Spot drilling : When drilling holes , a small chamfer – or even small corner break – is very desrible at the top of the hole . A chamfer will eliminate burrs or sharp edges that are the natural result of drilling , allowing a smoother entry of a tap , or just eliminating the burrs for easier handling . Spot drill is a tool that is used fort his purpose ; it resembels a regular drill and has two main pruposes:
· To start-up a hole with a small dmple at its exact location
· To machine a chamfer on a hole , contolling the depth ıf cut.
(CNC programming techniques , Peter Smid ; pg. 8 , First edition)
Maximum material condition : MMc is that condition of a par tor feature which contains the maximum amounth of materail ,e.g. minimum size hole , or a maximum size shaft. In certain cases its use allows an increase in the specified tolerances if it is indicated that the tolerance applied to the feature at its maximum material condition.
The maximum material principle take into account the mutual dependence of tolerances of size , form orientation and/or location and permits additional tolerance as the cosidered feature departs form its maximum material condition.
(Manual of engineering drawing , Colin H. Simmons,Dennis E. Maguire,Neil Phelps ; pg. 183 , first edition)
International Tolerance Grade (IT): A group of tolerances that vary depndending on the basic size , but that provide the same relative levels of accuracy within a given grade. An “international Tolerance grade “ establishes the magnitude of the tolerance zone or the amoungh of part size variation allowed for external and internal dimentions alike. Tolerances are expressed in grade number that are consistent with ınternational tolerance grades identified by the prefix IT , such as IT6 , IT11, etc. A smaller grade number provides a smaller tolerance zone.
( Shop reference for students and apprentices , Edward G. Hoffman,Christopher J. McCauley ; pg . 223. 2nd edition)
Uniform annunal cost method: The uniform annunal cost method involves axpressing all expenditures in terms of uniform annual costs. Some analysts prefer this method for pavement because routine maintenance exprenditures can be conveniently expressed as uniform annual costs without the need for a discounting process . however , one –time expenditures such as rehabilitation cost must be first converted to a present value , then to an annunal cost.
(Portland cement concrete resurfacing ; Kenneth H. McGhee , pg 36 . )
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