The number of decimal places used in a dimension normally refers back to the drawing's title sheet where there is often a note (either in the Note block or imbedded in the format) saying something like:
UNLESS OTHERWISE SPECIFIED:
DIMENSIONS ARE IN INCHES.
TOLERANCES:
.X±.1
.XX±.01 (could be .XX±.015 too!)
.XXX±.005
.XXXX±.0005
ANGLES±1º
Once you understand the functionality of the part then you'll know which dimensions have to be held tightly and which ones can be held loosely. To keep costs down you want to keep tolerances as coarse as possible.
In any given view you may have any combination of four-place, three-place, two-place, or one-place dimensions depending on the tolerance you wish to impart on a particular feature. These tolerances are often called the "block tolerances" and apply throughout the drawing regardless of the sheet, dimensioning method, or view. If a feature's tolerance has a geometric control (per ASME Y14.5-1994) then those rules say that the dimension be identified as a basic (theoretically exact) dimension by placing a box around it. In this case the number of decimal places has no significance. I think that if the size of the feature is presented with a three-place dimension then the convention is to write the basic dims with three places.
You could apply a set of block tolerances to a particular view by placing them in the view's caption (unusual but OK) and these would override the tolerances appearing on the title sheet.
As for ordinate dimensioning, this method may be used alone or in combination with any other method (chain, basic, baseline) and the type of part has nothing to do with it. This is VERY useful by increasing the number of dimensions you can place in one view.
After a while you learn to draw views that look "clean." In such a view there aren't so many dimensions as to make it look cluttered. Add a second identical view elsewhere in the drawing if you need to provide more dims.
Believe me I've seen drawings where EVERY dimension was held to a three-place decimal. This makes for an unecessarily expensive part while the "draftsman" just didn't care enought to give any thought to tolerances. Don't ever make statements such as "This part will be machined. They can hold every dimension this tightly." DO NOT LET THE MANUFACTURING PROCESS DICTATE HOW THE PART IS DRAWN. This practice tends to generate designs that are intolerant of changes in mfg process. For example, a part that is machined in the product's early production may end up being die-cast later on as the demand greatly increases. Who wants to be stuck with dozens of drawings of parts that can only be machined? You would be cursing the people who drew those drawings!
Tunalover
