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單面多軸鉆孔組合機床動力滑臺液壓系統(tǒng)
一、設計要求及工況分析
1．設計要求
要求設計的動力滑臺實現(xiàn)的工作循環(huán)是：快進 工進 快退 停止。主要性能參數(shù)與性能要求如下：切削阻力FL=30000N；運動部件所受重力G=10000N；快進、快退速度1= 3=0.07m/s，工進速度2=0.83×10-3m/s；快進行程L1=150mm，工進行程L2=30mm；往復運動的加速時間Δt=0.2s；動力滑臺采用平導軌，靜摩擦系數(shù)μs=0.2，動摩擦系數(shù)μd=0.1。液壓系統(tǒng)執(zhí)行元件選為液壓缸。
2．負載與運動分析
(1) 工作負載 工作負載即為切削阻力FL=3000N。
(2) 摩擦負載 摩擦負載即為導軌的摩擦阻力：
靜摩擦阻力 N
2000
10000
2
.
0
s
fs
=
′
=
=
G
F
m
動摩擦阻力 N
1000
10000
1
.
0
d
fd
=
′
=
=
G
F
m
(3) 慣性負載
N
357.1
N
2
.
0
07
.
0
8
.
9
10000
i
=
′
=
D
D
=
t
g
G
F
u
(4) 運動時間
快進 s
2．1
s
07
.
0
10
150
3
1
1
1
=
′
=
=
-
u
L
t
工進 s
1
.
36
s
10
83
.
0
10
30
3
3
2
2
2
=
′
′
=
=
-
-
u
L
t
快退 s
6
.
2
s
07
.
0
10
)
30
150
(
3
3
2
1
3
=
′
+
=
+
=
-
u
L
L
t
設液壓缸的機械效率ηcm=0.9，得出液壓缸在各工作階段的負載和推力，如表1所列。
表1液壓缸各階段的負載和推力
工況
負載組成
液壓缸負載F/N
液壓缸推力F0=F/ηcm/N
啟 動
加 速
快 進
工 進
反向啟動
加 速
快 退
2000
1357.1
1000
31000
2000
1357.1
1000
2222.2
1507.9
1111.1
34444.4
2222.2
1507.9
1111.1
圖1 F-t與-t圖
根據(jù)液壓缸在上述各階段內(nèi)的負載和運動時間，即可繪制出負載循環(huán)圖F-t 和速度循環(huán)圖-t，如圖1所示。
二、確定液壓系統(tǒng)主要參數(shù)
1．初選液壓缸工作壓力
所設計的動力滑臺在工進時負載最大，在其它工況負載都不太高，參考表2和表3，初選液壓缸的工作壓力p1=4MPa。
2．計算液壓缸主要尺寸
鑒于動力滑臺快進和快退速度相等，這里的液壓缸可選用單活塞桿式差動液壓缸（A1=2A2），快進時液壓缸差動連接。工進時為防止孔鉆通時負載突然消失發(fā)生前沖現(xiàn)象，液壓缸的回油腔應有背壓，參考表4選此背壓為p2=0.6MPa。
表2 按負載選擇工作壓力
負載/ KN
<5
5~10
10~20
20~30
30~50
>50
工作壓力/MPa
<0.8~1
1.5~2
2.5~3
3~4
4~5
≥5
表3 各種機械常用的系統(tǒng)工作壓力
機械類型
機 床
農(nóng)業(yè)機械
小型工程機械
建筑機械
液壓鑿巖機
液壓機
大中型挖掘機
重型機械
起重運輸機械
磨床
組合機床
龍門刨床
拉床
工作壓力/MPa
0.8~2
3~5
2~8
8~10
10~18
20~32
表4 執(zhí)行元件背壓力
系統(tǒng)類型
背壓力/MPa
簡單系統(tǒng)或輕載節(jié)流調(diào)速系統(tǒng)
0.2~0.5
回油路帶調(diào)速閥的系統(tǒng)
0.4~0.6
回油路設置有背壓閥的系統(tǒng)
0.5~1.5
用補油泵的閉式回路
0.8~1.5
回油路較復雜的工程機械
1.2~3
回油路較短且直接回油
可忽略不計
表5 按工作壓力選取d/D
工作壓力/MPa
≤5.0
5.0~7.0
≥7.0
d/D
0.5~0.55
0.62~0.70
0.7
表6 按速比要求確定d/D
2/1
1.15
1.25
1.33
1.46
1.61
2
d/D
0.3
0.4
0.5
0.55
0.62
0.71
注：1—無桿腔進油時活塞運動速度；
2—有桿腔進油時活塞運動速度。
由式得
2
4
2
6
2
1
cm
1
m
10
93
m
10
)
2
6
.
0
4
(
9
.
0
31000
)
2
(
-
′
=
′
-
′
=
-
=
p
p
F
A
h
則活塞直徑 mm
109
m
109
.
0
m
10
93
4
4
4
1
=
=
′
′
=
=
-
p
p
A
D
參考表5及表6，得d0.71D =77mm，圓整后取標準數(shù)值得 D=110mm， d=80mm。
由此求得液壓缸兩腔的實際有效面積為
根據(jù)計算出的液壓缸的尺寸，可估算出液壓缸在工作循環(huán)中各階段的壓力、流量和功率，如表7所列，由此繪制的液壓缸工況圖如圖2所示。
表7液壓缸在各階段的壓力、流量和功率值
工況
推力
F0/N
回油腔壓力
p2/MPa
進油腔壓力
p1/MPa
輸入流量
q×10-3/m3/s
輸入功率
P/KW
計算公式
快進
啟動
2222.2
—
0.44
—
—
加速
1507.9
p1+Δp
0.74
—
—
恒速
1111.1
p1+Δp
0.66
0.35
0.23
工進
34444.4
0.6
3.91
0.79×10-2
0.031
快退
啟動
2222.2
—
0.50
—
—
加速
1507.9
0.5
1.40
—
—
恒速
1111.1
0.5
1.31
0.45
0.59
注：1. Δp為液壓缸差動連接時，回油口到進油口之間的壓力損失，取Δp=0.5MPa。
2． 快退時，液壓缸有桿腔進油，壓力為p1，無桿腔回油，壓力為p2。
三、擬定液壓系統(tǒng)原理圖
1．選擇基本回路
圖2 液壓缸工況圖
(1) 選擇調(diào)速回路 由圖2可知，這臺機床液壓系統(tǒng)功率較小，滑臺運動速度低，工作負載為阻力負載且工作中變化小，故可選用進口節(jié)流調(diào)速回路。為防止孔鉆通時負載突然消失引起運動部件前沖，在回油路上加背壓閥。由于系統(tǒng)選用節(jié)流調(diào)速方式，系統(tǒng)必然為開式循環(huán)系統(tǒng)。
(2) 選擇油源形式 從工況圖可以清楚看出，在工作循環(huán)內(nèi)，液壓缸要求油源提供快進、快退行程的低壓大流量和工進行程的高壓小流量的油液。最大流量與最小流量之比qmax/qmin=0.35/(0.79×10-2)44；其相應的時間之比(t1+t3)/t2=(2.1+2.6)/36.1=0.13。這表明在一個工作循環(huán)中的大部分時間都處于高壓小流量工作。從提高系統(tǒng)效率、節(jié)省能量角度來看，選用單定量泵油源顯然是不合理的，為此可選用限壓式變量泵或雙聯(lián)葉片泵作為油源?？紤]到前者流量突變時液壓沖擊較大，工作平穩(wěn)性差，且后者可雙泵同時向液壓缸供油實現(xiàn)快速運動，最后確定選用雙聯(lián)葉片泵方案，如圖2a所示。
(3) 選擇快速運動和換向回路 本系統(tǒng)已選定液壓缸差動連接和雙泵供油兩種快速運動回路實現(xiàn)快速運動?？紤]到從工進轉(zhuǎn)快退時回油路流量較大，故選用換向時間可調(diào)的電液換向閥式換向回路，以減小液壓沖擊。由于要實現(xiàn)液壓缸差動連接，所以選用三位五通電液換向閥，如圖2b所示。
(4) 選擇速度換接回路 由于本系統(tǒng)滑臺由快進轉(zhuǎn)為工進時，速度變化大（1/2=0.07/(0.83×10-3)84），為減少速度換接時的液壓沖擊，選用行程閥控制的換接回路，如圖2c所示。
(5) 選擇調(diào)壓和卸荷回路 在雙泵供油的油源形式確定后，調(diào)壓和卸荷問題都已基本解決。即滑臺工進時，高壓小流量泵的出口壓力由油源中的溢流閥調(diào)定，無需另設調(diào)壓回路。在滑臺工進和停止時，低壓大流量泵通過液控順序閥卸荷，高壓小流量泵在滑臺停止時雖未卸荷，但功率損失較小，故可不需再設卸荷回路。
圖3 整理后的液壓系統(tǒng)原理圖
圖2 選擇的基本回路
2．組成液壓系統(tǒng)
將上面選出的液壓基本回路組合在一起，并經(jīng)修改和完善，就可得到完整的液壓系統(tǒng)工作原理圖，如圖3所示。在圖3中，為了解決滑臺工進時進、回油路串通使系統(tǒng)壓力無法建立的問題，增設了單向閥6。為了避免機床停止工作時回路中的油液流回油箱，導致空氣進入系統(tǒng)，影響滑臺運動的平穩(wěn)性，圖中添置了一個單向閥13?？紤]到這臺機床用于鉆孔（通孔與不通孔）加工，對位置定位精度要求較高，圖中增設了一個壓力繼電器14。當滑臺碰上死擋塊后，系統(tǒng)壓力升高，它發(fā)出快退信號，操縱電液換向閥換向。
四、計算和選擇液壓件
1．確定液壓泵的規(guī)格和電動機功率
(1) 計算液壓泵的最大工作壓力
小流量泵在快進和工進時都向液壓缸供油，由表7可知，液壓缸在工進時工作壓力最大，最大工作壓力為p1=3.96MPa，如在調(diào)速閥進口節(jié)流調(diào)速回路中，選取進油路上的總壓力損失∑?p=0.6MPa，考慮到壓力繼電器的可靠動作要求壓差Dpe=0.5MPa，則小流量泵的最高工作壓力估算為
(
)
MPa
01
.
5
MPa
5
.
0
6
.
0
96
.
3
e
1
1
p
=
+
+
=
D
+
D
+
3
?
p
p
p
p
大流量泵只在快進和快退時向液壓缸供油，由表7可見，快退時液壓缸的工作壓力為p1=1.43MPa，比快進時大。考慮到快退時進油不通過調(diào)速閥，故其進油路壓力損失比前者小，現(xiàn)取進油路上的總壓力損失∑?p=0.3MPa，則大流量泵的最高工作壓力估算為
(
)
MPa
70
.
1
MPa
3
.
0
43
.
1
1
2
p
=
+
=
D
+
3
?
p
p
p
(2) 計算液壓泵的流量
由表7可知，油源向液壓缸輸入的最大流量為0.45×10-3 m3/s ，若取回路泄漏系數(shù)K=1.1，則兩個泵的總流量為
L/min
33
/s
m
10
55
.
0
/s
m
10
45
.
0
1
.
1
3
3
3
3
1
p
=
′
=
′
′
=
3
-
-
Kq
q
考慮到溢流閥的最小穩(wěn)定流量為3L/min，工進時的流量為0.79×10-5 m3/s =0.47L/min，則小流量泵的流量最少應為3.47L/min。
(3) 確定液壓泵的規(guī)格和電動機功率
根據(jù)以上壓力和流量數(shù)值查閱產(chǎn)品樣本，并考慮液壓泵存在容積損失，最后確定選取PV2R12-6/33型雙聯(lián)葉片泵。其小流量泵和大流量泵的排量分別為6mL/r和33mL/r，當液壓泵的轉(zhuǎn)速np=940r/min時，其理論流量分別為5.6 L/min和31L/min，若取液壓泵容積效率ηv=0.9，則液壓泵的實際輸出流量為
由于液壓缸在快退時輸入功率最大，若取液壓泵總效率ηp=0.8，這時液壓泵的驅(qū)動電動機功率為
KW
17
.
1
KW
10
8
.
0
60
10
33
10
70
.
1
3
3
6
p
p
p
=
′
′
′
′
′
=
3
-
h
q
p
P
根據(jù)此數(shù)值查閱產(chǎn)品樣本，選用規(guī)格相近的Y100L—6型電動機，其額定功率為1.5KW，額定轉(zhuǎn)速為940r/min。
2．確定其它元件及輔件
(1) 確定閥類元件及輔件
根據(jù)系統(tǒng)的最高工作壓力和通過各閥類元件及輔件的實際流量，查閱產(chǎn)品樣本，選出的閥類元件和輔件規(guī)格如表8所列。其中，溢流閥9按小流量泵的額定流量選取，調(diào)速閥4選用Q—6B型，其最小穩(wěn)定流量為0.03 L/min，小于本系統(tǒng)工進時的流量0.47L/min。
表8液壓元件規(guī)格及型號
序號
元件名稱
通過的最大流量q/L/min
規(guī)格
型號
額定流量qn/L/min
額定壓力Pn/MPa
額定壓降?Pn/MPa
1
雙聯(lián)葉片泵
—
PV2R12-6/33
5.1/27.9*
16
—
2
三位五通電液換向閥
70
35DY—100BY
100
6.3
0.3
3
行程閥
62.3
22C—100BH
100
6.3
0.3
4
調(diào)速閥
<1
Q—6B
6
6.3
—
5
單向閥
70
I—100B
100
6.3
0.2
6
單向閥
29.3
I—100B
100
6.3
0.2
7
液控順序閥
28.1
XY —63B
63
6.3
0.3
8
背壓閥
<1
B—10B
10
6.3
—
9
溢流閥
5.1
Y—10B
10
6.3
—
10
單向閥
27.9
I—100B
100
6.3
0.2
11
濾油器
36.6
XU—80×200
80
6.3
0.02
12
壓力表開關(guān)
—
K—6B
—
—
—
13
單向閥
70
I—100B
100
6.3
0.2
14
壓力繼電器
—
PF—B8L
—
14
—
*注：此為電動機額定轉(zhuǎn)速為940r/min時的流量。
(2) 確定油管
在選定了液壓泵后，液壓缸在實際快進、工進和快退運動階段的運動速度、時間以及進入和流出液壓缸的流量，與原定數(shù)值不同，重新計算的結(jié)果如表9所列。
表9各工況實際運動速度、時間和流量
快進
工進
快退
L/min
24
.
0
L/min
95
7
.
44
5
.
0
1
2
1
2
=
*
=
=
A
A
q
q
m/s
10
824
.
0
m/s
10
95
60
10
47
.
0
3
4
3
1
1
2
-3
-
-3
5′
=
′
′
′
=
=
A
q
u
s
38
.
1
s
109
.
0
10
150
3
1
=
′
=
-
t
s
1
.
34
s
10
88
.
0
10
30
3
3
2
=
′
′
=
-
-
t
s
46
.
1
s
123
.
0
10
180
3
3
=
′
=
-
t
表10允許流速推薦值
管道
推薦流速/(m/s)
吸油管道
0. 5～1.5，一般取1以下
壓油管道
3～6，壓力高，管道短，粘度小取大值
回油管道
1. 5～3
由表9可以看出，液壓缸在各階段的實際運動速度符合設計要求。
根據(jù)表9數(shù)值，按表10推薦的管道內(nèi)允許速度取=4 m/s，由式計算得與液壓缸無桿腔和有桿腔相連的油管內(nèi)徑分別為
為了統(tǒng)一規(guī)格，按產(chǎn)品樣本選取所有管子均為內(nèi)徑20mm、外徑28mm的10號冷拔鋼管。
(3) 確定油箱
油箱的容量按式估算，其中α為經(jīng)驗系數(shù)，低壓系統(tǒng)，α=2～4；中壓系統(tǒng)，α=5～7；高壓系統(tǒng)，α=6～12?，F(xiàn)取α=6，得
五 液壓缸設計基礎(chǔ)
5.1液壓缸的軸向尺寸
液壓缸軸向長度取決于負載運行的有效長度（活塞在缸筒內(nèi)能夠移動的極限距離）、導向套長度、活塞寬度、缸底、缸蓋聯(lián)結(jié)形式及其固定安裝形式。圖示出了液壓缸各主要零件軸向尺寸之間的關(guān)系。活塞寬度。活塞有效行程取決于主機運動機構(gòu)的最大行程，m
18
.
0
3
.0
0
15
.
0
1
=
+
=
L
。導向套滑動面長度C的取值：當，。導向長度，
缸筒長度。
5.2主要零件強度校核
5.2.1缸筒壁厚δ=4㎜
因為方案是低壓系統(tǒng)，校核公式,
式中： -缸筒壁厚（）
-實驗壓力 ,其中是液壓缸的額定工作壓力
D-缸筒內(nèi)徑 m
D
11
.
0
=
· -缸筒材料的許用應力。，為材料抗拉強度（MPa）,n為安全系數(shù)，取n=5。
對于P1<16MPa.材料選45號調(diào)質(zhì)鋼，對于低壓系統(tǒng)
因此滿足要求。
5.2.2缸底厚度δ1=11㎜
對于平缸底，厚度 有兩種算法
1.缸底有孔時：
其中
2.缸底無孔時，用于液壓缸快進和快退；
其中
5.2.3桿徑d
· ,式中F是桿承受的負載（N）F=34444.4N
是桿材料的許用應力，=100
5.2.4缸蓋和缸筒聯(lián)接螺栓的底徑d1
式中 K------擰緊系數(shù)，一般取K=1.25~1.5；
F-------缸筒承受的最大負載（N）；
z-------螺栓個數(shù)；
----螺栓材料的許用應力， ，為螺栓材料的屈服點（MPa），安全系數(shù)n=1.2~2.5
5.2.5液壓缸穩(wěn)定性計算
液壓缸承受的負載F超過某臨界值時將會失去穩(wěn)定性。穩(wěn)定性可用下式校核：
式中 nc---------- 穩(wěn)定性安全系數(shù) ，-4，取nc=3;
由于缸筒固定活塞動，，由桿材料知硬鋼，因此
式中 l-------------安裝長度（m）；
Rc-----------活塞桿橫截面的最小回轉(zhuǎn)半徑（m）； -----------材料柔性系數(shù)，取=115；
-----------液壓缸支承末端系數(shù)，取=； E------------活塞桿材料的彈性模量，可取E=； J-------------活塞桿橫截面慣性矩，對于實心桿；對于空心桿，D為桿的外徑，d為桿的內(nèi)徑；
------------材料強度決定的試驗值，=； A-------------活塞桿橫截面積；
-------------系數(shù)，取=；
因此滿足穩(wěn)定性要求。
5.2.6液壓缸緩沖壓力
液壓缸設置緩沖壓力裝置時要計算緩緩從壓力，當值超過缸筒、缸底強度計算的時，則以取代。在緩沖時，緩沖腔的機械能力為，活塞運動的機械能為?；钊跈C械能守恒中運行至終點。
式中：
通過驗算，液壓缸強度和穩(wěn)定性足以滿足要求。
六、驗算液壓系統(tǒng)性能
1．驗算系統(tǒng)壓力損失
由于系統(tǒng)管路布置尚未確定，所以只能估算系統(tǒng)壓力損失。估算時，首先確定管道內(nèi)液體的流動狀態(tài)，然后計算各種工況下總的壓力損失?，F(xiàn)取進、回油管道長為l=2m，油液的運動粘度取=1′10-4m2/s，油液的密度取r=0.9174′103kg/m3。
(1) 判斷流動狀態(tài)
在快進、工進和快退三種工況下，進、回油管路中所通過的流量以快退時回油流量q2=70L/min為最大，此時，油液流動的雷諾數(shù)
也為最大。因為最大的雷諾數(shù)小于臨界雷諾數(shù)（2000），故可推出：各工況下的進、回油路中的油液的流動狀態(tài)全為層流。
(2) 計算系統(tǒng)壓力損失
將層流流動狀態(tài)沿程阻力系數(shù)
和油液在管道內(nèi)流速
同時代入沿程壓力損失計算公式，并將已知數(shù)據(jù)代入后，得
可見，沿程壓力損失的大小與流量成正比，這是由層流流動所決定的。
在管道結(jié)構(gòu)尚未確定的情況下，管道的局部壓力損失?pζ常按下式作經(jīng)驗計算
各工況下的閥類元件的局部壓力損失可根據(jù)下式計算
其中的Dpn由產(chǎn)品樣本查出，qn和q數(shù)值由表8和表9列出?；_在快進、工進和快退工況下的壓力損失計算如下：
1．快進
滑臺快進時，液壓缸通過電液換向閥差動連接。在進油路上，油液通過單向閥10、電液換向閥2，然后與液壓缸有桿腔的回油匯合通過行程閥3進入無桿腔。在進油路上，壓力損失分別為
在回油路上，壓力損失分別為
將回油路上的壓力損失折算到進油路上去，便得出差動快速運動時的總的壓力損失
2．工進
滑臺工進時，在進油路上，油液通過電液換向閥2、調(diào)速閥4進入液壓缸無桿腔，在調(diào)速閥4處的壓力損失為0.5MPa。在回油路上，油液通過電液換向閥2、背壓閥8和大流量泵的卸荷油液一起經(jīng)液控順序閥7返回油箱，在背壓閥8處的壓力損失為0.6MPa。若忽略管路的沿程壓力損失和局部壓力損失，則在進油路上總的壓力損失為
此值略小于估計值。
在回油路上總的壓力損失為
該值即為液壓缸的回油腔壓力p2=0.66MPa，可見此值與初算時參考表4選取的背壓值基本相符。
按表7的公式重新計算液壓缸的工作壓力為
此略高于表7數(shù)值。
考慮到壓力繼電器的可靠動作要求壓差Dpe=0.5MPa，則小流量泵的工作壓力為
此值與估算值基本相符，是調(diào)整溢流閥10的調(diào)整壓力的主要參考數(shù)據(jù)。
3．快退
滑臺快退時，在進油路上，油液通過單向閥10、電液換向閥2進入液壓缸有桿腔。在回油路上，油液通過單向閥5、電液換向閥2和單向閥13返回油箱。在進油路上總的壓力損失為
此值遠小于估計值，因此液壓泵的驅(qū)動電動機的功率是足夠的。
在回油路上總的壓力損失為
此值與表7的數(shù)值基本相符，故不必重算。
大流量泵的工作壓力為
此值是調(diào)整液控順序閥7的調(diào)整壓力的主要參考數(shù)據(jù)。
2．驗算系統(tǒng)發(fā)熱與溫升
由于工進在整個工作循環(huán)中占96%，所以系統(tǒng)的發(fā)熱與溫升可按工進工況來計算。在工進時，大流量泵經(jīng)液控順序閥7卸荷，其出口壓力即為油液通過液控順序閥的壓力損失
液壓系統(tǒng)的總輸入功率即為液壓泵的輸入功率
W
4
.
564
W
8
.
0
60
10
9
.
27
10
0588
.
0
60
10
1
.
5
10
99
.
4
3
6
3
6
p
2
p
2
p
1
p
1
p
r
=
′
′
′
+
′
′
′
=
+
=
-
-
h
q
p
q
p
P
液壓系統(tǒng)輸出的有效功率即為液壓缸輸出的有效功率
由此可計算出系統(tǒng)的發(fā)熱功率為
按式計算工進時系統(tǒng)中的油液溫升，即
°C
其中傳熱系數(shù)K=15 W/（m2·°C）。
設環(huán)境溫T2=25°C，則熱平衡溫度為
°C
油溫在允許范圍內(nèi)，油箱散熱面積符合要求，不必設置冷卻器。
六、設計小結(jié)
課程設計是機械設計當中的非常重要的一環(huán)，本次課程設計時間一周略顯得倉促一些。但是通過本次每天都過得很充實的課程設計，從中得到的收獲還是非常多的。?
? 這次課程設計，由于理論知識的不足，再加上平時沒有什么設計經(jīng)驗，一開始的時候有些手忙腳亂，不知從何入手。在老師的諄諄教導，和同學們的熱情幫助下，使我找到了信心?，F(xiàn)在想想其實課程設計當中的每一天都是很累的，其實正向老師說得一樣，機械設計的課程設計沒有那么簡單，你想copy或者你想自己胡亂蒙兩個數(shù)據(jù)上去來騙騙老師都不行，因為你的每一個數(shù)據(jù)都要從機械設計書上或者機械設計手冊上找到出處。雖然種種困難我都已經(jīng)克服，但是還是難免我有些疏忽和遺漏的地方。完美總是可望而不可求的，不在同一個地方跌倒兩次才是最重要的。抱著這個心理我一步步走了過來，最終完成了我的任務。
Fundamentals of Machine Tools
In many cases products from the primary forming processes must undergo further refinements in size and surface finish to meet their design specifications.To meet such precise tolerance the removal of small amounts of material is needed.Usually machine tools are used for such operation.
In the United States material removal is a big business—in excess of ＄36×109 per year,including material,labor,overhead,and machine-tools shipments,is spent.Since 60 percent of the machanical and industrial engineering and technology graduate have something connection with the machining industry either through sale,design,or operation of machine shops,or working in related industry,it is wise for an engineering student to devote some time in his curriculum to studying material removal and machine tools.
A machine tool provide the means for cutting tools to shape a workpiece to required dimensions;the machine supports the tool and the workpiece in a controlled relationship through the functioning of its basic members,which are as follows:
(a)Bed,Structure or Frame.This is the main member which provides a basis for,and a connection between,the spindles and slides;the distorion and vibration under load must be kept to a minimum.
(b)Slides and Slideways.The translation of a machine element(e.g. the slide) is normally achieved by straight-line motion under the constraint of accurate guiding surface(the slideways).
(c)Spindles and Bearings.Angular displacement take place about an axis of rotation;the position of this axis must be constant within extremely fine limits in machine tools,and is ensured by the provision of precision spindles and bearings.
(d)Power Unit.The electric motor is the universally adopted power unit for machine tools.By suitably positioning individual motors,belt and gear transmissions and reduced to a minimum.
(e)Transmission Linkage.Linkage is the general term used to denote the mechanical,hydraulic,pneumatic or electric mechanisms which connect angular and linear displacements in defined relationship.
There are two broad divisions of machining operations:
(a)Roughing,for which the metal removal rate,and consequently the cutting force,is high,but the required dimensional accuracy relatively low.
(b)Finishing,for which the metal removal rate,and consequently the cutting force,is low,but the required dimensional accuracy and surface finish relatively high.
It follows that static loads and dynamic loads,such as result from an unbalanced grindingwheel,are rmore significant in finishing operations than in roughing operations.The degree of precision achieved in any machining process will usually be influenced by the magnitude of the deflections,which occur as a result of the force acting.
Machine tool frames are generally made in cast iron,although some may be steel casting or mild-steel fabrications.Cast iron is chosen because of its cheapness,rigidity,compressive strength and capacity for damping the vibrations set-up in machine operations.To avoid massive sections in castings,carefully designed systems of ribbing are used to offer the maximum resistance to bending and torsional stresses.Two basic types of ribbing are box and diagonal.The box formation is convenient to produce,apertures in walls permitting the positioning and extraction of cores.Diagonal ribbing provides greater torsional stiffness and yet permits swarf to fall between the sections;it is frequently used for lathe beds.
The slides and slideways of a machine tool locate and guide members which move relative to each other,usually changing the position of the tool relative to the workpiece.The movement genenally takes the forms of translation in a straight line,but is sometimes angulai rotation,e.g. tilting the wheel-head of a universal thread-grinding machine to an angle corresponding with the helix angle of the workpiece thread.The basic geometric elements of slides are flat,vee,dovetail and cylinder.These elements may be used separately or combined in various ways according to the applications.Features of slideways are as follows:
(a)Accuracy of Movement.Where a slide is to be displaced in a straight line,this line must lie in two mutually perpendicular planes and there must be no slide rotation.The general tolerance for straightness of machine tool slideways is 0—0.02mm per 1000mm;on horizontal surfaces this tolerance may be disposed so that a convex surface results,thus countering the effect of “sag”of the slideway.
(b)Means of Adjustment.To facilitate assembly,maintain accuracy and eliminate “play” between slideing members after wear has taken place,a strip is something inserted in the slides.This is called a gib-strip.Usually,the grib is retained by socket-head screw passing through elongated slots;and is adjusted by grub-screws secured by lock nuts.
(c)Lubrication.Slideways may be lubricated by either of the following systems:
1)Intermittently through grease or oil nipples,a method suitable where movements are infrequent and speed low.
2)Continuously,e.g. by pumping through a metering value and pipe-work to the point of application;the film of oil introduced between surfaces by these means must be extremely thin to avoid the slide “floating”.If sliding surfaces were optically flat oil would be squeezed out,resulting in the surfaces sticking.Hence in practice slide surfaces are either ground using the edge of a cup wheel,or scraped.Both processes produce minute surface depresssions,which retain “pocket” of oil,and complete separation of the parts may not occur at all points;positive location of the slides is thus retained.
(d)Protection.To maintain slideways in good order,the following conditions must be met:
1)Ingress of foreign matter,e.g. swarf,must be prevented.Where this is no possible,it is desirable to have a form of slideway,which does not retain swarf,e.g. the inverted vee.
2)Lubricating oil must be retained.The adhensive property of oil for use on vertical or inclined slide surface is important;oils are available which have been specially developed for this purpose.The adhesiveness of oil also prevents it being washed away by cutting fluids.
3)Accidental damage must be prevented by protective guards.
A machine tool performs three major functions:1)it rigidly supports the workpiece or its holder and the cutting tool; 2)it provides relative motion between the workpiece and the cutting tool; 3)it provides a range of feeds and speeds.Machines used to remove metal in the form of chips are classified in four general groups:those using single-point tools,those using multipoint tools,those using random-point tools(abrasive),and those that considered special.
Machines using basically the single-point cutting tools include:1)engine lathes, 2)turret lathes , 3)tracing and duplicating lathes, 4)single-spindle automatic lathes, 5)multi-single automatic lathes , 6)shapers and planers, 7)boring machines.
Machines using multipoint cutting tools include:1)drilling machines, 2)milling machines, 3)broaching machines, 4)sawing machines, 5)gear-cutting machines.
Machines using random-point cutting tools include:1)cylindrical grinder, 2)centreless grinders, 3)surface grinders.Special metal removal methods include:1)chemical milling, 2)electrical discharge machining, 3)ultrasonic machining.
The lathe removes material by rotating the workpiece against a cutter to produce external or internal cylindrical or conical surfaces.It is also commonly used for the production of flat surfaces by faing,in which the workpiece is rotated while the cutting tool is moved perpendicularly to the axis of rotation.
The engine lathe is the basic turning machine from which other turning machines have been developed.The drive motor is located in the base and drives the spindle through a combination of belts and gears,which provides the spindle speeds from 25 to 1500 rpm.The spindle is a sturdy hollow shaft,mounted between heavy-duty bearings,with the forward end used for mounting a drive plate to impart positive motion to the workpiece.The drive plate may be fastened to the spindle by threads,by a cam lock mechanism,or by a thread collar and key.
The lathe bed is cast iron and provides accurately ground sliding surfaces(way)on which the carriage rides.The lathe carriage is a H-shaped casting on which the cutting tool is mounted in a tool holder.The apron hangs from the front of the carriage and contains the driving gears that move the tool and carriage along or across the way to provide the desired tool motion.
A compound rest,located above the carriage provides for rotation of the tool holder through any desired angle.A hand wheel and feed screw are provided with a hand wheel and feed screw for moving the compound rest perpendicular to the lathe way.A gear train in the apron provides power feed for the carriage both along and across the way.The feed box contains gears to impart motion to the carriage and control the rate at which the tool moves relative to the workpiece.On a typical lathe feeds range from 0.002 to 0.160 in. per revolution of the spindle,in about 50 steps.Since the transmission in the feed box is driven from the spindle gears,the feeds are directly related to the spindle speed.The feed box gearing is also used in thread cutting and provides from 4 to 224 threads per in.
The connecting shaft between the feed box and the lathe apron are the feed rod and the lead screw.Many lathe manufacturers combine these two rods in one,a practice that reduces the cost of the machine at the expense of accuracy.The feed rod is used to provide tool motion essential for accurate workpiece and good surface finishes.The lead screw is used to provide the accurate lead necessary for the thread cutting.The feed rod is driven through a friction clutch that allows slippage in case the tool is overloaded.This safety device is not provided in the lead screw,since thread cutting cannot tolerate slippage.Since the full depth of the thread is seldom cut in one pass,a chasing dial is provided to realign the tool for subsequent passes.
The lathe tailtock is fitted with an accurate spindle that has a tapered hole for mounting drills,drill chucks,reamers,and lathe centers.The tailstock can be moved along the lathe ways to accommodate various lengths of workpieces as well as to advance a tool into contact with the worpiece.The tailstock can be offset relative to the lathe ways to cut tapers or conical surfaces.
The turret lathe is basically an engine lathe with certain additional features to provide for semiautomatic operation and to reduce the opportunity for human error.The carriage of the turret lathe is provided with T-slots for mounting a tool-holding device on both sides of the lathe ways with tools properly set for cutting when rotated into position.The carriage is also equipped with automatic stops that control the tool travel and provide good reproduction of cuts.The tailstock of the turret lathe is of hexagonal design,in which six tools can be mounted.Althogh a large amount of time is consumed in setting up the tools and stops for operation,the turret lathe,once set,can continue to duplicate operations with a minimum of operator skill until the tools become dulled and need replacing.Thus,the turret lathe is economically feasible only for production work,where the amount of time necessary to prepare the machine for operation is justifiable in terms of the number of part to be made.
Tracing and duplicating lathes are equipped with a duplicating device to automatically control the longitudinal and cross feed motions of the single-point cutting tool and provide a finished part of required shape and size in one or two passes of the tools.
The single-spindle automatic lathe uses a vertical turret as well as two cross slids.The work is fed through the machine spindle into the chuck,and the tools are operated automatically by cams.
The multispindle automatic lathe is provided with four,five,six,or eight spindles,with one workpiece mounted in each spindle.The spindles index around a central shaft,with the main tools slide accessible to all spindles.Each spindle position is provided with a side tool-slide operated independently.Since all of the slides are operated by cams,the preparation of this machine may take several days,and a production run of at least 5000 parts is needed to justify its use.The principal advantage of this machine is that all tools work simultaneously,and one operator can handle several machines.For relatively simple parts,multispindle automatic lathes can turn out finished products at the rate of 1 every 5 sec.
A shapers utilizes a single-point tool in a tool holder mounted on the end of the ram.Cutting is generally done on the forward stroke.The tool is lifted slightly by the clapper box to prevent excessive drag across the work,which is fed under the tool during the return stroke in preparation for the next cut.The column house the operating mechanisms of the shaper and also serves as a mounting unit for the work-supporting table.The table can be moved in two directions mutually perpendicular to the ram.The tool slide is used to control the depth of the cut and is manually fed.It can be rotated through 90 deg. On either side of its normal vertical position,which allows feeding the tool at an angle to the surface of the table.
Two types of the driving mechanisms for shapers are a modified Whitworth quick-return mechanism and a hydraulic drive.For the Whitworth mechanism,the motor drives the bull gear,which drives a crank arm with an adjustable crank pin to control the length of the stroke.As the bull gear rotates,the rocker arm is forced to reciprocate,imparting this motion to the shaper ram.
The motor on a hydraulic shaper is used only to drive the hydraulic pump.The remainder of the shaper motions are controlled by the direction of the flow of the hydraulic oil.The cutting stroke of the mechanically driven shaper uses 220 deg. of rotation of the bull gear,while the return stroke uses 140 deg..This gives a cutting stroke to return stroke ratio of 1.6 to 1.The velocity diagram shows that the velocity of the tool during the cutting stroke is never constant,while the velocity diagram for a hydraulic shaper shows that for most of the cutting stroke the cutting speed is constant.The hydraulic shaper has an added advantage of infinitely variable cutting speeds.The principal disadvantage of this type of machine is the lack of a definite limit at the end of the ram stroke,which may allow a few thousandths of an inch variation in stroke length.
A duplicating device that makes possible the reproduction of contours from a sheet-metal template is available.The sheet metal template is used in conjunction with hydraulic control.
Upright drilling machines or drill presses are available in a variety of sizes and types,and are equipped with a sufficient range of spindle speeds and automatic feeds to fit the needs of most industries.Speed ranges on a typical machine are from 76 to 2025 rpm.,with drill feed from 0.002 to 0.20 in. per revolution of the spindle.
Radial drilling machines are used to drill workpieces that are too large or cumbersome to conveniently move.The spindle with the speed and feed changing mechanism is mounted on the radial arm;by combing the movement of the radial arm around column and the movement of the spindle assembly along the arm,it is possible to align the spindle and the drill to any position within reach of the machine.For work that is too large to conveniently support on the base,the spindle assembly can be swung out over the floor and the workpiece set on the floor beside the machine.
Plain radial drilling machine provide only for vertical movement of the spindle;universal machines allow the spindle to swive about an axis normal to the radial arm and the radial arm to rotate about a horizontal axis,thus permitting drilling at any angle.
A mutispindle drilling machine has one or more heads that drive the spindles through universal joints and telescoping splined shafts.All spindles are usually driven by the same motor and fed simultaneously to drill the desired number of holes.In most machines each spindle is held in an adjustable plate so that it can be moved relative to the others.The area covered by adjacent spindles overlap so that the machine can be set to drill holes at any location within its range.
The milling operation involves metal removal with a rotating cutter.It includes removal of metal from the surface of a workspiece,enlarging holes,and form cutting,such as threads and gear teeth.
Within an knee and column type of milling machine the column is the main supporting member for the other components,and includes the base cotaining the drive motor,the spindle,and the cutter.The cutter is mounted on an arbor held in the spindle,and supported on its outer extremity by a bearing in the overarm.The knee is held on the column in dovetail slots,the saddle is fastened to the knee in dovetail slots,and the table is attached to the saddle.Thus,the build-up of the knee and column machine provide three motions relative to the cutter.A four motion may be provided by swiveling the table around a vertical axis provided on the saddle.
Fixed-bed milling machines are designed to provide more rigidity than the knee and column type.The table is mounted directly on the machine base,which provides the rigidity necessary for absorbing heavy cutting load,and allows only longitudinal motion to the table.Vertical motion is obtained by moving the entire cutting head.
Tracer milling is characterized by coordinated or synchronized movements of either the paths of the cutter and tracing elements,or the paths of the workpiece and model.In a typical tracer mill the tracing finger follow the shape of the master pattern,and the cutter heads duplicate the tracer motion.
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