Dimension And Tolerancing Flashcards
What is the rockwell hardness test?
It takes into account the considerable difference in various materials by scaling the testing applications. Uses hard penetrator in the form of a ball for soft materials (hrb), and a penetrator in the form of a cone for hard materials (hrc). The hardness number is given by the depth of indentation. The harder the material, the smaller the indentation and the higher the rockwell number.
What is hardness?
Hardness is the ability of a material to withstand deformation when subjected to outer forces. Commonly, it is expressed as a value of how the material can withstand indentation.
What is the brinell hardness test?
A ball penetrates into the material surface by a specific force at a specific time. The diameter of the indentation is measured, and the brinell hardness number is read from a table showing the relationship between the forced applied and the area of indentation measured (hb=f/a).
This measurement is used for softer materials, hence why it is a ball.
What is the vickers hardness test?
A pyramid is pressed into the material surface by a known force during a specific time. The difference between this and the brinell test is that the diamond penetrator means that the hardness number is independent of the force used jn the test. Again values read off a table, once force, area and diagonal length are found. Harness values expressed as hv.
What is roughness (R)?
The small irregularities in the material surface texture. These are finely spaced microgeometric deviations measures along the smallest sampling length on the workpiece by a stylish in micrometers or microns.
Roughness is used to help determine fatigue and lubrication. Roughness does not take anomalies into account. To do this use Rz which gives the biggest deviation eg. Highest to lowest point. For all parts that are sensitive to stress and fatigue, Ra (roughness average) and Rz need to be specified.
A surface can be engineered to take a load and provide sufficient space between highest and lowest point to absorb lubrication.
What is waviness (W)?
These are the large irregularities measured along the next level up of sampling length on the surface. The roughness irregularities are superimposed on the macrogeometric wavy profile.
What is surface lay (L)?
This is the orientation of surface pattern. It is the direction that the dominating pattern follows, which is generated by the machining method.
What are flaws (F)?
These are faults not included in the actual measurement of the surface, but indicated separately as regards to design and inspection. These include material inclusions, scratches, cracks, holes and other unintentional deformations of the surface.
What is a clearance fit?
Tolerances that don’t overlap, there is always a small bit of extra space.
What is an interference fit?
Shaft can be bigger or smaller than hole, normally tolerances overlap.
What is a shrink fit?
Shaft is bigger than hole, so need to heat it up to squish it in.
What is the difference between microhardness and nanohardness?
Microhardness is the indentation over multiple grains
Nanohardness is the indentation in one grain
