Machining

Goal

Machining has a cost: working time, extra thickness of matter to be removed, wear of the machine tool, consumable (tool, Lubrifiant, Electric current), cost of the Stock (the part in the course of manufacture is an expenditure which will be compensated only at the time of the sale, and it requires surface of workshop for storage, the advance)… One thus practices only machinings necessary. Machining is selected according to the form to reach (for example drilling for a cylindrical hole, turning for a form having a symmetry of revolution, milling for a plane surface) and final surface quality, all this depend on the function which must fill the machined part (for example to receive a screw, to ensure a slipping contact, to ensure a tight contact). But it also depends on the Procédé of development: a process can give a functional surface without requiring very well completion, another to envisage on the contrary an extra thickness intended to be removed by machining.

One typically distinguishes sixteen functions princiales which surface can fill of a part:

surface of contact with another part:

Friction of slip lubricated (FG),
dry friction (FS),
rolling friction (FR),
fluid Friction (FF),
resistance to the Hammering (RM),
dynamic sealing with and jointless ED),
sealing static with and jointless (ES),
fixed adjustment with Forced (AC),
Adherence, Joining (AD);

free, independent face:

face of cut of a Tool (OC),
load-carrying capacity alternate (EA),
resistance to the Corrosion (RC),
intended to receive a coating, Painting (RE),
intended to receive a electrolytic Deposit (OF),
Measurement (ME),
aspect (AS).

These functions will define:

final dimensions of the part with the tolerances;
the surface quality necessary (Roughness);

what will determine the type of machining and its parameters, the completion necessary, control to be carried out.

Let us take the case of a cylinder having to pass by a hole. One can consider several situations:

the cylinder must be able to slide or to turn freely, the play is not a problem and the sealing is not necessary; it is the case for example of a tree passing through the Carter of the engine which drives it; one can envisage a hole whose nominal diameter is higher than that of the tree, one does not need a precise machining;
the cylinder must be gone up in force with the hand in the hole in order to solidarize the parts, but the system must be dismountable: the hole and the cylinder have the same nominal diameter, but one envisages in the tolerances that the cylinder can be slightly thicker and that the hole (Alésage) can be slightly narrower, of some Micromètre S (for example designation ISO H7/f6);
the cylinder must freely slide but with a weak play: the hole and the cylinder have the same nominal diameter, but one envisages in the tolerances that the cylinder can be slightly narrower and that the hole (Alésage) can be slightly broader, of some Micromètre S (for example designation ISO H8/f7).

The precision of the adjustment thus depends on the function of the part.

The surface quality as for him conditions the visual aspect (reflection, Diffraction of the light), adherence, friction,…

Chips

The Copeau is the part of matter which is detached at the time of the cut in process of machining. The surface quality machined finished depends on the conditions of obtaining the chip. There exists a depth of cut in lower part of which one should not go down under penalty of not having any formation of chip. This depth of cut depends on material, the speed and the type of tool used, and defines the minimum chip. If the depth of cut is lower, there is no more cut, but a work hardening which creates a heating prejudicial with the surface quality of the part and the tool.

The processes of machining produce chips, plus the Limaille which, mixed with the Huile S of cut, forms Boue. A certain quantity of material which was produced (extracted, run…) will not be in the part, it should be collected and the to recycle.

The chips thus have a cost. If machining can be necessary for the functional parts (i.e. having a particular function in the final mechanical assembly, possibly in esthetics), it is advisable to reduce them to the maximum, in particular by obtaining a blank nearest possible to the final part. That can be done:

on the basis of a part moulded and by optimizing casting and cooling (to control the Withdrawal, to avoid the Shrinkage pipe S)
by using the Forging
by using the Stamping
on the basis of a tack welded part, together of elements of standard form welded between them.
by Metallurgy of the powders, proceeded of Sintering, without fusion, therefore with a great control of dimensions and microstructure.

Various processes of machining

The various processes of machining are (the symbols are those of standard NFE 05-019 of 1992):

the Boring (Al);
the Stitching (Br);
the Milling: end milling (frb), up milling (frr);
the Slotting;
the Facing;
the Drilling (PC), Threading (fl), Tapping;
the Honing (pi)
the Planing (Rb);
the Correction: plane correction (CCP), cylindrical correction (rcc);
the Planing;
the Grinding (rd);
the turning (to): Raising (Dr.), Slide-lathing;
the flashing (I.E.(internal excitation));
the electro-erosion (éé);
the Undercutting;
the Polishing (Po);
the electropolishing (ep);
the Super completion (sf);
the Sanding: dry sanding (hopper), wet sanding (sah);
the Scraping (gr.);
the Grinding (me);
the Cutting (die): Cutting plasma, Cutting jet of water, laser Cutting;
the Shot-blasting: spherical shot-blasting (gns), angular shot-blasting (gna);
the Rolling (ga);
the Polygonnage;

Machining can be with Numerical control (CN), with high speed (UGV).

Elements of cut and master key

The parameters of Coupe are:

the Cutting speed $v_c$ in meter per minute (m/min): characterize speed relative between the part and the tool to the contact point
the Avance by turn $f$ in millimetre per turn (mm/tr): characterize the surface quality obtained

The parameters of cut are selected according to the mechanical characteristics of the matter to machine and the tool. They are independent of the machine used and the geometrical characteristics of the part and the tool.

The parameters Machine are:

the Rotational frequency of the pin $N$ out of turns per minutes (tr/min)
the Speed in advance $V_f$ in millimetre per minute (mm/min)

The parameters machines are calculated starting from the parameters of cut and the geometrical characteristics of the part and the tool.

The element of Passe are:

the Depth of cut $a_p$ in millimetre (mm)
the Many master keys $n_p$
the machined Length $l$ in millimetre (mm)
the machined Overall length $L$ in millimetre (mm)

Derived direction

Machining is the period of integration of the pupils to the 3Ecole Nationale Sup3erieure of arts and trades.

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