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page 66

7.4 DRILLING PROCESS PARAMETERS

• The parameters for drilling are found in almost the same way as for lathes,

CS = rpm ×

C

 

 

 

 

C = π-------D

=

-----------π D

 

 

 

 

12

 

1000

 

 

 

 

rpm = 12------------------×

CS

=

1000-------------------------

×

CS

 

π ×

D

 

π ×

 

D

 

imperial

 

 

 

metric

where,

 

 

 

 

 

 

CS = cutting speed (fpm or m/s) - can be selected from tables rpm = revolutions per minute of the machine spindle

C = circumference of the drill bit (ft. or m) D = diameter of drill bit (in. or mm)

L L

T = -- = -------------------

F rpm × F

C = T × R

where,

L = length of cut (in. or mm)

F = feed rate (in./rev. or mm/rev.) - found in tables

R= Machine cost ($/min.)

Typical high speed drill speeds are, [Krar]

page 67

Drill dia. (in.)

steel casting

tool steel

cast iron

machine steel

brass/aluminum

 

40 fpm

60 fpm

80 fpm

100 fpm

200 fpm

 

 

 

 

 

 

 

1/16

2445

3665

4890

6110

12225

 

1/

1220

1835

2445

3055

6110

 

3/16

815

1220

1630

2035

4075

 

1/4

610

915

1220

1530

3055

 

5/16

490

735

980

1220

2445

 

3/8

405

610

815

1020

2035

 

7/16

350

525

700

875

1745

 

1/2

305

460

610

765

1530

 

5/8

245

365

490

610

1220

 

3/4

205

305

405

510

1020

 

7/8

175

260

350

435

875

 

1

155

230

305

380

765

 

 

 

 

 

 

 

 

• Consider also the typical feeds for drilling, [Krar]

Drill dia. (in.)

Feed per Rev. (in.)

 

 

1/8 or less

0.001 to 0.002

1/8 to 1/4

0.002 to 0.004

1/4 to 1/3

0.004 to 0.007

1/2 to 1

0.007 to 0.015

1 to 1.5

0.015 to 0.025

 

 

7.4.1 The mrr For Drilling

• considering the parameters defined in the discussion of speeds and feeds, etc, the mrr is given below,

mrr = A F rpm = --------- F rpm

× × π D2 × ×

4

where,

A = cutting area of the drill bit (a cross section)