Understanding ASME Y14.43 1

[illini8181]

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!

 

[Belanger]

I only have time for a quick reply right now, but it is often helpful to specify the position tolerance as zero with the MMC modifier. In this way, any tolerance that you juggle will only happen on the size of the pin or hole. (It makes the allocation of tolerances on the gage a lot easier!) But I hope others will jump in with a fuller answer for you.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[illini8181]

Belanger, thank you for the quick reply. If I understand correctly though, if I did specify the position tolerance as zero with the MMC modifier, there would still be a tolerance of position (TOP) of .013 that I would need to address. Example A2-b of the standard seems to add this TOP to the pin diameter, and Example A2-a does not. Or am I missing something? Thanks again.

 

[SeasonLee]

Here is the rough calculation for the pin size and its TOP

Season

 

[SeasonLee]

The final pin size need to be corrected to Ø.717+.00215-.0000

Season

 

[illini8181]

I can't tell you how much it helped me to see how you worked that out. I'm pretty sure I followed it completely. However, in reading what you did, I realized that I did a terrible job with my two figures and have probably confused anyone who has looked at this. The pin is not located on the fixture. The fixture actually has four holes (not four pins as it probably looked in my first attachment). The pin is pushed through the part and the fixture, while the part and fixture are clamped together. I am actually trying right now to simplify a pretty complicated fixture (for the purpose of asking this question) which includes many many more holes (why I have to use a single pin gage, instead of a pin in place of each hole) and a more complex shape. I updated the figures in the attached file, and have included a view showing the part/fixture/pin assembly.

 

[SeasonLee]

It just a method tell you how to calculate the size an its tolerance, you may follow it and try do it by yourself.

Season

 

[illini8181]

Thank you. Honestly, I am not trying to just "ask for the answer", I am trying to understand. Following the same steps you followed, I get the pin size shown in the attachment. But, this is without considering the .010 position tolerance applied to the fixture holes (see attachment in my above post). My question is still regarding that position tolerance. If I want to design the fixture according to the "absolute tolerancing policy", do I have to add this tolerance to the size of my pin? Or can the fixture/pin design still be considered to be designed to the absolute tolerancing policy if this is not added to the pin diameter. It seems to me that the standard allows room here for either choice, but I just want to make sure I am not misunderstanding the standard.

 

[SeasonLee]

If I understand your figure correctly, the gauge pin should be inserted from the fixture side as shown on the attached, I am wondering what the hole size 4X Ø.750 and the TOP is zero position, isn’t it?

Season

 

[illini8181]

The .750 is the size of the four holes in the fixture, which the pin passes through to check the holes in the part. You are right that the pin is inserted from the fixture side, as you have it shown.

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.

 

[illini8181]

Here is an isometric view of the fixture that might make things clearer.

 

[CheckerHater]

I probably didn’t' wake up completely yet, but how do you check position of 4 holes with 1 pin?

 

[illini8181]

Well I've made this really confusing, but in reality, the part is fixed in place with two pins in two diagonal corners, and then it is clamped to the fixture with multiple clamps, and all of the holes (many more than 4) are checked one at a time with the pin. I tried my best to simplify the fixture to ask this one question (whether the TOP of the holes on the fixture has to be included in the size of the pin), but it looks like maybe it can't be simplified.

 

[SeasonLee]

I don’t know how do you get this figure |POS|Ø.010RFS|X|Y|Z ? You said this is applied to the ID of the bushing, then what is the tolerance applied to the pin?

If the part is fixed by clamps on the fixture, then how do you check the hole location?

Season

 

[pmarc]

illini8181,
Is this thread still alive? I have some thoughts to share, but I am not sure if any input is still needed.

 

[illini8181]

It is still alive. I would definitely appreciate more input.

 

[pmarc]

Okay, so allow me to start from saying that there is something wrong going on with your approach. And I am not thinkinking about "tiny" differences between figs. A-2a and A-2c on one hand and A-2b and A-2d on the other.

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.

 

[illini8181]

Thank you. It is correct that I am using a push pin design. I actually have 2011, not 2003 (R2008), but I know the section that you are talking about which deals with push pin design (5.2.6b of 2011).

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.

 

[pmarc]

I agree with your understanding of 4.5.1 in 2011 (2.5 in 2003), however to me one basic requirement specified in this paragraph is not met in your case - it is stated that "Each feature of the gage that represents a feature on the workpiece is recommended to receive a tolerance between 5% and 10% of the tolerance assigned to that particular workpiece feature.. If you just compare position tolerance values for the gage holes and for the workpiece holes at MMC you will get 0.1/0.3 = 33%. That is way too much in my opinion.

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?

 

[illini8181]

Wow, thank you so much! I had not yet seen that figure. That is exactly what I need to look at. Very helpful.

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!