CNC Part Setup

Gentlemen, here is a post I just placed at the shopmaster web site. I think
it fits in here and hope it helps. My response was in regards to a new guy
wondering about different ways to set up a part in a CNC.

Greetings,
Your head is swimming and you are wondering how to set up parts. This is
just the beginning! I taught CNC machining and injection mold manufacturing
for five years at a university and I have had the good fortune to work with
awsome toolmakers from all over the world. What I've found is that there
are a lot of ways to set up a part for milling. Some always set up (zero)
on the lower left corner so all numbers are positive. Others indicate in on
the middle of the part. Some on the middle, closest edge to you.

Here's what I do on critical things and why. I throw my part on the vise
crooked. Intentionally it is mounted so it looks like I walked by, tossed
it down, and bolted or clamped it to the table where it landed. I then
verify the top is flat (parallel) table. From there I use a wiggler (A
spinning shaft with a spring) and touch off on two locations on each of the
four vertical edges (assuming a square or rectangular part). Each time I
touch off I write these numbers down using the machine absolute coordinate
system. When finished "wiggling off" I have 8 points which I plot in
CAD/CAM, as mentioned using the absolute coordinate system from the mill.
In CAD I make a square using these 8 points by connecting each set of
points. Of course you have to extend the lines once connected. Once I have
an outline of the raw part in CAD, I take the PART model in cad and move
and rotate it to make it match the outline of the box I just drew (DON'T
MOVE THE REFERENCE BOX!!!). Typically I keep the part model and the
reference square-(frozen) on two different layers to make manipulation
easier.

I then generate my G-Code using a cam package. Cool thing is the numbers in
the CAD/CAM model
match the absolute numbers on the mill. No worry about loosing your G54
(home) position.

This method is by far the most time intensive method for setting up. But in
the five years I used it, I didn't screw up any parts because of being
located in the wrong position or from being .002" or so off in the wrong
direction. Reason being by indicating on all four sides you are halving (at
a minimum) the error from only wiggling on two edges. Plus if you make a
typo keying in your numbers it shows up in CAD as the reference square you
draw comes out goofy looking.

Authors note: I didn't say there were no screw ups in five years, only that
I was not indicated or homed wrong. There was still plenty of scrap, this
was just one less worry. :-)

The advantages to this system are big. Less mistakes and no "influence"
from you with regards to
"thinking" the part is "close enough". By this I mean when you indicate a
part in as parallel with an x or y axis, at some point you say ok that small
amount of error is acceptable. The way I mentioned circumvents this.

Disadvantages: Slower, more work, and you can not use the handles on the
mill in manual mode to make a slot in the x or y axis as your part is
sitting crooked (unless you want a slot running at an angle across your part
:-)

I didn't always use the above described method, only when I needed it to be
right and really accurate the first time. - which is frequently the case
with injection molds (or home made gun parts :-)

Hope that helps
Rob Anderson
Mechanical Engineer Tritronics
(And really curious guy in general)