TYP and REF.dimension definition on drawings

TYP means that the dimension shows up in some (unspecified) number of places. It might, for instance, be applied to a fillet that shows up on all the edges of a parts: R.25 TYP

REF is a dimension that is of interest, but which the machinist has no control over. For instance, if you're modifying an existing shaft by adding a keyway, you might list the OD of the shaft as a REF dimension

A "REF" (reference) dimension is one that could have been determined from other existing dimensions and geometry. If such a redundant dimension did not include the mark "REF" the tolerancing of the part could be ambiguous.

A good example of the use of "REF" is that of a right triangle. If the two legs of a right triangle are dimensioned, this should be sufficient to construct the triangle. The designer might wish to add a dimension for one of the angles of the triangle, or its hypotenuse, if these dimensions are critical or germane to the design. Now you have three dimensions for an object that only requires two, so one of them should become a "REF" dimension.

Why? Let's say the default tolerances in the title block of the drawing call for the leg dimensions to be +/-.005", but angle dimensions are only accurate to +/- one degree. This causes a contradictory situation. A full degree difference in the angle is almost certain to result in more than a .005" change in the opposite leg for any but the smallest triangles.

A "REF" dimension is usually added as a courtesy, as an additional "check" during manufacture. It is redundant and is not intended as a serious dimension to be "held", and tolerances do not apply to it.

Don Kansas City

"eromlignod" wrote in news:1112884805.325560.42170 @z14g2000cwz.googlegroups.com:

These are also called 'helper' dimensions, and in ISO format drawings, will typically be shown in parenthesis. These should not be relied upon for any actual machining or fabrication of the part. Many times these dimensions may be rounded off to a rough dimension.

Typ = Typical, which would mean it applies any place on the drawing you see that feature that is not otherwise dimensioned. Good example of TYP would be threaded hole callouts, where you only call out one hole, but the TYP means it applies to all holes not specifically called otherwise. 'M5x0.8 TYP' Some may dimension 'M5x0.8 Typ 5 plcs'