SB CG1/Feed & Speeds

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Introduction

The tool moves through the material at a specified rotational speed, defined in revolutions per minute (RPM), and feed rate, defined in millimeters per minute (or inches per minute - IPM). Probably the most vexing problem for the beginning CNC machinist is selecting proper cutting speeds and feeds. This selection is actually more difficult on a CNC than a manual mill because, with a manual mill, the operator can feel the cutting pressure and alter the feed based in part on the cutting force.

CNC mills require calculating speeds and feeds in advance. These speeds and feeds can, and often are, adjusted at the machine based on chip shape and color, cutting sound, and machine horsepower meter readings.

The best source of data about cutting speeds and feeds for a specific tool, application, and material is the tool supplier. Much of this data is found on manufacturer's web sites or printed tooling catalogs. Tool sales representatives can be a valuable resource, so if you do a lot of machining, develop a good relationship with a knowledgeable representative.

Another source of speeds and feeds data is CAD/CAM software. These have become increasingly sophisticated and often provide good cutting data.

Yet even the best speed and feed data is just a starting point. Speeds and feeds require adjustment due to many factors including the maximum spindle speed or horsepower of the machine, rigidity of work holding, and the quality and condition of the machine tool itself.

The following pages provide cutting data for the most commonly machined materials and a methodology for calculating speeds and feeds. As always, use common sense. If the part is held by double sided tape, feeds based on vise work holding are probably too high. If the tool is very long and thin, speeds and feeds will likely require reduction.


Cutting speed

Cutting speed (also called surface speed or simply speed) may be defined as the rate (or speed) that the material moves past the cutting edge of the tool, irrespective of the machining operation used. A cutting speed for mild steel, of 100 ft/min (or approx 30 meters/min) is the same whether it is the speed of the (stationary) cutter passing over the (moving) workpiece, such as in a turning operation, or the speed of the (rotating) cutter moving past a (stationary) workpiece, such as in a milling operation. What will affect the value of this surface speed for mild steel, is the cutting conditions:

For a given material there will be an optimum cutting speed for a certain set of machining conditions, and from this speed the spindle speed RPM can be calculated. Factors affecting the calculation of cutting speed are:

  • The material being machined (steel, brass, tool steel, plastic, wood) (see table below)
  • The material the cutter is made from (Carbon steel, high speed steel (HSS), carbide, ceramics)
  • The economical life of the cutter (the cost to regrind or purchase new, compared to the quantity of parts produced)

Cutting speeds are calculated on the assumption that optimum cutting conditions exist, these include:

  • Metal removal rate (finishing cuts that remove a small amount of material may be run at increased speeds)
  • Full and constant flow of cutting fluid (adequate cooling and chip flushing)
  • Rigidity of the machine and tooling setup (reduction in vibration or chatter)
  • Continuity of cut (as compared to an interrupted cut, such as machining square section material in a lathe)
  • Condition of material (mill scale, hard spots due to white cast iron forming in castings)

The cutting speed is given as a set of constants that are available from the material manufacturer or supplier, the most common materials are available in reference books, or charts but will always be subject to adjustment depending on the cutting conditions. The following table gives the cutting speeds for a selection of common materials under one set of conditions. The conditions are a tool life of 1 hour, dry cutting (no coolant) and at medium feeds so they may appear to be incorrect depending on circumstances. These cutting speeds may change if, for instance, adequate coolant is available or an improved grade of HSS is used (such as one that includes cobalt).

Cutting speeds for various materials using a plain high speed steel cutter
Material type Meters per min (MPM) Surface feet per min (SFM)
Steel (tough) 15–18 50–60
Mild steel 30–38 100–125
Cast iron (medium) 18–24 60–80
Alloy steels (1320–9262) 20–37 65–120
Carbon steels (C1008–C1095) 21–40 70–130
Free cutting steels (B1111–B1113 & C1108–C1213) 35–69 115–225
Stainless steels (300 & 400 series) 23–40 75–130
Bronzes 24–45 80–150
Leaded steel (Leadloy 12L14) 91 300
Aluminium 75–105 250–350
Brass 90–210 300–700 (Max. spindle speed)
Plastics (Derlin) 150–210 500-700


Milling Formulas

Example of machining an alloy steel (V_c = 30m/min) with a tool diameter of 10mm

Milling machine cutting speeds are derived from the following formula:


RPM = {1000 \times V_c \over \pi \times Diameter} = { 1000 \times 30 m/min \over 3 \times 10 mm} = {1000 revs/min}


where:

  • RPM is the rotational speed of the cutter or workpiece in revolutions per minute
  • V_c Cutting speed is the recommended cutting speed of the material in meters/minute or feet/min
  • Diameter in millimeters


Feed

V_f = f_z \times z \times rpm (mm/min)


  • V_f = Feed speed
  •  z = number of teeth
  •  rpm = spindle speed