Controlling coaxial features
Controlling coaxial features
(OP)
I stumbled across this dusty thread: thread1103-200480: DIAMETERS SHOWN IN LINE NOT NECESSARILY COAXIAL
I found it quite educational. Basically I've been assuming implied coaxial features on the same center is acceptable.
I'll be correcting my practices regarding this in the future.
My question is: What is an acceptable practical way to govern the coaxiality of multiple cylindrical features (on a turned part for example) without using GD&T?
A general title block or drawing note?
Adding a datum and a FCF to something like a round knob seems to be overkill. I understand that doing that with a appropriately loose tolerance shouldn't be a problem.
The reality with a majority of our vendors may cause a cost increase when they see that.
I found it quite educational. Basically I've been assuming implied coaxial features on the same center is acceptable.
I'll be correcting my practices regarding this in the future.
My question is: What is an acceptable practical way to govern the coaxiality of multiple cylindrical features (on a turned part for example) without using GD&T?
A general title block or drawing note?
Adding a datum and a FCF to something like a round knob seems to be overkill. I understand that doing that with a appropriately loose tolerance shouldn't be a problem.
The reality with a majority of our vendors may cause a cost increase when they see that.





RE: Controlling coaxial features
Believe it if you need it or leave it if you dare. - Robert Hunter
RE: Controlling coaxial features
Apply a sloppy tolerance.
Sometimes, I apply an FCF because the part looks precise, and I want to communicate to the fabricator that I do not care. Do not take the time and effort...
JHG
RE: Controlling coaxial features
As FOOTNOTE and for thought, I recently ran across a ASME instructor who had it 'right'. "One of the reasons for using Geometric Dimensioning is that it makes the drawing more clear and AMBIGUOUS."
Think on that for a while.
RE: Controlling coaxial features
KENAT, probably the least qualified checker you'll ever meet...
RE: Controlling coaxial features
A couple of comments and questions about your last post.
The note "Diameters indicated to be concentric within .00X TIR" probably covers the coaxiality requirement, but as a former QA person I'm compelled to point out the inspection issues.
The TIR suggests something similar to a runout tolerance, but some important details are left out. For one, what are the diameters to be concentric to? Each other? Some other datum axis of the inspector's choosing? Also, there is no indication whether the TIR applies to the entire feature (like total runout) or to individual cross sections (like circular runout).
Regarding the ASME instructor's comment, did he/she really say that GD&T makes a drawing more ambiguous? If so, I would have to disagree. ASME Y14.5 has its share of ambiguities and gray areas, but they're nothing compared to plus/minus tolerancing. Did the instructor give any reasons for that opinion?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Controlling coaxial features
T.I.R. - Total Indicator Runout. Yes?
As to what is the datum, Could you pick a convenient round feature for a datum, make sure the other coaxial features meet the requirement and satisfy the callout?
After seeing the light from this thread. I've added a general note addressing this to our format. Do wee need to do more?
RE: Controlling coaxial features
ASME Y14.5M-1994 2.7.3(c)says:
cwdaniel, have you got 14.5 and if so have you read 2.7.3?
KENAT, probably the least qualified checker you'll ever meet...
RE: Controlling coaxial features
Send them a draft of each and see what happens. If they raise the price, start asking questions. Maybe teach them a thing or three.
Obviously, you need runout control. If the only thing driving the price change is the fact that the spec is GDT and not some convoluted nearly-adequate scheme, you need to have a conversation with your vendors. You also need a conversation with new vendors.
This is the price one pays for low grade vendors.
RE: Controlling coaxial features
Fine, this works for me too:
Still, how does one verify perfect coaxiality without a datum? Same question applied to the T.I.R.
RE: Controlling coaxial features
KENAT, probably the least qualified checker you'll ever meet...