Flatness V/S Taper

[Madhu454]

Hi All,

I have a question on effect of Rule#1 on Flatness.

I have understood that, when there is no form control is specified, the size limits controls the form.

Please refer to the attached file.

Refer to figure-1 of the attached file. as per Rule#1, the flatness allowed on the top surface is 1.0

Assume a case, as shown in figure 2, the top surface is having only taper (no zig zag, perfect taper) within the specified limits (One side 10.0 and other side 11.0)

Question is
1) When I try to measure flatness, by keeping the top surface on the surface plate. Even though there is a taper on the surface. The entire surface lies on the surface plate.

In this case I will get flatness error reading as Zero.

Please correct me if I am wrong in understanding this concept.

 

[pmarc]

Madhu454,

There is nothing wrong with this inspection method and the flatness error has to be zero because you assumed surface to be perfectly flat even though there is a taper defined by limits of size.

 

[drstrole]

The part in figure 1 does not have a flatness tolerance specified, so you do not have to measure it. As long as you are inspecting the limits of size, that is the only requirement that needs verified.

Drstrole

 

[dingy2]

I agree with drstole. No flatness is called out so one only has to measure the size.

Rule #1 is not understood well in the field and is rarely checked. If an Inspector saw your sketch, they would only check size and not surface condition complying with rule #1.

Dave D.

http://www.qmsi.ca

 

[pmarc]

I believe the intent of OP's question was to understand the concept of indirect surface flatness control by size tolerance. The example with inspection was only to visualize his concern.

Dave,

Rule #1 is not understood well in the field and is rarely checked. If an Inspector saw your sketch, they would only check size and not surface condition complying with rule #1.

Although I have similar impression about inspection practices on rule #1, your statement somehow implies that correct applicability of GD&T according to Y14.5 really struggles in industry. Many tolerancing concepts are based on rule#1, actual mating envelope, etc - so the concepts not directly visible on a drawing. To give one example: bonus tolerance for position callout should be calculated basing on a size of related actual mating envelope of a FOS. If this envelope is not captured properly by inspection, bonus and in consequence position tolerance may not be verified according to the standard.

 

[ptruitt]

Perhaps using 'profile of a surface', composite or not, would get beyond the Rule #1 confusion.

Peter Truitt
Minnesota

 

[dingy2]

pmarc:

When I asked a CMM Operator about checking rule #1, he said that if it isn't shown on the drawing, he doesn't check it. Positional tolerances at MMC are on the drawing and so they are confirmed. One must remember that Quality places a number by each requirement on the drawing including feature control frames. Rule #1just isn't there.

If a Designer is feels that rule #1 covers a requirement, it just isn't going to happen on the shop floor. I even developed a web page on such a subject at

http://www.qmsi.ca/Rule#1.html

GD&T is complex enough and I wish that ASME took the ISO approach and deleted Rule #1 then the Designer would have to place the geometric requirement in a feature control frame.

Dave D.

http://www.qmsi.ca

 

[pmarc]

Dave... I understand your standpoint to some extent.

For me however it is not an explanation that if rule#1 is not visible directly on a drawing, an inspector is free to leave it unverified. If OP's drawing of a shaft is governed by Y14.5, any inspector is obliged to verify two separate requirements: two-point measurement and the envelope (Rule #1) priniciple. If he does not do it and there is something wrong with the part later on, designer is clear and all the burden goes on the inspector. I believe no court in the world is going to accept inspector's explanation that because it was not shown on the drawing, he did not have to check it.

It is a matter of willingness to be familiar with Y14.5's fundamental rules.

 

[fsincox]

Dave,
I agree.
pmarc,
I understand that you are on solid ground by "the book". If the law is ignored, as is being done where I work too, I question what have we really have gained. Automatically implied was safe for engineering but since it is being ignored in practice what good does it do me? So you will win the few legal battles but every day we lose because we build our house of cards on a shakey foundation.
I do believe that this is why the ISO chose to go the other way.
Frank

 

[powerhound]

I'm with pmarc on this one. If Y14.5 is specified on a drawing then rule #1 is shown and the part must comply with it...inspection or not. Just the act of specifying the standard is the same as saying you want the part to conform to rule #1, rule #2, all the fundamental rules, separate requirements, threaded holes located by a TOP are checked to the pitch diameter NOT the minor diameter, and so on.

That being said, I'm not trying to insist that an inspector check every single aspect of every single part. When using a CMM to check profile, there are an infinite number of points that can be taken in order to ensure part compliance but that just wouldn't be feasible so the inspector takes as many points as he or she feels comfortable with. The same goes for the general and fundamental rules. If the inspector is comfortable leaving certain things unverified, fine, an inspector probably doesn't have to do anything he or she doesn't want to do, but that doesn't mean it's okay to ignore the requirement. It definitely doesn't mean that the requirement is not there.


Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[DeanD3W]

Since the reason for Rule #1 is to enable size data to verity that two mating features of size will fit with each other, then it sure does seem like a fundamental thing that some of industry are either not aware of or are not giving enough importance to.

Checking only local sizes on a given feature does not enable a determination of whether it will fit with its mating feature... Only verification that the perfect form MMC boundary has not been violated can do that.

Measurement is never absolute verification and at times the risk of a fit problem may be low, but to always ignore a requirement that is on a drawing is simply wrong. The requirement for a size tolerance on a drawing with reference to Y14.5 is that the measured mating size and the worst local size both be within the specified size range. If local size or best-fit size data is all that is gathered, then that may not be good enough, so I hope at least an informal risk assessment with someone with the needed knowledge is going into a decision to not address part of the size tolerance requirement. Otherwise, this is like driving fast through the woods at night with your headlights off. You might make it OK, but you also might hit a tree (a fit problem).

Dean

http://www.d3w-engineering.com

 

[dingy2]

Gentlemen:

I wonder how many companies represented here divide the drawing information into 2 segments - features of size and features of location in addition to the feature control frames?

On features of size, there is a number for the dimension but, theoretically, there must also be a number for the form size. Is it happening? NO!

Designers can still rely on the fact that they place "complies with ASME Y14.5M-94 (or 2009) to cover form and they are theoretically correct. When things go down the toilet, they can always say that it is not my fault if Quality doesn't know GD&T. I have worked as a Quality Manager many years ago when this was the approach. Each department covering their butts but does it help the company? I don't think so.

If there is a design need to cover the form on a feature of size, please place it in a feature control frame. Could this be called redundant? Yes but it WILL be confirmed and it could helps your company in the long run.



Dave D.

http://www.qmsi.ca

 

[powerhound]

Dave,
I don't think placing redundant requirements on a drawing is the way to go. Referring to a recent tip by Don Day, should a 90 degree angle be specified everywhere there is a 90 degree angle shown? Why not just re-list all the fundamental rules in the notes section along with the general rules?

I understand your beef with checking flatness when flatness has not been explicitly specified, but rule #1 is in the standard and thus the part must comply with it.

Why is it your opinion that a designer relying on the default rules of the standard is "covering their butts"? Where I work, drawing specifications are a matter of function, not self-defense. But at the same time if a designer specifies something--whether explicit or implied--that QC doesn't check, and the part fails because of it, it would not unreasonable to look to QC for some answers as to why something wasn't checked.

Putting your scenario in terms of "covering their butts" is a cynical viewpoint but it's probably accurate. Isn't that what documentation is all about though? If a designer can just tell a machinist what he/she wants and the machinist makes it, and then it goes to the customer and it works, and this happens every time, there would be no need for drawings, engineers, or inspectors...just machinists...haha.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II