Understanding ASME Y14.43
Understanding ASME Y14.43
(OP)
Hello, I am working on the design of a gage. This is the first time I have designed a gage, and it seems as though there is no one in my company who is knowledgeable about doing so. I have read through ASME Y14.43 several times. Each time I read it, it becomes clearer. I have a fundamental question that is spelled out in the attached document. I have been going over and over this in my head, and am pretty stumped. I would appreciate any help! Note that the "fixture" I show is not the actual fixture I am designing. It is just a simplified version for the sake of asking the question.
Thanks!
Thanks!





RE: Understanding ASME Y14.43
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Understanding ASME Y14.43
RE: Understanding ASME Y14.43
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RE: Understanding ASME Y14.43
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RE: Understanding ASME Y14.43
The .750 hole in the fixture has a tolerance of +0.001 (ie the size is .750/.751). It is actually a bushing. The TOP applied to the hole is not zero, it is 0.010 RFS, which is applied to the ID of the bushing. This is how we dimension fixtures. (similar to Examples A-2c and A-2d in the standard, although the example is a pin, not a hole).
This is actually a very large fixture with many more than 4 holes, which is why we cannot hold a TOP of zero. I did my best to simplify our fixture, in order to ask this question, but I think I ended up causing a lot of confusion. Sorry about that.
RE: Understanding ASME Y14.43
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If the part is fixed by clamps on the fixture, then how do you check the hole location?
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RE: Understanding ASME Y14.43
Is this thread still alive? I have some thoughts to share, but I am not sure if any input is still needed.
RE: Understanding ASME Y14.43
RE: Understanding ASME Y14.43
First of all, I would suggest reading paragraph 3.2.6(b) of Y14.43 (I assume you use 2003(R2008) edition of the standard. I do not have 2011 edition with me, so am not able to give corresponding paragraph number in this document). It is about one of the gage pin types called the pushpins, which you are going to use in your gage, as far as I undestood you correctly. You can find a following statement there: "If the pushpin gage design is employed, the part tolerance shall be divided between the gage pin size limits and its counterpart gage hole's positional tolerance. Consideration shall also be given to the fit between the gage pin and its counterpart gage hole.
In other words, if total part tolerance is .043, you have already introduced approximately 23% of part tolerance to gage design by assigning .010(S) positional tolerance to holes in the gage plate (0.010/0.043 = 0.232). If you add gage pin size tolerance and a possible loose between gage pin and the holes in the plate to that, I do not think you will end up with total gage tolerance below 25% of part tolerance. This will not be satisfying any Y14.43's gaging tolerance policy.
Another thing -- the very same paragraph of Y14.43 states: It is recommended that projected tolerance be used on these types of gage holes, since the gage hole gives orientation to the gage pin. I do not know how thick your part is (sometimes, especially when inspected part is thin, it may not be reasonable to go with projected tolerance concept), but I would at least recommend taking this aspect into consideration.
And finally -- the size of the holes in the gage plate. In my opinion the holes should not be of the same nominal size as the holes in inspected part, but rather bigger. Look at fig. 3 and especially at fig. 4. The part of gage pin that mates with the plate hole, called pilot diameter, is bigger in diameter (by approximately 30%) than the part of the gage pin that checks the part hole, called the gage diameter. Unfortunately, from tolerance perspective this would mean that some portion of total gage tolerance would have to be consumed by positional relationship between pilot dia. and gage dia. of the pin.
RE: Understanding ASME Y14.43
It is interesting that you bring up the gage tolerance as a percentage of part tolerance, because Section 4.5.1 of 2011 states “If 5% to 10% of the tolerance on all features being gaged is represented in the gage, consideration should be given to the entire gage tolerance that has accumulated. It is recommended that this tolerance not exceed 50% of the tolerance for the specific workpiece feature being gaged. For example, if a pattern of holes is being gaged for a position tolerance and the maximum position tolerance for the holes is 0.3 (which includes any possible bonus tolerance drawn from the size limits of the holes), then the gage that inspects that hole pattern (which may include the gage flatness tolerance on the primary datum feature simulator, the size tolerance and the perpendicularity tolerance on the secondary datum feature simulator, the size tolerance and the position tolerance on the tertiary datum feature simulator, and the gage pins size tolerance and position tolerance for the hole pattern), when added should not exceed 50% of the 0.3 part tolerance on the holes."
Am I misunderstanding this section, or has the recommendation of the standard changed?
Part of my issue is that I am in a group which has designed many gages in the past, but we have lost a significant amount of experience due to retirements. I spend a lot of time looking at gages we have designed, and wondering why certain things are done, but I cannot seem to find answers. One of the things I have noticed is that, with pushpin designs, there is always a diameter difference between the part of the push pin that engages with the gage, and the part that engages with the part, exactly as you have recommended. My initial thought was that that would be undesirable, for the reason that you gave - it adds an additional tolerance (the positional tolerance between the two diameters) into the pushpin.
Could you more fully explain why you think this is desirable? I am not disagreeing with you - I don't have enough knowledge to disagree with you! I just want to understand.
RE: Understanding ASME Y14.43
As for recommended pushpin stepped geometry -- have a look at fig. B18 (I am not sure about corresponding number in 2011 edition). Its title is: "Pushpin Gages for Part Clearance Holes". There is a note on that figure saying that the fit between pilot diameter of gage pin and gage hole should be "tight but sliding" based on ASME B4.2. I do not have B4.2 with me right now, but do have ISO standard for limits and fits and based on it I can tell that the diameter tolerance in the pilot portion of gage pin would have to be approximately 0.010 mm (for diameter range 6 to 10 mm). This in consequence would mean that in case of "single diameter" pin you would have to have two different areas of the pin with two different size tolerances -- 0.010 for pilot portion and 0.020 (8.46-8.44) for gage portion. So such kind of gage pin would still have to be treated as containing two separate cylinders, and would still require a positional control between the two, even though the nominal sizes of the cylinders would be the same.
Does this help?
RE: Understanding ASME Y14.43
I see your point about the different size tolerances for the two parts of the cylinder (pilot portion and gage portion), but in my mind, if you can hold 0.010 tolerance on the pilot portion, why not just apply the same (tighter) tolerance to the gage portion? As I understand it, the tighter your gage tolerance is, the less likely you will be to reject good parts.
I could see the argument that it is going to be more expensive to apply a tighter tolerance to the whole gage than to just the pilot portion, but is that the only argument in favor of two different diameters for the two sections of the gage? That's my last question, I promise!
RE: Understanding ASME Y14.43
And one more argument, according to the chapter about pushpins, the pilot diameter should be of a standard size. In most case this would be very difficult to fulfill with a single diameter pin, don't you think? (This probably boils down to mentioned cost aspect).
RE: Understanding ASME Y14.43
Thanks so much for your help. Thanks everyone else also who posted thoughts. I've learned a lot from this thread. Hopefully someday I'll know enough to answer other people's questions.