Re: Z velocity

[Jon Elson <elson-at-pico-systems.com>]

Richard wrote:

>Hi,
>
>I'm CNC'ing a knee mill with a 56" x 10" (36" x 18" travel)
>table and 18" knee travel, 5" quill travel. I'm planning on
>around 200IPM for the table but haven't a clue how to gear the z
>axis quill.
>
>I'm working on bracketry to hold the 0.2" pitch ballscrew and
>pulleys & belts. I'm using AC Servo motors and drives and have
>one of around 3.5Nm(500oz in) constant stall / 25Nm(3500 oz in)
>abs max peak reserved for the Quill drive. Current thinking is
>double HTD pully reduction of 127/30 and 72/30 giving 0.5mm (20
>thou) quill movement per motor rev and a max quill speed of
>25mm/sec (~1"sec).
>
>I've no idea what the norm is for home conversions. How do those
>figures sound?. I guess the mill is similar to a large
>Bridgeport. It's a Beaver MKII.
>
>  
>
I think the motor is too large.  On the Bridgeport 1J head the 
power-feed clutch is set at 200
Lbs of linear force.  Maybe this is to protect the drive components, and 
the spindle bearings
can take more, but it seemed like a good limit for me.  I assume the 3J 
has a proportionally
higher limit, but 400 Lbs of linear force would seem like a good figure. 
 Using a belt reduction
on 25 Nm peak motors sounds like you could demolish the spindle bearings 
on your first
crash.  I'm not going to run the whole computation, but I'm guessing 
over 2000 Lbs of
linear force there!  I'm using a VERY small servo motor on my quill.  It 
delivers the
200 Lbs of linear force and then some, but not a great deal more, and 
that's fine with me.
I have crashed all the axes of my machine at one time or another, and 
although my coordinate
systems may have slipped a little, it never did any damage to the 
machine, itself.  The ball nuts
are bolted in place, and a good crash may shift them slightly.  I have 
broken tools off in the
spindle, too.

I think you should think about this and calculate the linear force this 
system could deliver
before going any further.

Jon