Precision Question

[Ritch7]

Question.

 

You are drawing at a precise 1:1 scale, you have a hexagon and across flats you are told the size has to be strictly 1.255 - 1.260 so you have 5 thou work with, If the drawing had to be bang on 1:1 with no exceptions what would you input the exact distance in CAD,

Normally I know you could meet half way and then set the tolerances but what is the proper way.

It’s not a big issue and I know it's still correct as long as you keep in the tolerances, but I’ve always just wondered for clarification purposes.

 

Cheers

[Tiger]

I always thought that you were given an exact dimension (say 1.2575) with a tolerance ( +-0.0025).

 

And that is also how I would interpret it if I would just be given a tolerance - the middle is the exact measuerment

[Ritch7]

yeh Tiger that's what I always thought, sometimes I dont get given a dimension just a range it has to be in, I always go by the middle amount but wonderd if this is normal professional practice

[dbroada]

you should set up a dimension style to suit. You can have a symetrical tolerance or a non symetrical tolerance to suit.

 

Typically a hole will have -0 +0,005 tolerance so in that case I would draw the hole on bottom limit and let the dim style take care of the rest. You can have as many dim styles as you have dimensions!

 

An old term which may still apply is MMC - maximum material condition. Thinking about that will determine how you should make/draw/dimension a component.

[rkent]

Question.

 

You are drawing at a precise 1:1 scale, you have a hexagon and across flats you are told the size has to be strictly 1.255 - 1.260 so you have 5 thou work with, If the drawing had to be bang on 1:1 with no exceptions what would you input the exact distance in CAD,

Normally I know you could meet half way and then set the tolerances but what is the proper way.

It’s not a big issue and I know it's still correct as long as you keep in the tolerances, but I’ve always just wondered for clarification purposes.

 

Cheers

 

You could dim three different ways.

1.255 +0.005/-0.000

1.260 +0.000/-0.005

1.2575 +0.0025/-0.0025

 

But the third choice now has you dealing in ten thousands which may be way to tight of a tolerance for the part being made and may drive the cost up unnecessarily.

 

The best way to dim is to know how the part relates to the whole and what the other parts tolerances are. Since it sounds like you are not given any of that information I would pick the first one and let them tell me different.

 

If I am simply drafting something for someone with little additional information then I do what I am instructed.

 

When I am actually designing something on my own then I know I have to take the whole into account.

I follow ASME Geometric dimensioning and tolerancing standards for my machine drawings and I have a thorough understanding of what the parts are needed for and how it will be used.

[Strix]

I'm with Rkent on this one - especially given the fact that there is no sensible mid point in that range

 

my guess is that this is a hole/insert arrangement, in which case your tolerance will include a 0.000 in one direction, with the corresponding part having its 0.000 in the other direction - there's no point in having a nut which won't fit into a spanner for instance! For that case the nut will have a tolerance of x +0/-y, but the spanner would be x +y/-0