Re: CNC Part Setup

Hi Rob,

Interesting.... I don't care for this method for the same reason
Marcus mentioned. You can't manually debug the code easily.

We have a fairly sophisticated controller on one of my mills. For
irregular workpieces we position the part as you do, without having
undo concern for it's relationship to the X or Y axis. By touching
off one edge we find an angular value of how far off from true the
part is fixtured.

Generate Gcode as if the part was oriented with one or the other
axis. Then as the first part of the Gcode we insert an axis rotate
command of the previously determnined angle which allows the
controller to treat all further Gcode as if the part was perfectly
oriented.

I realize this method is not available to most CNC'ers since their
controls don't support axis rotation. I only mention it to show a
possible use for it. It has very many other uses, especially
combined with a repeat function. So, if you're ever in the position
of buying a CNC and axis rotation is an extra cost option, I advise
taking a hard look at it. It's an incredibily powerful option.
Also, IMHO, those of you writing control software should include axis
rotation.

Doug


-- In CAD_CAM_EDM_DRO@y..., "Rob Anderson" <randerson@t...> wrote:

> 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)