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?
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
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
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
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
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
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
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
Tunalover
RE: Coaxiality and concentricity
This one may be useful as well.
RE: Coaxiality and concentricity
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
RE: Coaxiality and concentricity
Frank
RE: Coaxiality and concentricity
Frank
RE: Coaxiality and concentricity
RE: Coaxiality and concentricity
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
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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
dgallup,
If it passes they are fine.
Frank
RE: Coaxiality and concentricity
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
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
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
RE: Coaxiality and concentricity
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
RE: Coaxiality and concentricity
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
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
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
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
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
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
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
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
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
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
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.