Drill Bushes

Slender and cantilever type cutting tools, mainly drills, usually suffer from run – out due to possible errors in the drill, sockets and drilling machine spindle and finally in the overall alignment. Such run out causes over sizing, out of roundness and surface roughening of the drilled holes. Such run out aggravates further with the increase in drill speed (rpm) and the thrust force, especially if the drill is not geometrically symmetrical. This often leads to, in addition to poor product quality, breakage of the drill by bending and / or buckling. To reduce such problems, bushes are used in the jigs to guide the drill bits as indicated in Fig - The bushes, used to guide and properly locate drills, reamers etc. are generally made of carbon or alloy steel and made wear resistive by hardening to RC 60 and above. Often bushes are also made from grey cast iron for antifriction and protection of the tools. - The hardened jig bushes are finished outside by grinding and inside by grinding and lapping if high precision is insisted. - The bush’s length should be sufficient (≥ twice drill diameter) and its diameter should be slightly larger than the drill diameter - Design and construction should enable easy and quick proper fitting and removal or replacement of the bushes - Bushes should not come out from its seat along with the drill during its return.

Types of Drill bushes

Depending upon nature of fitting, quick mounting and replacement, job requirement etc. jig bushes are classified into several types. Bushes may be

• Press fitted type
• Slip type
• Screwed type

Press fitted thin sleeve type bushes are generally used for shorter runs and are not renewable. Renewable type slip bushes are used with liner. But screw bushes, though renewable may be used without or with liner. Bushes may be

• without head
• with head
• with a flange being screwed on the bracket

Fig. Locking of frequently replaceable bushes

• Frequently replaceable bushes are provided with some locking system as shown in Fig
• Some special jig bushings are often designed and used as and when required as indicated in Fig.

• Many other types are possible and made depending upon the working situation and critical requirements

Locating Pins

• The pins of various design and made of hardened steel are the most common locating devices used to locate a work piece in a jig or fixture.
• The shank of the pin is press fitted or driven into the body of jig or fixture.
• The locating diameter of the pin is made larger than the shank to prevent it from being forced into the jig or fixture body due to the weight of the work piece or cutting forces.
• Depending upon the mutual relation between the work piece and the pin.
• The pin may be classify as follows:

Locating Pins:

When reamed or finally finished holes are available in work piece, these can be used for locating purpose of the manner as shown, these are two types of locating pins:

• Conical locating pins
• Cylindrical locating pins

Support Locating Pins:

• With these pins (also known as rest pins) buttons or pads the work piece with flat surfaces supported at convenient.
• In the fixed support pins the locating face is either ground flat or curved.
• Support pins with flat head are usually employed and provided location and support to machine surface, because more contact area is available during location. It would insure accurate and stable location.
• The spherical head or round head rest buttons are used for supporting rough surfaces (unmachined and cast surfaces) because they provide a point support which may be stable under these circumstances.
• Adjustable type support pins are used for work piece whose dimension can vary. For example sand casting, forging or unmachined faces.

Jack Pins:

• Jack pins or spring pins are also used to locate the work piece whose dimension are subjected to variation.
• The pin is allow to come up under spring pressure or conversely is pressed down by the work piece.
• When the location of the work piece is secured the pin is locked in this position by means of locking screw.