General tolerances

[mobostians]

Can I specify general tolerances variably to A-B-C? Like Surface Profiles | 2| A | 4 | B |8 | C where allsurfaces are +/-1 to A - +/-2 to B and +/-4 to C.

 

[Belanger]

No, because a geometric tolerance (profile, in your case) establishes one tolerance, and that one tolerance is held to a single datum reference frame. In other words, the datums A, B, and C work together to create one "system" in space.

To get what you want, you'd have to chop it into three separate feature control frames (although there'd be other issues in that about relating the datum features to each other in this separated format).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[mobostians]

Thanks. Then how do I deal with a material that has a wider tolerance?

0 to 150mm = +/-1.5mm
151 to 250 = +/-2.5mm
250 and above = +/-1%

If I have part that is flat and sort of narrow but quite long - I don't want to have to use the largest possible tolerance as the general tolerance because there are alot of dims that will need to be tighter. And I don't want to call out every dim.

 

[MechNorth]

I hate when people follow those tabletized general tolerances. They are antiques. Unless your manufacturing process is also antique, it probably isn't what you really want. The standards were developed decades-plus ago, using established manufacturing techniques. That means screw mechanisms which magnified the manufacturing error with increased travel. Today's machines, with glass scales, encoders, lasers, and so many other means of accurately tracking cutting tool location, can hold the same tolerances for a part of 0.25mm as it can at 5000mm. Please rethink why you are using this poor method of tolerancing.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

I'm not quite sure what you mean (in how having profile to three separate datums would solve the problem).

Plus, isn't the general tolerance meant to apply to displayed dimensions anyway? Or are they meant for unstated lengths of material?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[mobostians]

My process is unique. I'm molding foam which will grow or shrink at a variable great. 1% of a 300mm width would be +/-3mm but the length may be 1000mm which I could expect +/-10mm of variation.

 

[fsincox]

No more antique than tolerances based on decimal places!

 

[CheckerHater]

Or Inch system...

 

[fsincox]

CH,

 

[ctopher]

Add dimensions/notes as needed on the drawing indicating your tolerances.
If the tolerances are variable, then they are not "general tolerances".

Chris
SolidWorks 11
ctopher's home
SolidWorks Legion

 

[CheckerHater]

NOTE:
THE CAD MODEL IS BASIC
EVERY DIMENSION YOU CAN POSSIBLY MEASURE
IS SUBJECT TO +/-3% VARIATION (NON-ACCUMULATIVE)

 

[MechNorth]

Ok, I see why you want the variable tolerances. Chris has the right idea; CH's note seems reasonable for the task.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[MintJulep]

1% is not a variable rate. It is a constant rate.

And there is no reason that tolerances have to be a fixed distance.

If within 1% of nominal is what you need, then there is no reason to not specify tolerances as +/- 1%.

 

[powerhound]

I was trying to find a good way to say what MJ just said. Thanks MJ.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[johnchrc]

Checkerhater,

You always break these GD&T questions down to a simple logical solution. Just a quick aknowledgemenet of your skill, so when I come uo with a GD&T dilema, you will feel compelled to help.

- CJ