Saturday, September 8, 2012

Hole Callouts & To draw a countersunk hole in cad

There are four industrialized processes that produce hole shape characteristics that are used so often that they are defined using standardized drawing callouts: Ream, Counter bore, countersink and spot face.
Reaming is a procedure that smooths out the inside of a drilled hole, holes created using twist drills having spiral shaped machine mark on their surfaces. Reaming is used to remove the spiral machine marks & to increase the roundness of the hole. Ream callout generally include a much tighter tolerance than do drill diameter callouts.
A counter bore consists of holes drilled along the same centreline, counter bore are used to allow fasteners or other objects to be recessed, thus keeping the top surface uniform in height.
A counter bore drawing callout specifies the diameter of the small hole, the diameter of the large hole, & the depth of the large hole.
The information is given in this sequence because it is also the sequence of manufacture. Note that the hidden lines used in the front view of the counter bored hole clearly show the intersection between the two holes.
A countersink is a cone-shaped hole used primarily for flat –head fasteners. The drawing callout specifies the diameter of the hole, the included angle of the countersinks & the diameter of the countersinks as measured on the surface of the object. Almost all countersinks are 82° but some are drawn at 45°.
A spotface is a very shallow counterbored hole generally used on cast surfaces.cast surfaces is more porous than machine surfaces. Rather than machine an entire surface flat, it is cheaper to manufacture a spotface because only a small portion of the surface is machined. Spotfaces are usually used for bearing surfaces of fasteners.

A spotface callout defines the diameter of the hole & the diameter of the spotface. A depth need not be given, when machining was done mostly by hand, the machinist would make the spotface just deep enough to produce a shiny surface (most cast surfaces are more gray in colour) so no depth was needed. Automated machines require a spotface depth specification. Usually the depth is very shallow.
 To draw a countersunk hole in cad

1.     Draw a circle 10 mm diameter hole in the top view and project the hole’s diameter into the front view.

2.     Draw another concentric circle 20 mm diameter hole in the top view and project its diameter into the front view. In the front view draw 45° lines from the intersection of the 20 mm diameter circle hole & the top surface of the front view so that the lines intersect the 10 mm diameter circle hole’s projection lines. Use OSNAP (Shift + mouse right click), intersection to ensure accuracy.

3.     Draw a horizontal line between the intersection created as per above mention, once again use osnap intersection.

4.     Trim & erase the excess lines.

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