| Milling machines -
Cutting Speed and Feed Rate |
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Video
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| The phrase speeds and
feeds or feeds and speeds refers to two separate velocities in
machine tool practice, cutting speed and feed rate. They are often
considered as a pair because of their combined effect on the cutting
process. Each, however, can also be considered and analyzed in its own
right. |
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| Cutting Speed
Calculations |
Cutting speed may be defined as
the rate at the workpiece surface, irrespective of the machining
operation used. A cutting speed for mild steel of 30 m/min is the same
whether it is the speed of the cutter passing over the workpiece, such
as in a turning operation, or the speed of the cutter moving past a
workpiece, such as in a milling operation. The cutting conditions will
affect the value of this surface speed for mild steel.
Schematically, speed at the workpiece surface can be thought of as the
tangential speed at the tool-cutter interface, that is, how fast the
material moves past the cutting edge of the tool, although "which
surface to focus on" is a topic with several valid answers. In drilling
and milling, the outside diameter of the tool is the widely agreed
surface. In turning and boring, the surface can be defined on either
side of the depth of cut, that is, either the starting surface or the
ending surface, with neither definition being "wrong" as long as the
people involved understand the difference. An experienced machinist
summed this up succinctly as "the diameter I am turning from" versus
"the diameter I am turning to." He uses the "from", not the "to", and
explains why, while acknowledging that some others do not. The logic of
focusing on the largest diameter involved (OD of drill or end mill,
starting diameter of turned workpiece) is that this is where the highest
tangential speed is, with the most heat generation, which is the main
driver of tool wear.
There will be an optimum cutting speed for each material and set of
machining conditions, and the spindle speed (RPM) can be calculated from
this speed. Factors affecting the calculation of cutting speed are: |
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-
The material being machined (steel, brass,
tool steel, plastic, wood) (see table below)
- The material the cutter is made
from (High-Carbon
Steel, high
speed steel (HSS), Carbide, Ceramics,
and Diamond
tools)
- 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]). |
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Cutting speeds for
various materials using a plain high speed steel cutter
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Material type |
Meters per min (MPM) |
Surface feet per min (SFM) |
| Steel (tough) |
18–50 |
60–100 |
| Mild Steel |
3–38 |
10–125 |
| Mild Steel
(with coolant) |
6–7 |
20–25 |
| Cast Iron
(medium) |
1–2 |
6–8 |
| Alloy Steels
(1320–9262) |
3–20 |
12–65 |
| Carbon Steels
(C1008–C1095) |
4–51 |
0–70 |
| Free Cutting
Steels (B1111–B1113 & C1108–C1213) |
35–69 |
115–225 |
| Stainless
Steels (300 & 400 series) |
23–40 |
30–75 |
| Bronzes |
24–45 |
10–80 |
| Leaded Steel (Leadloy
12L14) |
91 |
30 |
| Aluminium |
122-305 |
400-1000 |
| Brass |
90–210 |
300–700 |
| Machinabil Wax |
6 |
20 |
| Acetal
Copolymer (Delrin) |
11 |
35 |
| Polyethylene |
12 |
40 |
| Acrylic (with
coolant) |
15 |
50 |
| Wood |
183–305 |
600–1000 |
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Feed rate is the velocity at which the cutter is fed, that is, advanced
against the workpiece. It is expressed in units of distance per
revolution for turning and boring (typically inches per revolution [ipr]
or millimetres per revolution). It can be expressed thus for milling
also, but it is often expressed in units of distance per time for
milling (typically inches per minute [ipm] or millimetres per minute),
with considerations of how many teeth (or flutes) the cutter has then
determined what that means for each tooth.
Feed rate is dependent on the:
-
Type of tool (a small drill or a large drill,
high speed or carbide, a box tool or recess, a thin form tool or wide
form tool, a slide knurl or a turret straddle knurl).
- Surface finish desired.
- Power available at the spindle (to
prevent stalling of the cutter or workpiece).
- Rigidity of the machine and
tooling setup (ability to withstand vibration or chatter).
- Strength of the workpiece (high
feed rates will collapse thin wall tubing)
- Characteristics of the material
being cut, chip flow depends on material type and feed rate. The
ideal chip shape is small and breaks free early, carrying heat away
from the tool and work.
-
Threads per inch (TPI) for taps, die heads
and threading tools.
- Cut Width. Any time the width of
cut is less than half the diameter, a geometric phenomenon called
Chip Thinning reduces the actual chipload. Feedrates need to be
increased to offset the effects of chip thinning, both for
productivity and to avoid rubbing which reduces tool life.
When deciding what feed rate to use for a certain cutting operation, the
calculation is fairly straightforward for single-point cutting tools,
because all of the cutting work is done at one point (done by "one
tooth", as it were). With a milling machine or jointer, where
multi-tipped/multi-fluted cutting tools are involved, then the desired
feed rate becomes dependent on the number of teeth on the cutter, as
well as the desired amount of material per tooth to cut (expressed as
chip load). The greater the number of cutting edges, the higher the feed
rate permissible: for a cutting edge to work efficiently it must remove
sufficient material to cut rather than rub; it also must do its fair
share of work.
The ratio of the spindle speed and the feed rate
controls how aggressive the cut is, and the nature of the swarf formed. |
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