Finishing and Microfinishing Processes in Machining of Metals

 

Tolerances. In the metal working industry, especially in large-scale production, the work-pieces are machined to specified sizes. The size mentioned in technical instruction, called nominal size, must be obtained.

But when several workpieces are measured their actual sizes will vary from the nominal size by more or less significant differences. For example, it is impossible to make two work-pieces to measure exactly 40 mm (40.000 mm). When exact measurements are taken, one workpiece may have a dimension of 40.070 mm, the other one of 39.999 mm.

The following principle has been adopted by the metalworking industry: the size of a workpiece should not be as precise as possible but as precise as necessary.

The principle of tolerance led to the establishment of certain tolerances also known as permissible variations or limits of dimensions (Fig. 6.25), for workpieces in accordance with their intended use. For example, working parts of an engine must have a higher dimensional accuracy than components of agricultural machines. If a length in technical drawing is 50±0.1 mm, the permissible variation of ±0.1 mm means that a work-pieces having an actual size of 49,9 mm or 50,1mm are still useful. Any actual size between these limits also fits for service.

Fig. 6.25. The principle of tolerance: D₁-maximum size; D₂-minimum size; D₃- nominal size; t – tolerance

 

Surface roughness. On the other hand, any method of machining cannot provide an absolutely smooth surface of a workpiece. Every surface of the workpiece has some departures from the nominal surface. These departures or deviations have a form of peak and valley, named microirregularities, and characterize the degree of surface roughness.

The surface roughness is characterized by the middle average height H of microirregularities within a base length 1 (Fig. 6.26).

Fig. 6.26. Surface roughness assessment

 

Surface finish (roughness) has a vital effect on the most important functional properties of machine parts. These include wear resistance, fatigue strength, corrosion resistance and power losses by friction in motion.

Therefore, finishing and microfinishing processes are employed in machining the surfaces of many parts to obtain minimum tolerances and minimum roughness of surfaces (Table 6.2). Such processes include finish cutting, finish grinding, honing, lapping, supper finishing and other ones.

To obtain the roughness as small as possible tools must have special forms (Fig.6.27) and travel with small feed motion, removing small allowance to decrease cutting forces and workpiece deformation.

Wheels of a coarse grain are used for rough-grinding operations, whereas wheels of a fine grain (micropowders) are used for finishing. In the last case the feeding motion is very small, or equal to zero during last passage (reciprocation).

Honing (Fig. 6.28a) is the application of bonded abrasive sticks, held on a honing tool, to surfaces for the purpose of limited stock removal and to obtain a suitable surface finish. The honing tool rotates continuously in one direction and simultaneously reciprocates axially.

Fig. 6.27. Lathe tools for finish cutting

Fig. 6.28. Finishing methods of machining: a – honing; b – superfinish;

1-work; 2-tool holder; 3 – abrasive tool; 4 - mineral oil

 

The most honing is done on internal surfaces, or holes, such as automobile cylinders. Since it can remove up to 0.7 mm of stock, honing can correct some out of roughness, taper, and axial distortion. It cannot correct hole location or perpendicularity.

The usual amount of stock removal is from 0.10 to 0.25 mm, and as low as 0.01 mm. Honing is applicable in a wide size range: holes of a diameter as small as 3 mm and as large as 1000 mm can be successfully honed. The length of the honed surface may in some cases reach
30 m.

Lapping is final machining operation, applied to produce a very smooth surface. It consists in charging the surfaces of the work or tool (called a lap) with special abrasive pastes or loose-grain abrasive flours mixed with oil or similar fluid. The abrasive is applied to the work by means of lapping belts, shoes, or disks, which are made of a material softer than the work to be lapped (cast iron, copper, wood, felt).

The purpose of lapping is to correct minor surface imperfections, to provide a close fit between mating parts, and to produce a smooth surface. The object of lapping is not to remove metal, since it is not economical as a material-removal operation, but rather to finish to some size or surface finish. The material removal depth is usually less than 0.025 mm although rough lapping may remove as much as 0.075 mm and finish lapping as little as 0.0025 mm.

Superfinishing (Fig. 6.28b) is used to obtain a surface of the highest class of finish. This process is similar to honing with small allowances but differs in the large variation of different tools (abrasive stones and wheels), work motions involved (sometimes up to 12) and the lower cutting speed at lower pressure of the stones on the work surface.

A notable feature of superfinishing is the sharp reduction in metal removal after the ridges and peaks, produced by previous machining, have been removed.

 

Table 6.2 - Comparative data of surface roughness achieved by various machining methods

Process	Average height of microirregulatarities Z, mkm	Designations
		Rz, mkm	RA, mkm
Rough turning, milling, planing	320...80	RZ 320	Ra40
Finish turning	40...12.5	RZ 40	Ra 5
Precision turning	12.5...1.25	RZ 12.5	Ra 4
Drilling	160...80	RZ 160	Ra 40
Counterboring	40...12.5	RZ 40	Ra 10
Grinding	12.5...1.25	RZ 12.5	Ra 4
Honing	1.25...0.13	RZ 1.25	Ra 0.4
Lapping	0.25...0.08	RZ 0.6	Ra 0.04
Superfinishing	0.25...0.01	RZ 0.6	Ra 0.04