2.7.4. Designing of Detail, Produced by Machining.
Technological processes of this class are the most labour consuming and expensive ones. They may compose up to 70% of total cost of a product. That's why, choosing the material and production operations, with which labour input and cost would be minimum, becomes the most important factor in designing of such details. In designing of machined details, the following rules should be observed:
-extent of machined surfaces must be reduced up to constructively
required level;
-to try to maximally reduce amount of metal, removed during ma
chining;
-to foresee production of details by the most effective methods of
machining;
-to apply bar and profiled bar, keeping maximum number of rough
surfaces;
-to foresee production of details from castings with the form
close to the one of a final product;
-to facilitate production of labour-consuming details by application
of composite constructions;
-to foresee possibility of on-pass machining, which is the main con
dition of labour productivity increase, of obtaining high accuracy and mini
mum roughness of machined surfaces;
-when it is impossible to perform on-pass machining , to foresee
providing of going out of machined tool for a distance, sufficient for obtaining
accurate surfaces;
-to provide convenient coming up of a cutting instrument to ma
chined surfaces;
-to foresee the possibility of machining of maximum number of
surfaces in one operation at one machine-tool, from one installation, by one
and the same tool;
-to impart to details of multiple and mass application the forms, al
lowing batch processing with the help of combined tool;
-to provide possibility of machining accurate aligned and parallel
holes from one installation, which facilitates obtaining of alignment and re
quired accuracy of spacing on centers;
-to foresee strict separation of surfaces, machined at different op
erations, by different tools and with different degree of accuracy;
-to foresee such distance between machined and nearest non-
machined surfaces, that provide machining with maximum variations of size.»
of blanks, available due to production conditions;
-to avoid joint machining in assembly , breaking down in continuity
of production flow, reducing interchangeability and hampering replacement of
details during operation;
-to reduce nomenclature of machining instrument by means of uni
fication of sizes and shapes of machined elements;
-to reduce to minimum application of special cutting tools in indi
vidual and small-lot production;
-to impart to machined surfaces the form, providing uniform and
shock-free work of a cutting tool;
-tc unload cylindrical multiblade tool (drills, reamers, multifiute
drills) from one-sided pressure during machining;
-to impart to machined sections high and uniform rigidity, providing
accurate machining and promoting application of productive types of machin
ing;
-to provide convenient bases for testing of dimensions with appli
cation of universal measuring tool, if it is possible.
The fact that during machining detail strength is reduced at the expense of cutting of material fibers must be taken into account too. Strength of details is also reduced at the expense of holes, transitions from one meter to another, key slots, splints, which are strong stress concentrators. If it is impossible to fully eliminate stress concentrators, then strong concentrators should be replaced by moderately acting ones. Concentrators should be taken away from the stressed sections of a detail, and, if the construction allows, to transfer them to the zones of the least stresses. In order to reduce nominal stresses it is expedient to increase cross section of details at the places, where concentrators are located. Most often, in order to reduce concentration of stresses, fillets are introduced at the sections of transition. Concentration of tension is lowered with reduction of diameter difference and increase of relative radius of a fillet. Details, having great cyclic loads, must have fillets at all reentering angles. Fillets are to be introduced into key, spline and gear transmissions.
During designing of machined details, very often in order to change their mechanical properties, different kinds of heat treatment are prescribed. The fact that during heat treatment the stresses of a detail, reducing its strength take place should be taken into account. Design of a detail must consider possible thermal deformations, which can be compensated by means of tolerances.
Thus, we can come to the conclusion, that during designing details, produced by machining, problems of their adaptability to manufacture and strength play the most important role.
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