Re: [CAD_CAM_EDM_DRO] Knee or Quill Drive?

Hi Bernard,

I drive the knee on my BP clone mill. You cannot beat the benefits of a
Z axis with 16" of travel. I hardly ever use the quill. Tool changes
(R8 tooling) under complete CNC control are a breeze - not counting
manually changing the tool!

I had a LOT of trouble getting everything to work correctly. Firstly, I
had to fit a ball screw to the knee to reduce friction and eliminate
backlash. With the acme screw, I found when I rapidly drove the knee
down, I could observe the backlash. Now, I only rapid Z up/down at
about 30 IPM and about 60 IPM in X & Y. I have a NEMA42 stepper fitted
directly to the "hand" crank shaft as I have minimal backlash through
the the bevel gears after a bit of shimming. My motor is ~1750oz-in and
is driven by a Gecko. To eliminate the risk of the knee falling, I
installed two 75lb gas-lifts (from McMaster Carr) although I would
recommend two 150lb or more gas lifts to give a better safety margin on
a servo system. One commercial company that powers the knee uses servos
and they fit a brake for safety.

Powered down, the knee doesn't move at all. But remember, I have a
stepper installed - which has much greater rotational friction, unlike a
servo, when powered down. I also had to fiddle with the gib adjustment
because at speed in the down direction, the tapered gib slides would
vibrate - until I had them snugged, but not too snug. Trial and error
will fix this - just remember there are screws at both ends (top and
bottom) of the gib slide.

Once you change to a ball screw, your servo motor power should be
sufficient. I would also recommend yo do the other mecanical mods
before upgrading your motor.

I've posted a couple of pics in the folder: BP Clone CNC Retro One pic
shows the Y&Z motors fitted and the other blurred pic shows the mod you
will have to make to the inside hole of the knee column - to provide
clearence for the ball nut return tubes. I couldn't find a pic of the
gas springs - but let me know if you want to see that, too. FYI, I got
my ballscrews from Rockford Ballscrew Co. - they had to be custom made
for my (First Brand) model of clone...

Cheers, Peter



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?
>
> 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?
>
> 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?
>
> 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?
>
> All comments / suggestions gratefully received.
>
> Bernard Randall
>
>
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