Information Sheet No.

11
Finishing Processes—Reaming,
Broaching, and Lapping
Objective

After completing this unit, you will be able to:

1. Identify and explain the purpose of several types of hand reamers
2. Ream a hole accurately with a hand reamer
3. Cut a keyway in a workpiece using a broach and arbor press
4. Lap a hole or an external diameter of a workpiece to size and finish

Introduction

Hand cutting tools are generally used to remove only small amounts of metal and are
designed to do specific operations.
Reamers, available in a wide variety of types and sizes, are used to bring a hole to size
and produce a good fish.
Broaches, when used in a machine shop, are generally used with an arbor press to
produce special shapes in the workpiece. The broach, which is a multi-tooth cutting tool of the
exact shape and size desired, is forced through a hole in the workpiece to reproduce its shape
in the metal.
Lapping is a process whereby very fine abrasive powder, embedded in a proper tool, is
used to remove minute amounts of material from a surface.

HAND REAMERS

A hand reamer is a tool used to finish drilled holes accurately and provide a good finish.
Reaming is generally performed by machine, but there are times when a hand reamer must be
used to finish a hole. Hand reamers, when used properly, will produce holes accurate to size,
shape, and finish.

Types of Hand Reamers Figure 1 A solid hand reamer

The solid hand reamer

(Figure 1) may be made of carbon
steel or high-speed steel. These
straight reamers are available in inch sizes from .125 to 1.500 in. in diameter and in metric
sizes from 1 to 26 mm in diameter. For easy starting, the cutting end of the reamer is ground
to a slight taper for a distance equal to the diameter of the reamer. Solid reamers are not
adjustable and may have straight or helical flutes. Straight-fluted reamers should not be used
on work with a keyway or any other interruption, since chatter and poor finish will result.
Since hand reamers are designed to remove only small amounts of metal, no more than .005
in., or 0.12 mm, should be left for reaming, depending on the diameter of the hole. A square
on the end of the shank provides the means of driving the reamer with a tap wrench.
The expansion hand reamer is designed to permit an adjustment of approximately
.006 in. (0.15 mm) above the nominal diameter. The reamer is made hollow and has slots along
the length of the cutting section. A tapered threaded plug fitted into the end of the reamer
provides for limited expansion. If the reamer is expanded too much, it will break easily. For
inch expansion hand reamers, the limit of adjustment is .006 in. over the nominal size on
reamers up to .500 in. and about .015 in. on reamers over .500 in. Metric expansion hand
reamers are available in sizes from 4 to 25 mm. The maximum amount of expansion on these
reamers is 1 % over the nominal size. For example, a 10-mm diameter reamer can be expanded
to 10.01 mm (10+1 %). The cutting end of the reamer is ground to a slight taper for easy
starting.
The adjustable hand reamer has tapered slots along the entire length of the body. The
inner edges of the cutting blades have a corresponding taper so that the blades remain parallel
for any setting. The blades are adjusted to size by upper and lower adjusting nuts.
Taper reamers are made to standard tapers and are used to finish tapered holes
accurately and smoothly. They may be made with either spiral or straight teeth. Because of
the shearing action and its tendency to reduce chatter, the spiral-fluted reamer is superior to
the straight one.
A roughing reamer with nicks ground at intervals along the teeth, is used for more
rapid removal of surplus metal.
The finishing taper reamer is used after the roughing reamer to finish the hole
smoothly and to size. This reamer, which has either straight or left-hand spiral-flutes, is
designed to remove only a small amount of metal (about .010 in. (0.25 mm)) from the hole.
Since taper reamers do not clear themselves readily, they should be removed frequently from
the hole and the chips cleared from their flutes.

Reaming Precautions
1. Never turn a reamer backward (counterclockwise) because it will dull the cutting teeth.
2. Use a cutting lubricant where required.
3. Always use a helical-fluted reamer in a hole that has a keyway or an oil groove cut in it.
4. Never attempt to remove too much material with a hand reamer; about .010 in. (0.25
mm) is the maximum.
5. Frequently clear a taper reamer (and the hole) of chips.

To Ream a Hole with a Straight Hand Reamer

1. Check the size of the frilled hole. It should be between

.004 and .005 in. (0.10 and 0.12 mm) smaller than the
finished hole size.
2. Place the end of the reamer in the hole and place the
tap wrench on the square end of the reamer.
3. Rotate the reamer clockwise to allow it to align itself
with the hole (Figure 2).
4. Check the reamer for squareness with the work by
testing it with a square at several points on the
circumference.
5. Brush cutting fluid over the end of the reamer if
required.
6. Rotate the reamer slowly in a clockwise direction and
apply downward pressure. Feed should be fairly rapid
and steady to prevent the reamer from chattering.
NOTE: The rate of feed should be about one-quarter the
diameter of the reamer of each turn.
Figure 2 Turn the reamer clockwise
when starting it in a hole.
BROACHING

Broaching is a process in which a special tapered multi-

toothed cutter is forced through an opening or along the outside of
a piece of work to enlarge or change the shape of the hole or to form
the outside to a desired shape.
Broaching was first used for producing internal shapes, such
as keyways, splines, and other odd internal shapes (Figure 3). Fig 3—Examples of
Internal Broaching
 Its application has been extended to exterior surfaces, such as the flat face on
automotive engine blocks and cylinder heads.

The cutting action of a broach is performed by a

series of successive teeth, each protruding about .003 in.
(0.07 mm) farther than the preceding tooth (Figure 4).
The last three teeth are generally of the same depth
and provide the finish cut.
Broaching has many advantages and an extremely
wide range of applications:
1. Machining almost any irregular shape is possible,
providing it is parallel to the broach axis.
Fig 4—The cutting action of a Broach
2. It is rapid; the entire machining process is usually completed in one pass.
3. Roughing and finishing cuts are generally combined in the same operation.
4. A variety of forms, either internal or external, may be cut simultaneously and the
entire width of a surface may be machined in one pass. Thus, eliminating the need
for a machining operation.

Fig 5—Using an arbor press Fig 6—A broach set for cutting Fig 7—Two shims are used
to cut a keyway with a broach internal keyways for making the final pass
with a broach
Cutting a Keyway with a Broach

Keyways may be cut by hand in the machine shop quickly and accurately by means of
a broach set and an arbor press (Figure 5). A broach set (Figure 6) covers a wide range of
keyways and is a particularly useful piece of equipment when many keyways must be cut. The
equipment necessary to cut a keyway is a bushing (Figure 6a) to suit the hole size in the
workpiece, a broach (Figure 6b) the size of the keyway to be cut, and shims (Figure 6c) to
increase the depth of the cut of the broach.
Follow this procedure:
1. Determine the keyway size required for the size of the workpiece.
2. Select the proper broach, bushing, and shims.
3. Place the workpiece on the arbor press. Use an opening on the base smaller than the
opening in the workpiece so that the bushing will be properly supported.
4. Insert the bushing and the broach into the opening. Apply cutting fluid if the workpiece
is made of steel.
5. Check the broach to be sure that it has started squarely in the hole.
6. Press the broach through the workpiece, maintaining constant pressure on the arbor-
press handle.
7. Remove the broach, insert one shim, and press the broach through the hole.
8. Insert the second shim, if required, and press the broach through again. This will cut
the keyway to the proper depth (Figure 7).
9. Remove the bushing, broach, and shims.
 LAPPING

Lapping is an abrading process used to remove minute amounts of metal from a surface
that must be flat, accurate, and smooth. Lapping may be performed for any of the following
reasons:
1. To increase the wear life of a part
2. To improve accuracy and surface finish
3. To improve surface flatness
4. To provide better seals and eliminate the need for gaskets

Lapping may be performed by hand or machine, depending on the nature of the job. Lapping
is intended to remove only about .0005 in. (0.01 mm) of material. Lapping by hand is a long,
tedious process and should be avoided unless absolutely necessary.

Lapping Abrasives

Both natural and artificial abrasives are used for lapping. Flour of emery and fine
powders made of silicon carbide or aluminum oxide are used extensively. Abrasives used for
rough lapping should be no coarser than 150 grit; fine powders used for finishing run up to
about 600 grit. For fine work, diamond dust, generally in paste form, is used.

TYPES OF LAPS

Laps may be used to finish flat surfaces, holes, or the outside of cylinders. In each case,
the lap material must be softer than the workpiece.

Flat Laps
Laps for producing flat surfaces are made from
close-grained cast iron. For the roughing operation or
“blocking down”, the lapping plate should be scored
with narrow grooves about .500 in. (13 mm) apart,
both lengthwise and crosswise or diagonally to form a
square or diamond pattern (Figure 8a). Finish lapping
(b) is done on a smooth cast-iron plate (Figure 8b).
Fig 8 (a) A roughing lapping plate
(b) a finishing lapping plate

Charging the Flat Lapping Plate

Spread a thin coating of abrasive powder over the surface of the plate and press the particles
into the surface of the lap with a hardened steel block or roll. Rub as little as possible. When
the entire surface appears to be charged, clean the surface with varsol and examine it for bright
spots. If any bright spots appear, recharge the lap and continue until the entire surface
assumes a gray appearance after it has been cleaned.

Lapping a Flat Surface

If work is to be roughed down, oil should be used on the roughing plate as a lubricant. As the
work is rubbed over the lap, the abrasive powder will be washed from the grooves and act
between the surface of the work and the lap. If the work has been surface-ground, rough
lapping or “blocking down” is not required.
Follow this procedure:
1. Place a little varsol on a finish-lapping plate that has been properly charged.
2. Place the work on top of the plate and gently push it back and forth over the full surface
of the lap using an irregular movement. Do not stay in one spot.
3. Continue this movement with a light pressure until the desired surface finish is
obtained.
Precautions to Be Observed
1. Do not stay in one area; cover the full surface of the lap.
2. Never add a fresh supply of loose abrasive. If required, recharge the lap.
3. Never press too hard on the work because the lap will become stripped in places.
4. Always keep the lap moist.

Internal Laps
Holes may be accurately finished to size and
smoothness by lapping. Internal laps may be made
of brass, copper, or lead and may be of three types.
The lead lap (Figure 9a) is made by pouring lead
around a tapered mandrel that has a groove along
its length. The lap is turned to a running fit into the
hole and is then sometimes slit on the outside to
trap the loose abrasive during the lapping operation.
Adjust by lightly tapping the large end of the
mandrel on a soft block. This will cause the lead
sleeve to move along the mandrel and expand.

Fig 9—Various types of internal laps:

(a) Lead lap; (b) copper lap; (c) adjustable lap

The internal lap (Figure 9b) may be made of copper, brass, or cast-iron. A threaded-
taper plug fits into the end of the lap, which is slit for almost its entire length. The lap diameter
may be adjusted by the threaded-taper plug.
The adjustable lap (Figure 9c) may be made from copper or brass. The lap is split for
almost its full length, but both ends remain solid. Slight adjustment is provided by means of
two setscrews in the center section of the lap.

Charging and Using an Internal Lap

Before charging, the lap should be a running fit into the hole. Follow this procedure:
1. Sprinkle some lapping powder evenly on a flat plate.
2. Roll the lap over the powder, applying sufficient pressure to embed the abrasive into the
surface of the lap.
3. Remove any excess powder.
4. Mount a lathe dog on the end of the lap.
5. Fit the workpiece over the end of the lap.

NOTE: The lap should know be a wringing fit in the hole of the work and about 2.5 times the
length of the workpiece.

6. Place some oil or varsol on the lap.

7. Mount the lap and the work between lathe centers.
8. Set the machine to run at a slow speed,150 to 200 r/min for a 1-in. (25-mm) diameter.
9. Hold the work securely and start the machine.
10. Run the work back and forth along the entire length of the lap.
11. Remove the work and rinse it in varsol to remove the abrasive and to bring it to room
temperature.
12. Gage the hole for size.

NOTE: Always keep the lap moist and never add loose abrasive to the lap. Loose abrasive will
cause the work to become bell-mouthed at the ends. If more abrasive is necessary, recharge
the lap and adjust as required.
External Laps

External laps are used to finish the outside of cylindrical workpieces. They may be of
several forms (Figure 10); however, the basic design is the same. External laps may be made
of cast iron or they may have a split brass bushing mounted inside by means of a setscrew.
There must be some provision for adjusting the lap.

Fig 10—External Laps

Charging and Using an External Lap

1. Mount the workpiece in a three-jaw chuck on the lathe drill press.

2. Adjust the lap until it is a running fit on the workpiece.
3. Grip the end of the lap in a vise.
4. Sprinkle abrasive powder in the hole.
5. With a hardened steel pin, roll the abrasive evenly around the inside surface of the lap.
6. Remove any excess lapping powder.
7. Place the lap on the workpiece. It should now be a wringing fit.
8. Set the machine to run at a slow speed [150 to 200 r/min for a 1-in. diameter (25-mm)
workpiece].
9. Add some varsol to the workpiece and the lap.
10. Hold the lap securely and start the machine.
11. Move the lap back and forth along the work.

NOTE: Always keep the lap moist.

12. To gage the work, remove the lap and clean the workpiece with varsol.