![]() I agree that there are different (better or worse) ways to tackle the problem. Of course pmarc's suggestion would work if as you say the size tolerance (over pins/balls - and any associated taper between each side of the dovetail) can be as tight as the form tolerance and vice-versa. Or each part of the dovetail could be constructed separately utilizing angle tolerances.ĭoes anyone have an example of a "best practice" or a tried and true application showing an example of how to properly dimension geometry like this? ![]() Or dispense with the gauge pins/balls and directly dimension between each side of the dovetail at a defined point along the taper (think something like Figures 8-17/8-18 or 8-27 from Y14.5-2009 except applied to a dovetail). A few options come to mind like specifying an "over gauge pins/balls" toleranced dimension in conjunction with a profile callout - with or without datum targets (as long as they're not self referencing). It seems to me that, like many things in GDnT, there is many ways to skin this cat. I know I'm probably opening a can of worms just mentioning it but it seems to me that the 2x examples semiond provided are flirting with being self-referencing if not already there. I would remove the reference to datum A (formed by datum targets) no matter what - someone please correct me if I'm wrong as there has been quite a bit of debate over its definition recently - but isn't this an example of another self-referencing datum? Datum A defined by the datum targets could be used to define another feature on the part but not the dovetail itself I would think. That is why the directly toleranced dimension over the cylinders does not look good to me. The idea of SIM REQT added to both profile callouts was to control both sides of the dovetail relative to each other in terms of orientation AND location, meaning that the size of the dovetail would also be controlled. RE: Datum target cylinders? pmarc (Mechanical) 5 Jun 18 11:33 The datums that should be established from the cylinders is a center plane in the middle of the distance between them, and a plane that passes through their axes perpendicular to the center plane.īelow are some modifications i made following your and the others' comments.ģDDave, i took notice of your comment regarding the fixed distance between cylinders, the current sketch adresses this too. Thanks for bringing up those references from Y14.8 - they are very helpful. In any case, even if it is acceptable, i'm not in a hurry to adopt this as a dimensioning scheme. As CheckerHater noted, they don't look quite like datum targets (and could be taken for datum target line simulators being displayed on a part drawing when they shouldn't). RE: Datum target cylinders? AndrewTT (Mechanical) 4 Jun 18 15:49įor now i live in peace with the chicken and the egg dilemma, i'm more concerened about the question if this is acceptable according to ASME or not, because of the unusual use of cylindrical datum targets. (c) a spherical datum feature establishes a datumįeature simulator that creates a datum center point andĬonstrains three translational degrees of freedom. The spheres may only contact at 3 points, not even circles. ![]() The datum targets in 4-49 are the centers of the spheres. Remember to include errors in the form of the gage pins as part of the tolerance analysis. ![]() I avoided the problem by measuring the tangent to the cylinder at a fixed location along the cylinder rather than taking a 5 DOF measurement of multiple features and trying to pick out a single DRF from them. What you will find to be difficult is if there is the slightest lack of perfect parallelism between the cylinders. I have used a cylinder where the mating part was an actual cylinder to determine a location. It is used to verify pitch diameters for screw threads and the backlash cut into involute gears. Measurement over cylinders is often used to make size measurements, which the dovetail example shows. ![]()
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