Re: rE:Re:RE: programming by hand

Smoke asked:

>It appears to me your saying a drawing for a part should not have any
>tolerance at all Bill.
>Is that what you mean to convey...or did I mis-read something there?
>From an engineering standpoint there are very good reasons for using
>a nominal dimension and then adding a plus or minus (as required)
>tolerance on the part.

Smoke,

I'm not Bill, but I believe you misunderstood his point.

He is saying that using a 3 inch dimension (contained in the
electronic file of the drawing) as the BASIS for CUTTING the part
WILL result in bad parts IF the part tolerance (in the TEXT
DIMENSIONS of the CAD DWG) is not FIGURED INTO THE CUTTING PATH.

Using the numbers given; 3 inches +0/-.010 we have a tolerance range
of 2.990 to 3.000. If the CNC programmer USES the "drawn" line (which
is 3.000) to generate a toolpath, standard machining variations will
mean that a lot of parts will not meet tolerance (they'll most likely
be too big).

The 'generally considered' CORRECT way to do this tool path will
split the tolerance given, and program a 2.995 length. This way the
machining variations will fall on either side of the "programmed"
path, and MOST(if not all) parts will meet spec.

It is a VERY GOOD point to have made on this forum! I'm glad he
brought this up...

Hope this helps.

Ballendo