Coaxiality and concentricity 5

[kakalee1]

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?

 

[TheTick]

You probably want position for this.

 

[powerhound]

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

 

[pmarc]

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?

 

[pmarc]

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.

 

[powerhound]

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

 

[pmarc]

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.

 

[powerhound]

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

 

[pmarc]

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.

 

[3DDave]

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.

 

[3DDave]

How the S-cam operates, for those not into over-the-road trucks.

http://www.youtube.com/watch?feature=player_detailpage&v=3mrUMTP4thI#t=234

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.

 

[CheckerHater]

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).

 

[CheckerHater]

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.

 

[powerhound]

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

 

[pmarc]

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.