Re: Z-axis: Quill vs Knee

Bertho Boman wrote:

> From: Bertho Boman <boman@...>
>
> What are pros and cons for using the knee versus the Quill for feeding
> the Z-axis?

If you use the knee as the Z axis, then you can't use the quill tilted at some
special angle to drill holes at an angle, without a LOT of tricky work.
If the knee slide is worn (like mine) the top of the table moves about .001"
in the Y direction, when the knee direction is reversed. If you want to have
'feel' of the progress of a drill, tap, etc. the knee is too heavy for that.
If you need quick motion to retract a drill, or to feed a very small drill in
soft work, the knee may not be able to move that fast.

> I realize the obvious con of much larger mass and force required and
> massive "weight compensating springs" for the knee. On the other hand,
> locking the quill will give a much more rigid system.

Well, on a 'mill-drill' this is most likely true. I have a WELL-worn Bridgeport
J head, and the quill looks like somebody used it for a hammer (or anvil).
But, it is as tight as I can measure. With the quill retracted or at full extension,
I can't see any deflection of the quill relative to the spindle housing - none
whatsoever - so it must be less than .0001" Now, on the spindle housing, itself,
I can deflect it .001" or so just by leaning on it hard - maybe 100 Lbs or so.
And, I can deflect the spindle more than that, but it is NOT the quill that is the
weak link. I have about .001" backlash in my whole Z axis drive, but that is
a combination of MANY stacked components.

>
> My mill already has built in DRO. I intend to use the DRO encoders for
> the CNC function. The Z-axis is monitoring the knee, not the quill. I
> can always switch to the quill but I like to evaluate my options before
> digging into the conversion.

Yes, someday I want to at least put crude power on my knee, too.

> My goal is to do the conversion so that I can also manually run the
> mill. Although, that could be under joy-stick mode and I let the servos
> do the hard work

Yup, that's how I do manual work, now -- from keyboard buttons.
Someday, maybe, we'll have a joystick in EMC.

Jon