Feed back requested on drawing - angularity& position questions 2

[AndrewTT]

Background: We are implementing a GD&T initiative at my company. The Engineers have been studying the ASME Y14.5-2009 standard and using the Cogorno text book to learn GD&T. We are now beginning to practice applying GD&T to some of our simpler parts. The attached image represents one of our engineer’s first practice drawing.

Discussion: I would like to first discuss two particular items and then any other aspects of this drawing can be addressed.

Item #1: Is this a legit use of angularity to control the center plane? I believe that it is not because the angularity FCF should be associated with the slot width size dimension (.106 & .204). Please comment.

Item #2: Is this a legit use of position to control the slot? I believe that it is not because the radius of the slot is not a FOS. Position is used to control FOS. Our discussion of this drawing centered on if you can consider the radius an irregular FOS, because if you could then maybe position could be used to locate it. However, if the radius is an IFOS then I believe that you would have to do something similar to figure 8-24 to employ position. I do not believe that you can directly apply position to an IFOS. Please comment.

Rationale for this GD&T scheme: We were attempting to reproduce the tolerance zones that are created by the coordinate dimensioning scheme that is currently employed on this drawing. The design intent of this part and the current coordinate dimensioning print allow the slots to move laterally .005” to either side of true position and also to rotate ½° from true orientation. This movement was not possible to reproduce exactly using a simple position tolerance associated with the slot width dimension. Our goal was not to increase part cost by having GD&T tolerance zones that are smaller than the current tolerance zones. We also did not want to potentially decrease the function of the part by opening up tolerances beyond what they currently are. The attached image represents, I believe, a creative attempt to match coordinate tolerance zones that cannot be easily/at all re-created using GD&T. However, I do not believe that the FCF usage agrees with the wording or the spirit of the standard.

Other Schemes: We also had engineers create drawings that used 1) position associated with the slot width dimension, 2) profile, 3) composite profile, 4) combined controls (profile/position like figure 8-24). These all had their own pros and cons that I do not want to discuss at this time. I would like to simply address the dimensioning scheme of the attached print.

Additional topics: This part is produced via stamping. I believe that flatness of the derived median plane would be a better choice of form control than surface flatness in this instance. It should make for easier manual inspection at least. You could measure the thickness with calipers and then build a bridge gage of the appropriate height to check the flatness (our design intent would not allow for the MMC modifier to be used with FDMP). This should be easier than manually inspecting datum surface A by means of sweeping with a dial indicator. Please comment.

Thank you!!

 

[powerhound]

#1. No. The only thing angularity will do is ensure the angle is 36 degrees. Use position on the slot width to locate it and refine with angularity if necessary.
#2. No. Refer to #1. Use position to locate the slot width. Angularity does not locate.

Change out your angularity callouts with position and get rid of the existing position callouts except for the one on the OD. Use another method to indicate slot depths.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[3DDave]

I would drop all the angularity and position tolerances and use profile. Your design so far does not benefit from allowing increased location or orientation variation when the size of the slots or the center hole change.

It's good to establish a condition for [A] but it isn't going to work to use position on the ends of slots; the diameter of the slot end does not exist in most directions and therefore one can't determine the location of the center in the radial direction. Some will argue they can assume a full diameter from the arc; that's an interpretation that is not (yet) inside the Y14.5 standard.

 

[powerhound]

Well, I guess that'll teach me. I should make it a habit to read the entire post rather than just the thread title and attached drawing.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[AndrewTT]

Thank you powerhound. The engineer who made this print was hoping that the drawn tangent between the radius and the slot would let the position callout on the radius also control the location of the slot. We have no one here certified so sometimes it is difficult to say something is wrong with certainty even if we feel that it is not correct.

Do you have an opinion on the last part of my original post concerning surface flatness vs flatness of the DMP?

Thank you.

 

[mkcski]

AndrewTT:

I agree with 3DDave, use profile to control the size, form, orientation, and position of the slots and, I would suggest, all part features in the plan view except the datum B. Unless something assembles into the slots, I would define the slots and OD with BSC dims and apply a profile to the entire "shape".

Flatness error of a surface cannot be more than the size tolerance due to Rule #1 for FOS. So let Rule #1 control the flatness, which would provide .003 error at LMC. Flatness applied to the thickness dimension - a FOS - can be larger than the size tolerance because Rule #1 is over-ridden and boundary conditions are in effect (virtual condition if MMC is applied as in fig 5-9).

The .106 arc at the base of the slots is not a FOS so position cannot be used to control their location. See 1.8.4 for dimensioning slots.

Regarding datum C, the radius (rounded end) at the base of the slot is not a FOS, so position cannot be used to control its location. The 10X slots appear to be a pattern that as a group can rotate around datum axis B without affecting function. So would be no functional requirement to have Datum C to restrict the rotation of one slot relative to others - they all rotate as a group.

The CF should have 10X in front of the size

For visual clarity it would be better to define datum B in the larger scale view on the left.

Certified Sr. GD&T Professional

 

[powerhound]

The CF should have 10X in front of the size

I'm pretty sure you omit the number of times when CF is used since it is now considered a single feature. Also, none of the examples in the standard use number of times.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[mkcski]

powerhound:

I agree. Most times we use it so there is no-question as to how many places included in the control. We sometimes even label the features and state "X thru X" to eliminatee any confusion and assumptions.

Certified Sr. GD&T Professional

 

[AndrewTT]

You'll notice that circle I has been used to break rule #1 for the thickness dimension. Now flatness can be called out larger than rule #1 would allow. My question is if flatness of the derived median plane is more appropriate in this instance (also free of rule #1)?

Also, profile will not work for us because of other reasons that I do not want to get into at this point. My questions on proper use of angularity and position have been answered. I am now looking for feedback on surface flatness vs. FDMP.

Thank you!

 

[pmarc]

Some comments from my side:

Answering to OP's two questions first:
Q1. It is perfectly legit to apply angularity callout like this. FCF does not have to be associated with the slot width dimension. Refer to fig. 7-28 in Y14.5. The figure is not showing angularity tolerance, but position, but it does not really matter. What matters however, is whether these angularity callouts make functional sense, and that should be the question you should ask yourself or designer first.

Q2. I would say it is incorrect use of position tolerance. The radius may or may not have an arc length over 180 degrees, and because there is a chance it will not be a feature of size (will not have opposed elements), the position callout, as is, should be dropped. Side note - the fact that something is not a regular FOS, does not mean it automatically becomes irregular FOS. And the radius in question is not a IFOS too.

Q3. What kind of flatness callout would be better? Well, assuming you do not want to apply DMP flatness callout at MMC, your idea of bridge gage will work only if the plate has uniform thickness all over it. But if that is not the case, you are not able to build a bridge gage for the entire part.

Additional remarks:
1. Whether the slots should be solely controlled with profile, should depend on functional requirements, because there are other ways to control slots like these, that may work as well. One might, for example, use position and profile combination, as shown in fig. 8-24, or even a composite profile.

2. Flatness tolerance applied to datum feature A, as shown at the moment, is correct, because there is an Independency modifier attached to the plate thickness callout.

3. While I like the idea of perpendicularity callout applied to an inscribed cylinder derived from double-hex hole B, the shape and "outer" size of this hole is uncontrolled. The 24X 15 deg dimension is not capable of doing it by its own.

4. There are also different options to control OD of the part. The position callout is one, but profile, as already suggested, could work to. If position is still a choice, it is worth to remember that CF modifier, as specified right now, may lead to some difficulties in interpretation, because the Y14.5 standard does not actually tell how to deal with this kind of continuous features (it only shows examples of continuous features interrupted lengthwise/axially). In this very case most likely there will not be a problem, because for each arc between two slots there is exactly opposed arc on the other side of the diameter. But if the number of slots were odd (so that not everywhere opposed elements would exist), the interpretation could not be that easy.

5. Currently there is nothing on the drawing to control mutual orientation relationship (clocking) between slots and the double-hex. If that is needed, either the double hex needs to be controlled by a callout referencing clocking datum feature, OR a feature on the double-hex should be defined as clocking datum feature, and other features should be controlled relative to it, OR a simultaneous requirement (SIM REQT) concept could be deployed and then the tertiary datum feature might not be needed at all. If the clocking between the double-hex and the slots is not needed, a SEP REQT can be used - again most likely you would not need tertiary datum feature at all then.

 

[mkcski]

AndrewTT:

Woops. I am guilty of NOT looking at every detail. I am at work so I just scan of content more than detail. This is rude. Sorry.

Regarding the I vs DMP. If the part mounts or contacts a mating part on one surface only, use I (which in my travels is rarely applicable). If the parts fits between two parts I would use the DMP concept. I ask why is the thc

Certified Sr. GD&T Professional

 

[mkcski]

AndrewTT:

Having a bad day - i press the submit box too soon. Let me finish the last sentence

Might I ask why is the thickness size tolerance so small and the flatness larger?

Certified Sr. GD&T Professional

 

[AndrewTT]

All this feedback is great. Thanks to everyone. I am in training today so not a lot of time to respond to lengthy questions. I'll try to hit a few quick answers here.

mkcski - We measured parts and found the flatness within .004 inches. The coil stock thickness is repeated for the stamped part thickness. We are working under the assumption that parts we make work as intended and we are reflecting their actual measurements on this print. This assumption is based off of performance testing and field results.

pmarc - The part assembles against datum surface A. There is no guarantee of uniform thickness so I now agree FDMP without the MMC modifier is probably not a good choice over surface flatness.

pmarc - There is no orientation requirement between the double hex and the rest of the part. We have a note (not shown on my pdf) that states this.

pmarc - I'm not sure how figure 7-28 (2009) relates to angularity. Can you elaborate here, please?

 

[pmarc]

AndrewTT,

I was merely trying to say that such method of FCF attachment/placement is perfectly legal, so I used fig. 7-28 as an example to back up my statement.

In your case no slot width dimensions are given (which is incorrect, by the way). If they were specified, as +/- dimensions for example, the angularity FCF would most likely be attached to those size callouts.

 

[AndrewTT]

pmarc - OK, now I understand what you are saying. But I believe that figure 7-28 can not be used to argue that angularity of the center plane can be applied to the slot un-associated from the width size dimension as my engineer did. Figure 7-28 is a special case, I believe, due to the bi-directional aspect of the tolerance zone. Also, imo, any figure in chapter 6 that shows angularity of the center plane (or axis) not associated with the size dimension is a figure that is intentionally incomplete. I am still of the opinion that this is not a legitimate use of angularity due to the slot not being positioned correctly (positioned on the radius feature and implied to be held tangent to said radius).

 

[AndrewTT]

pmarc - What would you suggest be added to control the double hex, w.r.t. size and shape? There is no orientation required w.r.t. the rest of the part. We have a note stating this that was not shown on my pdf.

 

[pmarc]

AndrewTT,

I agree. Because the slots are not positioned correctly, it is not a legitimate use of angularity tolerance. But again, in my first reply I was just merely trying to say that there is nothing wrong in placing FCF as it is done right now.

As for your second question, my first choice would be to use profile all around relative to A. Then I would drop the +/- size dimension for the inscribed cylinder B (most likely would leave only the diameter symbol, as shown in fig. 4-35), and remove the zero at MMC perpendicularity callout.

The alternative could be applying profile all around with no datum feature references, dropping +/- size dimension for the inscribed cylinder B (leaving only the diameter symbol), and leaving the zero at MMC perpendicularity callout unchanged.

 

[pylfrm]

AndrewTT,

Regarding Item #1: This symbology fits the description for method (d) in para. 3.5, and I think the intent is clear. It's perhaps a bit questionable because the included angle of the slot is uncontrolled, but that's not much different from the situation you'd have if the tolerance were applied on an assembly drawing.

We are working under the assumption that parts we make work as intended and we are reflecting their actual measurements on this print. This assumption is based off of performance testing and field results.

I would be inclined to develop a new tolerance scheme based only on the functionality of the part, ignoring the old drawing. Actual measurements of existing good parts could then be used to help set tolerance values if desired. Duplicating an old tolerance scheme with new methods seems like a rather questionable goal.


pylfrm

 

[AndrewTT]

This part has approvals of outside testing agencies. Any change to the tolerance zones of the part could result in expensive re-testing of the part. We want to avoid this if at all possible. This is why we are trying to mimic the existing tolerance zones as much as possible. And yes, we are measuring parts, that function properly, as well as we can to determine what we actually have.

 

[AndrewTT]

pmarc - I have attached a portion of a catalog for standard shape punches. The highlighted punch is what is used to create the double hex hole feature. The only dimension specified for this punch is the "w" dimension. The "w" dimension is represented by the 6X (.305) dimension on our print. The design intent of this feature is to receive a stem/boss of the joining part in assembly. This stem is then riveted to ensure that the subject part cannot rotate about its datum axis B w.r.t. the assembly. The exact shape of the double hex is not so important. What is needed is simply space for stem/boss material to move into during the rivet process so that our part bites into it, thus eliminating any rotation. We are more concerned with specifying a cylindrical shaped VC for the double hex such that it will always mate with the joining part's stem/boss.

So, is this where reality deviates from theory? The double hex feature may not be fully controlled for size, shape, location, orientation but is it good enough for my purpose, as drawn? Does everyone who uses GD&T fully define every feature on every drawing? The standard would say that you need to, and I agree in theory, but in the real world is this how GD&T is actually used? I do not think our QA dept. has ever measured anything more than the size of the inscribed cylinder on this part since its launch.

Thanks!