Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

MMB / LMB sizes

Status
Not open for further replies.
greenimi

greenimi

Mechanical
Nov 30, 2011
2,259
IF (again IF)
datum features B and C are to be modified at MMB or LMB
and
Note 3 is changed to "DRAFT ADDS MATERIAL"

what would be the appropriate sizes of the applicable boundaries? (sizes of datum feature simulators)
(from ASME Y14.8-2009)

MMB(B)=? MMB(C)=?
LMB(B)=? LMB(C)=?

QFET_xp6w7j.jpg
 
Replies continue below

Recommended for you

Sort by date Sort by votes
No one would like to take up this challange? I don't have a correct answer, just curious about opinions.

 
Upvote 0 Downvote
We already have note 2, which seems to answer the question.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
Upvote 0 Downvote
So which are the answers?
MMB for B is how much?
 
Upvote 0 Downvote
greenimi, the answer is that had note no. 3 not been specified, these datum features would not be appropriate to be applied at MMB over the inability to calculate the boundary due to "lack of control".

See the caution note in the end of para. 3.6.1 in ASME Y14.8-2008:

The standard said:
CAUTION: The symbols for “+DFT” and “–DFT” or “DRAFT ADDS MATERIAL” or “DRAFT REDUCES MATERIAL” should not be used for critical features due to the limitations of
(a) measurements being only near, rather than at, a given end of the feature
(b) the lack of control over the rest of the feature geometry
For critical features, a geometric control such as a profi le of a surface tolerance should be used.
Click to expand...
 
Upvote 0 Downvote
What I meant in my reply is that note 2 tells us that any draft is totally ignored in your GD&T calculations, so the MMB (for B) would be 8.51.
If you want to include the draft idea into the MMB/LMB calculations -- which might be a worthy academic discussion -- you'd have to change the notes, but that still leads to the problems mentioned by Burunduk.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
Upvote 0 Downvote
Just for my understanding. I have done molds. I have never seen notes before indicating "Draft adds/removes material".
Is this standard for the mold vendor? If it's in a ASME standard, I will look it up.
I have used the +DFT and -DFT, but as ref.
Note 5 is not needed.

Chris, CSWP
SolidWorks '20
ctophers home
SolidWorks Legion
 
Upvote 0 Downvote
Chris,
See Y14.8-2009 ¶3.6.1 Methods for Specifying Dimensions Affected by Draft
(latest edition I have access to)

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
 
Upvote 0 Downvote
ctopher said:
Is this standard for the mold vendor? If it's in a ASME standard, I will look it up.
I have used the +DFT and -DFT, but as ref.
Note 5 is not needed.
Click to expand...

If you want to use -DFT or +DFT I am fine with that and my inquiry and questions on how to calculate MMB's, LMB's stay the same.

Belanger said:
What I meant in my reply is that note 2 tells us that any draft is totally ignored in your GD&T calculations, so the MMB (for B) would be 8.51.
Click to expand...

J-P,

If this is the case then the datum feature simulator values (for those internal holes) will be bigger than the values allowed for the size+draft specifications therefore, the part might not fit into the applicable gages....Am I right?
So, how we can circumvent this issue?

 
Upvote 0 Downvote
Greenimi, yes it seems that the simulator may be larger than the physical boundary of the hole (since material is added).
Two things I would say... first, the Y14.8 standard does recognize that draft angle and tolerance "may exceed the perfect form boundary at MMC" unless otherwise specified (para. 3.6). That's not a direct answer to your issue, but they acknowledge that draft throws a wrench into the ideal mathematics.
More to your question, though, the caution that Burunduk already quoted seems to be the solution: "For critical features, a geometric control such as a profile of a surface tolerance should be used."


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
Upvote 0 Downvote
(2009) 4.11.6.3 - Clarifying Applicable MMB - In cases where the boundary is not clear, or another boundary is desired, the value of the boundary shall be stated, enclosed in brackets, following the applicable datum feature reference and any modifier in the feature control frame.

2018 covers this in section 7.11.10 - Explicit Specifications of True Geometric Counterpart Boundaries.

If this was my design I would make my intended boundary clear my placing the value in brackets.
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

Burunduk
  • Locked
  • Question
Secondary datum MMB, tertiary RMB
Replies
4
Views
78
Burunduk
Burunduk
sendithard
  • Locked
  • Question
Can you apply MMB with orientation tolerances?
Replies
5
Views
87
Belanger
Belanger
sendithard
  • Locked
  • Question
LMB datum shift on an exterior diameter 1
2
Replies
22
Views
290
Burunduk
Burunduk
Burunduk
  • Locked
  • Question
MMB datum feature simulators used in a restrained condition?
Replies
14
Views
120
3DDave
3DDave
rollingcloud
  • Locked
  • Question
Is datum A-B still practical when the corresponding bearing surfaces have different sizes? 2
Replies
5
Views
74
rollingcloud
rollingcloud

Part and Inventory Search

Sponsor

Back
Top Bottom