concept of tolerance

[snaroz]

hello to all, I had been thinking a little about the concept of tolerance and I wanted to know if what I write is correct. from what I read, the concept of tolerance was first put into practice by a German company in 1905 in order to reduce the number of pieces discarded after the various stages of processing. before the introduction of this concept the dimensions of the pieces that were drawn were established by simply assigning a number to the size in question. if the dimensions of the piece coming out of the workshop were equal to those written and drawn by the designer, this was accepted; if the size of the piece was greater, this subdued adjustment operations in order to reach the expected values; Finally, if this was smaller than those of the project, it was obviously discarded. the problem that soon came out was that respecting the nominal dimensions imposed on drawing was very difficult, and therefore many pieces were discarded or had to suffer expensive adjustment operations, resulting in levitation of costs, waste of material and time. strictly respect the nominal dimensions was difficult because of the inevitable inaccuracies of the processing processes, imperfections of the machine tools etc...the idea therefore that lies at the base of the concept of tolerance and that will solve the problem of the waste of pieces however good, of time, and of money is the following: the designers sensed that being rigorous in the assignment of the size of the pieces was a useless complication: mechanical parts perform their function even if the dimensions are slightly different from the nominal ones. Let's make an example. Suppose we draw a tree and a hub that must mate slightly forced. Suppose we assign to the diameter of the shaft the value 30 mm, and to that of the hole the value 29,8. Now when the workshop goes to work and produce the piece, it will hardly be able to respect values 30 and 29,8. You can go to [Bleep] and you can match the required values, or not. In that case, or would throw the pieces or would work them further, resulting in cost levitation. But let's note that the coupling suits me the same even if the tree coming out of the workshop has a diameter of 30,1 and the hub of 29,7. the coupling is forced the same, so I'm fine. the designers then realized that instead of assigning to a dimension only one numerical value needed, to solve the problem of waste etc..., assigning them a range of values, compatibly with the function that the piece planned had to have. It is evident that for the workshop it is much easier than a piece has a length falling in range 29,8-30,5, than do not respect the value 30,1. These concepts were then put into practice by various companies to extend to various countries, and to make order since probably each company quotated the pieces in its own way intervened whose purpose was basically to uniform the notation at the graphic level.

therefore submitting a project without dimensional or geometric tolerances would mean to subject the workshop to which the piece is asked to be manufactured to a high stress, as well as to increase considerably times and production costs. of each mechanical component you must understand the function well; Do this, you have to understand what the range inside which can vary the size and shape of the object, but always guarantee the functionality of the piece....then that you say, there are according to you?
Good evening

 

[PierArg]

You can see that you have to do the drawing exam.:wink:

 

[snaroz]

Yes, but do you think I understand the concept about tolerances?

 

[MBT]

is not a question of "stress" of the workshop
the workshop is there to make the piece, be stressed or not is a problem of its own, not of the designer
the concept is that it must also be considered what I use to make the piece and especially with what I mix it.

I'll explain.
I find a pin with indicated diameter 30
What precision do I want? Absolute? to the size of austenitic wheat? What if there's too much grain? Discard the piece?
or do I mix it with the wall meter, accuracy of about one mm??? ?
What happens if I mix it with a 20th-gauge and I find it 30,05? I'll throw it?
The same piece I mix it with a micrometer and I see it's 30,01... still to throw? ? ?
and to realize it, what machine do I have to use? because if I have to have the precision of the austenitic wheat, after the lathe and the rectification I will have to equip myself with lappings, but of those thrusts, and maybe even controlled acid attacks. . .
If instead it is +/- 1 mm... just the lathe "made in china" that I buy in the brico...

if instead I place 30 +/- 0.01 then things become clear for everyone.. .
I know I can control the piece with a micrometer centesimal, I don't need the scanning microscope and the meter I leave it to the mason
I know I can do the part at the lathe, but I have to go through it. This time the lapping machine will remain off.

As a result, I head-workshop stressed, can give precise indications to the pre-emptive Bed, which will give a price "played" and not an absurd cabbage.. .

I don't know if I explained

On the other hand, you, designer, need to consider what you need to make that piece work. . .
So once again, do you really need to know how many Austrian grains are in that piece? or if it is also 30,1 is ok the same, knowing that the piece with which you mate can have a diameter ranging from 30,1 to 30,3 ?? ?

 

[meccanicamg]

tolerances are the fundamental thing of mechanics. It can't be okay. If you planner (capable) are studying an object that must have specific characteristics you have to use tolerances for the manufactured product to remain in the fields of operation of the planned thing. That always applies.

if you work pieces in series then the matter becomes even more special attention. for the product to be interchangeable, it must have geometric and shape tolerances, therefore it is possible to produce in large series of interchangeable and mating pieces under all points of view worldwide.

Another noteworthy thing is that to facilitate life was invented basic hole or base tree, i.e. in h7 or h7, i.e. 0/+ or 0/-. It's practically zero. everything else is related to this and for convenience you can do free, interfering or blocked couplings.

 

[Er Presidente]

...
if you work pieces in series then the matter becomes even more special attention. for the product to be interchangeable, it must have geometric and shape tolerances, therefore it is possible to produce in large series of interchangeable and mating pieces under all points of view worldwide.
...

This particular aspect is fundamental.
if the "tolerances" were born to "reduce costs", soon we realized that they solved another big problem that was facing in the industrial world, the "globalization" of production then only a hypothesis.
the management of tolerance allowed us to consider usable pieces with controlled shocks compared to the nominal dimensions, each piece designed acquired a "snap" that shaded the shapes and managed the clouds and not the "pieces".
magically you could send a design anywhere and you could be reasonably sure to be able to mount it coupled with other pieces built in different places.
it was possible to supply spare parts and exchange pieces between different assemblies and everything continued to work, a revolution that, unfortunately not yet finished, in the sense that not all still exploit the advantages.