Dowels are used to obtain precise positioning between mating parts. Dowels provide a press fit in one component and a tight push fit in the mating component. When assembling parts with blind dowel holes, a slight flat is provided along the dowel's length to prevent air locks. For ease of assembly, one dowel is typically made longer than the other by about one diameter. To ensure precise alignment of dowel holes, the holes are drilled and reamed with the components screwed together.

1. DESIGN FOR MANUFACTURING
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DOWELS AND DOWELLING PROCEDURE
Dowels are used to obtain location between two or more mating parts. Usually
the requirement is to obtain a precise location between the mating parts
although sometimes dowels are used for an approximate location. The
machining procedurefor each of these requirements can differ. For a precise
location, when the two parts must assembly together repeatedly with a complete
absence of play or shake between them, the machining, which consists of
drilling and reaming, is individual to each unit as distinct from mass production
method.
Consider a bevel gear unit, where the bevel pinion is mounted in the box.
The bevel gear is mounted in the removable cover and the teeth meshing
requirements call for a precise dowel location. The dowels will be needed, two
being the maximum ever used; sometimes one is sufficient depending upon
the particular design involved.
Fig 1: Dowels ensurea precise positioning of cover with
box

2. DESIGN FOR MANUFACTURING
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The dowels are made a press fit in one component –in this design, the cover
plate –and a tight push fit in the mating component. When the dowels holes
in one of the components are blind holes, as in the gear box, means of
avoiding an air lock should provided: in this design a slight flat ,either filed
or ground, along the dowel length satisfactory .
Fig 2 : Design feature for Dowel

3. DESIGN FOR MANUFACTURING
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A further design feature which facilitates the segment of the dowels into the
dowel holes during Assembly is to one dowel longer, by approximately one
diameter; it is much easier to engage one tight push fit dowel than two dowels
simultaneously.
Fig 3: dowel positions shown on outer components
To achieve the required locational precision of the dowel holes a both
components, the reaming to size must be done when both components are
screwed together and the usual practice is to drill holes 0.5 mm. Less than
final size in the upper or outer componentonly (in this example the holes are
drilled in the cover plate ).Then , with the upper (outer )component secured
in the working position and the previously drilled holes serving as guidance,
the same size holes are drilled into the lower component, followed by the
final reaming to size.

4. DESIGN FOR MANUFACTURING
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 SCREW:
- A screw is a type of fastener characterized by a helical ridge,
known as thread, wrapped around a cylinder.
- The most common uses of screws areto hold objects together and
to position objects.
Screw Head Feature

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COUNTERSUNK HEAD SCREW
 Countersunk Head screws are the screw that are fitted into a
countersink.
 A countersink is a conical hole cut into a manufactured object, or
the cutter used to cut such a hole.
 A common use is to allow the head of a countersunk bolt or screw,
when placed in the hole, to sit flush with or below the surface of
the surrounding material.
Cross-sections of countersunk holes of various chamfer angles

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USES
 B.S. 4183 states that “The basic requirement of Countersunk head
screws is that the head should fit into the counter sunk hole with as
great a degree of flushness as possible…to achieve this it is
necessary for the dimensions of the both the heads of the screw and
the countersunk hole to be controlled within prescribed limits”.
 When total flatness of surface is required without any projections,
these type of screws are used.
 When there is space limitation.
 For nicer appearance , countersunk screws are used.

7. DESIGN FOR MANUFACTURING
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 LIMITATIONS
 Degree of accuracy in the fitting require very special conditions.
 Least nos. of countersunk screws are to be used for a component.
Multiple heads leads to misalignment and poorfitting.
 Spacerestriction areas.
 Cheese-head screws are used to overcome these limitations.
 The plate holes give clearance with the screw bodyand head
diameter

8. DESIGN FOR MANUFACTURING
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References:
 Designing For Manufacture by Harry Peck
 www.wikipedia.com