Re: Knee or Quill Drive?

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

Jon,

I measured the voltage across the motor terminals with a scope, on
raising the load the trough of the voltage falls to 30 volts as the
bulk capacitor discharges and rises to 32.5 as it charges. The
supply would benefit from some additonal volts and the bulk capacitor
need increasing from its present 10milliFarad.

I do agree with Peter's earlier comments that it is very convenient
having the long travel available on the knee as the movement is 0.1"
per revolution and tool changing with R8 tooling more often than not
involves droping the knee.

You are correct in your assumption that drive is through bevel gears,
backlash has been suprisingly good at .003" and the limited tests I
have made shows no significant cyclical errors.

I obtained my XY ball screws as off the shelf replacement from Hiwin
and it looks as though a Hiwin screw for a 12" Y axis is the same
pitch and direction as the existing acme but slightly longer. I need
to investigate whether there would be much modification needed to the
working end.

If I decide to go along the knee route I guess the order of work will
be to check the slides, install the ballscrew and then remeasure the
torque and change the drive ratio as needed. In my talk to Mariss he
did run through the recommended motor torque to required torque, it
would be very helpful if he could publish it for us....

Thanks for all the help.

Bernard


I'll be following up
> bwrfromuk wrote:
>
> > Hi Group,
> >
> > I am in the process of converting an Enco 9x42 mill to CNC. X & Y
> > axis are working fine and I recently connected drive to the
knee. I
> > am using 600oz-in peak servos with G320s, all axis are geared down
> > 2:1. Using Master5 X and Y feed limits at 46 in/min, Z loses
track at
> > 3 ins/min. The torque measured to lift the Z axis is 40lb-in
(belt
> > off). The measured motor current (lifting)is 8 amps, measured
across
> > a 0.1ohm resistor with a 'scope, ripple across the 37 VDC supply
is
> > 2.5Vp-p. From the limited data on the motors the peak current
rating
> > is 12 amps max, nominal voltage is 38 volts, RPM is 900.
> >
> > The question I am now facing, and asking the groups advice on, is
it
> > best to carry on working on the knee drive or would I be better
off
> > somehow driving the quill?
>
> Depending on the work planned, the only reason to power the knee is
if
> you will have to move greater than the quill travel in one move.
If not,
> then the quill is a LOT easier to move, and can be moved a lot
faster,
> too.
>
> I would make a guess that the motor should be geared down much more,
> as the stall ocurrs at such a low speed. You may also have much
better
> results with a higher voltage power supply. Much of the 38 V may
be lost
> in the Gecko and armature resistance. When 8 A is delivered to the
motor
> lifting the knee, what is the voltage across the motor terminals?
>
> > To date I have not replaced the Z acme screw, how much improvement
> > would a ball-screw give and would it add further complications
like
> > the knee dropping if the drive kicked out?
>
> A fine pitch screw may not drop when unpowered, it depends on the
knee
> weight and the drag of the motor. A power-off brake could probably
be
> built very easily.
>
> But, the real question with the current setup is what accuracy do
you get?
> I am assuming the knee is driven through a bevel gear arrangement
like
> the Bridgeport. This can lead to all sorts of cyclical errors and
backlash
> between the motor and the knee.
>
> > How large a motor do I need for the measured torque? From the
> > measurements and talk with Marriss I could and should improve the
> > supply with additional capacitance and the supply voltage could be
> > increased some, but the fundemental problem seems that the motor
is
> > too small. Does anyone have a source for reasonably priced brush
> > servos?
>
> 40 in-lb is 640 in-Oz. So, with a 2:1 speed reduction, you should
need
> only 320 In-Oz. That doesn't sound like a big motor. (This is
neglecting
> cutting forces, which might add a significant amount.) With a 4:1
> reduction, you'd only need 160 in-Oz. That is the first thing I'd
try.
> You could even use V-belts and pulleys just to test the performance,
> and then buy the belt and sprockets after finding a ratio that
works.
>
> > What strategies do you recommend for driving the quill? I've
looked
> > at Jon's method of driving a dog through the stop mechanism in the
> > quill and also seen a pulley on the quill coarse feed lever. Any
> > others?
>
> The coarse feed handle drives the quill through a rack and pinion.
> This allows huge backlash on the quill, so it is literally a joke,
unless
> you'll only be using it to drill through holes. I've removed my
> pinion entirely, so it won't be clunking around as the quill moves.
>
> Jon