A pneumatic extractor is a good choice for
small-diameter tubing. This is an environmentally friendly piece of equipment, but the typical shop air (80 pounds per square inch [PSI])
found in most fabrication shops isn’t enough
to handle large diameters.
More common on modern tube bending
machines is a hydraulic extractor. With the
right hydraulic cylinder size, a hydraulic
mandrel extractor is an efficient way to
extract the mandrel. While a pneumatic
extractor is limited in capacity, a hydraulic
extractor has no such limit.
A chief characteristic of hydraulic
extractors is that they don’t hesitate when
A proximity sensor on a hydraulic mandrel
extractor sends feedback to a machine’s
controller to confirm that the mandrel has been
extracted.
they receive the command to retract. Some
pneumatic extractors delay before the mandrel starts to retract from the bend zone,
which leads to a longer cycle time. Even if
the delay is short, it can add up over a year,
a month, or even a week.
Another type of extractor uses an electric servo drive. Greener than a hydraulic
extractor, an electric servo extractor can
work to shave the cycle time from the overall bending process. Electric servo extractors aren’t as limited in capacity as pneumatic extractors are; the drawback concerns the costs, both the initial investment
and replacement parts. It is never fun to
purchase a configured servo drive from the
original machine tool builder when the bender is down.
Mandrel extractors are as varied as tube
bender tooling itself. Variants include
extractors for nonround tubing, multistack
vertical positioning, and those that position
a mandrel in more than two horizontal
locations. Instead of having just two positions (inserted and retracted), some tube
loaders require that the mandrel start in
one location (before the bend starts); move
to a second location to support the tube
during the bending process; and retract to a
third location after the bend is finished.
Early
Mandrel
Retraction
(Deg.):
CONTROLLING A
MANDREL EXTRACTOR
ON A CNC TUBE BENDER
The goal of the machine’s controller is to
get the mandrel into and out of the bend
zone at the right time in the bending
process. For the most part, this is not too
difficult. A mandrel extractor typically
pushes the mandrel into a bend zone before
the clamp closes. Then, after a bend is finished, the mandrel extractor pulls the mandrel out of the bend zone before the clamp
and pressure dies open. It is most important
for the machine’s controller to confirm that
the mandrel is extracted from the bend
zone before the clamp and pressure dies
open (see Figure 2).
In some tube bending applications, the
extractor must retract the mandrel from the
bend zone before the bend is complete.
When the bend is formed at 120 degrees per
second, the machine’s controller must be
able to react quickly to retract the mandrel
consistently from bend to bend, part to part.
This technique is known as early mandrel
extraction. High-end tube bending controls
usually allow for this feature through the
operator interface (see Figure 3).
ALTERNATIVES TO A
MANDREL EXTRACTOR
Some mandrels don’t need to be extracted.
For example, some bending applications
use a low-melting-point alloy for the mandrel. The operator heats the alloy, pours it
into the tube, and allows it to cool before
bending the tube. After making the bend,
the operator heats the tube and pours the
mandrel out.
Another approach is pinch-roller tooling. Pinch rollers are designed to control
the ovality of a tube in the bend zone and
do not require a mandrel.
Figure 3
To program the controller for early mandrel extraction, the operator enters an angle. This angle
determines how far before the end of the bend the mandrel is to be extracted. A sophisticated
controller allows the operator to enter a unique value for each bend.
George Winton, P.E., is the founder and
president of Winton Machine Co., 3644
Burnette Road, Suwanee, GA 30024, 888-
321-1499, gwinton@wintonmachine.com,
www.wintonmachine.com.
