Fit Between Square Hole and Tab

[Madhu454]

Hi All,

I am very new to GD&T and started learning from few months back. This is my first drawing for which I tried to apply GD&T.This is just a practice drawing where I am trying to tolerance two parts to achieve a clearance fit. (to be assembled manually, there should not be much play but easily removable.).
Please see the attached file. I have shown “Plate with rectangular tab” in pictorial view for better visualization.

I have used Profile of a surface to achieve the fit.
The minimum clearance is zero (Virtual size for both is 19.9)
The maximum clearance is 0.4 (Max hole size 20.1 - Min tab size 19.7), i.e., 0.2 each side.


I got following doubts while creating the drawings.
1) Whether my approach is right?
2) Is there any other way can I achieve the same fit?
3) I have used flatness and perpendicularity controls to qualify the datums. To what accuracy I need to control these surfaces? I mean how to decide the tolerances for these datum qualifications? Is there any guideline or best practice to calculate this?
4) To loacte the square tab and hole, the basic dimension 35 is given to one of the side face. Is that Okay? or Do I need to locate the tab to the mid line (center line). which is the best practice.

Thanks for your time.


Madhusudhan Veerappa
Mechanical Engineer

 

[pmarc]

Hi Madhusudhan,

Answering to your questions:
1. The approach you took is one of couple possible but in my opinion is absolutely acceptable. Moreover I would say it is the easiest method to interpret, understand and calculate any clearences between considered features.

2. Like I said, there are other ways to achieve the same fit. I will give you two as an example:
a) you can put two directly toleranced size dimensions for height and width of the square and assign two positional tolerances for each of the dimension. This will control position of center planes derived from two pairs of opposite sides of the square. It would look similar to fig. 5-41 in Y14.5-1994M except the toleranced feature is square not round. This method gives you a possibility of applying material modifiers to positional tolerance if needed.
b) you can go for a "boundary" method - mix of position and profile tlerance - as desrcibed in fig. 6-19.

3. I do not believe there is any guide or best practice for controlling mutual relationship between datum features. Geometric tolerance values should be chosen based on the function of the part with additional take of manufacturing process capabilities into account. It is very good that you have this relationship defined at all. Lot of drawings do not have it since designers / drafters are not aware that this can cause serious implications on inspection report results at certain conditions.

4. For your dimensioning scheme (with profile tolerance) it does not really matter where the basic dimensions 35 end. Basic dimensions are non-cummulative, so as long as shape of squared feature is defined by basic dimensions and there is a clear path between datum features and toleanced feature depicted by other basic dimensions, there is no need for applying basic dimensions to center lines.

Side note: instead of writing "SQUARE" next to basic dimension value I would recommend applying square symbol as shown in fig. 3-15 of 1994 edition.

 

[Madhu454]

Hi Pmarc,

Thanks for the explanation.
I have question on your point 4, The end faces of hole/tab are located wrt datums. In our drawing the datums are aligned together. I mean datum feature B mates with datum feature E. datum feature C mates with datum feature F. Since there is a difference in size of both square tab and square hole. As per the present dimensions don’t you think there will be clearance only to the one side when the square tab and hole are assembled? (Since both starts at same distance).? I feel the tabs should be located at a basic distance of 35,1mm w.r.t to datums by doing so there will be clearance distributed all-round the features when they mate.

As per your suggestion I tried creating the drawings. Please see the attached drawing. Please refer to the Drawing Number-2, I have used 0 tolerance at MMC to locate the mid plane of the tab and hole. I have a question here; since the tolerance value is same in both the directions can I use a single position control with boundary concept? (as shown inside a circle on the drawing towards the left corner). Will there be any difference?



Madhusudhan Veerappa
Mechanical Engineer

 

[pmarc]

Hi Madhusudhan,

I was out of the town for the weekend so I could not respond to your post earlier...

I do not think you need basic 35.1 dimensions for tab location wrt A, B, C, because the basic dimensions define location of true profile of the feature, not the boundary of profile of surface tolerance zone. For both tab and hole true profile is located at 35 wrt respective datums so in perfect situation there will be 0.1 clearance per side.

Drawing Number-2 is just what I meant in my previous post. Although I would rather reserve boundary concept for features of more irreagular shape, I think your proposal is more or less correct - I would probably only replace '4X' by square symbol. But since this method gives the same result as 2 separate positional tolerances, I would rather stay with 2 positional tolerances as they seem to be much easier to understand and interpret by an average drawing user.

 

[Madhu454]

Hi Pmarc,
Thanks for the reply and suggestion.

Madhusudhan Veerappa
Mechanical Engineer