Hi Eng-Tip community. I am a young

[arandy]

Hi Eng-Tip community. I am a young production/mechanical engineer and have decided improve my drawing reading/creating skills, since the knowledge and practice obtained from my school program is a bit restrained. I have now read several textbooks and lots of ISO standards, but the actual experience is only achievable with practice. I have looking for a forum where people with experience could answer my questions, which I am having while practicing. I don't expect somebody to teach me, but sometimes You just need a little explanation or advice to move on Your progress. I am sure most of You have been in a situation like me. Hopefully there is helpful people here, who doesn't mind beginners questions.

Note. I am using ISO standards, but most of the questions I am having are about principles anyway.
Note. My native language is not English. I apologize for any misunderstandings and please let me know if I am not clear enough or something.

My first questions are quite a basic and newby, but it is important for me to make sure I am getting it right. So that was the introduction. I am going to skip it next time
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1. We have a plate with one circular and two semicircular holes in it. If the location of the holes is not determined with dimensions, is it then assumed that the circular hole lies exactly in the middle of the plate and the two semicircular holes in the middle of the shorter side of the plate?

2. There is couple of questions for the next drawing:

1) Lets assume that the two flanges (A) has to be with equivalent width and the two elements (B) also with the same width. Should this requirement get solved by geometrical tolerancing rather than trying to imply it with dimensions?

2) Diameter of shaft different parts has dimensioned on the left side of the model. What if the shaft diameter on the right side is identical to the diameters on the left side. Could I just expand the extension line to the other side of the shaft so the diameter applies on both features (for example diameter 60 applies to both flanges A)?

3) There are 7 holes in both flange (A). Is the alignment and positioning of the holes designated unambiguously? If there is evenly distributed holes in any case, is it just enough to show only the distribution corner or there is additional annotations required?

4) The holes in the flange (A) are represented on both flange (A). Which are the possible and convenient ways to bring this out? A text annotation?

Any other advice and suggestions are of course more than welcome. Thank You.
arandy

 

[arandy]

I apologize for the first a bit messy post. I am missing a headline and the second picture didn't appear. For now I am going to add the pictures as a links, which are much easier to follow at and zoom. Here's the second drawing.

http://oi48.tinypic.com/wi3c2.jpg

 

[pmarc]

Hi arandy,
Welcome to the forum.
1-1. Yes, it is assumed that for perfect (as-drawn) part those features are exactly in the middle of the plate. However, as you most likely know, there are no perfect parts in real life, so there always has to be something (a tolerance) on a drawing defining how far from ideal the feautres can be and still stay functionally acceptable. That being said, your first print does not give such info for vertical location of center hole and two semi-circular slots.

2-1. Simple width dimensions for flanges should be enough.
2-2. "2x" annotation in front of dimension value can be used to indicate that the dimension applies to 2 features and not 1.
2-3. Unfortunately it is not (see last part of my post).
2-4. Again, "2x" annotation can be used.

Please do not take it personally - I know beginnings are hard - but your both sketches are beautiful classic examples of ambiguity of what is called "coordinate dimensioning". It is simply impossible to give full and clear definition of part's geometry with the use of linear and angular dimensions only. You need much more - set of symbols, rules and definitions commonly called GD&T (Geometrical Dimensioning and Tolerancing) or in ISO world GPS (Geometrical Product Specifications). Have you tried to look for any handbook on this topic? I think most of them at initial sections will offer plain explanation of shortcomings of coordinate dimensioning and will show why GD&T should be used instead.

 

[arandy]

Thanks for Your time pmarc

So I am going to work trough your post. Yes I am actually aware that there are no perfect part in the real life. So let's say I have to add dimensions with tolerances for hole alignment to define them fully. But if there isn't such information like on my initial drawing, then the allowance from ideal feature is designated by tolerances that the manufacturer uses in such a cases(i don't really know how its called)? I have read articles and texts of dimensional tolerancing, but I haven't really practiced them yet.

What concerns the GD&T then month ago I was looking for a good reference to get the basic knowledge of geometrical dimensioning and tolerancing. I found a book which I pretty much liked, cause there is lots of examples in it:

http://www.amazon.ca/Geometrical-Tolerancing-Desk-Reference-Interpreting/dp/0750668210

I have now covered the book, but I haven't really tried to implement the knowledge yet. I thought it would be nice to sort out the questions which I have concerning to linear and angular dimensioning. Anyway I try to redo the drawings and fully define them with my existing knowledge. Let's see how it works out.

I am using lots of Google anyway, but if there is any good reference that I could use to improve myself then just let me know.

 

[pmarc]

arandy,
I think you are referring to general geometrical tolerances concept as defined in ISO 2768 for example. If that standard was invoked on your print, then yes, I would say there is general symmetry tolerance between holes and the middle of the plate specified.

As for good ISO-based GD&T books, there are couple authors dealing with the topic, e.g.: A. Krulikowski or G. Henzold. Green's one is fine too. Just keep in mind that a lot has changed in ISO GPS in last 2-3 years, so if you are really interested in most current state of knowledge, you should be looking for something relatively new.

 

[fsincox]

Green's book is very basic stuff.

 

[ctopher]

Also, the "2x R2 min" can be very big if the machinist desires. You may want to make it "max"?

Chris
SolidWorks 11
ctopher's home
SolidWorks Legion

 

[arandy]

Thanks ctopher, your comment makes sense.


Umm, this thing is more complicated then I ever thought, but that's why I love it. I really want to build solid basis in GD&T, so I can fully concentrate to other stuff like product design and improvement. Green's book really is basic, but it gave a good overview. It only took 4 days to read it anyway. Looks like the books Pmarc suggested are more appropriate:

http://www.amazon.com/Krulikowskis-Geometrical-Tolerancing-Reference-Guide/dp/0924520175

http://www.amazon.com/Geometrical-Dimensioning-Tolerancing-Manufacturing-Inspection/dp/0750667389

Krulikowski's book looks especially good, it is also published in this century. The problem is these books cost a fortune for me and there is no hope to find books like these in Estonian libraries. Of course there is possibility to use Google if You know what I mean, but I doubt if you can find these books out there. Anyway I am constantly reading explanations and articles from web to work my way up in understanding the GD&T. Your comments and critics have been very useful also.

I tried to redo the first drawing a bit. Does this make at least bit more sense?:

http://oi50.tinypic.com/2e5v4sx.jpg

 

[pmarc]

arandy,
It looks better, however there are still some things that could be improved:

- Since you selected faces A, B, C as your only datum features, 4 smaller holes in corners should all be located from that faces. In other words you may want to consider use of basic dimensions from faces assigned as B and C to centers of those holes (basic 13 and 187 in horizontal direction and basic 13 and 87 in vertical direction). Then you should put position tolerance frame, not perpendicularity, as perp. is not powerful enough to control location of features. It should contain A, B, C datum references;

- For perpendicularity callout on center hole, diameter symbol in front of tolerance value is missing;

- Although according to ISO it is absolutely legal to use position callout on cylindrical features being smaller than 180 degrees (two semi-circular slots), I would recommend use of profile of surface instead in order to reduce measurement uncertainity. Profile of surface control does not require finding centers of the slots which is very unrepeatable inspectional operation on that kind of features. If you switch to profile, do not forget to switch dia. 50 to basic;

- Unless you have something like title block angular tolerance, nothing controls mutual geometrical relationship between datum features A, B, C at the moment. Common practice is to apply: 1) perpendicularity tolerance of B to |A|; 2) perpendiculariy tolerance of C to |A|B|;

- In ISO GPS symbology there is an abbreviation for "NOT CONVEX" - it is NC and it is recommended to be put beneath, not above, tolerance frame;

- Generally you do not need trailing zeros for dimensional and geometrical tolerances;

 

[arandy]

It took a while to go over your tips, but again I learned something. It was very useful to bring out the title block tolerance subject, cause I was considering about geometric relationship between datum planes other day.

http://oi46.tinypic.com/14jyz5w.jpg

Standards are usually straightforward and they are not too hard to remember. Once You have used them, You probably remember them next time as well. More confusing is to understand the GD&T in overall. I have to spend more time reading books that explain it in detail and keep training myself.

I am wondering if there are explanations out somewhere that explains how GD&T requirements are validated in real life. For example how the manufacturer controls the allowances of this part in real life. Maybe this would give me a better realization of GD&T. To be honest I don't really have the unambiguous understanding of the GD&T. For example the basic dimensions on this plate represent the theoretically exact dimensions and thus the geometric tolerances apply to the features determined by them. But the dimensions of the plate itself have been determined by toleranced dimensions. So for me it seems like the variation in plate dimensions also affects the final position of the small holes, cause the basic dimension is dimensioned from the side of the plate. I believe I really need to sort out the basis of GD&T first and then move to the technical drawings.

Thanks again



Thanks again

http://oi46.tinypic.com/14jyz5w.jpg

 

[pmarc]

- Basic dimensions 13, 50, 87, 100 and 187 should originate at datum features B and C, not at opposite faces. This will solve the problem that location of holes are dependent on width and height of the plate;

- Profile tolerance frame on semi-circular holes should be attached to the contour of the hole by a leader, and not placed right beneath basic dimension;

 

[SeasonLee]

Profile control can be used with datum feature referenced and without datum feature references, profile control with datum feature reference is used to control size, location, orientation and form, while profile control without datum feature reference is used to control form only, so I think it would be better to have a datum references on your latest sketch. |profile|0.01|A|B|C|.

And I will recommend to view the link below, it will tell you why pmarc suggested to apply the profile control on the semi-circle feature.
Position FOS

SeasonLee

 

[pmarc]

SeasonLee,
This is not exactly the reason why I suggested profile of surface instead of position control on semi-circular slots.

Tec-Ease's tip is based on Y14.5M standard and I am absolutely with Don in claiming that position tolerance shall not be used on non-features of size in ASME world. However, as some ISO-based handbooks say, position tolerance is acceptable on NFOS (even on curved surfaces), so knowing that OP works according to ISO, I could not simply say that position tolerance was illegal in his case.

Instead of it I indicated that position control on such kind of features does not offer repeatable and reproducible measurement results, so profile of surface is much better choice.

 

[pmarc]

Ah, and by the way there is a leader line from FCF to hole's contour missing in profile of surface callout in Tec-Ease's tip, so this example has to be used with some caution.