Hello Scott, The 'True Position' dimension, (boxed, sometimes also called 'basic') is without tolerance. The positional tolerance (in the case for example of a drilled hole) is expressed as a diameter, within which, the centre of the actual hole must lie. This positional tolerance diameter is 'normal' to a reference datum as well. I usually specify a diameter positional tolerance of 0.08mm for the pattern of drilled holes on the board, and use a diameter positional tolerance of 0.2mm to position the drilled hole pattern on the PCB profile. These values are based on our manufacturers' typical capabilities for good yield. hope this helps a bit.. regards, Phil. -----Original Message----- From: Scott Kauling [mailto:[log in to unmask]] Sent: Saturday, 8 December 2001 06:31 To: [log in to unmask] Subject: [TN] PWB Fab Note Question - Diameter True Position In IPC 2221 section 9.2.2 describes hole location tolerance, it is expressed as "diameter about true position". IPC T-50 defines "true position", not diameter about true position. My questions are: 1. How does the diameter about come into play with the true position? 2. What tolerance of diameter about true position do you specify on your fab prints 3. What is the typical capability of diameter about true position that PWB fabricators hold? Thanks in advance for your input. Have a good weekend! Scott Kauling ---------------------------------------------------------------------------- ----- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------