huangyhg

a simple practical gd&t problem....help needed
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.
your drawing didn't come through. please post it again.
dave d.
ok...i hope it comes through this time.
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.
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
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.

these sites may help:
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
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...
promoting, selling, recruiting and student posting
are not allowed in the forums.
(add stickiness to your site by linking to this professionally managed technical forum)
title: drafting standards,
description: drafting standards, gd&t & tolerance analysis technical support forum and mutual help system for engineering professionals. selling and recruiting forbidden.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]