Position tolerancing on curved (circular) surface

[amracson]

How can I use position tolerance for the 4.76 hole in the attached drawing? I do not know where to place the datums. I want the 4.76 hole to be on the center using the given tolerance. Thank you.

 

[KENAT]

Please don't double post, redflag your other post for management to delete. My original response from over there.

amracsun I suggest re-posting over in forum1103: Drafting Standards, GD&T & Tolerance Analysis giving more detail on your function.

Based on your dimension scheme I'd maybe make the top surface your primary datum and one of the diameters your secondary. However, depending on function this may change. The 'center line' cannot be your datum feature. The axis derived from a cylinder dia can be.

What tolerance, you mean the +-0.25? You should probably convert this to equivalent dia, to use with position tol, again depending on function.

What drawing standard are you using?

From memory per ASME Y14.5 you shouldn't dimension to hidden lines.

Do not dual dimension, if you must put a dimension twice make the extra one reference by putting in parenthesis.

I don't believe you need the 15.00 dimension or is this just there for you to give the +- tol?

Millimeters is mm not MM.

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[SeasonLee]

1. Make Ø30 as primary datum
2. Make top surface as secondary datum( or vice versa)
3. Change both dim 15.00±0.25 and 6.00±0.25 to basic dim 15 and 6 respectively
4. Add POS|Ø0.707 circle M|A|B to the hole ( 0.5(±0.25) X 1.4142 = POS tolerance)

Be sure to specify the general tolerance on the print.

SeasonLee

 

[pmarc]

amracson,

1. Agree with Kenat that datum scheme should be based on function of the part - the qestion whether top surface should be primary and hole diameter secondary or vice-versa can be answered only if the function of component is known. Maybe completely different datum scheme must be considered?

2. No question that the location of 4.78 hole should be controlled by position tolerance, so dim. 6.00 +/-0.25 should be basic, and there is no need to put dim. 15.00 +/-0.25 at all.

3. Dimension dia. 30.00 for the biggest cylindrical feature should be placed only once.

4. You should remember that you must also somehow assure relationship between counterbored hole in the center and dia. 30.00 feature. Showing them coaxial on the drawing does not mean they will be coaxial in reality. A tolerance (probably positional) have to be defined. Which of these two features will be a datum feature for measuring this relationship also strictly depends on the function of whole component.

5. Nevertheless I think we must know more details about functioning of this part, otherwise all of our considerations will be only a kind of assumption. Maybe any counterpart could be shown? Or more detailed description delivered?

 

[fsincox]

Are there tolerances on the other dimensions implied in some way that is not shown? Ditto on the functional comments, that will be the final key. This is a good example of how confusing the old system can be: is the toleranceon the 15.00 dimension intended from the outer edge to the inner hole, the cross hole, both or the other way around?
Frank

 

[pmarc]

Agree, Frank. Perfect one. Such examples could be nicely used in a discussion with somebody who will be trying to say that coordinate ('old') dimensioning is the best and sufficient way of defining design requirements.

 

[ewh]

I realize that you are getting more than you asked for, but all have been valid comments thus far.

You call out a chamfer with only an angle; what is the linear dimension?

Sorry for getting so pendantic about your example, but the best way to learn this stuff is to be continually corrected.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[fsincox]

amracson,
To your OP,
"I want the 4.76 hole to be on the center using the given tolerance."
Making the 12.00 and the 30.00 dimensions datums and locating the 4.76 hole to them with a position call out will essentually say that. I am not saying it matches the functional requirements, just says what you wish. The problem is the "given tolerance" part. As I mentioned before the 12.00 and other dimensions must have some tolerance but you have not told us what they are. Also once we have a tolerance on these dimensions, your current scheme has the hole location biased to one side implied by tolerance stack-up so which tolerance will you want?
Frank

 

[KENAT]

One possible dimension & tolerance scheme is shown in my sketch. However, since you've never come back with clarification whether it reflects function or your local drawing standards is anyones guess.

http://files.engineering.com/downlo...088-4fd7-8c35-46c48d8fbd95&file=amracson_.tif

I didn't draw in the section but would probably put one to clarify the hole goes through to the central dia and I'd then get rid of the hidden detail.

(I don't do many mm drawings so appologies if there's any dp errors or the like.)

ASME Y14.5M-1994 doesn't really have any very simple figures of your situation. However, 4-19 is a fairly simple example, and may be very relevant if your c'bore hole is actually the datum.

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[pmarc]

Kenat,

Three minor remarks:
1. The depth of counterbore should be 3 not 6.
2. Why did you put different datum precedence order in FCF's (A|B and B|A)?
3. I would put dia. 30 dimension on the leader connected to the circumference of the part and datum B symbol attached to it. This will precisely say that datum feature simulator is a cylinder, not two parallel planes 29.75-30.25 apart that establish horizontal centerplane only.

 

[KENAT]

1. OK
2. Based on the title and limited info in the OP I thought the relation to the dia was perhaps the primary driver for the 4.76 hole. For the c'bore I just followed the typical tolerance precedence for holes of first datum being perpendicular. However, I'll admit I haven't fully thought through the implications so if you want to elucidate feel free. Though without more information on function from the OP even my sketch is arguably a waste of time.
3. ? What I put is not ambiguous and fully complies with ASME Y14.5M-1994 3.3.2 & figure 3-4(e) as I understand it. Given the dia symbol infront of the 30 dimension I don't see how it could be interpreted as 2 parallel planes by anyone with any familiarity with GD&T.

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[pmarc]

Kenat,

2. I was just only asking out of curiousity. I agree that since we do not have any further info about part's function we are only guessing and any datum precedence can be fine.

3. Sorry for this, I was too fast. You are right, such way of assigning datum feature fully complies with standard. I don't know why, but I originally thought this can be confusing with bi-directional position tolerance scheme. Silly thing. Your way of assigning datum feature is also presented in fig. 5-18 of 1994 standard.

One more thing - what do you think about defining any (perpendicularity?) relationship between both datum features? The standard also requires that.

 

[Belanger]

Would that be covered by his Note #4?

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

 

[SeasonLee]

Kenat

The position tolerance of the hole Ø4.76 should be:
POS|Ø.7 circle M|B|A or POS|Ø0.7 circle M|B|A
I think we can’t omit the zero before the decimal point

SeasonLee

 

[KENAT]

Indeed leading 0 is required, mea culpa, I kind of hoped my disclaimer covered this though.

(I don't do many mm drawings so appologies if there's any dp errors or the like.)

However, Pmarc & SeasonLee thanks for reminding me why I quit doing sketches like that for anything other than my own questions.

(Thanks Belanger, I'd think #4 covered it though of course function if we knew it might dictate another value.)

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[pmarc]

Kenat,

My intention was not to discourage you on preparing such sketches. Appologies for this.

 

[KENAT]

I wasn't being as grouchy as I perhaps came across.

However on a number of occasions I've put sketches to illustrate a specific point, in the spirit of ASME Y14.5M-1994 1.1.4, and people have picked apart completely irrelevant points that don't pertain to the point being made. A picture paints a thousand words and yet in the GD&T forums here it also attracts nit picking, so I at least think twice before posting one.

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