Smithy Midas 1220 XL - Manual (Page 36)

The modern lathe offers the following:
• The strength to cut hard, tough materials
• The means to apply power
• The means to hold the cutting point tight
• The means to regulate operating speed
• The means to feed the tool into or across, or into and across the work, either
manually or by engine power, under precise control
• The means to maintain a predetermined ratio between the rates of rotating works and
the travel of the cutting point or points.
Turning Speeds
When metal cuts metal at too a high speed, the tool burns up. You can machine soft
metals like aluminum at fast speeds without danger or trouble, but you must cut hard
steels and other metals slowly.
You must also consider the diameter of the workpiece (Figure 7.1). A point on a 3”
diameter shaft will pass the cutting tool three times as fast as a point on a 1” diameter
shaft rotating at the same speed. This is because the point travels a tripled
circumference. For work in any given material, the larger the diameter, the slower the
speed in spindle revolutions needed to get the desired feet per minute (fpm) cutting
speed.
Lathes cut threads in various numbers per inch of material threaded, according to the
operator's needs. The Midas 1220 XL cuts threads to metric or inch standards.
In thread cutting, the carriage carries the thread-cutting tool and moves by rotating the
leadscrew (Figure 5.1). The basic principle is that the revolving leadscrew pulls the
carriage in the desired direction and at the desired speed. The carriage transports the
toolpost and the threading tool, which cut the thread into the metal being machined.
The faster the leadscrew revolves in relation to the spindle, the coarser the thread. This
is because the threading tool moves farther across the revolving metal with each
workpiece revolution.
The lathe spindle holding the workpiece revolves at a selected speed (revolution per
minute, or rpm) according to the type and size of the workpiece. The leadscrew, which
runs the length of the lathe bed, also revolves at the desired rpm. There is a definite and
changeable ratio between spindle and leadscrew speeds. Figure 7.2 shows belt positions
for various speeds.
7-2
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Midas 1220 XL Operator’s Manual