Smithy Midas 1220 XL - Manual (Page 40)

Heat
The energy expanded at the lathe's cutting point converts largely into heat, and because
the energy is great, the heat is intense. Before today's HSS, carbide and ceramic tools,
this heat created a serious machining problem. Machining could be done only under a
steady flow of coolant, which kept the tool from heating to its annealing point, softening
and breaking down.
With HSS, you can usually cut dry unless your machine is running at extremely high
speeds on continuous, heavy-duty production work. HSS tools are self-hardening even
when red hot. They do not dissipate the heat, however, or in any way prevent the
workpiece from heating up. Because steel expands when heated, it is a good idea,
especially with long shafts, to check the tightness of the lathe centers often and make
sure workpiece expansion does not cause centers to bind.
In everyday lathe operations like thread cutting and knurling, always use cutting oil or
other lubricant. On such work, especially if the cuts is light and lathe speed low, dipping
a brush in oil occasionally and holding it against the workpiece will provide sufficient
lubrication. For continuous high-speed, heavy-duty production work, however, especially
on tough alloy steels, using a cutting oil or coolant will increase cutting efficiency. It's
essential if you're using a non HSS cutting tool. When you use coolant, direct it against
the cutting point and cutter. Consider installing a coolant system if you don't have one.
Table 8.1 Cutting speeds and feeds for High Speed Steel Tools
Table 8.1 list cutting speeds and feeds for HSS cutters so you can set up safe rpm rates.
The formula is as follows:
rpm = CS x 4 / D”
where CS = cutting speed in surface feet per minute (sfm) and D” = diameter of
the workpiece in inches.
Low-Carbon
Steel
High-
Carbon
Steel
Annealead
Alloy Steel
Normalized
Aluminum
Alloys
Cast Iron Bronze
Speed (sfm)
Roughing
Finishing
90
120
50
65
45
60
200
300
70
80
100
130
Feed (ipr)
Roughing
Finishing
0.010-0.202
0.003-0.005
0.101-0.020
0.003-0.005
0.010-0.020
0.003-0.005
0.015-0.030
0.005-0.010
0.010-0.020
0.003-0.010
0.010-0.020
0.003-0.010
8-3
For Assistance: Call Toll Free 1-800-476-4849
Midas 1220 XL Operator’s Manual