Re: knee mill router

Jon,

I did not mean knee mill. What I was referring to is using a drill
press and converting it to cnc (hobby stuff). The X & Y table is all
metal using dovetail grooves to slide back and forth (usually comes
with hand turnable handles). I already have a self made gantry and
found the gantry moves alot. I will limit my projects to just wood
projects for my gantry cnc. As for metal projects that require more
precision, I plan to build another cnc using a drill press and X&Y
tables (dovetail groove style). I know the aluminum extrusion of the
non moving gantry system will bend, but I plan to build mine using
mild steel. I am hoping I can get as nearly the same precision for
the fixed gantry as compared with the drill press conversion.

Thanks,

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> 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