A simple practical GD&T problem....help needed

[BullmanOz]

Hello

Although I have done a course several years ago on GD&T and have a understanding of some basic concepts, I as of yet have never really had to practically apply what I had learnt, much of which has since been forgotten. However I still retain all the notes and books and I intend to now relearn and become proficient at GD&T so as to apply it to my own design work.

My first question relates to datum definition. I have provided an illustrative example of a part which is meant to represent a simple premachined rectangular component and the required machine cuts to be made, with reference to datums. I inderstand that the dimensions given on the drawing provided are incomplete (eg. dimension X needs reference to the datum defined by the underside plane of the part somehow) but hopefully it conveys the intent.

How would a correctly annotated/dimesnioned GD&T machining instruction drawing look for instructions to make the desired machining cuts.

Thanks

Bull

PS: If anyone can point me to a good source of practical worked examples that show how to create GD&T drawings to capture similar practical design intent, please post them here.

 

[dingy2]

Your drawing didn't come through. Please post it again.

Dave D.

http://www.qmsi.ca

 

[BullmanOz]

OK...I hope it comes through this time.

 

[BullmanOz]

OK. Seems the link syntax cant handle the character &.
My file was named GD&T.gif

I have renamed it to GDnT.gif and reposted it. Link is below.

 

[ctopher]

The pre-shape is not necessary. Only show on the dwg the final part and the fianl dims/GD&T.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Apr 30, 2008)

 

[BullmanOz]

OK I do undertand that the machining drawing for this part wouldn't show the pre-machined shape and the drawing would only show the final part and dims/GD&T.

I am wondering how then it would be dimensioned/GD&Ted to capture the design intent/sequence/use of datums I have described.

 

[ctopher]

These sites may help:

http://www.efunda.com/designstandards/gdt/3d_datums.cfm

http://www.tec-ease.com/gdt-tips.htm

Depending on your design...you may have datums A, B & C (Bottom and two sides).

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Apr 30, 2008)

 

[MechEng2005]

Bullman,

I think maybe your intent is to have the datum be a machined surface to reference. In that case, you could just use a surface roughness to indicate that the surface must be machined and flat. Then use that surface as your datum and you don't need to worry about having a stock edge as a datum.

I think this is what you were asking about, but could be wrong. You probably know a lot more about GD&T than I do. I haven't used it in practice and haven't had courses on it ever. I've done a little "exploring" and deciphering when it appeared on others drawings. That said, I think you may also be able to achieve the tolerance machined surface (as opposed to a stock edge) as a datum by using GD&T for a flatness of the surface.

- MechEng2005

 

[KENAT]

I think MechEng has more or less got it.

There isn't really a distinction between your datum A and B.

The face may need cleaning up and this would be achieved probably with a flatness and/or similar tolerance.

Essentially/simplistically, assuming the face is your secondary datum surface then the datum will be derived from the 2 high spots. If it's your primary datum it will be from the 3 high spots.

KENAT, probably the least qualified checker you'll ever meet...