March 28, 2020

Speeds available with the 12x36 Atlas Lathe

The goal in setting the spindle speed is to obtain a desired cutting rate for a given material.

Some recommended cutting rates are:

Aluminum and Brass -- 400 sfm
Mild Steel -- 90 sfm

Alan uses:

Alloy Steel             -  60 sfm
CRS - cold rolled steel -  80 sfm
mild steel              - 120 sfm
Bronze                  - 150 sfm
Aluminum                - 300 sfm
Brass                   - 600 sfm

Mild steel can be hot or cold rolled (mild refers to a steel without alloying elements, and fairly low in carbon). Cold is stronger and has a lower recommended cutting speed.

Many other recommendations can be found. These recommendations are for an alloy steel cutter. Speeds for carbide should be higher.

Setting speed on the Atlas

There are 16 speeds possible with this lathe. These are selected by fiddling with two belts and by switching the back gear in and out.

The double pulley on the motor shaft should have the small pulley on the right (closest to the motor).

The other belt can be placed in 4 positions, which we call "4" on the left (the small spindle pulley) to "1" on the right (the large spindle pulley).

Page 45 on the Atlas lathe manual presents the following tables, for use with the standard 1725 RPM motor:

Here are the speeds with direct drive:

Motor Belt
Spindle Belt Position
Position
4
3
2
1
A
685
418
266
164
B
2072
1270
805
500

And, here are the speeds with the back gear engaged:

Motor Belt
Spindle Belt Position
Position
4
3
2
1
A
112
70
45
28
B
345
211
134
83

It is simple math to calculate the spindle speed to obtain a desired cutting speed. Given the diameter of the piece being turned (in inches) and the desired cutting speed (in SFM -- surface feet per minute), the spindle rpm is given by:

rpm = sfm * (12/pi) / d
What many books do is to let 12/pi be approximated by 4, which is entirely adequate, yielding slightly higher speeds, but not enough to matter. I generated the following table using the proper value for pi, namely 3.14159.

Here is the ruby program I used to generate the following table:

Diameter of Work (inches)
Desired surface speed (feet per minute)
30
40
50
60
70
80
90
100
120
150
200
300
400
500
0.062
1833
2445
3056
3667
4278
4889
---
---
---
---
---
---
---
---
0.125
917
1222
1528
1833
2139
2445
2750
3056
3667
4584
---
---
---
---
0.188
611
815
1019
1222
1426
1630
1833
2037
2445
3056
4074
---
---
---
0.250
458
611
764
917
1070
1222
1375
1528
1833
2292
3056
4584
---
---
0.375
306
407
509
611
713
815
917
1019
1222
1528
2037
3056
4074
---
0.500
229
306
382
458
535
611
688
764
917
1146
1528
2292
3056
3820
0.625
183
244
306
367
428
489
550
611
733
917
1222
1833
2445
3056
0.750
153
204
255
306
357
407
458
509
611
764
1019
1528
2037
2546
1.000
115
153
191
229
267
306
344
382
458
573
764
1146
1528
1910
1.250
92
122
153
183
214
244
275
306
367
458
611
917
1222
1528
1.500
76
102
127
153
178
204
229
255
306
382
509
764
1019
1273
1.750
65
87
109
131
153
175
196
218
262
327
437
655
873
1091
2.000
57
76
95
115
134
153
172
191
229
286
382
573
764
955
2.500
46
61
76
92
107
122
138
153
183
229
306
458
611
764
3.000
38
51
64
76
89
102
115
127
153
191
255
382
509
637
3.500
33
44
55
65
76
87
98
109
131
164
218
327
437
546
4.000
29
38
48
57
67
76
86
95
115
143
191
286
382
477
5.000
23
31
38
46
53
61
69
76
92
115
153
229
306
382
6.000
19
25
32
38
45
51
57
64
76
95
127
191
255
318
7.000
16
22
27
33
38
44
49
55
65
82
109
164
218
273
8.000
14
19
24
29
33
38
43
48
57
72
95
143
191
239
9.000
13
17
21
25
30
34
38
42
51
64
85
127
170
212
10.000
11
15
19
23
27
31
34
38
46
57
76
115
153
191

The following table appears on page 47 of the Atlas lathe manual. It is like the above, but shows exact speeds that can be achieved by moving the belts on the Atlas lathe. Speeds were selected in some way that would be "closest" to the exact speed shown above. The selection is usually, but not always, lower than the exact speed.

Diameter of Work
Surface Speed in Feet Per Minute
Inches
30
40
50
60
70
80
100
120
150
200
300
500
0.0625
2072
--------------
--------------
--------------
--------------
--------------
--------------
--------------
--------------
--------------
--------------
--------------
0.125
805
1270
1270
2072
2072
2072
--------------
--------------
--------------
--------------
--------------
--------------
0.1875
685
805
805
1270
1270
1270
2072
2072
--------------
--------------
--------------
--------------
0.25
418
685
805
805
805
1270
1270
2072
2072
--------------
--------------
--------------
0.375
266
418
500
685
685
805
805
1270
1270
2072
--------------
--------------
0.5
266
266
418
418
500
685
685
805
1270
1270
2072
--------------
0.625
164
266
266
418
418
500
685
685
805
1270
2072
--------------
0.75
164
164
266
266
418
418
500
685
805
1270
1270
2072
1
112
164
164
266
266
266
418
418
500
805
1270
2072
1.25
83
112
164
164
164
266
266
418
418
685
805
1270
1.5
70
112
112
164
164
164
266
266
418
500
805
1270
1.75
70
83
112
112
164
164
164
266
266
418
685
1270
2
45
70
83
112
112
164
164
266
266
418
500
805
2.5
45
70
70
83
112
112
164
164
266
266
418
805
3
45
45
70
70
83
112
112
164
164
266
418
685
3.5
28
45
45
70
70
83
112
112
164
164
266
500
4
28
45
45
45
70
70
83
112
164
164
266
500
5
28
28
45
45
45
70
70
83
112
164
266
418
6
28
28
28
45
45
45
70
70
83
112
164
266
7
28
28
28
28
45
45
45
70
83
112
164
266
8
28
28
28
28
28
45
45
45
70
83
164
266
9
28
28
28
28
28
28
45
45
70
83
112
164
10
28
28
28
28
28
28
45
45
45
70
112
164


Have any comments? Questions? Drop me a line!

Tom's home page / tom@mmto.org