datum location
 

datum location
does anyone know the proper way to add a datum point to an imaginary point?
find a job or post a job opening
and just how is the imaginary point being established please?
it will be a center point(not centerline)of a closed slot.
patatmoog,
your datum must be a real feature. a basic rule of gd&t is that the fabricator and inspector must use your datums for fixturing. it helps to visualize what this fixture looks like.
the width and length of a slot can be picked up by a cross shaped pin. this is necessary if you have a separate datum that prevents your part from rotating.
on your drawing, the width and the length of your slot would be two separate datums, i.e. you will need four datums to locate your part. both datums need to be specified at mmc. whenever i specify a slot, i make the length sloppy. sloppy features are not good feature-of-size datums.
if you need an x/y locating feature, you should make it round, and slot all the other sloppy features. this makes the gd&t on your drawings, and your fabricator's and inspector's lives, easier.
another possibility is to use your slot to control rotation as well as x and y. this will work if your slot has an accurate width and it is long. otherwise, this is a bad idea. your maximum angle error in radians is your maximum clearance divided by your slot length.
jhg
there is no proper way to do it, because it isn't proper to do.
datums have to be real, tangible features.
v
you can add an imaginary datum to it.
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
this will maybe confuse, but it is a critical distinction. a datum is a theoretical point, line (axis) or plane. a datum feature is a real, physical feature on the workpiece, which is used to establish the datum (point, axis or plane). the datum simulator would be the pin or key (in this case) or other representative component that would be used to represent the datum.
jim sykes, p.eng, gdtp-s
gentlemen,
are we exercising proper descriptions in our discussion? if not shouldn't we?
i was taught that datums are theoretical points, planes and lines. that is rather imaginary is it not?
datum features on the other hand do exist on the parts.

ringster,
the op is asking how to add a datum point. i am interpreting this as how to specify such a datum on a drawing. my cross-shaped pin will locate the centre of a closed slot, at mmc, with the limitations noted above.
i suppose my first paragraph is not well written. how about...
your datum must be defined by a real feature.
jhg
my guess is that the orig post was to inquire how to identify a datum point. if that is the case i believe it is covered by the standard.
asme y14.5m-1994 should cover how and where to create datum points.
matt lorono
cad engineer/ecn analyst
silicon valley, ca