March 14, 2019 What's the diameter? How far apart are the two circles? Is the intersecting plane between the circles, or outside at some distance? Link to comment Share on other sites More sharing options...
[An...] Posted March 15, 2019 Share Posted March 15, 2019 Please sign in to view this quote. Just playing around ... See attached.Contribution_15_03_2019.pdf Link to comment Share on other sites More sharing options...
[Ch...] Posted March 18, 2019 Author Share Posted March 18, 2019 Thanks for the input guys. When formulating the thoughts in my head, it really helped me seeing how everyone else explained the same issue. Sorry, for just asking a question and not responding, but I had to go to Chicago for my son's surgery so I was out for a bit. Although I didn't have time to respond, you guys provided me with some great reading material in the hospital rooms. Link to comment Share on other sites More sharing options...
[Ow...] Posted March 18, 2019 Share Posted March 18, 2019 I always throw out a caution flag anytime somebody mentions a few microns and a lot of money. Replicating a functional fit with different evaluation techniques is often important to accepting or rejecting a part but, it can reduce repeatability and reproducibility when reporting back to the manufacturing machine. This is because manufacturing variation, chatter, distortion, ect, is not repeatable and you end up chasing your tail making adjustments for it off a CMM report. Furthermore, the CMM isn’t going to touch 100% of the surface. It can be an arguable fact that there is NO perfect way to analyze functional fitting on a CMM, a granite table/indicator or hand gauges. You can get close but, honestly, there are too many variables, thermal expansion, pressure, balance, distortion, inadequate alignment and design structure can all affect fit, form and function EVEN IF EVERYTHING IS MADE PERFECT DIMENSIONALLY. Godspeed to your Son’s recovery. Link to comment Share on other sites More sharing options...
[Ch...] Posted March 18, 2019 Author Share Posted March 18, 2019 Please sign in to view this quote. I agree Owen. This is hard enough when everything is in house. Is is near impossible to do on two different brand of CMM's, two different programmers, two different measurement strategies, and two different countries. All you can do is hope they are cooperative and you both can agree on the same measuring strategy. On a separate rant, I think too much weight is given to ASME 14.5Y when it comes to stuff like this. 14.5Y is great for theory, but doesn't always translate well when you are working with form and microns all day. I have attached an article that I love that I think is roughly associated with this thread. It is the one about the CMM programmer that said "So What".CMM-GDT_Measurement_Planning_Hand-Out.pdf Link to comment Share on other sites More sharing options...
[Br...] Posted March 18, 2019 Share Posted March 18, 2019 Please sign in to view this quote. I don't know about that. Y14.5 focuses on design intent and part function. Its currently the best thing out there that can give the clearest meaning to how each part should function. ISO on the other hand puts more emphasis on metrology instead of function which is why most companies in America use Y14.5. That article you posted is true though. You must take into consideration the uncertainty of measurement on each instrument you use, and make the call as to what is the best way to validate. There will always be a balance that must be made between accuracy and efficiency. And if you're squabbling over micron, remember, those Zeiss machines have an uncertainty of a few micron.(Our Contura is 1.9µ+L/300) Most engineers don't tolerance line-to-line, so this really shouldn't even be an issue. Link to comment Share on other sites More sharing options...
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