Mehmet Can ÇAPAR 030070131 11th week definitions, Bonus week

1-Clearance (sheet metal cutting parameter)

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The clearance c in a shearing operation is the distance betwen the punch
and die, as shown Figure 20.1(a). Typical clearances in conventional
pressworking range between 4% and 8% of the sheet-metal thickness t.
The effect of improper clearances is illustrated in Figure 20.5. If the
clearance is too small, then the fracture lines tend to pass each other,
causing a double burnishing and larger cutting forces. If the clearance
is too large, the metal becomes pinched between the cutting edges and an
excessive burr results. In special operations requiring very straight
edges, such as shaving and fine blanking, clearance is only about 1% of
stock thickness.
The correct clearance depends on sheet-metal type and thickness. The
recommended clearance can be calculated by the following formula
c=Acxt
where c=clearance mm (in); Ac=clearance allowance; and t=stock thickness mm(in).
The clearance allowance is determined according to type of metal. For convenience,
metals are classified into three groups given in Table 20.1, with an associated
allowance value for each group.
The calculated clearance values can be applied to conventional blanking and hole-
punching operations to determine the proper punch and die sizes. The die opening
must always be larger than the punch size (obviously). Whether to add the clearance
value to the die size or subtract it from the punch size depends on whether the part
being cut out is a blank or a slugi as illustrated in Figure 20.6 for a circular part.
Because of the geometry of the sheared edge, the outer dimension of the part cut out
of the sheet will be larger than the hole size. Thus punch and die sizes for a round
blank of diameter Db are determined as
Blanking punch diameter=Db-2c
Blanking die diameter=Db
Punch and die sizes for a round hole of diameter Dh are determined as:
Hole punch diameter=Dh
Hole die diameter=Dh+2c
In order for the slug or blank to drop through the die, the die opening must have an
angular clearance (see Figure 20.7) of 0.25 to 1.5 degrees on each size.

(Mikell P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, pg:44-48)

2-Drill Helix Angle (A term related with drill bits)


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 A helix angle is found on any tool or work piece with a flute or thread. This angle is
the angle formed between the slant of the flute or thread and the centre line axis of
the tool. Examples of these tools or work pieces are twist drill bits and end mills,
which have spiral flutes, and shafts machined with threads... The twist drill bit is
the most common type of drill bit, as it can drill most materials, such as metal,
plastic and wood.
 The helix angle on a twist drill bit is the same as the rake angle
on other cutting tools. The standard helix angle is 30 degrees and the point angle is
118 degrees. The point angle is formed by the lip length and angles at the end of the
cutting edge of the drill bit.
 The main types of helix angles are:


-Slow helix angles, which are abgles of 20 degrees (smaller than normal helix angles)
-Quick helix angles, which are angles of 40 degrees (larger than normal helix angles)


 Drill bits with slow, standard or quick helix angles are usually called slow, standard
or quick helix drill bits.
 A slow helix drill bit has a point angle 118 degrees and is used on harder materials
such as brass and bronze. Aslow helix on a drill bit lengthens the lead of the helix.
A quick helix drill bit has a point angle 100 degrees and is used on softer materials
such as aluminium and copper. A quick helix on a drill bit shortens the lead of the helix.

(Schnetler, Gleeson & Kohlhofer, FCS Fitting & Turning L3, pg:35,36)