7.6 Fits and Clearances Flashcards
What determines the size/diameter of a drilled hole?
The size/diameter of a drilled hole is determined by the type of fastener fitted in that hole.
How does the hole size differ for various fasteners?
A hole drilled for an aluminium solid rivet differs from a hole drilled for a threaded screw or a plain shank bolt of the same diameter.
What determines the hole size for a metal shaft?
The size/diameter of the shaft and the application in which the hole/shaft combination is to be used.
What must be done if a hole is required to be reamed?
A smaller hole size/diameter must be drilled to allow for material removal during the reaming process.
What is a drill bit?
A drill bit is a circular cutting tool used to remove material to produce a certain sized hole in different materials.
How are drill bits used?
Drill bits are attached to a powered (electric or pneumatic) drill, which cuts through the material by rotation.
Where can standard drill sizes be found?
Standard drill sizes can be found on charts that list each size and include equivalent sizes in metric, fractional, letter, and number/letter systems.
What are the common imperial and metric drill sizes?
In imperial measurements, common drill sizes range from 1/16 - 1 in. In metric measurements, they range from 0.5 - 25 mm.
What is the largest and smallest letter size in the letter system?
The largest drill diameter is given the letter ‘Z’ (10.5 mm), and the smallest letter ‘A’ (5.85 mm).
What does the number system for drill sizes start and end with?
The number system starts from 1 (5.79 mm) and reduces to 80 (0.34 mm).
What is the tolerance for the diameter of a drilled hole?
The diameter of a drilled hole has no tolerance; it is assumed to be as close to the drill size as possible.
What factors affect the accuracy of a drilled hole?
Factors include the accuracy of the drill point, size of the drill, use of a drill guide bushing, material being drilled, length of the drill, accuracy of the spindle and chuck, rigidity of the drill and part, and cutting fluid used.
What can cause a drilled hole to be undersized?
Materials like light alloys and plastics have a high coefficient of thermal expansion, causing the hole to be smaller when the material cools.
What does the term ‘fit’ refer to in engineering?
The term ‘fit’ refers to the grade of tightness or looseness between mating parts.
What are the three basic classes of fit?
Clearance Fits, Interference Fits, and Transition Fits.
What characterizes a clearance fit?
There is always some play after assembly, allowing for movement between parts.
What is an interference fit?
An interference fit allows no movement after assembly because the inner part is larger than the part into which it fits.
What are the three classes of interference fit?
Driving Fit, Force Fit, and Shrink Fit.
What is a transition fit?
A transition fit occurs when the mating parts are almost the same size, with very small differences in size.
How can the type of fit be determined?
The type of fit can be determined by selective assembly of pieces on either side of the allowance.
What is meant by ‘hole-based’ and ‘shaft-based’ systems?
‘Hole-based’ means the hole is constant and the shaft varies in size, while ‘shaft-based’ means the shaft is constant and the hole varies in size.
What does the term ‘fitting’ mean in hand and machine fitting?
Fitting means putting parts together, where they either touch or join each other.
How are parts machined to achieve a specific fit?
Parts must be machined, filed, or ground to a desired size.
What has improved the accuracy and uniformity of fitted parts?
The advancement of precision machinery has significantly improved the accuracy and uniformity of fitted parts.