Saturday, July 31, 2021

Surface Finish - Industrial Engineering and Productivity Aspects

Lesson 57 of Industrial Engineering ONLINE Course

Lesson 14 of Process Industrial Engineering FREE ONLINE Course (Module)


Surface Finish - An Area of Inquiry for Industrial Engineers


Industrial engineers have to find alternative ways that reliably produce to the specification of the design but give productivity. They have to first know alternatives to produce to specification.

Industrial engineers have to examine, whether cutting conditions are being kept substantially below the maximum cutting parameters recommended in theory or best practice by shop personnel because of the intention to reach the surface finish specified. They have to understand the theory behind surface finish achievement and the theory of cutting conditions that give maximum material removal rate and then come out with the best cutting conditions. They they have to organize the experiment to do most productive combination and if any problem persists, they have to find out whether any improvement in other machine variables is going to eliminate the problem. It is the industrial engineer's job not to give up utilizing the best cutting variables and achieve maximum productivity while achieving the specified surface finish. Hence, industrial engineers have to be aware of various solutions to improve surface finish.

Surface Finish, Integrity and Flatness

Many parts are machined to produce surfaces with specified finishes for locating, sealing, or similar applications. In finishing cuts,  surface flatness, and finish requirements restrict the range of tool sizes, geometries, and feed rates that can be used. Understanding the surface finish issues, can help in increasing the productivity of machining operations.

MEASUREMENT OF SURFACE FINISH

The surface finish  is most commonly measured with a stylus-type profile meter or profilometer, an instrument similar to a phonograph that amplifies the vertical motion of a stylus as it is drawn across the surface.

Types of measurements

For nominally flat surfaces, the shape is referred to as the slope or lay of the surface.

Waviness refers to variations in the surface with relatively long wavelengths or, equivalently, lower frequencies. Waviness may result from clamping errors, errors in the tool or cutter geometry, or vibrations of the system. Spindle tilt in face-milling operations also produces a waviness or shape error.

Roughness is the term for surface profile variations with wavelengths shorter than those characteristic of waviness.  Roughness has a geometric component dependent on the feed rate, tool nose radius, tool lead angle, and cutting speed, as well as a natural component resulting from tool wear, inhomogeneities in the work material, higher frequency vibrations of the machining system, and damage to the surface caused by chip contact.

The commonly used roughness measurements for machined surfaces: the average roughness Ra, maximum peak height Rp, maximum valley depth Rv, peak to valley height Rt, average maximum profile height Rz, maximum roughness depth Rmax, and bearing ratio tp.

The parameters Ra, Rv, Rp, and Rt  are all defined with respect to a centerline of a filtered stylus trace. Filtering is performed to remove the slope and waviness components of the trace. Once filtering is performed, the centerline is determined as the mean line of the surface profile.

The average roughness Ra is defined as the average absolute deviation of the workpiece from the centerline:

Rp is the maximum deviation of a peak above the centerline encountered within the sampling length

Similarly, Rv, the maximum depth of valley below the centerline,

Rt, the maximum peak to valley deviation or total profile height, is equal to
 Rt =  Rp  + Rv

The average maximum profile height Rz and maximum roughness depth Rmax  are  defined in ASME B46.1  as the average and maximum values of the profile heights over five successive sampling intervals:

The bearing ratio tp  is a function of the depth p below the highest peak and is
defined as the ratio of the total length of the profile below the depth p to the total trace length L:

Ra is the most commonly specified roughness parameter and is well suited for monitoring the
consistency of a machining process.

SURFACE FINISH IN TURNING AND BORING
For turning and single-point boring, the geometric roughness is easily calculated from the tool angles and feed.

For a tool with a nose (or corner) radius, the depth of cut is often smaller than the nose radius, especially in finish turning and boring. In this case the geometric roughness is independent of the tool angles κre and k¢re and is determined by the feed per revolution f and nose radius rn:


These equations provide lower bound for the roughness obtained in practice and indicate that smoother surfaces can be generated by using a smaller feed, larger tool nose radius, and larger tool
lead angle. 

SURFACE FINISH IN MILLING

When face milling with radiused inserts, the surface finish depends on the insert radius and on the effective feed rate. The geometric component of the average peak to valley cusp height, Rtg, and average roughness, Rag, measured in the feed direction can be calculated approximately using the formulas.

When face milling with radiused inserts, the surface finish depends on the insert radius and on the effective feed rate.



SURFACE FINISH IN GRINDING

Measured roughness values in cylindrical plunge grinding are usually well characterized by an
equation using

vw: the workpiece velocity
vs: the wheel velocity
a:  the depth of cut


RESIDUAL STRESSES IN MACHINED SURFACES
Machined surfaces often exhibit residual stresses  induced both by differential plastic deformation and by surface thermal gradients . Stresses due to plastic deformation are obviously mechanically induced, but those due to thermal gradients may reflect phase transformations or chemical reactions. These stresses increase with tool wear, since both deformation forces and tool–workpiece frictional heating increase. For a sharp tool, significant residual stresses typically do not occur at depths much greater than 50 μm below the surface; for worn tools, however, significant stresses may occur at 5 or 10 times this depth.

WHITE LAYER FORMATION
White layer formation occurs when cutting ferrous materials, especially steels . The white layer is a surface layer, which has undergone microstructural alterations caused by excessive surface temperatures and air hardening. It is resistant to standard etchings, so that it appears white under an optical microscope (or featureless in a scanning electron microscope.) The white layer has the same chemical composition as the substrate, but due to its different microstructure it has different mechanical properties, and most significantly increased hardness.

SURFACE BURNING IN GRINDING
Surface burning concerns often limit the maximum wheel speed or stock removal in grinding operations. Surface burning has been studied in detail mainly for carbon and alloy steel workpieces. Burning is accompanied by metallurgical and chemical phenomena such as oxidation (which produced the characteristic burn marks on the surface), tempering, residual stresses, and phase transformations; apart from aesthetic concerns, burning should be avoided because these phenomena lead to a reduction in fatigue life.

MEASUREMENT OF SURFACE FLATNESS
Machined surfaces used for sealing or motion control often have a specified flatness tolerance.  Flatness is defined as the minimum separation between two parallel planes containing the entire surface profile. Since the entire profile is not normally measured with high resolution, the sampling method and resolution of individual measurements both influence measured flatness values. Therefore, the flatness and profile/waviness requirements on the part should be considered in the selection of the process even though the proper surface finish can be achieved.

There are several contact and non-contact methods for measuring the flatness or profile of a surface. Automated flexible methods use coordinate measuring machines (CMM). All CMMs have a mechanical or optical probe attached to the third moving axis of the machine. 

SURFACE FLATNESS COMPENSATION IN FACE MILLING

Flat surfaces are often machined by face milling.  The standard methods of controlling flatness in these applications include fixture and cutter optimization and the use of multiple passes, including a low DOC finishing pass. Limited feed rate optimization, in which the feed rate is slowed over sections of the tool path with low part stiffness, is also used.

Industrial engineer have to examine, whether cutting conditions are being kept substantially below the maximum cutting parameters recommended in theory or best practice by shop personnel because of the intention to reach the surface finish specified. They have to understand the theory behind surface finish achievement and the theory of cutting conditions that give maximum material removal rate and then come out with the best cutting conditions. They they have to organize the experiment to do most productive combination and if any problem persists, they have to find out whether any improvement in other machine variables is going to eliminate the problem. It is the industrial engineer's job not to give up utilizing the best cutting variables and achieve maximum productivity while achieving the specified surface finish. Hence, industrial engineers have to be aware of various solutions to improve surface finish.

Other compensation methods

Tool Path Direction Compensation
The tool path direction has an important influence on both cycle time and surface flatness when face milling large surfaces. Cutting forces interact with the part and fixture to cause deflections, and due to spatial and directional variations in part and fixture stiffness, applying forces from different direction (by changing the tool path) results in different flatness and profile errors. The milling process, up or down milling, also affects the forces on the part for a similar reason. The removal of residual stress is a progressive process, which is affected by the tool path direction and local DOC. Tool path direction compensation (TPDC) is performed on a macroscale because the face mill cutter diameter is usually large in order to cover as much surface area as possible.

Depth of Cut Compensation
The milling path along a plane can be compensated normal to the plane by cutting shallower on
convex surface sections (high spots) and deeper on concave surface sections (low spots).

Tool Feed Compensation
The tool feed compensation (TFC) method focuses on cutting force–induced errors, which can be directly controlled by the optimization of feed and speed in relation to the cycle time. The feed per tooth can be optimized to improve the surface flatness because cutting forces exerted on the part surface are proportional to feed.

Spindle-Part Tilt Compensation
The spindle-part tilt compensation (SPTC) method is used to prevent contact of the face-milling
cutter trailing edges with the machined surface.



Updated on 31 July 2021, 22 January 2021
First published on 11 July 2020


No comments:

Post a Comment