How To Select The RIGHT Drill Bushing - Page 4
INSTALLATION
SUGGESTIONS
Cast Iron or unhardened steel jig plates.
To maintain accuracy in the production
operation, extra care must be taken in mounting hole preparation and press fit bushing and
liner installation. Factors to consider are diametral interference fits, alignment, chip
clearance, and proximity of bushing to workpiece.
| MOUNTING HOLE. Diametral interference fits of .0005 to .0008 inch (headless press
fit) and .0003 to .0005 inch (head press fit) are usually recommended between bushing O.D.
and mounting hole I.D. In most cases, this interference adequately holds the bushing in
the jig plate, yet does not distort the jig plate or close the bushing enough to cause
bellmouthing or tool seizure, nor does it prevent a slip bushing from being inserted into
a liner. Enough interference is necessary, also, to prevent loose-fitting bushings, as
these will usually result in inaccurately drilled holes.
These interferences, however, must be considered in terms
of jig plate thickness and material, wall thickness of the bushing, and length of mounting
hole. Generally, the thicker the plate and the longer the bushing the less interference
necessary.
We also recommend that all mounting holes be jig bored or
sized with a reamer to assure roundness. Twist drills cannot be relied upon to produce
perfectly round or accurately sized holes. |
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| Diametral
Interference |
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| Bellmouthing |
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| Jig Plate
Distortion |
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BUSHING INSTALLATION
Use a hand arbor to press the bushing or liner
into the jig plate, whenever possible. If not, use some other means, such as drawing the
bushing into the jig plate with two steel plates and a nut and bolt. A hammer should be
used only if no other methods are possible - and then, never strike the bushing directly;
use a block to take the hammer blows. Whichever method is used, care should be taken to
maintain centerline perpendicularity, otherwise inaccuracies may result in the production
operation. Use of a lubricant will aid in bushing installation and will make it easier to
remove the bushing. |
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CHIP CLEARANCE Locate the bushing a sufficient distance from the
workpiece to allow the chips to clear. Less than a drill diameter is recommended for small
chips, one to one and one-half for large chips. Careful consideration of this point will
aid in producing accurate holes. For example, when drilling the original hole use a slip
renewable bushing that provides adequate chip clearance. Then insert a longer bushing for
accurate reaming to the required tolerance. |
DRILLING
FOR ACCURACY
Bring the bushing close to the work surface
for holes that must be drilled with great accuracy. You achieve the greatest accuracy when
you butt the bushing directly against the workpiece. Remember, the greater the clearance
the greater the chance for error. |
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Imbedment Materials.
When selecting the right type of knurled
bushing suited to a particular application, attention should be paid to the following
points:
Bushings may be either molded or pressed into
the plastic fixture. Whether molded in or pressed in, sufficient fixture material should
be present around the bushing to handle the thermal stresses which may develop (during the
production operation) due to the different thermal coefficients of expansion between the
steel bushing and the plastic fixture material. Other points to be considered are:
INSTALLATION OF MOLDED
KNURLED BUSHINGS.
Knurled bushings should be located in the mold
prior to the molding operation. Care should be taken to insure centerline
perpendicularity, and avoid molding material into the bushing I.D. Grooved diamond knurled
bushings are superior in applications that place a load on the bushing.
INSTALLATION OF
PRESSED-IN KNURLED BUSHINGS
Knurled bushings can be used in molded or
drilled holes in imbedment materials. Hole size will vary according to hardness or
ductility of the plastic. Therefore, specific hole size recommendations cannot be made.
Sufficient interference is necessary, however, to assure a firm hold of the bushing. Care
should be taken not to have too much interference, which may crack or distort the fixture
because of the displaced material. Reacting much like a drive screw when forced into the
molded or drilled hole, the high points of the knurled cut into the plastic and force
material into the adjacent low points, producing a tight fit. Too much interference will
cause too much material to be displaced, resulting in a cracked or distorted fixture.
Heating the plastic during installation aids in assembling. The expanded hole allows easy
insertion of the bushing, and shrinkage of the plastic during cooling causes the material
to flow around the knurled edges and provide a shrink fit.
Economy Bushing Co.
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