Re: [CAD_CAM_EDM_DRO] Auto tool changer

> Hello,
>
> I was wondering if someone could please explain how an automatic tool

changer on an industrial cnc mill works?

> Is there still a drawbar and are there sensors which tell me machine the

tool is correctly engaged?

[...]

Yes, there is still a drawbar, but typically it is not a threaded rod that
engages a toolholder.

An ATC must quickly change tools, so there is usually a retention knob
on the end of the toolholder. The following should explain this a bit more.
It comes from
http://www.compumachine.com/Products/VirbraFree/VFhsm/VFhsm2.htm
It was written by William Popoli, President, IBAG North America:

"
The most common technique used in drawbar construction is to stack
belleville washers to
create a long tension ring. The end of the drawbar grips the toolholder
retention knob,
and holds the toolholder in position in the taper. When a tool change must
occur, a hydraulic
or pneumatic cylinder compresses the drawbar, and the toolholder is released

With regard to spindle design, the drawbar presents some challenges. A
drawbar is a movable
device, and with each actuation the springs may end up in slightly different
locations. This can c
reate a balance problem, which could cause unwanted vibration at high
speeds. To overcome this,
drawbar components are manufactured to close tolerances, and guide bushings
are used internally.

Also, as speeds increase, the holding force required also increases. It is
not practical to increase
the holding force by simply increasing the number of washers, as this would
require that the spindle
shaft be longer (remember bending modes?). It is also not always practical
to increase the diameter
of the washers, as this may require the shaft to be larger (larger bearings,
lower speed!).

To satisfy the holding force requirement, mechanical locking systems are
sometimes used.
The drawbar uses belleville washers to pull the toolholder into the taper.
Once seated, however,
a mechanical locking system then is actuated. The locking components may be
small balls or cams.
After the locking mechanism is in place, all cutting forces are directed
against the solid steel shaft,
not against the belleville washers. This system provides very high holding
force and rigidity, which
is critical to the high speed cutting process.

As the spindle will be used with an ATC magazine, it is necessary to have
electronic sensors or switches
built-in to indicate to the control logic when a tool is clamped, unclamped,
or missing. These signals
must be derived from monitoring the position of the drawbar.

"

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robert weiss bobweiss@...