Re: [CAD_CAM_EDM_DRO] ...but how much precision????

Hi Bill:
You've just shed a new light on what you were hoping to find out about.
Basically, precision in machining is like precision anywhere else...it cost
a lot to get there.
It is possible, even on worn out equipment, to achieve very high levels of
precision, but it requires a quite different machining strategy than just
moving the thingy to where the number says it will be right and then
charging ahead.
This is true even on the most up-to-date equipment.
Where a ballscrew equipped CNC shines, is that the reliable, backlash "free"
movement takes one of the many variables and reduces its importance to the
extent that it can often be ignored.
Let me give you a simple example:
Suppose I want to mill a 1" square stump sticking up from a block of
aluminum.
By just programming the nominal size and grabbing a stock new endmill and
walking around the job with one roughing cut and one finishing cut, I can
get within 0.002" of what I wanted on my Haas VMC.
The stump will almost always be oversize and will almost always be 0.0002"
wider in the Y direction than in the X direction because my mill happens to
be more rigid in X.
I don't need to do anything special to get there...I just program and go.
For lots of applications this is plenty good enough.
To get the block reliably within 0.0002" I have to rough the block within
0.002" oversize and then invoke cutter compensation and either a probing
routine, or measurement protocol to nibble my way down to my goal (actually
0.0002" is less than the ripples produced by the cutter and ain't really
measureable on a milled part anyway!!).
To make it exactly square I have to set up two cuts in the X direction and
then 2 cuts in the Y direction rather than just driving around the periphery
of the block.
To make hundreds all the same within 0.0005" I have to set up flood cooling,
an in process gaging routine, a compensation routine, and a cutter changeout
cycle at intervals determined by cutter wear.
So you can see, that even on modern equipment, you can't just set up and go
if you require super precision.
My mill will position within 0.0002" and is repeatable to within half of
that (or so the manufacturer claims).
In my work (most often injection molds) I need to be able to get within
0.0005" reliably on features such as shutoffs, 0.0002" on locations of
features such as leader pin and taperlock bores, and 0.0005" on pocket
dimensions and locations.
These are not "plug in the number and go" kinds of features.
When I rout plastic housings for the electronic fab houses, I can just bang
out the program, assume the cutter will cut nominal size and go.
A routed housing with 20 cutouts typically goes for under 5 bucks Canadian,
and a small mold typically goes for twenty grand.

To bring all this back to your mill conversion...yes you can hit exquisite
precision without ballscrews if you are willing to do what it takes to
compensate for the machine's limitations, but it will cost you both time and
programming complexity.
If you're knocking out bumper brackets it really doesn't matter, and the
autonomy of the machine is its biggest asset.
If you're expecting to make precision parts at high production rates, you
require a very robust machine, and even then it's a lot more than just plug
in and go.
Hope all this helps.
Cheers

Marcus

----- Original Message -----
From: "Bill Kichman" <billkichman@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, May 09, 2003 3:23 PM
Subject: Re: [CAD_CAM_EDM_DRO] ...but how much precision????


> Thanks for sharing your experiences guys...what I was basically after, is,
> how precise must my machine's backlash be, to attain a reasonable amount
of
> precision, all other things being equal, without having to resort to
> ballscrew conversion. And additionally, how much precision is typically
> merited in machining work As I understand it now, most professional cnc
> machines are lucky to attain 0.0005 to .0015 accuracy. Sound correct?
> I spent a part of last evening adjusting the acme screws on the old 1945
Van
> Norman mill that I will be converting to cnc, and using a dial indicator,
> was able to reduce the backlash on all 3 axes to within (1) thou. The
> handwheels are a bit tighter than I would want for manual use now, but I
> don't intend to do a LOT of hand turning :-) This machine is in great
> shape, they don't make things like they used to.
>
>
> Bill Kichman
> 103 Old Furnace Road
> Cornwall, PA 17016-0643
> tel/fax 717/270-0714