Re: [CAD_CAM_EDM_DRO] knee mill router

george_barr wrote:

>I would like to know the advantages of on CNC mill design over the
>other. The first machine is a Knee Mill (e.g. drill press converted
>to a CNC machine)
>

A knee mill is NOT a drill press converted to anything. The knee of even
a small milling machine weighs several times more than an entire drill
press. A drill press, as I know it, has only one controllable axis,
that is the
quill, and it goes up and down.

A knee mill has a very accurately made XYZ orthogonal motion system
in addition to generally having a quill for convenient drilling and tapping.
These axes are capable of maintaining position and orthogonality under
high cutting loads, and the capability of delivering substantial forces
to drive work into the cutting tool. 1000 Lbs of linear force is not
unusual in heavy rough cutting.

> and the other is where the gantry is stationary and
>does not move (see sample picture at the bottom of
>http://www.campbelldesigns.com). If I want to mill precision metal
>projects where precision is very important, how much accuracy will a
>Knee Mill provide over the stationary gantry design? Also, what is
>the advantages/disadvantages of one over the other?
>
>

Can you jump on your gantry without deflecting it more than a few thousanths
of an inch? (From the looks of it, jumping on it would permanently warp the
extrusions.) One of the deflections I know (as I use it) is that
pulling toward
me on the quill with about 50 Lbs force deflects the cutter about .001".
This is enough to allow a stopped boring tool to not leave a scribe mark
on the
work when I retract it before the next pass. (This is on a round ram
Bridgeport
retrofitted with the J head.)

I know that the table and knee are stiffer than the above "measurement".
Your gantry might be able to do very light milling of aluminum with 1/8"
end mills, but not handle much bigger tools without excessive vibration.
I would expect a 3/8" end mill to get it vibrating .050" or more at the
center
of the span. How do you set the axes to be precisely orthogonal? Do you
use a precision scraped master square? Or a laser interferometer? How
about
a Renishaw ball-bar tester? If you don't use some precision tools to
set the axes
square, nothing you cut will come out the right shape or size.

Jon