Broaching

—Broaching is a multiple-tooth cutting operation with the tool reciprocating.

—Since in broaching the machining operation is completed in a single-stroke as the teeth on the cutting tool, called broach, are at gradually increasing height corresponding to the feed per tooth of a milling cutter.

—The shape of the broach determines the shape of the machined part.

—Broaching was originally developed for machining internal keyways, but looking at the advantages, it has been extensively used in the mass production of automobile component manufacture for various other surfaces as well.

Questions on BORING

The main purpose of boring operation, as compared to drilling is to:

  (a)  Drill a hole  

  (b)  Finish the drilled hole

  (c)  Correct the hole 

  (d)  Enlarge the existing hole

Ans-d

Enlarging an existing circular hole with a rotating single point tool is called   

  (a)  Boring        (b)  Drilling

   (c)  Reaming   (d)  Internal turning.

Ans-a

—Boring

—Boring always involves the enlarging of an existing hole, which may have been made by a drill or may be the result of a core in a casting.

—An equally important and concurrent purpose of boring may be to make the hole concentric with the axis of rotation of the workpiece and thus correct any eccentricity that may have resulted from the drill drifting off the centerline. Concentricity is an important attribute of bored holes.

—When boring is done in a lathe, the work usually is held in a chuck or on a faceplate. Holes may be bored straight, tapered, or to irregular contours.

—Boring is essentially internal turning while feeding the tool parallel to the rotation axis of the workpiece.

Question on REAMER

For reaming operation of blind hole, the type of reamer required is

  (a)  Straight flute reamer

  (b)  Right hand spiral fluted reamer

  (c)  Left hand spiral fluted reamer

  (d)  None of the above

Ans-b

Shell reamers are mounted on

  (a)  Tool holders  (b)  Amour plates

  (c)  Arbor              (d)  Shanks

Ans-c

Types of Reamers

The principal types of reamers are:

  1.  Hand reamers

       a.  Straight

  b.  Taper

  2.  Machine or chucking reamers

  a.  Rose

  b.  Fluted

  3.  Shell reamers

  4.  Expansion reamers

  5.  Adjustable reamers

Reamer Flutes

—The reamer flutes are either straight or helical.

—The helical flutes promote smoother cutting and should be used specifically for holes that are not continuous, such as those with keyways parallel to the axis of the hole.

—The cutting action of the helical flutes is smoother and helps in preventing chatter.

—The reamers are termed as left hand or right hand, depending upon the direction in which they are moved, looking from the shank to the cutting portion.

—The right-hand reamer with right-hand helix is used for roughing cuts, since the tool tends to go into the workpiece more efficiently and thereby promotes the material removal.

—A right-hand reamer with left-hand flutes is used for finishing cuts.

Reaming

—Reaming removes a small amount of material from the surface of holes.

—It is done for two purposes: to bring holes to a more exact size and to improve the finish of an existing hole.

—Multiage cutting tools that has many flutes, which may be straight or in a helix are used. 

—No special machines are built for reaming. The same machine that was employed for drilling the hole can be used for reaming by changing the cutting tool.

—Only a minimum amount of materials should be left for removal by reaming. As little as 0.1 mm is desirable, and in no case should the amount exceed 0.4 mm.

—A properly reamed hole will be within 0.025 mm of the correct size and have a fine finish.

Question on BUFFING

Buffing wheels are made of 

  (a)  Softer metals  (b)  Cotton fabric

  (c)  Carbon             (d)  Graphite

Ans- b

Question on cutting fluid

Dry and compressed air is used as cutting fluid for machining

  (a)  Steel         (b)  Aluminium 

  (c)  Cast iron  (d)  Brass

Ans-c

Question on grinding

Tool life in the case of a grinding wheel is the time

  (a)  Between two successive regrinds of the wheel

  (b)  Taken for the wheel to be balanced

  (c)  Taken between two successive wheel dressings

  (d)  Taken for a wear of 1mm on its diameter

Ans-c

question on grinding.

In the grinding wheel of A 60 G 7 B 23, B stands for

  (a)  Resinoid bond         (b)  Rubber bond

  (c)  Shellac bond           (d)  Silicate bond

Honing

—Honing is a finishing process, in which a tool called hone carries out a combined rotary and reciprocating motion while the workpiece does not perform any working motion.

—Most honing is done on internal cylindrical surface, such as automobile cylindrical walls. The honing stones are held against the workpiece with controlled light pressure. The honing head is not guided externally but, instead, floats in the hole, being guided by the work surface.

—It is desired that

1. Honing stones should not leave the work surface

2. Stroke length must cover the entire work length.

3. In honing rotary and oscillatory motions are combined to produce a cross hatched lay pattern.

—The honing stones are given a complex motion so as to prevent every single grit from repeating its path over the work surface. 

Is transverse feed provided in vertical spindle reciprocating table surface grinder?

—Usually no transverse feed is provided in such machine. The wheel diameter is kept larger than the width of the workpiece surface to be ground.

State the disadvantages of centreless cylindrical grinding machine?

  •  It does not grind concentrically with centres.

  •  Large diameter short work piece are difficult to   control in the process

  •  It may not improve work piece perpendicularity.

Centerless Grinding

—Centerless grinding makes it possible to grind both external and internal cylindrical surfaces without requiring the workpiece to be mounted between centers or in a chuck.

—This eliminates the requirement of center holes in some workpieces and the necessity for mounting the workpiece, thereby reducing the cycle time.

—Two wheels are used. The larger one operates at regular grinding speeds and does the actual grinding. The smaller wheel is the regulating wheel. It is mounted at an angle to the plane of the grinding wheel.

—The regulating wheel controls the rotation and longitudinal motion of the workpiece and usually is a plastic- or rubber-bonded wheel with a fairly wide face.

—The workpiece is held against the work-rest blade by the cutting forces exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel.

What are the characteristic features of a universal cylindrical grinder?

—Characteristic features of a universal cylindrical grinder not possessed by plain cylindrical grinder are:

  •  Swivelling wheel head

  •  Swivelling wheel head slide

  •  Swivelling head stock

Is dressing necessary for single layer wheel?

—Conventional macro level dressing is not required because the wheel inherently has an open structure. However, touch dressing is carried out to obtain better uniformity in grit height in order to improve surface finish of the workpiece.

Can a resin bonded CBN wheel be electrochemically dressed?

—Electrochemical dressing is not possible with resin bonded wheel because it is not electrically conducting.

What is the main short coming of vitrified bond?

Vitrified bond is brittle and can not with stand high impact loads. This bond can not be used for high wheel speed due to risk of wheel breakage under centrifugal force.

Why is coarse grain and open structured wheel is preferred for stock removal grinding?

—Coarse grit allows large grit protrusion and open structure provides large inter grit chip space. Thus in combination those two provide large space for chip accommodation during stock removal grinding and risk of wheel loading is minimized.