Engineering Question - Tolerances

[anksta]

Sorry for posting a "homework" thread but I'm not askin someone to do somethin for me. For my Detail Design and Manufacturing project, I have to calculate the tolerances for these little things, all I have of them is a picture with the dimensions on.

I looked in the booklets we got last year explainin them but all it has is like a format of putting them on the detail drawing. Does anyone know how to calculate them? I'd really appreciate any help.

Cheers

 

[touch]

There is no formula for calculating tolerances.
For example, if you draw a small childs toy or something, the tollerances are not as important and could be up to a few millimeters.
Drawing pistons for a car engine, the tollerances will be a lot tighter, and usually to withing tenths of millimetres (would also be negative tollerances meaning it can be slightly smaller than the hole it is meant to fit in, but cannot be any bigger)

edit: re-read question - do you have to make up your own tollerances?

 

[Visage]

Sorry for posting a "homework" thread but I'm not askin someone to do somethin for me. For my Detail Design and Manufacturing project, I have to calculate the tolerances for these little things, all I have of them is a picture with the dimensions on.

I looked in the booklets we got last year explainin them but all it has is like a format of putting them on the detail drawing. Does anyone know how to calculate them? I'd really appreciate any help.

Cheers

You cant calculate tolerances without knoing what the factors are that will determine them, which in turn depend on the context.....

 

[Jokester]

Tolerances are something you would specify.

Jokester

 

[Slam62]

Well there are std tolerences for different types of surface and component, it depends what its being used for, an important locating surface will have a tight tolerence, something such as a casting surface a lose one, you have to use common sense to make it as easy to manufacture as you can without losing any function.

 

[touch]

Tolerances are something you would specify.

Yep, you'll prolly have to make up your own tollerances. Just remember that if something has to fit inside something else, it cant be any bigger than you've designed it and the hole cant be any smaller

 

[anksta]

I don't get what that means though. I mean I know what they are and what they're for, but I don't get where they come from. Is it something to do with the way they're made, with what tools or somethin?

 

[anksta]

Cheers, the context is a crane, and this is a stub that something rotates on, I think, it definitely goes inside something, but I dont think I've got the dimensions for the hole. Do I need those?

And then can I just put +/- 0.02mm or somethin very accurate because its an important component?

Thanks by the way.

 

[touch]

Is it something to do with the way they're made, with what tools or somethin?

Its to do with the job they are designed for. If you are designing small parts for mechanical engines or something you want a very tight fit with other parts around it. This will tell the manufacturers that they need to make them precise.

 

[Jokester]

I don't get what that means though. I mean I know what they are and what they're for, but I don't get where they come from. Is it something to do with the way they're made, with what tools or somethin?

It's to do with the limits of manufacturing technology and cost. You can't make something precisely to the dimensions or whatever as there's always a level of uncertainty.

It's a balancing act between what's required and cost of getting that.

What ever it is you're designing you have to live with some level of uncertainty and ultimately it's upto the designer to specify what is required to meet the function, bearing in mind what's actually possible in terms of manufacturing technology.

Jokester

 

[touch]

I think, it definitely goes inside something, but I dont think I've got the dimensions for the hole.
And then can I just put +/- 0.02mm or somethin very accurate because its an important component?

I would say the hold would be: (usuing your 0.02 as a reasonable tollerance)
+0.02
-0.00 (the hole cannot be any smaller than your design or the thing may not fit inside it)
and the other thing would be:
+0.00
-0.02 (it can be slightly smaller than designed because it will still fit in the hole, but no bigger)

 

[anksta]

What ever it is you're designing you have to live with some level of uncertainty and ultimately it's upto the designer to specify what is required to meet the function, bearing in mind what's actually possible in terms of manufacturing technology.

From that, I think I know what kind of lathe it'll be made with, if I find out what the tool that'll be doin it is, then how accurate that can get, can I pull off that kind of accuracy or is there another factor affecting it?

Bearing in mind that we're not actually making any of this, its all just design.

 

[anksta]

I would say the hold would be: (usuing your 0.02 as a reasonable tollerance)
+0.02
-0.00 (the hole cannot be any smaller than your design or the thing may not fit inside it)
and the other thing would be:
+0.00
-0.02 (it can be slightly smaller than designed because it will still fit in the hole, but no bigger)

See I completely understand that, its clear and its common sense, but seeing that on paper, I wouldn't have a clue whether or not someone would go "na thats wrong".

 

[touch]

Tools shouldnt have anything to do with tollerances.
You dont design things around tools, you use tools that fit the design.

 

[anksta]

Tools shouldnt have anything to do with tollerances.
You dont design things around tools, you use tools that fit the design.

so could I (for example) just whack similar tolerances on what you just said, is there a wrong answer? Or is it just a matter of practicality, I presume that because its just design and none of our methods or designs are actually goin to be put to the test we have a bit of room to manoeuvre with things as most little errors and things would be ironed out in production.

I have a picture with the dimensions on, do I just draw this again and write the tolerances on? I swear it seemed like there was more to it than this when I was given this to do.

 

[Jokester]

Tools shouldnt have anything to do with tollerances.
You dont design things around tools, you use tools that fit the design.

I wouldn't go that far, cost plays a key part in design and as such can drive you towards the type of manufacturing used. There's no point blindly specifying a tight tolerance without having an idea of the cost of achieving it.

Jokester

 

[anksta]

I wouldn't go that far, cost plays a key part in design and as such can drive you towards the type of manufacturing used. There's no point blindly specifying a tight tolerance without having an idea of the cost of achieving it.

Jokester

but for that though, where are you supposed to get the cost of doin that from? I mean it cant be cost = power x time or whatever, every lathe would be different surely.

I dont see where you could get an accurate estimation of the cost from.

 

[Jokester]

but for that though, where are you supposed to get the cost of doin that from? I mean it cant be cost = power x time or whatever, every lathe would be different surely.

I dont see where you could get an accurate estimation of the cost from.

From the point of view of your exercise it probably won't play a part as you won't be actually making it.

At a guess typical tolerances for physical dimensions would be +/- 0.1% of the overall size (could be wrong though!).

Jokester

 

[touch]

where are you supposed to get the cost of doin that from?

I think we are both talking about mass production here, whereas you are talking about a 1-off school project(?). So there wont be any cost implications for you.
(things like the extra time it takes to be more accurate and therefor lower production rate, longer machining times, etc are how you work out cost)

I think that as long as you can justify your tollerances to your teacher (and they are not ridiculous) then you cant be wrong.

 

[anksta]

Ok, if I work to that, is the work just a case of drawing it with the tolerances on? Am I not supposed to prove that they're appropriate tolerances or anythin?