Double plus tolerance - Why?

[keego]

I've seen double unilateral tolerances many times. I've always been taught that they're used in interferance fit applications and the purpose is to give the machinist a tighter tolerance than the inspector. For example:

A print has Ø14.02 +0.01 +0.04

The machinist must shoot for 14.03-14.04 but if it happens to be cut to 14.219 it will pass inspection.

Is this right or wrong? I'm beginning to have my doubts but no one I talk to can give me a logical explanation. Some say they've been taught the same and some say a part below 14.03 is a bad part.

[craiger_ny]

I remember seeing that before and I think you are on the right track with it being relative to a press fit feature. Then again I almost want to say I've seen it on a casting drawing too. Kevein, where are you?

[cmmguy]

Its a tolerance of fit. Used by them forenners. It's ISO.

ID might be 25 +.005 +.025
and mating part would be
OD might be 25 -.005 -.025

So they are right about the tolerance. In the example above. The min for the ID is 25.005 and the max for the OD is 24.995

[bw_bob]

I've been doing this for 20 years and this is the 1st time i've seen that called & it also has to be one of the dumbest & most confusing ways to dimension something.

[cmmguy]

I've been doing this for 20 years and this is the 1st time i've seen that called & it also has to be one of the dumbest & most confusing ways to dimension something.

Weeellllll, the top three automakers in the world use it. There might be somethin to it.

It's only confusing at first. It actually makes alot of sense from both the design side and the manufacturing side. I think that we are just used to having the nominal as the target dimension.

[craiger_ny]

I've been doing this for 20 years and this is the 1st time i've seen that called & it also has to be one of the dumbest & most confusing ways to dimension something.

What it is is nominal size then the tolerance. Here in the US a shaft is nominal and the female is adjusted per tolerance class. Say a customer orders a motor and says "I need a 1-1/4" interface". What they want is a motor with a 1-1/4" shaft. All shafts in the 1-1/4" range will be the same size and tolerance (the male is nominal), we would spec it as 1.250 +0 -0.001. Then the hub would be spec'd according to tolerance class (females are variable) for example one class might be 1.249 +0.001 -0, and another tolerance class might be 1.249 +0.0005 -0. Regardless the interface is called a 1-1/4" interface. This is ISOs way of putting the nominal interface size on the drawing. I forgot all about this until CMMguy posted. It is funky but the way they do it. Someone else may be able to chime in what the advantages are to puting nominal interface dimensions on the detail drawing.

[Andrew Gardner]

I see it quite a bit around here. Personally, I think its a bunch of B.S. I think the engineering world is trying to complicate things more than they need to.

[why me]

++

I've been told By our engineering dept. that it has to do with tolerance stacking on fitting parts I see it allot on our prints. mostly on diameters

[bw_bob]

Guys I understand what everybody is saying. The big 3 use it, fine.
All i'm saying is why make it harder then it needs to be.
Still a stupid callout.

[gasper22]

my opinion

in our company(Slovenia, EU), we operate under this procedure:

Ø14.02 +0.01 +0.04= Ø14.03(14.02+0.01) lower allowance
Ø14.06(14.02+0.03) upper allowance

so we operate betwen these two measures

[rebeldude]

Thats almost like saying the nominal is actually about 14.04

[Wes Cisco]

in our company(Slovenia, EU), we operate under this procedure:

Ø14.02 +0.01 +0.04= Ø14.03(14.02+0.01) lower allowance
Ø14.06(14.02+0.03) upper allowance

so we operate betwen these two measures

Do EU calculators use some form of math unkown in America? My Texas Instruments backs up my brian. 14.02+0.03 = 14.05!

[cmmguy]

Do EU calculators use some form of math unkown in America? My Texas Instruments backs up my brian. 14.02+0.03 = 14.05!

typo.... look higher.... your BRIAN must be heat affected

[Wes Cisco]

typo.... look higher.... your BRIAN must be heat affected

My brain is well protected along with my CMM inside my airconditioned square bubble. I was pointing out the typo, and/or the fact that not all of his numbers made sense. Yes I think I understood what he meant, but to paraphase Socrates: 'If you can't say what you mean, you will never mean what you say.'

[djayne]

We have this on almost all of our suspension parts because of, like what was mentioned before, they are press fit parts. And I've seen the drawings for mating parts, because we assemble some of our suspension knuckles with bearings and ball joints, and they are the opposite polarity in tolerance like was mentioned. I guess I'm just used to it - it doesn't really bother me either way. One thing it does affect is calculating SPC. Sometimes we modify the nominal to make the equations easier or so the SPC software doesn't get confused. We have Ford, Honda, Subaru, BMW, and Chrysler parts that all use this type of tolerancing.