GD&T with a complex form (holes on planes not parallel/perpendicular)

[Ospif1]

Hi all,

Required to do a GD&T drawing for the shape attached below. As you can see all the holes lie on different planes and I am confused as to how I do datum references and so forth whilst producing the drawing with all holes shown normal to the page when showing their dimensions.

The drawing needs to be in 3rd angle projection but I am unsure as how to comply with this whilst effectively showing the datum referencing and showing each hole normal to the page.

Help would be greatly appreciated.

 

[MechNorth]

1. identify which GD&T standard you applying to the drawing
2. without assuming too much, it seems that you believe that the datums change for each feature or face ... this is generally NOT the case. Typically, one datum reference frame does the entire part. Usually, you look at the design features which interact with other components. I normally select mating features going up the assembly chain rather than down. The feature which takes away the most dof is a good candidate for primary; second most dof, gets secondary datum, and finally the tertiary datum.
In this way, all features are related to higher-priority features, not just the face on which they originate.

As for how to get views normal to the features, CAD systems can generate auxiliary views normal to any face you select. There is no limit to the number of views used, as long as they add value.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Ospif1]

Thanks for the reply. I do not believe the datums change at all, I understand the primary, secondary and tertiary however not as well as i probably should.

For example I plan to make the top left drawing's hole the primary, the hole below the secondary and the last hole tertiary. My confusion is with how I dimension the drawing to show distance from hole to hole when they do not lie on perpendicular or normal planes to eachother. Creating some auxillary views to show the tolerancing of the secondary and tertiary holes is not issue either but once again just unsure as to how I am supposed to reference hole to hole.

 

[MechNorth]

Had to ask because lots of users feel they need to change the datum reference frame for each new face. Glad you're not one of them.

Looks like you'll need to dimension in sections; is that doable? You can use a combination of linear and angle basic dimensions.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Jim,
I agree with your analysis of parts,above. I have found people see the simple part examples in the books and take it way too literally. This is one of the reasons I sometimes wonder if the ISO’s seeming method of just show'em and let'em go is an actual strategy on their part.

Having separate datum frameworks is OK if they really function that way, but usually, (insert required wiggle room here) there is some functional requirement that ties the separate features together.
Frank

 

[drawoh]

Ospif1,

You need to pick primary, secondary and tertiary datums. 0nce you have done this, the GD&T is easy. Call up each feature with respect to your datums.

The practical stuff is hard. How will the part work? How accurate is your fabrication process? How will you inspect the thing?

The best primary datum is your primary mounting surface. You may have to design inspection tools.

JHG

 

[MechNorth]

I need to spend more time on ISO, Frank. It's on my list along with a bunch of other really important things!

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com