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423 Chapter 22 Dimensioning and Tolerancing of Gages [per the ASME Y14.43-2003 Dimensioning and Tolerancing Principles for Gages and Fixtures Standard] Steps for Writing a Dimensional Inspection Plan Chapter Objectives Readers will learn; 1. To design, dimension and tolerance GO gages for MMC, NOGO gages for LMC and Functional gages for geometric tolerances per ASME Y14.43-2003. 2. How to calculate whether the gage is likely to accept borderline out-of-tolerance parts, reject borderline parts that are in-tolerance, or if the possibility exists that the gage might do either. 3. The ramifications of using different modifiers (MMC, LMC or RFS implied) on gage tolerances. 4. The differences between Absolute, Practical Absolute, Optimistic and Tolerant gages and which policies are preferred per ASME Y 14.43. 5. The steps necessary in writing a Dimensional Inspection Plan Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 424 Chapter Twenty-Two Dimensioning and Tolerancing of Gages [per ASME Y14.43-2003] In 2003, an ASME standard was approved called ASME Yl4.43-2003 Dimensioning and Tolerancing Principles for Gages and Fixtures. It marked the first time a nation had issued a standard (ANSI and Department of Defense approved) on the proper design, dimensioning and tolerancing of gages and fixtures for the inspection of geometric tolerances. This standard not only governs the principles for the appropriate procedures for the creation of gages for geometric tolerances (called functional gages) but also continues the practices to measure maximum material conditions with GO gages and least material conditions with NOGO gages that were originally shown in ANSI B4.4. B4.4 has been retired, but its principles were absorbed into Y14.43 and extended to apply to the more difficult Functional Gages (to inspect geometric tolerances). The basic premise of all gaging is to reject all bad parts (those that violate the tolerances) and to accept all of the good parts (those parts that are in compliance with specified tolerances). But since all gages need to be toleranced, it is understood that they will fail to achieve these lofty goals. They will either reject a small percentage of technically ‘in-tolerance’ parts or they will accept a small percentage of technically ‘out-of-tolerance’ parts. The parts that are on the borderline of exceeding their tolerances (whether just barely exceeding tolerances or just barely in tolerance) are the ones in question. What is critical is that companies decide which side they would rather error on. Would you rather ‘buy’ a few bad parts or reject a few good ones? This is the question whose answer will determine whether gage pins will have a plus tolerance or a minus tolerance. It will also determine whether gage holes are toleranced on the plus or the minus side of their acceptable boundaries. For example, if a GO gage pin designed to check a maximum material condition is dimensioned at the MMC of the hole to be gaged, but then toleranced with a plus only tolerance, the plus only tolerance will infringe on and, therefore, subtract from the tolerance assigned to the hole being gaged. Therefore, some of the borderline, but in-tolerance, holes being gaged could be rejected. This could have the effect of increasing manufacturing costs for the parts containing the holes, but increase the quality of the parts. Conversely, if the GO gage pins are sized at MMC and then toleranced with a minus only tolerance, some of the borderline, but technically ‘out-of-tolerance’, holes being gaged could be accepted. This could have the effect of decreasing manufacturing costs but also decreasing the quality of the parts. Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 425 Dimensioning and Tolerancing of Gages So, a company must choose which they will do--take the risk of rejecting a few borderline good parts or accepting a few borderline bad parts. Their decision will commonly set the course for all gages and fixtures the company designs (or has designed for them) in the future. The ASME Y14.43 standard has taken as its preferred practice two policies on gage and fixture tolerancing. These policies are called Absolute (also called pessimistic) Gage Tolerancing and Practical Absolute Gage Tolerancing. For GO gages that inspect the maximum material condition sizes of features, the Absolute Gage Tolerancing policy is preferred. It sets as the goal never to accept an out-of-tolerance part. Therefore, all GO gage pins are designed at the MMC and toleranced to have only a plus tolerance on the size (no minus tolerance). All GO gage holes are dimensioned at the MMC of the pins being gaged and then toleranced so that the gage hole may only be produced at that size or smaller (all minus tolerance, no plus tolerance). This has the effect of never accepting features (holes, shafts, slots and tabs) that are outside of their tolerance range. It also has the effect of rejecting a small percentage of technically in-tolerance parts. For example: FIGURE 22-1 [Part with Hole] FIGURE 22-2 [GO Gage pin with Absolute Tolerancing] This gage is shown using 10% of the part tolerance. Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 426 Chapter Twenty-Two Both the GO gage and the NOGO gage have been toleranced so as to subtract tolerance from the hole being gaged. The GO gage pin is all plus tolerance, to accept no parts that are outside of the MMC size limit. This has the effect of also rejecting a very small percentage of in-tolerance holes. The NOGO gage also accepts no bad parts, but may reject a small percentage of borderline, but technically good, parts. Remember, the job of the NOGO gage is to ‘not go’ into the hole. By reducing the size of the gage (from the 051mm LMC) with a minus only tolerance, the gage is more likely to go into the hole and, therefore, reject the hole as being too large (in violation of the least material condition). FIGURE 22-3 [NOGO Gage] This gage uses 10% of the part tolerance. FIGURE 22-4 [Detail Drawing of a Workpiece to be Gaged] Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 427 Dimensioning and Tolerancing of Gages For the workpiece shown in FIGURE 22-4, a gage is constructed using 10% of the part tolerance for each element being represented on the gage-for example, 10% of the flatness tolerance, each perpendicularity tolerance, position tolerance and hole size tolerance. This gage is called a Functional Gage and is toleranced with the Practical Absolute Gage Tolerancing methodology. FIGURE 22-5 [Functional Gage using the Practical Absolute Gage Tolerance] GAGE Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 428 Chapter Twenty-Two As you can see, datum feature simulators are constructed to represent datum features A, B and C. Datum feature A is a portion of the entire surface, so the datum feature simulator is that large (70.5 x 100). It is assigned a flatness tolerance of 0.01 (10% of the 0.1 flatness tolerance on the workpiece). The gagemaker’s tolerance ideally ranges from 5% to 10% of the workpiece feature’s tolerance that is being simulated. Datum features B and C both have a tolerance on the workpiece of perpendicularity, so the datum feature simulators on the gage have been assigned perpendicularity tolerances of 10% of these tolerances. Datum feature simulator B references only datum A in its perpendicularity control, but datum feature simulator C references both datums A and B. The holes on the workpiece are represented by gage pins on the gage. These gage pins are sized at the virtual condition of the holes on the workpiece that are being gaged. 15.0 = MMC Holes - 0.2 = Geometric Tolerance at MMC 14.8 = Virtual Condition of Holes Functional gage pins are dimensioned to be the virtual condition of the holes being gaged. So, the two gage pins are sized at 14.8. With Absolute and Practical Absolute Gage Tolerancing methods, the gage pin tolerance is all on the plus side of the 14.8 virtual condition boundary size. Since the holes have a size tolerance of 0.2, the gage pins will have a plus only size tolerance of 10% of that, which is 0.02. The gage pins will be: ?14.8 +0.02 0 The gage pins are given a position tolerance. Since this gage is shown with fixed pins, the pins are given a position tolerance directly that is 10% of the position tolerance on the holes being gaged. The holes being gaged have a position tolerance of 0.2 at MMC, so the gage pins are given a position tolerance of 10%) of 0.2 at MMC, which is 0.02 at MMC. Now the control reads: 2X ?14.8 +0.02 0 s 1 ? 0.02 M A B C If this gage used push pins that are to be shown separate from the gage base, the gage pins would be dimensioned as 2X ?14.8 +0.02 0 for the portion of the pin diameter doing the gaging. Then the holes in the base of the gage that the pins would be pushed in to (once the workpiece was mounted onto datum feature simulators A, B and C appropriately) would be given a position tolerance of: s 1 ? 0.02 M A B C Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 429 Dimensioning and Tolerancing of Gages This position tolerance would govern the allowed movement of the holes in the base of the gage. The fit between the gage pins and these holes in the gage base is to be a ‘Sliding Fit’ as governed by ANSI B4.2 on Preferred Metric Limits and Fits. Had the workpiece and accompanying push pin gage been dimensioned and toleranced in inches, the fit between the gage pins and the holes in the base of the gage would have been a ‘Sliding Fit’ per ANSI B4.1. The way to determine the likelihood of a good part being rejected by this gage or a bad part being accepted is to construct a chart of the hole’s (being gaged) virtual condition boundary and the gage pin’s inner and outer boundaries. The virtual condition of the holes being gaged is 14.8. Any gage pin outer boundary larger than 14.8 runs the risk of rejecting good parts. Any gage pin inner boundary smaller than 14.8 runs the risk of accepting bad parts. The risk of rejecting good (but borderline) parts is very real. The risk of accepting bad parts is mostly theoretical in that the physical gage pin diameter is a minimum of 14.8. Any reduction of this number is caused by the position tolerance allowing the pin to move away from its perfect location (as shown by the basic dimensions on the gage drawing. But wherever the gage pin ends up in its location, it is still at least 14.8 in size. Also, remember that for every action, there is an equal and opposite reaction. So, as the gage pin moves in on one side (acting smaller on that side), it moves out on the opposite side (acting larger on that side). This means that even though this movement may generate an inner boundary smaller than 14.8, it will (because of its movement) simultaneously generate an outer boundary larger than 14.8. Think of yourself (as the gage pin) trying to walk through a door (the hole being gaged). If you center yourself to the middle of the door, you walk easily through it. But if you move a step to the right of center, your left shoulder easily clears the left side of the door. You are acting as though you are smaller on the left side of your body. But at the same time, your right shoulder bangs into the door frame and you don’t fit through the door. While your left side might be occupying less than its half of the door entrance, your right side is occupying more than its half of the door entrance (acting as though you have grown on your right side). So, you are really the same size as you always were, but because you have moved to the right, the left side of your body acts smaller and the right side of your body acts bigger. Now the important part... You don’t fit through the door. And, likewise, the gage pin doesn’t fit into the hole being gaged. If the gage pin moves, it is more likely to reject a good part than accept a bad one. When could it accept a bad one in this scenario? ...when the door (hole) moves in the same direction, by the same amount, as you (gage pin) move. This, in a practical sense, is most unlikely to happen. That is why this type of gage tolerancing is called Practical Absolute Gage Tolerancing. It means that a gage toleranced in this manner will practically absolutely not accept a bad part. Now that the practicality has been explained, we can look at the numbers and not panic when we see them wander down into the ‘a(chǎn)ccepts bad parts’ range. Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 430 Chapter Twenty-Two Outer Boundary of the Gage Pins 14.82 = MMCPins +0.02 = Geometric Tolerance at MMC 14.84 = Outer Boundary Gage Pins Inner Boundary of the Gage Pins 14.80 = LMCPins - 0.04 = Geometric Tolerance at LMC 14.76 = Inner Boundary Gage Pins FIGURE 22-6 [Graph] The graph would seem to imply that there was just as much of a possibility of accepting bad parts as rejecting good ones, until we remember that the physical diameter of the gage pins are a minimum of 14.8 and a maximum diameter of 14.82. The rest is movement. Moving the pin to the left or right is rarely going to allow a 14.80-14.82 gage pin to fit into a hole acting smaller than that. You will probably reject a very small percentage of technically good, but borderline, parts. You will absolutely, practically never accept any bad parts using this gage tolerancing policy. The pin gage dimension and its tolerances can be manipulated to get any result you desire. For example, if I wanted an Absolute Gage (instead of Practical Absolute) where, even in theory, no bad parts would be accepted, we could increase both gage pin size limits by the difference between them and the position tolerance. Since the difference between the gage pin MMC of 14.82 and the LMC of 14.80 is 0.02, we would take that 0.02 and add it to the position tolerance, which is also 0.02 at MMC for a total of 0.04. This 0.04 would then be added to the size limits as follows: 14.82 = MMC Gage Pins + 0.04 = Increase Factor 14.86 = New Gage Pin MMC Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 431 Dimensioning and Tolerancing of Gages and 14.80 = LMC Gage Pins + 0.04 = Increase Factor 14.84 = New Gage Pin LMC These new gage pins would be as follows: LMC MMC 2X 14.84-14.86 Gage Pins 0.02 A B C% M This would generate new boundaries: 14.86 = MMC + 0.02 = Geometric Tolerance at MMC 14.88 = Outer Boundary Gage Pins and 14.84 = LMC - 0.04 = Geometric Tolerance at LMC 14.80 = Inner Boundary Gage Pins So, our new graph would be as follows. FIGURE 22-7 [Graph] This graph shows that we can’t buy a bad part with these new gage pins, even in theory. However, it also shows that the chance of rejecting good parts is much greater. With the original gage drawing, we had only wandered into the ‘rejects good parts’ range to 14.84. Now, with the new gage pin dimensions, we have gone twice as deep into that range to 14.88. This potentially raises the cost of the workpiece being gaged, with more technically good parts being rejected. Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 432 Chapter Twenty-Two Another possibility that gives similar results is the use of the LMC modifier on the gage pins in the position control. For example: LMC MMC 2X 14.82-14.84 Gage Pins 0.02 A B C% L As you can see, the MMC and LMC of the gage pins have been increased by the 0.02 position tolerance. This is to keep us out of the ‘a(chǎn)ccepts bad parts’ (in theory) range. If the position tolerance had been zero, instead of 0.02, the gage pin MMC (14.82) and LMC (14.8) would have remained the same. But, with the MMC raised to 14.84 and the LMC raised to 14.82, the outer and inner boundaries are as follows: 14.84 = MMC Gage Pins + 0.04 = Geometric Tolerance at MMC (0.02 Geo. Tol.. plus 0.02. Bunos Tol.) 14.88 = Outer Boundary Gage Pins and 14.82 = LMC Gage Pins - 0.02 = Geometric Tolerance at MMC 14.80 = Outer Boundary Gage Pins As calculated, it becomes apparent that these boundaries are the same for both possibilities that follow. 2X 14.84-14.86 Gage Pins 0.02 A B C% M and 2X 14.82-14.84 Gage Pins 0.02 A B C% L Both generate gages categorized as Absolute and will never, even in theory, accept bad parts. But both run the risk of rejecting more in-tolerance parts than the original ASME Y14.43 favored method of Practical Absolute Gages toleranced as: 2X 14.80-14.82 Gage Pins 0.02 A B C% M FIGURE 22-8 is another example that uses the Practical Absolute Gage Tolerancing method. Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 433 Dimensioning and Tolerancing of Gages FIGURE 22-8 Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 434 Chapter Twenty-Two FIGURE 22-9 [Functional Gage for the 4-HoIe Pattern Position Tolerance] GAGE WORKPIECE APPLIED TO GAGE All gages in this section have used either the Absolute Tolerancing Method (shown on the GO and NOGO gages) or the Practical Absolute Tolerancing Methods (shown on both Functional Gages). These gaging practices use as the premise that all gage pins have plus tolerances and all gage holes have minus tolerances for all GO gages and Functional Gages. For NOGO gages, all gage pins have minus tolerances and all gage holes have plus tolerances. This is to achieve a gage that does not accept parts that are out of their tolerance ranges. Downloaded From: http://ebooks.asmedigitalcollection.asme.org/ on 04/11/2014 Terms of Use: http://asme.org/terms 435 Dimensioning and Tolerancing of Gages There are two other gage tolerancing practices that are NOT RECOMMENDED by the ASME Y14.43- 2003 standard on Dimensioning and Tolerancing Principles for Gages and Fixtures. One of these is called Optimistic Gage Tolerancing. This policy tolerances gages in ways that are the opposite of those described in this unit. GO gage pins and functional gage pins would begin at the same sizes shown in this unit but would have no plus tolerances. These gage pins would be tolerancing entirely in the minus direction. The 5% to 10% policy would still apply, just in the opposite direction as shown for Absolute and Practical Absolute Gage Tolerancing. For gage holes on Go gages and Functional Gages, the Optimistic Gage Tolerancing would be all plus and no minus. For NOGO gage pins, the Optimistic gage would have a plus tolerance and the Optimistic gage holes would have a minus tolerance. Optimistic Gage Tolerancing risks buying a small percentage of out-of-tolerance parts. Optimistic gages buy all parts within tolerance and also a few that are not. This is generally perceived as lowering production costs of parts but sacrificing a small portion of quality and the parts’ ability to function or mate with other parts in the assemblies. The third policy NOT RECOMMENDED by ASME Y14.43 is known as Tolerant Gaging. Tolerant Gaging sizes GO gages at MMC, NOGO gages at LMC and Functional Gages at virtual condition, just as do the Absolute, Practical Absolute and Optimistic methods. But instead of just tolerancing to either the plus or to the minus side only, the Tolerant Gaging policy gives gage pins and gage holes both a plus and a minus tolerance. The problem with this non- recommended practice is that it does not take a stance as to a company’s policy. It does not decide to reject a