The rotary broaching principle can also be applied to broaching on
a CNC machining center or vertical mill. The only difference being that the
broach holder is rotated in the machine spindle and the part is stationary
instead of the reverse condition on a turning machine.
As manufacturers of rotary broaching tools, we soon realized that
in order to produce parts successfully there are a few basic rules that must
be followed. These rules are based on understanding how and why rotary
broaching works.
1. Cutting Principle
The basic principle that makes this tool work is the same for all
tools regardless of the manufacturer: The centerline of the cutting tool is
offset at 1 degree from the centerline of the work piece. This causes the
broach to shear its way into the part with a scalloping effect as it is
advanced into the work piece. We have found that the easiest way to visualize
this cutting action is to picture it as if the work piece were stationary and
the broach holder were rotating, instead of having the work piece rotating
and the holder stationary as is normally the case with a screw machine.
FIGURE 1
As can be seen in Figure 1, the broaching cutter spindle is driven
into the part at a 2 degree included angle of its cone of rotation. This
causes the broach to cut only on its leading edge, not its full end surface
as it would if it did not have the 1 degree offset. This eases the load of
the cut and creates a shearing, rotational cutting action so that cutting
tool is actually spiraling its way into the part.
2.
Construction
Since all holders operate on the same principle, the only real
difference between broaching tools is holder and cutting tool life.
The prime cause of holder breakdown is bearing failure. Following
experimentation with many types of bearing, our current bearing types,
construction, and quality have resulted in longer bearing life than for any
other tool available. Because of superior bearing construction, our tool is
more rigid, contributing to longer cutting tool life and more accurate and
uniform parts.
3. Feed
Rate
The Somma broach tool is offset 1 degree, and it is essential that
the helix angle of the feed rate not exceed one degree. If it does, the
broach will bottom out, crowding the metal instead of cutting, and causing
tool wear or breakage. See Figure 2. Therefore, the best feed rate should be
the equivalent of 1 degree helical maximum. To maintain this 1 degree
helical feed rate, the feed rate per revolution can be calculated by
multiplying the diameter that is to be broached by .016. (EX: .250"
part diameter x .016 = .004" IPR) The feed rate, however, must
never be less than .004 IPR or else a counterboring effect will occur.
Rotational speed (RPM) has very little effect on cutter life, since
the cutter rotates with the work piece. However, life of the spindle bearings
is affected by the speed of rotation, therefore, we do not recommend speeds
over 1200 RPM. Cutting tool material and use of coolants is not critical for
this same reason.
4. Blank Preparation
For internal broaching, the hole should be drilled
0.005"-0.015" larger than smallest diameter of the broach, and
countersink it at 90 degrees to slightly larger than largest dimension of
broach. Drill the hole as deep as possible to leave room for chip
accumulation.
FIGURE 2
For external broaching, turn the O.D. to be broached
0.005"-0.015" smaller than the largest dimension of the broach and
form a 45 degree chamfer on the front end down to at least the smallest
dimension of the broach. This 45 degree angle is essential for easy starting
of the broach; if the part design does not permit it, it can be removed
after broaching.
5.
Broach Geometry
Internal broaches must be ground with a 1-1/4 degree maximum back clearance on all surfaces. The face
should be dished at a 7-10 degree clearance. This can be done with a carbide
centering-type drill. The larger end of the broach should be made to the
high side of the part tolerance since the broach gets smaller as it is
sharpened.
External
broach dies must be made with maximum 1-1/4 degree back taper (draft) on all
surfaces. The front end of the opening in the die should be made to the low
side of the part tolerance since the hole gets larger as the broach is
resharpened.
The face
of the die should be sharpened with a 10-15 degree face angle. When
broaching squares or hexes, this clearance angle should be surface ground in
facets to coincide with the points of the form, rather than ground
cylindrically. See Figure 3.
FIGURE 3
6. Part Configuration
Hex and Square shapes can usually be broached without much
difficulty. Problems may be encountered when broaching other shapes, such as
splines or keyways, especially when the depth of the cut is greater than
half its width. If the chip being cut is too thick and does not curl away
from the cutting edge of the broach, the broach can jam up.
Please note: On Internal Broaches, countersink the hole at 45° to a
diameter larger than the cross corners dimension of the broach.
On
external broaches, chamfer the bar end with a 45° angle smaller than either the
across flats dimension or the I.D. points of the broach.
7. Spiralling
Spiralling of the broached form is caused by the back taper on the
broach. Since the broach is driven by the leading edge of the hole (ID)
against the nearest surface of the broach (BB), the space between the broach
and the hole caused by the back clearance allows the broach to rotate
slightly and cut a spiral as shown in Figure 4.
The
greater the back clearance on the broach, the greater the spiralling action.
For this reason, a maximum 1-1/4 degree back clearance is recommended
instead of the 1-1/2 degree more commonly used. Ideally a 1 degree back
clearance would eliminate all spiralling, but, due to machine or holder
inaccuracies, dragging action might cause other problems.
Spiralling will also occur if the broach is not properly centered.
The more the form is shifted off to one side, the greater will be the amount
of spiralling.
If the job layout permits it, the spiralling can sometimes be
reduced by reversing the spindle rotation half way into the part. This
causes the broach to be driven by the opposite wall of the hole thereby
reversing the direction of the spiralling and making the hole straighter.
FIGURE 4
8.
Setting the Holder for Center
Nothing
is more important than having the cutter centered as close as possible to
the center of the work piece. Improper center setting will cause uneven hole
configurations, oversize holes, spiralling, excessive cutter wear and
excessive holder wear.
Therefore, it is essential that you observe the following
instructions when setting up internal or external broach cutters.
Centering
Internal Broaches:
Step
#1
1.
Clamp shank of broach holder in a V-Block or in the machine turret.
2.
Loosen the 3 (Part #5) broach holder spindle screws that hold the broach.
3.
Install the broach in the broach holder spindle & snug up the 3 (Part 5)
spindle screws.
4.
With the shank locked in a stationary position (not rotating), indicate the
broach (at the outermost point of the HSS broach) to within .001 TIR by
turning the spindle & broach assembly by hand and adjusting the 3 (Part
#5) spindle screws in or out and then tightening.
Step
#2
1.
Loosen the 6 (Part #8) body adj. screws and the 2 (Part #7) clamp screws.
2.
Loosen the shank of the holder in the V Block and rotate the complete holder
while indicating the broach to within .001 TIR by adjusting the 6 (Part 8)
body adj. screws.
3.
When .001 TIR is achieved, tighten the 2 (Part #7) clamp screws and lock in
place.
4.
Start Broaching!
Centering
External Broaches:
Same
steps as above, but use a gage pin for setup. Please note:
Concentricity must be checked at the same distance, with the indicator point
located at the same location as the cutting face of the external broach.
FIGURE 5
| |
To Fit |
To Fit |
To Fit |
| |
ORBX00 |
ORBX0S |
ORBXO/ORBX2
ORBX3/ORBX4 |
| A |
.9445 |
.9445 |
1.5745 |
| B |
3/8 |
1" |
1" |
| C |
.200 |
.200 |
.200 |
| D |
.750 |
.750 |
1.406 |
| E |
.094 |
.094 |
.188 |
| F |
1/16 |
1/16 |
3/32 |
| G |
3/8 |
3/8 |
3/8 |
| H |
--- |
1/2 |
1" |
| Catalog # |
ORBX00-20 |
ORBX0S-20 |
ORBX2-20 |
CENTERING THE INTERNAL ROTARY BROACH
CENTERING THE EXTERNAL ROTARY BROACH
If you have a specific broaching problem you need
help on, please do not hesitate to contact
us. [Back to
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