Smithy GN1300 - Manual (Page 103)

How to Determine Speeds and Feeds for Lathe
Turning (machine, materials, and tools)
The lathe rotates a workpiece against a cutting edge. With its versatility and numerous
attachments, accessories, and cutting tools, it can do almost any machining operation.
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 Speed
When metal cuts metal at too high a 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. A point on a 3” diameter shaft will
pass the cutting tool three times as fast as a point an 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 materials threaded, according to the
operator's needs. The Smithy Granite Series machine cuts threads to metric or inch
standards.
In thread cutting, the carriage carries the thread-cutting tool and moves by the rotating
leadscrew. The basic principle is that the revolving leadscrew pulls the carriage in the
desired direction at the desired speed. The carriage transports the toolrest and the
threading tool, which cuts the screw 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.
Appendix A
Machining Reference Guide
A-1 Or Visit www.smithy.com