Hard Broaching - inspire

Hard Broaching
Methods and Dimensioning Tools for Optimizing the Abrasive Layer on Hard Broaching
Introduction
Hard broaching is a finishing process to correct hardening
distortions and geometric deviations of hardened work
pieces (tipically over 60HRC) with any type of inner profile.
The high hardness of the work pieces requires the
application of special tools. The tool consists of a hardened
steel body overlaid with a single layer of metal-bonded
diamond grits and has essentially the opposite contour of
the work piece. The tool is divided into three parts (Figure
1): a centering, a conical roughing (material removal) and a
cylindrical finishing part (calibration).
Figure 2: Conventional pendulum strategy during hard
broaching
Figure 1: Hard broaching tool for internal gears
The internal profile is machined by the axial, oscillating
motion of the broaching tool. Between the start and the end
position, a sequence of numerous upward and return
strokes takes place (blue in Figure 2).
By each upward stroke the reached machine position is
increased by a constant amount (referred to as feed-perstroke
f stroke) until it reaches the end position. The upward
strokes are responsible for the material removal. Each
upward stroke is followed by a retreat to the level of the oil
nozzle. The retreat serves to clean the chipping from the
tool and happens only up to the line of the tool which is still
in contact with the work piece. As soon as the final position
is reached, additional calibrating strokes (red in Figure 2)
give the work piece its final dimensions and superficial
quality.
Objectives
The project aims to get a deeper understanding of the hard
broaching process. This allows to substitute the empirical
solution detection by scientifically supported determination
of the process parameters and of the tool dimensioning.
For this purpose it is necessary to determine the influence
of the removed chip volume in function of the process
parameters and tool’s geometry over the available chip
space on the abrasive layer. There the maximal admissible
load at the diamond grain should be determined without
damaging it. This is the basis of a force-controlled process
strategy. Furthermore the influence of the process
parameters along the accumulated machined length should
be analyzed, so that the lifetime of the tool can be better
controlled.
Approach
To easily determine the influence of the process and tool
parameters on the results, a stochastic model of the
broaching tool was developed at the hard broaching
computer simulation. This model takes into account the
differing shape, size, orientation and position of the single
grains over the tool’s body. The kinematics of the hard
broaching corresponds to the conventional peldulum
strategy.
With the mathematically generated abrasive layer is the
cutting phenomenon simulated. From this results the
position of the active grain edges and their respective
cutting areas, and from the active grains the available chip
space is calculated. The relation between chip space and
cutting area results into the theoretical maximal feed-perstroke,
which can be carried out without having blockage of
the chip spaces.
The chip space is defined by the enclosed volume between
active grain and neighborhood in the cutting direction.

chipping from the tool. This cicle is repeated up tp the
moment that the tool achieves the end position.
Figure 3: Simulation of the hard broaching tool
Figure 5: Influence of the feed-per-stroke on the cutting
force (u.) and cutting perpendicular force (l.)
Status
This project number CTI 8967.1, supported by the Swiss
Confederation’s innovation promotion agency (CTI) is
closed at 2009.
Figure 4: Evaluation of the available chip space
During hard broaching experiments was detected a strong
influence of the feed-per-stroke over the cutting force and
the cutting perpendicular force. Probably is the increase of
the forces caused by the blockage of the chip space on the
abrasive layer. A new experimental series will be carried
out to correlate the increase of the forces with the blockage
of the chip space.
The correlation between forces, chip space and maximal
admissible load on the grains build together the basis for
the force-controlled process strategy. Thereby the tool
should moves continuously into the work piece until the
specified force limit is achieved, so that no damage of the
abrasive layer happens. As soon as this force limit is
achieved a retreat of the tool should follow to clean the
Projektpartner
Fässler AG – www.faessler-ag-ch
Breu Diamantwerkzeug GmbH – www.breu-diawz.ch
Kontakt
Kuster Fredy
IWF Institute of Machine Tools and Manufacturing
ETH Zurich
Tannenstrasse 3, CLA G17.1
8092 Zurich Switzerland
Tel +41 1 632 24 24
Fax +41 1 632 11 25
e-mail kuster@iwf.mavt.ethz.ch
August 2009