220s15-ExpA-LabManual

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MECH 220 MECHANICAL ENGINEERING LABORATORY I

Experiment A

Surface Roughness Measurement

OBJECTIVE

The purpose of the experiment is to review the definitions on surface roughness and demonstrate the surface roughness measurement by using a tester. THEORY

In todays engineering applications, high-speed mechanisms under relatively higher loading are widely used. To withstand these severe operating conditions with minimum friction and wear, a particular surface finish is essential for the mating parts. It is necessary for the designer to accurately describe the required finish to the person who makes the parts. Surface finish control starts in the design and drafting room. The designer has the responsibility of specifying the quality of the surface to give maximum performance and service life at the lowest cost. There are two principal reasons for surface finish control: to reduce friction, and to control wear. Whenever a film of lubricant must be maintained between two moving parts, the surface irregularities must be small enough so that they will not penetrate the oil film under the most severe operating conditions. Bearings, journals, cylinder boxes, piston pins, bushings, pad bearings, helical and worm gears, seal surfaces, machine ways and so forth, are the types of items for which this condition must be fulfilled. Surface finish is also important to the wear of certain pieces that are subjected to dry friction, such as machine tool bits, threading dies, stamping dies, rolls, clutch plates and brake drums.

1. Surface Texture Surface texture is characterized and quantitatively expressed by surface roughness

height, surface waviness height and width, lay and flaws. Figure 1 shows these parameters schematically.

Figure 1. Surface texture characteristics.

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Roughness: Roughness is the relatively finely spaced surface irregularities that are produced by the cutting action of tool edges and abrasive grains on surfaces that are machined.

Roughness height: Roughness height is the average (arithmetical) deviation from the mean line of the profile. It is expressed in micrometers (m).

Roughness width: Roughness width is the distance between successive peaks or ridges, which constitute the predominant pattern of roughness. Roughness width is measured in millimeters (mm).

Roughness width cutoff: This term indicates the greatest spacing of repetitive surface irregularities to be included in the measurement of average roughness height. It is measured in millimeters (mm).

Waviness: Waviness is the surface undulations that are of much greater magnitude than the roughness irregularities. Waviness may result from machine or work deflections, vibrations, warping, strains, or similar causes.

Waviness height: Waviness height is the peak to valley distance rated in millimeters (mm).

Waviness width: Waviness width (rated in mm) is the spacing of successive wave valleys or wave peaks.

Flaws: Flaws are irregularities, such as cracks, checks, blowholes, scratches, and so forth, that occur at one place or at relatively infrequent or widely varying intervals on the surface.

Lay: Lay is the predominant direction of the tool marks of the surface pattern.

2. Designation of Surface Characteristics A surface whose finish is to be specified should be marked with the finish mark having

the general form of a check mark () so that the point of the symbol shall be on the line representing the surface, on the extension line, or on a leader pointing to the surface. Good practice dictates that the long leg and the extension shall be to the right as the drawing is read. Figure 2 shows an example on presenting the surface characteristics.

Figure 2. Surface texture symbol (unit for roughness height is [m], for the others [mm]).

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3. Definitions of Surface Roughness Measurement Parameters The following definitions are used in expressing surface roughness quantitatively: Ra, Arithmetical Mean Deviation Profile Ra is arithmetic mean of the absolute values of profile deviation (yi) from mean within

sampling length. Figure 3 shows the deviation in the surface roughness.

=1

||

=1 (1)

Figure 3. Deviation in the surface roughness.

Rq, Root-mean-square Deviation of Profile Rq is the square root of the arithmetic mean of the squares of profile deviation (yi)

from mean within sampling length, l.

= (1

2=1 )

1/2 (2)

Rz, Ten Point Height of Irregularities Rz is the sum of the mean height of the five highest profile peaks and the mean depth of the five deepest profile valleys from mean within the sampling length, l. Figure 4 shows the heights and depths to calculate Rz.

=1

5(

5=1 +

5=1 ) (3)

Figure 4. Ten point height of irregularities.

Rp, Maximum Height of Profile Peak Rp is the height from the highest profile peak line to mean line within sampling length, l as shown in Figure 5.

Rv, Maximum Depth of Profile Peak Rv is the depth from the deepest profile valley line to mean line within sampling length, l as shown in Figure 5.

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Ry (ISO), Maximum Height Profile Ry is the sum of height Rp of the highest profile peak from the mean line and depth Rv

of the deepest profile valley from the mean line within sampling length, l as shown in Figure 5.

Figure 5. Schematic indicating Rp, Ry and Rv.

Ry (DIN) Maximum Height Profile Ry (DIN) is calculated over the evaluation length and it is the maximum Ryi value

measured in each sampling length.

Rt, Total Peak-to-Valley Height Rt is the sum of the height of the highest peak and the depth of the deepest valley

over the evaluation length.

R3z, Third Maximum Peak-to-Valley Height R3z is the mean of the sum of the third profile peak height and the third profile valley depth of each sampling length over evaluation length.

Sm, Mean Spacing of Profile Elements Sm is the mean spacing between profile peaks at the mean line within sampling length,

l. Figure 6 shows parameters to calculate Sm.

= 1

=1 (4)

Figure 6. Schematic indicating parameters to calculate Sm.

S, Mean Spacing of Local Peaks of Profile S is the mean spacing of adjacent local peaks of the profile within sampling length, l. Figure 7 shows the parameters to calculate S.

= 1

=1 (5)

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Figure 7. Schematic indicating parameters to calculate S.

Sk, Skewness of the Profile Sk is the ratio of the mean cube value of the profile deviation (yi) and the cube of Rq within sampling length, l.

=1

3

1

()

3=1 (6)

tp, Profile Bearing Length Ratio tp is the ratio of the length of bearing profile to the sampling length at a depth c below the highest peak as shown in Figure 8.

(%) = 100 % 1

=1 (7)

Figure 8. Schematic indicating parameters to calculate tp.

4. Roughness Tester TR200 is a hand-held roughness tester. Figure 9a shows the schematic of the tester.

This tester applies to production site and can be used to measure surface roughness of various machinery-processed parts, calculate parameters according to selected measuring conditions and clearly display all measurement parameters and profile graphs on LCD.

Features of the device: - Multi-parameter measuring: Ra, Rz, Ry, Rq, Rp, Rm, Rt, R3z, Sk, S, Sm, tp - High accuracy inductance pickup - Four filtering methods of RC, PC-RC, GAUSS and D-P - Compatible with four standards of ISO, DIN, ANSI and JIS - 12864 dot matrix LCD displays all parameters and graphs. - DSP chip is used to control and process data with high speed and low power

consumption.

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- Built-in lithium ion chargeable battery and control circuit have high capacity without memory effect.

- Consecutive work time is longer than 20 hours. - Built-in standard RS232 interface enables communication with PC. Figure 9b shows

the connections. - Automatic switch off, memory and various prompt instructions

Figure 9. Schematics of (a) roughness tester TR 200, (b) sockets for connections.

4.1. Measurement Principle When measuring roughness of part surface, the pickup as schematically shown in

Figure 10a is placed on the surface of the part. Figure 10b shows the installation and removal process of the pickup, and Figure 9a shows the location of the pickup in the tester. Afterwards, data are collected by tracing the surface at constant rate. The pickup acquires the surface roughness by the sharp stylus in pickup. The roughness causes displacement of pickup which results in change of inductive value of induction coils thus generate analogue signal which is in proportion to surface roughness at output end of phase-sensitive rectifier. This signal enters data collection system after amplification and level conversion. Subsequently, those collected data are processed with digital filtering, parameter calculation is done by the processor, and the measuring result can be read on LCD, printed through printer and communicated with PC.

Figure 10. Schematics (a) of the pickup, (b) indicating the installation and removing of the pickup.

(a) (b)

(a) (b)

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4.2. Basic Connection Method and Charging of Battery

Installation and Removal of Pickup For installation, hold the main part of pickup with hand, push it into connection sheath

at the bottom of the instrument as shown in Figure 10b, and then slightly pushed it to the end of the sheath. To remove, hold the main part of pickup or the root of protective sheath with hand and slowly pull it out. Caution: 1. The stylus of pickup is key part of the tester and great attention should be paid to it. 2. During installation and removal, the stylus should not be touched in order to avoid damage and affecting measurement. 3. Connection of pickup should be reliable during installation. Adjustable supporter and sheath of pickup

When measured surface of part is smaller than the bottom surface of the instrument, sheath of pickup and adjustable supporter of TR200 options can be used for auxiliary support to complete the measurement as shown in Figure 11.

Figure 11. Connection and use of adjustable supporter and sheath of pickup.

Power Adapter and Charging of Battery When battery voltage is too low (i.e., battery voltage symbol flashes on screen to prompt low voltage), the instrument should be charged as soon as possible. Power adapter could be connected to mains electricity (220V, 50Hz) and charging will begin. Input voltage for power adapter is AC 220V with a DC 6V output, about 500mA of maximum charge current for a charging time of 2.5 hours. This instrument adopts lithium-ion chargeable battery without memory effect and charging can be fulfilled at any time without affecting normal operation of the instrument. Important notes on battery and its charging:

1. Layout of connection lines shall not affect measuring part under charging state. 2. Meanings of battery voltage prompts are:

indicates normal voltage and measurement can be carried out; the black part inside prompt shows capacity of battery;

indicates too low voltage and battery should be charged as soon as possible;

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indicates that battery is being charged;

indicates that battery is full and charging power should be cut off as soon as possible.

3. Relative high noises of power source may affect measurement of weak signal to some extent when battery is being charged.

4. The instrument needs to monitor the process of charging, so it is not necessary to turn it off. The instrument will turn on automatically even it is turned off.

5. Keep the battery switch on unless the TR200 will not be used for a long period of time (more than 2 to 3 weeks). If the battery switch is off, the measurement results will be lost.

6. When the TR200 is delivered the battery switch is off. User should set the switch on firstly before using it. 4.3. Measuring Operation

Preparation for Measurement a. Switch-on to check if battery voltage is normal; b. Clear the surface of part to be measured; c. Refer to Figure 12 to place the instrument correctly, stably and reliably on the surface

to be measured.

Figure 12. Some warnings about the orientation of the tester for stable and reliable measurements.

Basic Measurement Status Press and release Power key to switch on. The instrument automatically shows model,

name of tester and information of manufacturer, and then displays basic measurement status as shown in Figure 13. Contents of basic measurement status displayed in the first switch-on are default settings of the instrument. Settings and data of last switch-off will be displayed in the next switch-on. Basic measurement status will be shown automatically for each switch-on. Dont keep pressing for a long time to switch on the instrument.

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Figure 13. Display when the tester is turned on.

In basic measurement status, Figure 14 shows the steps to be performed.

Figure 14. Basic measurement process.

To display the other measurement parameters:

i) Press Menu key, to enter menu operation status.

ii) Press Parameter key, first to show all parameter values of the last

measurement. Press Scroll key, to scroll pages. Press parameter key,

second time to display profile graphs of the last measurement. Press Scroll

key, to obtain profile graphs with other sampling lengths. Press

Parameter key, third time to display tp curve and tp value of the last measurement. Pressing the Parameter key again will repeat above descriptions in

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a cycle. Press Esc key, in each status to return basic measurement status as shown in Figure 15.

Figure 15. Display of parameters.

5. Bibliography

[1] Engineering Drawing and Design, C. Jensen, J. D. Helsel, D. R. Short; 7th Edition, McGraw- Hill, 2008.

[2] TR200 Manual.

EXPERIMENTATION Two specimens will be supplied for the surface roughness measurement. A lathe and

a milling machine have been used for machining the top surface of these specimens. The surface roughness of the specimens will be measured at 8 different locations as shown in Figure 16. Pay utmost attention while putting the pickup above the specimens. The laboratory assistants will adjust the settings of the instrument. (a) (b) Figure 16. Lines showing the measurement locations on the top surface of the specimen machined with (a) milling machine; (b) lathe.

Procedure 1. Take the specimen machined with lathe and clear the top surface. 2. Gently, place the instrument along the first line drawn above the specimen. Use

the adjustment part at the back of the instrument to level the pickup axis. Make

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sure that the instrument is placed securely so that it is in a stable and reliable condition. Axis of the pickup must be parallel to the surface to be measured.

3. Press the Run key, to start the measurement. 4. When measurement finishes the result will be displayed on the screen. Record

the arithmetical mean deviation of profile, Ra, in the Table provided in your data sheet.

5. Press the parameter key, and record all parameters listed on the screen at the appropriate places in the Table in your data sheet.

6. Press Esc key, to return to basic measurement status and move the instrument on the next line drawn on the specimen.

7. Repeat items 2 through 6 for 7 more times. 8. Change the specimen and use the one machined with the milling machine. 9. Repeat items 2 through 6 for 7 more times for the second specimen.

Analysis 1. For both specimens, calculate the mean values of the parameters you measured and

set the limits for 99, 95 and 80 percent confidence levels. 2. Prepare a working drawing of one of the specimens indicating its surface roughness

values. Use general geometrical tolerances to control the form of the specimen in K class. For all of the dimensions, general size tolerance in m class is applicable.

CAUTION!

Before coming to the laboratory get familiarized with the instrument and the parameters to be measured.

You will have less than 30 minutes to complete the experiment.

Name: Date:

MECH 220 MECHANICAL ENGINEERING LABORATORY I EXPERIMENT A

SURFACE ROUGHNESS MEASUREMENT DATA SHEET

Group No:

Table 1. Surface roughness measurement of the specimen machined with lathe.

1 2 3 4 5 6 7 8

Ra (m)

Rq (m)

Rz (m)

Ry (m)

Rt (m)

Rp (m)

Rv (m)

S (mm)

Sm (mm)

Sk

Table 2. Surface roughness measurement of the specimen machined with the milling machine.

1 2 3 4 5 6 7 8

Ra (m)

Rq (m)

Rz (m)

Ry (m)

Rt (m)

Rp (m)

Rv (m)

S (mm)

Sm (mm)

Sk