Page 153 - Turner 1st sem Practicals

Proceed now to sharpen one edge, removing as little metal Hold the drill shank again in the left hand with the elbows
as possible. against the side. The drill will swing back against the wheel
face in the same position and at the same angle as before.
Procedure to obtain equal angles
Points to be considered when sharpening drills
Move the drill back, clear of the Wheel face.
Grind as little as possible from the drill. Remove only
Turn the drill over without moving the position. This presents
the second edge to the wheel face at the same angle as enough to sharpen the cutting edges.
the first cutting edge. Rough down the drill point with a coarse grit wheel when
the edges are badly chipped.(Fig 7)
Proceed to sharpen the second cutting edge, using the
same amount of drill movement as before. When these
actions are carried out carefully, the drill will be sharpened
with equal cutting angles. The lip clearance will be correct
and equal.
Use a drill angle gauge to check that the cutting angle is
correct (118° for mild steel), the cutting edges are of equal
length and the lip clearances are equal and correct (about
12°). (Fig 6)
Never re-sharpen a cracked or split drill.
Avoid overheating the drill.
Apply light pressure against the wheel face. Lift the edge
clear of the wheel face frequently. This allows the air stream
produced by the wheel to cool the drill point.(Fig 8)

Lift the drill off the wheel face. Retain the grip on the drill Cooling a drill rapidly by quenching in cold
water may cause cracking of the cutting edge.
with the right hand.
Make such inspection or checks as are necessary. Move Re-sharpening of very small drills requires great skill. They
the right hand back on-the tool-rest in the same position require proportionally less movement to produce the cut-
as before. ting angles.

Testing a re-sharpened twist drill for its performance

Objective : This shall help you to
• test the drill that has been re-sharpened by drilling a through hole.

Set the spindle revolution of the drilling machine to.give a • Require only moderate pressure to feed it into the work.
cutting speed of 25 to 30 meters per minute. A drill that
has been re-sharpened correctly will: When the hole has been drilled through, take the drill out
of the machine and try it by inserting into the hole.
• Produce two evenly curled chips from its cutting edges
(Fig 1) If the drill fits without any play it means that (Fig 2):

140 Producltion & Manufacturing : Turner - Exercise 1.3.42