Implied Centerline 2

[bryjarmoc]

(see attached figure)
The only requirement on the print for the keyway is the width and length (from opposite side of ID to flat of keyway).
here are the questions:
Is this keyway "implied to be on center"?
If so, what would be the tolerance?
If engineering does NOT need the keyway to be on the centerline, how would this be shown on the print?

 

[Belanger]

It is assumed to be centered because the centerline of the keyway is common with the centerline of the hole in the middle of the part.

There is no implied tolerance for that, however, and this is where position (GD&T) would be used.

If the keyway were to be designed to be off-center, a dimension would have to be shown indicated the offset between the keyway's centerline and the hole's centerline. (Again, position should probably be used, so that dimension would be basic.)

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

 

[bryjarmoc]

to clarify:
In this case, the location of the keyway does not matter. It can been at any location in relationship to the 8 holes or "notch".
So how would engineering show this?

 

[Belanger]

Probably a local note stating that the clocking of the keyway is not important. (Despite all the benefits of GD&T, sometimes an old-fashioned note is the way to go!)

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

 

[caseynick]

Around here it would be done with a note.
"orientation optional"

 

[MechNorth]

Position callout would still be ok, just don't reference the anti-rotation (clocking) datum, and make sure it's not part of a pattern established accidentally by simultaneous requirement. Additional notes aren't needed.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Good call, Jim. That would still maintain the centering effect over the hole, and avoid the note.

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

 

[KENAT]

It's not clear from the partial image but is the 'notch' on the edge of the part dimensioned from the key, or just from the top hole?

My initial response was the same as MechNorth but as the sketch isn't completely dimensioned it's difficult to say for sure.

The concept of 'implied centerline' has been discussed a few times, if you don't get a full answer in this thread you might try finding one of the old threads.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MechNorth]

I was trying to keep it short. Honest!

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringman1]

If you are using GD and T, Y14.5, and position the keyway with respect to only a primary and secondary datum feature, no specific orientation would be required, as I see it

 

[MechNorth]

That's what we're saying, yes.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DustyRhodes]

I have a block whose width is datum A and I place a pad on top of the block aligned with the block but not centered on it. There is a slot through the pad. If I want to position the slot true position to datum A, what would be the correct notation to specify this. Thanks for your help and guidance.

Al

 

[Belanger]

The tolerance would be shown in a feature control frame using the position symbol followed by the tolerance number (with any desired modifiers) and then reference datum A at the end of the feature control frame (again, with modifiers if needed).
The intended location of the slot relative to datum A would be by way of the centerlines of each. If the slot is to be centered on the block, then the location can be implied as zero by showing a common centerline through both. If there is an intended offset, then a basic dimension is given between the two centerlines.
Remember, even though you said that the width is datum A, it's really the center plane of that width that is the true datum.

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

 

[MechNorth]

continuing JP's post ... and the width is the datum feature of size.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringman1]

Dusty,

Could you post a sketch?. If I read it correctly you have an assembly. Not totally clear as to what you have.

 

[DustyRhodes]

Thanks for all the inputs. I have attached a little sketch. I believe I understand Belanger's reply and it make sense to me. That is how I had concluded it should be. The basic 1.0 dimension is coming off the implied (and shown) center plane of datum A and is indicating the drawn centerline of the slot feature. The slots dimension is true positioned to the datum. If I am misunderstanding please comment. Thanks for your clear and concise information.

Regards

 

[Belanger]

Yeah, I think you're on the right track. Someone's gonna say it, so I'll point out that the datum feature symbol shouldn't have the dashes flanking the letter A. Just a square box with the A in the middle.

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

 

[MechNorth]

... and the 0.5 width dimension shouldn't have the preceding 0, and the 1.0 basic shouldn't have the trailing zero.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DustyRhodes]

You've all been very helpful. Thanks very much for your help and advice.