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Coaxiality and concentricity
5

Coaxiality and concentricity

Coaxiality and concentricity

(OP)
After reading a lot about coaxiality and concentricity, I am still having trouble understanding them, and when to use one or the other.

From what I understand, concentricity seems to include coaxiality, and it is really difficult to measure. However, I still don't see what possible outcome that can happen to a concentricity callout that cannot happen to a coaxiality callout. If anyone can give me a specific example, that would be greatly appreciated.

Also, when would use concentricity rather than coaxiality?

RE: Coaxiality and concentricity

Concentricity is a specific GD&T symbol. Coaxiality is not a symbol, nor is it a very specific term; it is a general word used to describe the idea of two or more circles that share a common center.

The problem with that general word is that it leaves open the question of HOW to find the center of a given circle. Concentricity has very specific language that describes how to find the center. That's why you've read that concentricity includes coaxiality, and it can be difficult to measure (at least how it's defined in ASME Y14.5). On the other hand, there is no such thing as a "coaxiality callout."

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

RE: Coaxiality and concentricity

(OP)
Belanger,
Thanks for your input.
How exactly would you measure concentricity and coaxility? Assuming that you have a CMM machine.
Let's use the example I attached. Units are in mm. What would be the possible outcome of concentrity that can not happen with coaxiality? how would you measure them?

RE: Coaxiality and concentricity

Concentrically is measured by median points of diametrically opposed elements. The cylindrical tolerance zone would be located by the datum axis. You would need two opposed indicators to measure the median points. As the part is rotated about the simulated axis, the indicators would move back and forth (dueling indicators). CMM's have difficulty with this measurement as the two measurements have to be diametrically opposed.

Coaxial is an implied relationship of a FOS datum and a FOS. The center axis of the datum and the FOS is created with a CMM by a best fit (lease squares) from all of the probe touches.

For this application, I would recommend profile of the bore to datum A. An optical comparator could do this very quickly.

John

RE: Coaxiality and concentricity

Kakalee1 -- In the picture you supply, there is no "coaxiality" callout. There is "concentricity" on the left-hand drawing, and then "position" on the right-hand drawing. BOTH are controlling coaxiality, which is a general verbal term for any centering effect. That was my point earlier.

Now that we know it's position being compared to concentricity, we can assess the difference. See the graphic attached below...
If we are given the concentricity symbol, then we'd have to look at the red part of my picture: the red cross-hairs would have to be within the given tolerance zone around the datum.
But if we are given the position symbol, then we would look at the black cross-hairs, which are taken from a perfect circle. We are still measuring that red part, but based on its "actual mating envelope." Thus, the part will probably fail concentricity yet pass position. (John's explanation is the same idea.)

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

RE: Coaxiality and concentricity

Concentricity is a terrible control to use, not because the idea is so bad, but because verification is.

Moving along the original axis, one measures from the axis to the controlled surface in opposite directions. Take the difference in the two measurements, multiply by 2, then compare that number to the concentricity limit. It is a way to control the balance of volume around an axis. It is no substitute for actual balance controls.

In this regard concentricity is like planar symmetry, and is OK when confined to revolved features.

The troublesome part is as described in the '1994 version, which includes "correspondingly-located elements of two or more radially-disposed features" which is taken to apply to non-revolved features, such as hexagons. Can it apply to gear teeth, tri-lobed features, or fan blades? Who knows as the '1994 committee included no such explanation for why it was included.


Other 'co-axial' controls don't attempt to measure the distance between axes as their primary goal. All it means in terms of position tolerancing is that the distance from the datum axis and feature axis or between mutually defined feature axes is nominally zero. The '1994 committee left/made the description confusing without adding value. I don't recall or care to look to see if it was the same in the '1982 version, but coaxial position might have been there then.

RE: Coaxiality and concentricity

Dave said that concentricity is a terrible control to use and then gave a good reason why it stinks so bad. My opinion is that it is a terrible control to use because it doesn't do anything that runout or position doesn't do that really means anything. You get no bonus tolerance where it might be allowed if you were to use position and you get no datum shift where it might be allowed if you were to use position. Circular runout or total runout is way better than concentricity. The only argument I've ever heard for concentricity is for dynamic balance. I've actually never heard a good argument for symmetry. If dynamic balance is that big of an issue, it needs to be handled by a balancing process. Please, put the entire idea of concentricity out of your mind forever and forget it ever existed.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Coaxiality and concentricity

95% of the applications I run into can be handled by position, profile, or flatness. Since I don't work with rotating equipment, run out is not something I have to deal with. I always control coaxiality between cylindrical features using positional tolerance control. For reasons described in this thread, I have NEVER used cylindricity or concentricity.

Tunalover

RE: Coaxiality and concentricity

I think I posted this link before. Either way - enjoy.

This one may be useful as well.

RE: Coaxiality and concentricity

In order to have full picture of the situation, I think it would be good to ask one more question:
Which GD&T standard are you, kakalee1, using - ISO or ASME?

All the answers given so far assumed that this is ASME. If, however, by any chance ISO in charge, the answer will be different, that is, there is no difference between concentricity and position in ISO GD&T (GPS) world.

RE: Coaxiality and concentricity

Good point, especially considering that the term "coaxiality" actually does exist in GPS

RE: Coaxiality and concentricity

Exactly, pmarc beat me to it! ISO or ASME?
Frank

RE: Coaxiality and concentricity

The dimensioning style in his example suggests ISO.
Frank

RE: Coaxiality and concentricity

And opinion "From what I understand, concentricity ... is really difficult to measure." suggests ASME smile

RE: Coaxiality and concentricity

That's true -- things change if we are talking about ISO.
So the terminology of the OP might indeed be correct, but the difference illustrated in my graphic still applies.

I don't agree 100% with those who say that runout is better because it encompasses everything done by concentricity. Yes, but did you know that it actually does more than concentricity, thus it might be more expensive and difficult to hold?
Certainly runout is cheaper to inspect, but actually it's more expensive/confining to mfg.

Of course, the bottom line is that all this should be driven by the functional requirements.

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

RE: Coaxiality and concentricity

Frequently when I put concentricity or position on a drawing QA measures runout anyway! Doh!

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Coaxiality and concentricity

JP, thank you.
dgallup,
If it passes they are fine.
Frank

RE: Coaxiality and concentricity

The reasons I think runout is a better control is because:
1) Runout controls form, where concentricity doesn't.
2) Runout is what they want anyway.
3) Runout is what will be checked anyway.

The arguments to these will be:
1) What if form control isn't desired? Answer: The location of median points of diametrically opposed elements are not really desired either so may as well go with the easier check.
2) How do you know they really want runout and not concentricity? Answer: Neither in my experience, nor the experience of anyone else I've ever asked this question of (who really knew the difference), has concentricity ever been the real goal. It was always really runout or position.

Actually, this question is open to all of you too. Have you ever, in your experience, seen an instance where concentricity--as defined by ASME--was really what was needed and runout/position or even a balancing process was not a better option? Please leave out the "No, but that doesn't mean someone else hasn't." I'm aware of that and am still looking for that individual.

3)How do you knw they will check runout and not true concentricity? I refer you to dgallup's post. It has been my experience as well. We actually had a device in the QC department where I used to work called a "concentricity checker". It actually checked runout but no one knew the difference.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Coaxiality and concentricity

(OP)
To all,

Thanks for all your input especially Belanger and Powerhound. They are really helpful.

I don't really have any background as a QA inspector or drafting, so all of these info are a bit too much to digest at once. I'll do more research to understand concentricity completely.

As for standard, my company follow the ASME y14.5m-1994.

One main reason why I created this post is because I don't know if our QA department inspect concentricity correctly. I can't really say that they are wrong or right when I don't fully understand it myself, and I have a feeling that no one from my company do.

Reading all the feedbacks here, it seems that my customer's concentricity callout was really unnecessary, and I believe all what they really need is a position call out.

I'll comeback to this post later when I have more question.

RE: Coaxiality and concentricity

To powerhound:
How will runout work if a part's feature to which concentricity callout is applied is not nominally cylindrical (see attachment)?
http://files.engineering.com/getfile.aspx?folder=4...

To kakalee1:
First thing you need to understand the concept of concentricity (and especially to grasp the difference between concentricity and position [coaxiality] at RFS) is to analyze figures 5-55 through 5-58 and associated text in Y14.5M-1994.

RE: Coaxiality and concentricity

You probably want position for this.

RE: Coaxiality and concentricity

pmarc,
I will entertain your scenarios if they are something you would actually do. If you would choose concentricity over position for the figure on the left or if you would choose it over profile on the figure on the right then we can hash it out. If you are just playing stump the chump then I'm not interested.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Coaxiality and concentricity

Neither profile nor position control location of so called median points. What if for some different reasons I am not able to do the additional balancing operation? Furthermore in both cases I am not interested in controlling size of the features too restrictively - they can fluctuate in size relatively freely, but when rotated about datum axis they shouldn't introduce too much of unbalance. Will any kind of position (for the square) or any kind of profile (for the ellipse) do that for me?

RE: Coaxiality and concentricity

powerhound,
May I at least have a hope for simple: "No, your scenarios are not interesting"?

In the other thread about profile you look disappointed that someone did not answer your question, yet you seem to be doing the very same thing here.

RE: Coaxiality and concentricity

I'm actually on vacation in CA and have no access to my CAD station. I'm typing this out on my cell phone. What I was going to show was single segment profile on both drawings with both datum references to the datum axis and a large value in the upper segment to allow greater size variation and a tighter tolerance in the lower to control form and location of surface elements. I wasn't avoiding the question. I was just waiting until I got home so I could make a drawing because I wasn't sure if a verbal explanation would suffice. A few days delay is not the same as 3 weeks or so.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Coaxiality and concentricity

Okay, thank you for the explanation, powerhound. I hope you are having great time in CA.

One comment to your idea that immediately comes to my mind:
What makes you think that by specifying a profile FCF containing two segments you will separate size of the feature from its form? I am not sure I exactly understand your reply, but regardless of whether you are planning to use composite FCF or two single-segment FCFs, don't you think that form and size of the feature will be inherently tied to each other and controlled by the lower segment/second FCF of the callout?

I have an idea how to grasp my requirement in case of the square, but honestly speaking I see no solution for the ellipse.

RE: Coaxiality and concentricity

Well, as I understand single segment, the lower frame basically locates the true profile to the datum axis. My position is that the center of the oval is tightly constrained to the datum axis but the tight tolerance zone can still expand and contract within the looser tolerance zone in the upper frame. I don't guess there's a specific paragraph in the standard that addresses this exact situation but I think the logic is sound.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Coaxiality and concentricity

When the only difference between two single-segment profile callouts is the tolerance value, the second FCF simply overrides the requirement from the first FCF.

It is the same logic as in case of two single-segment position FCFs applied to pattern of features - you never repeat exactly the same datums in the second FCF. As soon as you do that, the first FCF automatically becomes overriden.

RE: Coaxiality and concentricity

2
One practical application for concentricity is on the S-cam that actuates air brakes.

It is less important what the exact shape is than that each brake pad be moved the same amount for a given rotation. Not a lot less inportant, but less. There is no need for dynamic balance, just that each of the surfaces be rotationally symmetric with respect to each other.

RE: Coaxiality and concentricity

How the S-cam operates, for those not into over-the-road trucks.
http://www.youtube.com/watch?feature=player_detail...

A project I was on had nit-pickers that noticed the left-hand automatic slack adjusters were all in a similar release position and the right-hand automatic slack adjusters were also in a similar release position, but the left-hand release position and right-hand release position weren't the same. Since the adjusters, the brake drums, and brake shoes and linings aren't handed, the only unique item was the S-cams which come in left-hand and right-hand versions.

It was the difference in cam pitch between the forgings. They are low-rate cams, large rotation for small rise, so any difference is highly magnified. As long as there is enough stroke on the brake canister to take up each click on the auto-adjuster, it's all good.

The nitpickers were complaining about "having to adjust" the automatic adjusters to make right and left release positions identical. Of course the auto-adjusters would certainly fix themselves. Always nice to have non-functional requirements added on an ad-hoc basis. Not being identical one side will no doubt suffer some failure before the other; whether that could happen before the ordinary failures of brake pad wear-out or diaphragm puncture or heat-death of the universe, who knows.

RE: Coaxiality and concentricity

Star to 3DDave.
First time in my life I see practical application for ASME concentricity (although I knew what S-cam was for long time).

RE: Coaxiality and concentricity

To think of it, S-cam example should be included in textbooks. because it looks more convincing than "balancing"

Another crazy idea would be to suggest to replace current "concentricity" symbol with "central/axial symmetry" symbol looking like S-cam.

RE: Coaxiality and concentricity

So the Grand Canyon is something not to be missed in a lifetime. Pictures just don't do it justice. I live in Texas so we stopped by Arizona on our way home.

Anyway, I guess you and 3DDave have convinced me that there is a use for concentricity after all. After considering your comments, I see that you are correct and I just wasn't thinking about a static tolerance zone. Somehow I had convinced myself that lower segment tolerance zone could expand and contract within the zone in the upper segment. I see that I was wrong about that. I do, however, think that that concept is useful but I can't think of how to do it.
In the course of all the thinking about this that I have done, I came up with a question. How would one actually dimension the oval shape? Would it just be radii and a concentricity callout with the tolerance of the radii either directly toleranced or through a general tolerance?

Should we go down this road with symmetry?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Coaxiality and concentricity

powerhound,
Personally I would keep directly toleranced dimensions as far away from the oval/elliptical shape as possible. If one is interested in controlling its size and form within the same tolerance, profile callout applied to the true (basic) contour should be the first choice. If one is interested in separating its size from form... then in my opinion current state of the art does not offer solely graphical/symbolic tool(s) to accomplish this.

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