The meaning of FRTZF without datum reference
The meaning of FRTZF without datum reference
(OP)
Hi :
The books I read say the lower segment without datum reference controls the sapcing, the perpendicularity and parallelism are controled by the upper segment. I am confused and I really don't know what is the real meaning of this example.
Thanks in advance for all inputs.
Season
The books I read say the lower segment without datum reference controls the sapcing, the perpendicularity and parallelism are controled by the upper segment. I am confused and I really don't know what is the real meaning of this example.
Thanks in advance for all inputs.
Season





RE: The meaning of FRTZF without datum reference
If the tolerance was specified with a MMC modifier then a pin of dia. 0.123 could be used to verify the coaxiality along with separate measurments for size. The collout, however is not modified MMC so the each hole's short axis must be acquired and measured for orientational displacement from an axis created by both.
Paul
RE: The meaning of FRTZF without datum reference
Your explanation appears to be logical enough. However, it seems quite out unorthodox to have a positional tolerance expressed without referencing it to a single datum as a minimun.
Are you not making an 'assumption" when you relate the holes to the axis created by both?
Is there a similar example in the Standard? It appears that Y14.5 is invoked on this drawing although there are a couple minor 'glitches'.
RE: The meaning of FRTZF without datum reference
It is not unusual or unorthodox to see a callout like this without a datum reference in the lower FRTZF. See Y14.5M-1994 paragraph 5.11.1.5 and figure 5-51. I have used these controls often to tolerance a series of coaxial hydraulic control valve bores.
It is unusual however, that the FRTZF's tolerance is not modified "MMC" since the coaxial bores typically mate with a pin-like mating part. I suppose that if these holes mated with an expanding roll-pin and alignment of the holes were critical to achieve surface contact for retention then RFS may be appropriate... but I doubt that... Some one probably just omitted the MMC because it wasn't on the previous design or they didn't consider its relationship to function or gaging.
Paul
RE: The meaning of FRTZF without datum reference
I agree with Paul on this one.
One really needs the modifier MMC to be be able to confirm the alignment. One would then make a straight pin of the virtual condition size (MMC minus the tolerance) and insert it into both holes. The feature meets requirement if the pins are able to go into both holes with the force of no more than 1 finger.
Without MMC, I really don't know how one would confirm the requirement.
Dave D.
www.qmsi.ca
RE: The meaning of FRTZF without datum reference
The answer to "why" you don't need a datum reference on the FRTZF is this; the PLTZF (first level of the composite positional control) establishes the general zones based on the datum reference frame where the axes of each feature in the pattern must be located. The second level (FRTZF) is a refinement of the first lefel (PLTZF), wherein the inter-feature positional relationship is being refined. Datum references are permitted on the PLTZF if you are trying to simultaneously restrict the position of the features to each other and the orientation wrt your datum structure.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
If we are to consider the 2 holes as a single axis, why would not perfect form at MMC be applied without the need for the lower segment of the composite tol?
RE: The meaning of FRTZF without datum reference
Per ringster's proposal, I could tighten up to a 1/4" zone on the top level, but why make things tighter than they need to be?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
A similar exercise occurred in the 1994 version when resolving the old figure 142 of the 1982 version. IMHO.
RE: The meaning of FRTZF without datum reference
Sorry for the late response I had to reply to Dave... It took a while after church and such.
I disagree that it would be overly restricting as MechNorth stated. It is simply that the two holes are two seperate features and each one's "perfect form at MMC" is considered individually.
GM (in their USCAR addendum) allows features to be delared "interrupted" which makes both one feature. Outside of that... the coaxial position tolerance as was done in this composite example would be required to refine the coaxiality beyond the position tolerance.
Paul
RE: The meaning of FRTZF without datum reference
I do not have at my disposal, currently, the 1994 version but the 1982 for coaxiality does ref a datum feature. Fig 161.
RE: The meaning of FRTZF without datum reference
Absolutely reassess your assembled relationship. GD&T should apply only to features where there is a function and relationship to another feature on the same part or its mating part. If there is no function and relationship, don't apply GD&T.
Dave D.
www.qmsi.ca
RE: The meaning of FRTZF without datum reference
We are just the OEM part supplier and I am not quite sure about the assembly and its function, sorry for I can't provide further information regarding the two holes feature.
But I am interesting to know the meanings, if:
1. Two datums reference A B on the second segment (FRTZF) in this example.
2. And with only one datum reference A on the FRTZF in this example.
Thanks again
Season
RE: The meaning of FRTZF without datum reference
I hope this is not misunderstand, but sometimes there just AINT NO good solution to a problem.
RE: The meaning of FRTZF without datum reference
The purpose of the composite positional control is that you don't have to overly restrict the general position of the pattern while allowing the features to be more tightly positioned wrt each other, whether a planar layout of features or coaxial. So, by using only the second level of the composite control, with the tighter tolerance, you would be unduly restricting the position of the pattern as a group. The "uninterrupted" or continuous feature point is valid in this particular application because the holes will likely be fabricated at the same time as a single tool pass, but what if they were different sizes? While they could be fabricated using a stepped drill, the probability of such tooling drops with the decrease in production batch size. As a result, you would need to use a composite control to first locate the two holes wrt the DRF, then second to control the coaxiality of the two holes to each other.
Dave, Dave, Dave my friend... every part needs GD&T.
Without invoking Y14.5, you don't even get Rule #1. With GD&T, you get the size controlling the form at least. With Y14.5, you get documented implied conditions; without it you make assumptions based on "common practice". With GD&T, you have a documented means of controlling size, form, orientation and location of every feature wrt a datum structure. With GD&T, you know how to set up the part for inspection. With GD&T, you minimize tolerance accumulation, and you control the feature under consideration, not the dimension to a point in space.
I've frequently heard people say that they only use GD&T on critical features,; those that they care about. My questions then are:
1) so you don't care how the part is set up for inspection?
2) if you don't care about the feature, why have it?
3) if the feature broaches another feature but meets the +/- spec, is it ok?
4) a square or rectangular position zone is ok, with its biases?
5) you don't care if a rod or pin is bowed, or a rectangular feature is twisted and warped?
6) do you want to use a general tolerance that doesn't conflict with the geometric controls?
7) which is the overriding control ... a conventional +/- tolerance or a geometric control, when both are applied on the drawing?
8) do you think that you don't have to verify any of the conventionally-toleranced features when geometric controls are present?
I agree that designers shouldn't be tossing controls onto the pile just because they're there; they should be conservative in using only the controls necessary to attain the functionality they need. That, however, does not mean not using GD&T, it means judicious application of controls. My perspective is that, as an engineer, I want to fully define all relationships whether they have a tight tolerance value, or a loose tolerance value. By defining all relationships, I can avoid wasted time explaining what the drawing means, and avoid civil action to resolve open interpretation of an engineering print. Now, that being said, there are certain "junk" features that are appropriate applications of conventional tolerances; fillets, rounds & chamfers are the most prevalent.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
I know the purpose of a composite tolerance! Maybe I mis-understood the purpose of your "overly restricting..." comment... sorry if I did.
You stated in the latest response "The "uninterrupted" or continuous feature point is valid in this particular application because the holes will likely be fabricated at the same time as a single tool pass" What are you inferring with this comment? According to the current Y14.5 standard the two "in-line" coaxial holes are two separate features. If refinement of coaxiality is necessary one would need a geometric control to do so. It does not matter how the features were processed... they cannot be considered one interrupted feature and therefore constrained (refined) to some degree for coaxiality by Rule #1 (perfect form at MMC).
BTW...Thanks for responding to Dave's comment... I agree.
Season lee,
You asked:
"But I am interesting to know the meanings, if:
1. Two datums reference A B on the second segment (FRTZF) in this example.
2. And with only one datum reference A on the FRTZF in this example."
The upper part of a composite callot controls as many degrees-of-freedom as the specified datum references are capable of constraining... up to three for translation and three for rotation. The lower portion of the composite control "the refinement control of the pattern" has two purposes:
1) it controls all feature's in the pattern for their relative location and rotation (6 DOF) to each other without regard to specified datum features.
2) If datum features are specified... it controls the "refined" pattern zones for as many orientation degrees-of-freedom (up to 3 DOF) as the specified datum features in the lower segment are capable of constraining.
In your case... if A was specified in the lower composite segment the two dia. 0.010 zones would need to be parallel to surface A.
If both A and B were specified in the lower composite segment, B would do nothing because the only rotation DOF that it is capable of restricting (perpendicular to B) is already constrained by A (parallel to A).
Added note, The control on the print refining the coaxiality could also be done with seperate single segment position control rather than a composite... as long as there were no other identical controls (position without specifed datums) on the drawing because if there were the simultaneous requirement rule would make all features with identical controls one pattern. Unless of course this was a GM print which unilaterally reverses that simultaneous requirement rule by invoking its USCAR addendum.
Paul
RE: The meaning of FRTZF without datum reference
Your comment on the GM addendum re "uninterrupted" feature is timely in that the next rev'n of the Y14.5 standard looks like it will include what are called "continuous features" or "CFs". Where features are intended to be fabricated and therefore considered as a single feature despite an interruption, then a single control may be applicable to both by invoking "CF" (if I recall correctly at least). That's where I was going with my comment. If the two holes in the example are fabricated in one step and are considered a continuous feature, then a single-segment FCF rather than a composite should do the trick. I've seen this being considered for other corporate addendums also. Also, by extension, "CF" on a size callout would include all interrupted surfaces within a single form limit rather than individually per segment. Without the "CF", you would have to invoke co-cylindiricity by using a surface profile to establish the cylindricity-limiting relationship between the two surfaces.
The final reviews & public comment phase of the Y14.5 revision are winding down, and the next version should be released relatively soon. Some of the "wish list" items for GD&T look like they'll be included ... we'll have to see how the new standard looks & reads, and then everyone gets to decide whether they'll adopt it or not.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
Welcome back again. I now have someone other than Paul for intense discussions in this subject.
When I stated that one does not need GD&T on every feature or characteristic, I didn't say that the drawing should not have the statement "complies with ASME Y14.5M-94". It certainly should - absolutely. Rule #1 is now covered.
I didn't say that datums should not be shown since they, again, should be reflected on each and every drawing.
When I learned the subject in the early, mid 80s, I used the book written by Lowell Foster. I always remember the statement about function and relationship and it did make sense.
Should one have positional tolerances on holes to lighten the product. I don't think so. There is no function and relationship whatsoever. Other holes have fixed fasteners and, of course, positional tolerances are applicable.
Should one have profile of a surface on all surfaces of which most have not function or relationship? I don't think so.
Having a drawing with each and every feature containing a feature control frame (or in notes) does not make sense to me. Check the ASME Y14.5M - 94 standard. Many of the drawings in the standard do not have a feature control frame application to all the features. Function and relationship is important.
Dave D.
www.qmsi.ca
RE: The meaning of FRTZF without datum reference
GD&T was not used, understood, nor evolved to the level that it is today. You may have used a book for the '66 standard or the '82 standard, and Lowell Foster's consideration of GD&T's uses has surely changed/evolved over those years. The technology that is GD&T has changed, and so must our thinking. To accept that the understanding or perceptions of the past are blindly valid in perpetuity is to say that the world is still flat, the gods show favor or displeasure by good/bad weather, and "Yes, Virginia, there is a Santa Claus". Also, the illustrations in the standard are intentionally left incomplete so that there is not an information overload to the user, as it says at the beginning of the standard.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
RE: The meaning of FRTZF without datum reference
The holes that are used for lightening the product have control through co-ordinate or possibly polar tolerancing. They do not need control with positional tolerancing since there is no function and relationship. One simply confirms the centre of the holes on X & Y axis using a CMM. The holes locations would probably not be checked on a continuous basis but certainly at sample submission. Their locations are not important.
Let's say that we place a positional tolerance at MMC on the pattern of holes to lighten the product. We would think that their location must be important so off we go and make a hard gauge. Now we have it in the Control Plan checking the hole locations hourly. What a waste of time and money and the costs go up.
I realize that the company that you now represent promote that all holes should have positional tolerance and all surfaces should be reflected in profiles whether or not they are needed. No wonder the drawings that are overloaded with GD&T are scaring the suppliers.
Function and relationship is important. GD&T is a tremendous value used on such features but not all features.
Dave D.
www.qmsi.ca
RE: The meaning of FRTZF without datum reference
Just using the circular tol zone increases permitted tolerance variation without changing the effective 'spacing' of the holes.
Now I agree calling up MMC may not be appropriate, in fact LMC may be more appropriate if you're concerned about the edges of holes getting too close to each other (or other features).
I'm also concernd by the very idea of trying to interperet 'what's important' based on just on whether GD&T was used or not, without considering function/how tight the tolerance is/how it fits in process tolerance etc.. I believe it's in part this kind of thinking that can lead to increased prices when we GD&T an old part even though we've relaxed tolerances.
KENAT, probably the least qualified checker you'll ever meet...
RE: The meaning of FRTZF without datum reference
RE: The meaning of FRTZF without datum reference
KENAT, probably the least qualified checker you'll ever meet...
RE: The meaning of FRTZF without datum reference
RE: The meaning of FRTZF without datum reference
As part of a significant project which included GD&T implementation and cost reduction as just two facets of the project, I rethought the tolerancing and finish requirements of a very simple piece shaped rather like a heavy-set "L". I grossly opened tolerances and finishes to the point where it should have reduced our costs by at least the targeted 15%. Quelle surprise, vendors quotes ranged from about E750 (Euros) to $350 USD to $85 CDN. The original price was about $225 USD. The European supplier claimed to be experts in GD&T (ASME & ISO), the US supplier claimed ASME expertise as evidenced by working in the aerospace industry, and the Canadian supplier had about 4 hours training but understood enough to adequately study the prints and understand the controls applied (including being able to explain them back to me for confirmation). It was the lack of proper training & understanding that had driven up the costs of the other suppliers. I find that repeatedly in large companies whether on the OEM or supplier side.
As for what features to inspect, that's subject to the tolerances and manufacturing capabilities. Similarly, how you measure something is subject to the tolerances, fequency of inspection, and inspection capabilities (hardware, software & operator). If you occasionally needed to check the clearance holes, you could do an open setup fairly quickly; far quicker than a CMM can be programmed and establish the datums. I don't like bureaucracy, but I do like documentation, and an Inspection Protocol is something that I regularly recommend to clients. It ties in the symbology on the drawing that's used to indicate inspection requirements, the equipment to be used, the sampling methods / rates, etc. Developed by engineering, manufacturing and quality together as a team, you can effectively establish your inspection requirements. That way there's no guessing or assumptions as to what needs to be inspected.
I occasionally get a student who feels that you don't need to control everything back to the datums because they aren't critical features. I lead the class through rectangular and wedge shaped tolerance zones, the impossibility of verifying a single center or single radius of a non-FOS, the typically conflicting conventional & geometric controls, and ask how much time their employer wants them to spend in court defending an inadequately documented design. IF FOR NO OTHER REASON, the avoidance of misinterpretation with resulting litigation should be enough to compel an engineer to completely control their drawings.
I recognize that from an inspection perspective, that seems pointless, but from an engineering perspective it is absolutely essential. From an inspection perspective, you need to know what features on the drawing need to be inspected and with what frequency, but the presence of a geometric control DOES NOT establish a requirement for inspection in & of itself, it only communicates the designer's intent for that feature.
What tends to be forgotten is that GD&T is an ENGINEERING COMMUNICATION TOOL; it tells everyone else involved in the production life of the item exactly what the designer intended in an unambiguous, consistent (subject to the designer's abilities of course) and legally defensible format basesd on a defensible standard. It DOES NOT tell anyone how to manufacture the part, nor how to inspect it.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
I was training in an automotive plant recently and their customer forced them to place in the notes section of their drawings that all surfaces must have a profile of a surface tolerance of a value or some sort. I also saw on the same drawing a feature control frame with a profile tolerance and exactly the same value. It, obviously, was redundant. I asked why they double dimensioned this feature.
I was told that the other features didn't really require the profile tolerance (no design requirement) while the feature shown with the feature control frame actually did. It WAS IMPORTANT to its function and relationship and the design intent of the part.
Should the Designer have reflected all surfaces with a profile tolerance when it wasn't needed or no design intent? It can and is quite legal to show co-ordinate tolerances in addition to GD&T. Not all dimensions must have a feature control frame and there is no where in the present standard that dictates this.
Design intent - absolutely. In my over 20 years of training in the subject, I would never ever suggest that ALL dimensions have a feature control frame. It is a valuable tool to reflect the design intent where applicable.
Don't kill the fly on the wall with a 12 gauge shotgun.
Dave D.
www.qmsi.ca
RE: The meaning of FRTZF without datum reference
Again, someone is making an assumption that a geometric control automatically requires verification on the final article ... please show me where it says that in Y14.5M-1994. Without a CMM, how would you be verifying features that aren't located by a geometric control? Please explain to me how you would find the center of a hole with the old +/- location tolerances and no datum reference frame. Applying the inverse of your logic, if the hole's position is controlled by conventional tolerances, then you'd never inspect it? So, if it is grossly out of position, how would you know?
Individual / job function perspectives determine how we see things. What seems irrelevant to manufacturing and inspection may be of low importance to the designer, but still of importance none the less. The weight reduction holes, for example, may seem irrelevant as to the exact location for manufacturing and inspection because they have somewhat loose tolerances, however a designer may have done FEA and other analyses and established that stress flow is optimized by the configuration being moderately restricted, or stress concentrations may be negatively impacted by a more significant shift of the geometries. An engineer's perspective cannot just include what manufacturing and inspection sees, but also must include consideration of legal implications of an incompletely defined product.
As for the use of chained/ordinate tolerances, they are not precluded in the current (1994) standard, however they are not recommended either. "Section 2.1.1.1 Positional Tolerancing Method Preferably, tolerances on dimensions that locate features of size are specified by the positional tolerancing method described in Section 5. In certain cases, such as locating irregular-shaped features, the profile tolerancing method described in Section 6 may be used.". I also couldn't locate a single example in the standard where conventional tolerancing was illustrated for position dimensions for a feature of size. Regardless, it is a rather shortsighted view to exclude all other relevant standards, particularly those that will dominate the automotive sector very soon...i.e. Y14.41. In ASME Y14.41-2003, Table 8-1, the accepted use of +/- tolerances shall be restricted to:
1) Fillets, Rounds & Chamfers
2) Reliefs, Step Surfaces
3) Countersinks
4) Oblique Surfaces
5) Entry Depth and Spotface
6) Remaining Thickness
7) Notches, Flats, and Pin Heights.
As Y14.5 & Y14.41 are supposed to be companion standards, don't be surprised if the forthcoming revision of Y14.5 reflects the same restrictions of use for conventional tolerancing.
Now, if you look at the Note in Section 8.2 of Y14.41, it indicates that the list is not exhaustive, but rather indicative to similar and other valid applications. While it doesn't explicitly preclude the applicability to position dimensions, such application would be contrary to the indicated application methods of such controls which are restricted to attachment to size callouts, directed leader to the feature (surface), or on an extension line from the feature. Would you have such a tolerance applied to a centerline which isn't even present on the model?
Industry, automotive and aerospace in particular, is migrating slowly but inevitably away from design documentation by drawings, and towards the use of CAD models for all aspects of manufacturing and verification. GD&T users can choose to fight it (& lose) or embrace it and move forward. I'm already seeing a slowly growing trend in small & mid-sized companies using GD&T toward minimally-dimensioned and dimensionless drawings by requiring the use of the CAD models.
You can't stop a swarm of wasps with a pair of chopsticks.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: The meaning of FRTZF without datum reference
All dimensions require verification whether through the customer or through your own company. That is not described in the ASME standard and the standard just does not get into confirmation methods at all. This is usually performed on samples and then manufacturing also perform ongoing verification on characteristics (features) deemed important to the function and relationship of the part.
I would suggest, as per 1.9.1 of the standard, that co-ordinate tolerancing of holes reference that datum reference planes so the datum structure must be present.
So many years ago, I measured parts for a living, both with Portage Layout Equipment (manual CMM) and on the surface plate. The holes in a pattern were reflected in co-ordinate tolerancing. We actually reviewed the machining of the part or its mating surfaces to develop the 3, 2 and 1 point set up on the datums. This, of course, was prior to GD&T.
One would measure the actual hole size and then calculate the dimension to the edge of the hole. From the secondary datum, measure to the side of a hole and reflect the actual calculated dimension. We would also measure from the tertiary datum in the same manner.
I will let you have the last kick at this one if you like.
Dave D.
www.qmsi.ca
RE: The meaning of FRTZF without datum reference
In fact, Section 1.9 deals with dimensioning of the Location of Features, not Tolerancing of the Location of Features. You seem to be reading more into that title than is indicated in the text. Indeed, coordinate dimensioning whether chain, ordinate, or polar, are all perfectly acceptable and are indicated in the standard.
Chapter 2 deals with Tolerancing and Related Principles, and Section 2.1.1.1 again deals with Positional Tolerancing Method. In that section, it stipulates two ways to position an enclosed boundary whether a feature of size or an irregular shape; position, and profile of a surface. At no time does it mention or allude to controlling the locations of such enclosed boundaries by means of conventional tolerances.
One of the basic tenets of GD&T is that you are controlling features rather than dimensions. You cannot verify a basic dimension. You can verify the controls applied to the feature whose theoretically perfect location is established by basic dimensions which are tied back to the datum features whether directly or though chaining.
For more fun, I started another thread on this topic ... full vs partial applicatiion of GD&T. See ya there, Dave!
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com