Question

1. What functions are performed by the flutes on a standard twist drill?
2. Basically, what determines what helix angle a drill should have?
3. When a large-diameter hole is to be drilled, why is a smaller diameter hole often drilled first?
4. What is the function of the margins on a twist drill?
5. What are the primary purposes of reaming?
6. What are the purposes of:
a. Spot facing
b. Counterboring
c. Countersinking
7. Suppose you want to drill a 2-in. thick plate of cast steel (0.4%C), 4-in. diameter hole. An indexable HSS insert drill has been selected. Consider the table below, select the appropriate cutting speed, feed rate. Calculate the spindle speed (rpm), the feed rate (in/min), the MRR, and cutting time. Assume the cutting process will be performed in 3 steps using 3 drills sizes in sequence 1”, then 2”, then 4”. Calculate the cutting total cutting time, considering a proper allowance each time.
8. How much time will be required for milling machine to face mill an AISI 1020 steel surface BHN 150, that is 12 in. long and 5 in. wide, using a 6 in. diameter, eight-tooth tungsten carbide inserted-tooth face mill cutter? Select values of feed per tooth and cutting speed from the provided table, Table 24-1.
9. Draw in two sketches the up milling (conventional milling) and down milling (climb cut) machining set up and show the shape of machined material.
10. Why does down milling dull the cutter more rapidly than up milling when machining sand casting iron?

Answer 1

Problems 1, 3, 5, 6 are answered here.

1) Functions performed by flutes on a standard twist drill:

• They will ensure for easy escaping and the flow of chips.
• They will cause chips to curl and provide a route/passages for their flow.
• They will form the lips and cutting edges on the point.
• They will allow the cutting fluid to reach the cutting edges and thus reducing friction.

3) When large diameter hole is to be drilled, why is smaller diameter hole often drilled first.

• If large diameter hole is to be drilled, then there is a requirement of large size drill bits.
• We know that, large drill bits will have large chisel edge and large web thickness.
• When using this large size drill bits for drilling, the forces acting on the drill bit is higher and vibrations induced in the drill bit is higher. Therefore, positional accuracy of the hole produced is poor (positional error is large)
• Hence, to minimize the positional error of the hole, we should first make a drill with small diameter hole and then go with large diameter hole.

5) Primary purpose of reaming:

• During drilling operation, the variation in size of the hole produced is quite large. So, there won't be sufficiently good finish and accuracy.
• Hence, first a hole is drilled little undersize and then it is reamed to a correct size.
• Thus, Reaming is a operation meant for finishing an existing hole with very smooth and accurate size.
• It's accuracy is approximately 0.005 mm

6) Purpose of spot facing, counterboring, countersinking:

i) Pupose of spot facing:

• Purpose of spot facing is to make the surface of the hole as flat and square by using milling cutter with drilling machine.
• It is done for proper seating of bolt head or nut on the structure.

ii) Purpose of counterboring:

• Purpose of counterboring is to enlarge the end of an existing hole.
• This is done to accommodate the socket head screws, nuts, or round head bolts (to keep bolt/nut head inside the structure)

iii) Purpose of countersinking:

• Purpose- Making of conical enlargement at the end of an existing hole.
• This is done for keeping the screw head inside the structure.

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