Hi Sug,

A Tertiary datum is the third datum, usually shown as Datum C on the
drawing.  "A" for Primary and B for secondary datum's.  Drilling datum
uses the center of a drilled hole as a datum, and that datum can be any
of A, B, or C datum's.

Datum's only serve to set up reference planes and zero points.  A three
dimensional object needs three coordinates to define a point, so three
axes must be defined (X,Y,Z).  Any letter of the alphabet can be used to
define those axes, but convention usually sticks with A,B and C.  It is
bad form to define a fourth datum along the same plane as another datum.

A Datum can be any ANY arbitrary spot on a board, but unless one wants
to really anger some people, you should pick points that make sense.
For example if your board bolts to a chassis using three holes, then one
of those holes should be a datum and the zero point.  The dimensions of
the board, locations of routed features etc, should dimension from that
zero, and not a corner of the board.  This is because the hole in the
PCB must mate with bolt from the chassis and mating features are more
importantly located than non-mating features.  Since there are three
holes, a line drawn from the zero hole to one of the other holes can be
used to establish another axis.  While that would be good for
establishing the "important" feature of another mating hole, it will
cause confusion establishing dimensions with the rest of the board if
the line to that hole does not parallel an edge of the board.  Otherwise
an edge of the board should be used to establish an datum/axis.

Another example: boards that have edge connectors mating with a socket
should use the edge of connector as a datum (x axis) and a feature
perpendicular to that edge, such as a slot, as another datum (y axis).

Another example: the aluminum wheel of your car would use the center of
an imaginary circle drawn through the 5 lug bolt holes as the center
datum (radial axis).  The mating surface on the back of the bolt holes
as another datum (surface plane).

There is a tremendous amount of technical rules and methods associated
with establishing and using datum's (a book worth), way too much to
write in email.  Really, you should take a college course on "Geometric
Dimensioning and Tolerancing" or at minimum, JEDEC offers that course.
Getting it wrong causes problems and can make a product
"unmanufacturable".

Ryan


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Suganthi
Sent: Thursday, March 24, 2005 4:39 AM
To: [log in to unmask]
Subject: [TN] Datum

Dear All, 



I would like to know which is Tertiary datum,Drilling Datum,Primary
Datum
and Secondary Datum in the boartd .Pls let me know if any more datum
points
there.



Thanks 


---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text
in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------
Sug

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------