geometric position tolerances

[Lera]

hello to all, I am preparing the drawing examination of machines and I have arisen a doubt about the position tolerances. on my text it is said to use theoretically exact odds only when you locate an axis or other than a reference element. However, I have noticed that a plan is taken to primary reference and then as a second reference the axis of a hole, stating it with theoretically accurate quotas referring to elements not marked with the reference symbol. is it correct to do?(allego example) Moreover I wondered if it was more correct as a general rule to use the three orthogonal planes and locate the holes, or to choose for example two floors and the axis of a hole, if the hole is important for the purposes of the operation of the piece? listing an axis compared to another taken as a reference,doesn't risk "to accumulate" any errors?

 

[ramjet]

from which book are those images taken?

 

[meccanicamg]

Fortunately they are only drawings from "drawing book" because if all the drawings were listed so you wouldn't even know how to make the pieces.
Unfortunately the operators of machine tools and their masters do not know very well ste things, complicate existence and sometimes are not even functional.
if hole with a new cnc I have the positioning of the centers to ± 0.01 mm, if I have an unrevised cnc expired I have ±0,05mm and if I draw the hand holes with the truschino, bulino and hole with flag drill I will get ±0,2mm to go well.
So much so that only on some pieces you put geometric tolerances otherwise you go crazy and you throw a lot of time.

As for your question, you must read the din en iso 1101.
I attach the image of the din rule that tries to clarify the way of using theoretical quotas and indications of geometric tolerance.

definitely realize and quotate even with geometric tolerances a piece using the three main floors it is less difficult to believe that it can be done so also by machine. It is also true that sometimes some processing depends on another processing regardless of where the reference is made, so sometimes an axis is used as a reference for another processing.
even with dimensional tolerances is so....because it is not said that I put the zero of the piece in a corner and all the processing must be referred to that, especially if there are rough surfaces.

 

[meccanicamg]

Since in these years they have changed the rule of gps, I recommend you to recover the iso 1101:2017 which is greatly changed, expanded and practically there is to become dumb because now there are many special letters and also the additional pane....there is to have fun.

 

[meccanicamg]

second. iso 1101:2017 the exact theoretical dimensions are used to define:
- nominal shape and size of features (works)
- the theoretical nominal dimensions of tef processing
- localization and size of machining portions including restricted tolerances
- lengths of tolerated work projections
- localization and partial orientation of two or more tolerance areas
- location and orientation of the datum also variable both relative and absolute
- direction of tolerance width

I think your book is obsolete. in intermediate standards it was a mess to use three datums and caused confusion. now you use two datums and a reference plan in the appropriate box of auxiliary indicators....even if 3 datum can still be used.

 

[welcome to the machine]

not within the merit of tolerances, mecanicamg is very competent, but I point out the beautiful temperino for pencils. metal and even double caliber! what nostalgia.. .

 

[Lera]

Fortunately they are only drawings from "drawing book" because if all the drawings were listed so you wouldn't even know how to make the pieces.
Unfortunately the operators of machine tools and their masters do not know very well ste things, complicate existence and sometimes are not even functional.
if hole with a new cnc I have the positioning of the centers to ± 0.01 mm, if I have an unrevised cnc expired I have ±0,05mm and if I draw the hand holes with the truschino, bulino and hole with flag drill I will get ±0,2mm to go well.
So much so that only on some pieces you put geometric tolerances otherwise you go crazy and you throw a lot of time.

As for your question, you must read the din en iso 1101.
I attach the image of the din rule that tries to clarify the way of using theoretical quotas and indications of geometric tolerance.

definitely realize and quotate even with geometric tolerances a piece using the three main floors it is less difficult to believe that it can be done so also by machine. It is also true that sometimes some processing depends on another processing regardless of where the reference is made, so sometimes an axis is used as a reference for another processing.
even with dimensional tolerances is so....because it is not said that I put the zero of the piece in a corner and all the processing must be referred to that, especially if there are rough surfaces.

my doubts were generated by the fact that in lesson the professor told us to use as reference elements that are important for the function that the equipment performs, while at exercise the assistant advised us to use three planes of reference to block the piece and then check all the other parts of the piece, and the two things according to me (maybe mistaken) are not always compatible.

 

[Lera]

second. iso 1101:2017 the exact theoretical dimensions are used to define:
- nominal shape and size of features (works)
- the theoretical nominal dimensions of tef processing
- localization and size of machining portions including restricted tolerances
- lengths of tolerated work projections
- localization and partial orientation of two or more tolerance areas
- location and orientation of the datum also variable both relative and absolute
- direction of tolerance width

I think your book is obsolete. in intermediate standards it was a mess to use three datums and caused confusion. now you use two datums and a reference plan in the appropriate box of auxiliary indicators....even if 3 datum can still be used.

I will read the norm carefully, thank you very much. more than anything, my doubt about the design I posted was: if I choose as reference to the support plan and then as b the hole, with only one reference the piece is not blocked. how do I verify that the design is actually the location of the hole? is it subtent that the piece is stuck on the other two edges? I apologize if they are stupid questions, but I need to understand well and do all my concepts to be able to apply them.

 

[MassiVonWeizen]

It is a somewhat confusing quotation.
the reference are
a: opposite floor to the site of the throat 22h7
b: upper hole
c: central asola

However,
a has a flatness of 0.2
b has no own position quotas, but is a hole dependent on hole 16h10 below which has no geometric reference and has a shape tolerance compared to b
c is not a hole. It's an asola. where do you take it from? from the center?

Still
a is a plan with tolerance of shape 0.2 and in the lower view requires parallelism of the throat 22h7 of 0.008
Looking at the view ina lto there are no form tolerances on external geometry or at least one perpendicularity between the lower face (from which part the 16th of the first hole) and the right one (from which part the 20th placed on the upper hole)

Ultimately there are dozens of tolerances, some absolutely useless as 6,6h13 for a hole that contains a 6-diameter screw, both shape and dimensional that have exaggerated or hyperprecise shocks.
does not hesitate a zero piece or an element from which all odds start (it should be the hole 22h9 at the top, but it is not so)
roughness, but also tolerances, are exaggerated; the piece is a mistyling to what it takes:
a 1,25 on a hole 16h10
un1.25 on a throat with 0.2 tolerance
a shape tolerance of any 0.4 on the outer perimeter

Moreover and then I close if the 20h7 throat is presumed to serve to make a locking with an opposite seat and in the middle will presumably go a stick or similar. the share of the depth cannot be given by the difference between the general thickness that has a tolerance between -0.2 and -0.4 and the 18-post quota on the face opposite the throat coincident with the reference to; it must be given by the opposite face because it is this that will stand on its counterpart.

a simple tensile in the end will cost like a medium-sized motor

 

[meccanicamg]

In fact in my first post I wrote that it is a book and not a real manufacturing design....in fact of nonsense there are a lot of them and described them all massivonweizen.

Young man, your questions are lawful, but your book is wrong. ...this is the point. Then we wonder why young people don't know things....it's because they don't know old people and teach them legs by air.

 

[meccanicamg]

references b and c have zero tolerance with principle of maximum material, so it would mean that they are absolutely accurate and can be used as a reference. a is the datum of the back floor and all right. but it is pure climbing theory on the glass.

other wrong things are:
- the bevel is listed along the main axis, then rotated 90° compared to the representation of the book (unless you use other notations different from the canonical standard....but it is not the case of illustration)
- the bevel does not induce 2x5 as a quota and then 15° because it is formally wrong. you only share the size along the axis and the angle quota. 2 doesn't exist.
- stiffness makes laugh.....nature wants 3.2 normal milled for all in tolerance of 0.1 mm, 1,6 just if we want to make alesed holes and offices for tabs, 0.8 that is grinding or electro-erosion dive....I do not see it for a tab. ra6.3 for roughing and ra12.5 for clean of super coarse milling. . ..practically never.

then....the iso 1302:2002 has established that the indications of roughness should no longer be adopted what your book sponsors but the triangle with the horizontal handle and below you write ra 3.2.... as from my attachment.

 

[meccanicamg]

between the many obsolescences in the design of the first post there is an indication to the old norm of general tolerances iso 2768 now replaced by en iso 22768 for many years http://store.uni.com/catalogo/index.php/uni-iso-2768-1-1991.htmlThen... seen that we are on the subject, there would be a little thought about the general tolerances, because now they are in many, especially in the asme norms that have noticed that they worsen and make ambiguous the control references. there are several publications on which you can find online.
therefore it is better to indicate tolerances on the pieces and therefore on the quotas rather than to put the note "general tolerances...." especially for geometric problems. for dimensional quotas and tolerances is easier.

about this problem, especially in Italian historical companies, we do not use this norm but we write on the cartiglio a table where we say that for the size included in a certain range tolerance and ± something....and for the corners is worth something....se carpenteria vale x e se per fresare vale y .
therefore refers to linear and angular quotas.
see the asme question here http://www.ilprogettistaindustriale.it/stop-alle-tolleranze-generali/

 

[Lera]

between the many obsolescences in the design of the first post there is an indication to the old norm of general tolerances iso 2768 now replaced by en iso 22768 for many years http://store.uni.com/catalogo/index.php/uni-iso-2768-1-1991.htmlThen... seen that we are on the subject, there would be a little thought about the general tolerances, because now they are in many, especially in the asme norms that have noticed that they worsen and make ambiguous the control references. there are several publications on which you can find online.
therefore it is better to indicate tolerances on the pieces and therefore on the quotas rather than to put the note "general tolerances...." especially for geometric problems. for dimensional quotas and tolerances is easier.

about this problem, especially in Italian historical companies, we do not use this norm but we write on the cartiglio a table where we say that for the size included in a certain range tolerance and ± something....and for the corners is worth something....se carpenteria vale x e se per fresare vale y .
therefore refers to linear and angular quotas.
see the asme question here http://www.ilprogettistaindustriale.it/stop-alle-tolleranze-generali/

thank you so much to you but also to the others who have answered me, I think I have understood a little better than before or at least to have enough material to reason on.