Experimental Research on Effect of CNC Machining Parameters on

Surface Roughness of Workpieces

Chang guo Lu

Jing bo Wang

，

Yingkou Institute of Technology

，

Bowen Road,Yingkou, China

Keywords: The surface roughness, CNC machining, the amount of cutting.

Abstract: The surface roughness of a mechanical part is one of the accuracy of parts processing. A Cutting parameter

in CNC machining is a important factor that would affect the surface roughness of parts. From an

experimental point of view, the writers have studied the influence of the cutting amount on the surface

roughness in CNC machining, and the result could be used to guide the machining.

1 INTRODUCTION

The term surface roughness refers to the smaller

pitch of the machined surface and the unevenness

between peaks and valleys. The lower the surface

roughness is, the smoother the surface is. The

surface roughness of a part is a parameter that

characterizes the micro-geometry of the part surface,

to measure the accuracy of the part surface.

According to GB/T 1031-2009"Surface Structure

Profile Method Surface Roughness Parameters and

Its Value", the arithmetic average deviation Ra and

the contour maximum height Rz are used as the

evaluation parameters in the sampling length. When

used, the part’ surface roughness will affect its wear

resistance, fatigue strength, corrosion resistance and

tolerances and fits and so on.

In the cutting process, the surface roughness of

the workpiece will be affected by a lot of factors,

such as cutting amount, tool geometric parameters,

workpiece material, cutting fluid, vibration during

cutting, formation of built-up edge or not, and so on.

In the numerical control machining process, some

influence factors are fixed relatively, such as the

workpiece material, cutting fluid, vibration and

built-up edge. The others are need to be selected

according to different processing conditions, such as

and the cutting amount and tool geometric

parameters etc. The standard tools are used in CNC

machining. When a certain tool type is selected, the

geometric parameters are also determined. So as to

the actual cutting parameters，then there is only one

is actually to be selected，that is the cutting amount

in the actual machining process.

The cutting amount includes the cutting speed

vc，the back engagement of the cutting edge ap, and

the feed amount f and so on. When increasing one of

them ， the cutting efficiency will be more and

influence by the tool condition and the machine tool

performance. Because changing the cutting amount

parameters will affect the quality of the part

machining surface, so it is more important for us to

study the relationship between the cutting

parameters and the surface roughness of the

workpiece for ensuring the quality of the NC

machining while improving the cutting efficiency.

2THE THEORETICAL CAUSE OF

SURFACE ROUGHNESS

The term theoretical roughness is the tool surface

geometry and the workpiece surface roughness

caused by cutting motion. That is residual areas

formed between the contours of the adjacent tool

paths due to the geometry of the tool during

machining. Here，we take turning processing as an

example as shown in Figure 1.

(a) (b)

Figure 1 Surface area remaining on turning workpiece.

The value of surface roughness in cutting process

is mainly based on the height of the cutting residual

area. When tools with corner radius



=0, the height

of the workpiece surface remained (Referring to

Figure 1a).









cotcot

（1）

Where

- Feed amount (mm/r);



- Lead angle (≠90°);



- Deflection angle (≠90°).

When tools with corner radius



≠0, the height

of the workpiece surface remained (Referring to

Figure 1b).





(2)

Based on the above equations (1) and (2) ，the

height of the residual area can be reduced by

decreasing

、



、



orincreasing



Certainly, the microscopic geometric

deformation generated on the workpiece surface is

more than the theoretical roughness in the actual

cutting process, because of the friction between the

tool and the workpiece surface, the plastic

deformation of the workpiece surface, and the high

frequency vibration in the process system.

3THE ESTABLISHMENT OF AN

EXPERIMENTAL MODEL

It is long for theoretical and experimental research

on the influence of the three factors of cutting

amount on the roughness of the workpiece surface. It

is shown that the amount of feed has a significant

effect on the value of the surface roughness by a lot

of studies and the Long-term production. That is the

value of the surface roughness will increase after the

feed increases. And the effect of cutting speed on the

surface roughness of the workpiece is related to the

formation of built-up edge in the cutting process.

When cutting plastic material in the medium-speed

(vc =20-40m/min)，the built-up edge is easy to be

formed, then the surface roughness value of the

workpiece increases；While in low-speed or high-

speed, it is not and the roughness value of workpiece

surface is less ；The effect of the amount of the

back engagement of the cutting edge on the

workpiece surface roughness is not very obvious. Of

course, the results are based on ordinary metal

cutting process. CNC cutting machining is similar to

it, but it still has some specific features. Here, the

actual conditions of modern CNC cutting processing

is be used to determine the corresponding

experimental methods and parameters in the paper.

We intend to machine the workpiece with CNC

machining and CNC milling separately, then

measure the value of the surface roughness of the

workpiece, and finally analyze the measurement data

and results from them in this experiment. Based on

the actual production habits, the parameters are set

as follows:

①CNC turning: Specimenφ18×40，Q235； 93

degree turning tool ； ap(mm)0.1,0.2,0.5,1.0 ；

f(mm/r)0.05,0.08,0.10,0.12,0.20 ；

n(r/min)500,800,1000,1200；

② CNC milling: Specimen 64×16 ， Q235 ；

φ80End mill，φ20、φ16、φ10 End mill，R5Ball

end mill；ap(mm)0.1，0.2，0.3，0.5，1.0，2.0；

F(mm/min)50 ， 100 ， 200 ， 300 ； n(r/min)500 ，

800，1000，1200

，1800，2000.

4THE EXPERIMENTAL DATA

After machining, wetested specimen in the way of

contour contact measurement using a 2205 Surface

Roughness Gauge. The data are listed in Table 1 and

Table 2 (Excerpts) respectively. The 2205 Surface

Roughness Gauge can measure multiple surface

roughness parameters. This experimental is only

based on the most widely used contour arithmetic

average deviation Ra.

Table 1: Surface Roughness Measurement Results of CNC

Lathe Machining Test Pieces (Extract).

Note: Spindle speed n, feed amount f, and the

back engagement of the cutting edge a

refer to the

common CNC turning parameters selection.The cutting

speed Vc is a converted value (Vc=πdn/1000).

Table 2 :Surface Roughness Measurement Results of NC

Milling Test Pieces (Excerpt).

Note: Spindle speed n, feed speed F, and the

back engagement of the cutting edge ap refer to the

common CNC milling parameters selection.The cutting

speed Vc is a converted value (Vc=2πRn/1000).

5THE ANALYSIS OF DATA

There are 83 valid samples of CNC turning and

milling. Some data are eliminated by taking into

account the effects of random errors and

measurement errors during cutting. After studying

the conclusions are as follows:

① The feed or feed rate has a significant effect

on the workpiece surface roughness. Under the same

cutting conditions, the workpiece surface roughness

value will increase while the feed or feed rate

increases. Referring the number 02, 03 and 08, 09

and11,12 in Table 1; the 05, 07 and 11 ,12 and15 ,16

in Table 2.In Fig. 2 and Fig. 3 are the measured

results of the surface roughness of the workpieces of

the CNC turning; In Fig. 4 and Fig. 5 are the CNC

milling.

Fig. 2 Surface roughness measurement results of CNC

turning workpiece 11(n1200,f0.05,ap0.2).

Fig. 3 Surface roughness measurement results of CNC

turning workpiece 12(n1200,f0.08,ap0.2).

Fig. 4 Surface roughness measurement result of NC

milling workpiece 05(R40，n800,F100,ap2).

Fig. 5 Surface roughness measurement result of NC

milling workpiece 07(R40，n800,F200,ap2).

②The parameter back engagement of the cutting

edge has not a significant effect on the value of the

surface roughness of the workpiece. When in the

same condition in addition to the back engagement

of the cutting edge, the surface roughness value of

the workpieces keep unchanged basically either

CNC turning or milling. Referring 01,02 and 03,04

in Table 1, the value the of surface roughness of the

workpiece decreases when the back engagement of

the cutting edge increases, because of the affecting

by the turning radius of tool cutting edge of the

turning tool.

③ The cutting speed has an effect on the surface

roughness of the workpiece, but it is less than the

feed or feed rate. During in high-speed, the value of

the surface roughness of the workpiece decreases

when increasing the cutting speed, referring to 14,15

in Table 2, and 14, 15 and 02, 03, 04.when the

cutting speed Vc> 200m/min, the average value of

the surface roughness of the workpiece is some

0.628μm; and when Vc = 50~60m/min, the value is

about 2.122μm. However, if the spindle speed of the

machine tool is too high, the value of the surface

roughness of the workpiece will increase due to the

accuracy of the machine itself and the effects of

cutting vibration,. Such as when the spindle speed of

the economical CNC lathe reaches 1200r/min and

the CNC milling machine reaches 2000r/min.

6CONCLUSIONS

After CNC machining and milling, we tested the

value of the surface roughness of the sample pieces

and then study the effects on them by the cutting

parameters (the cutting speed vc, the

back engagement of the cutting edgeap, the feed f )

in this experiment, the conclusions are as follows:

①The feed has a significant effect on the value

of the surface roughness of the workpiece. the more

the feed, the bigger the value.

②The cutting speed effects on the value of the

surface roughness of the workpiece. the more the

speed, the less the value during high-speed cutting.

③ Theback engagement of the cutting edge has

little effect on the value of the surface roughness of

the workpiece.

④Usually, if to make the value of the workpiece

surface roughness Ra= 1.6 ~ 3.2μm, the ideal

turning amount in CNC turning: vc = 30 ~ 60m/min,

ap = 0.5 ~ 1.0mm, f = 0.05 ~ 0.12mm / r; In CNC

milling: vc = 20 ~ 100m/min, ap = 0.2 ~ 2.0mm, F =

50 ~ 200mm/min. When a large diameter end is sued

in CNC milling, and F = 50 ~ 100mm/min, the Ra is

up to 0.8μm.

⑤When the spindle speed of the machine tool is

too high, the value of the surface roughness value of

the workpiece will increase due to machine tool

accuracy and cutting vibration. Otherwise, if

theback engagement of the cutting edge is too small,

the value will increase due to the influence of the

turning radius of tool cutting edge.

REFERENCES

1. Liu Hui. ,2006.Interchangeability and Technical

Measurement. East China University of Technology

Press.Shanghai.

2. Wang Qiping.,1999. Mechanical Manufacturing

Technology. Harbin University of Technology

Press.Harbin .

3. Eung-Suk Lee,Suk-Hwan Suh and Jin-wook

Shon.,1998.A comprehensive method for calibration of

volumetric positioning accuracy of cnc-

machines.Advanced manufacturing

technology,1998,14(1):43~49.

4. Dufour P,Groppetti R.,2000.Computer aided accuracy

improvement in larger NC machine

tools.M.T.D.R.Conferrence Proceedings,

5. Lu Changguo, Duan Zhenyun. ,2016.

6. Technical basis of Mechanical Manufacturing. Dalian

University of Technology Press.Dalian.