specific tolerances (geometrical or dimensional)?

[Baley]

I am preparing the mechanical drawing exam and I cannot understand or find how to decide the specific tolerances on a quota, whether geometric or dimensional. random example: I have to apply a cylindrical tolerance on a 30mm diameter shaft. What value do I have to put? in general the values of these specific tolerances where I find them? I often read about examples 0.2 or 0.08, however, always decimal, but according to what do they establish them? I would like to understand what is the rule for which a diameter of xmm corresponds to a geometric tolerance of xmm. thank you in advance to those who will want to answer!

 

[MassiVonWeizen]

depends on the function of the detail you have to build.
a diameter ø30 on which a bearing will be mounted will have a different tolerance of a ø30 that works from locking plug of a door.
also mount a bearing for a shopping cart and one for a car require different tolerances.
so first you have to understand what it takes and what the object you are drawing is to be coupled with. . .
I'm running and I can't stretch, but in the meantime it starts to think about this concept

 

[Baley]

Thank you first of all for the answer. I've been thinking about what you told me and I think I understand, but I don't know what values I have to do. in the sense that I have understood how there are components that need a closer tolerance and others of a wider one, however I still can not understand what values to insert... Is there a prospectus, a table, something to do with? If for example I had decided that on that hypothetical diameter of ø30 goes a tight tolerance, what values do I correspond to? which measures by corresponding to a tight tolerance and which to a wide one?

 

[linch]

post the full tree design, and you will see that we will find the solution.

 

[Tequila]

as responded by mass... you have to know the application of that detail that you will draw and quote.
If you have to extrapolate it from an already known assembly, that is, you already have an indicative design of where the detail is mounted, you will have well known and in view of which are the other components that will be coupled to this.
without knowing what is the use of the particular you have no obligation to put dimensional and/or geometric tolerances, because it can be quietly that the piece only serves as "fermacarte".

the final information on use will have to provide you with your professor in case you do not have a complete set, instead if you have it you can determine what the constructive quotas to be "tolerant" and what not.

to establish tolerances for example if you have to insert a bearing in your tree, on the bearing catalogs you can find which tolerances and roughness are recommended to adopt.

as told by linch, put a design so we can advise you or better make you think about what are the best solutions.

Hi.

 

[innocente]

I recommend that you use the tolerances un en 22768 as reference for work (printed, cold, etc.) "general", perhaps, with designation m (media).
with regard to specific couplings or mechanisms, the rule quoted by massivonweizen, depends on the use. . .

 

[Baley]

I put the text of the exam to which I referred as an example. at lesson the professor quickly carried out this examination at the board and said that a cylindrical tolerance on the ø30 diameter and a local oscillation on the outer surface of the semi-joint, without telling us the values to be inserted. I don't know if there are any other geometric tolerances, since he only spoke about these two.

 

[PierArg]

I think the professor only cares if you understand how to use geometric tolerances, not the values to be assigned to each of them.

I would say to put them "at random" but still with criterion.

 

[PierArg]

Anyway, I don't know why (I'm sure wrong) I would do the opposite:
- I would take the ø30 hole axis as reference and put the coaxiality of 0.05 (you get very easily) on the diameter from ø48

- the outer diameter, that ø150, if it is not coaxial with ø30 nobody dies. the oscillation tolerance can stand there and could settle on 0.2.
Of course, everything depends on whether the ø150 is obtained by processing (it is known that it is a circumference and not a cone) or remains raw (for example a fusion or molding).

 

[innocente]

In my opinion:
ø30 h7 = +21/0 micron (0.001mm)
therefore calculates for depth of 120 other tolerances.

It's an exercise, not reality. Mah!