In both milling and sawing, the cutting action is discontinuous as compared to
other processes such as turning.The tool and work piece are subjected to
alternating mechanical and thermal loads. Besides abrasion, thermal
degradation is a further cause for tool wear. Cutting speeds that are too high
relative to a low feed rate, melt the polymer along the cutting line. On the
other hand, if the chosen feed rate is too high, it can cause rough surfaces
or even lead to breakage, regardless of the cutting speed. To efficiently
remove heat that develops during milling, single-edge cutters or cutters with
a low number of edges are recommended so that sufficient chip space is
available. Mills with four cutting flutes produce good results for most
plastics.
The table below lists generic milling conditions. The data shown does not
represent optimal values, but are guidelines to achieve acceptable results.
Milling conditions
| Xantar PC | Stamylan UH | Other plastics |
|
Rake angle
|
0 - 15º
|
5-15°
|
15
|
|
Clearance angle
|
5 -20º
|
5-15°
|
5 -20º
|
|
cutting speed
|
30-60 m/min (100-200 ft/min)
|
200-800 m/min (650-2600 ft/min)
|
70-2000 m/min (230-6600 ft/min)
|
|
feed speed
|
50-250 mm/min (2-10 in/min)
|
about 0.3 mm/rev (12 mils/rev)
|
160-250 mm/min (6-10 in/min)
|
|
cutting depth
|
0.1-3.0 mm (5-125 mils)
| |
1.5-6.0 mm (60-230 mils)
|
Milling is often done in two steps. In the first step a high cutting depth is
used to remove material at a fast rate and in the second finishing step, a
small cutting depth and high cutting speed are used to produce a smooth
surface.
Turning and boring
Turning and boring of DSM’s thermoplastics can be done on a conventional
lathes, as used for metals and is often used to produce round parts from bar
stock. A minimum nose radius of the cutting tool of 0.4 mm (15 mils) is
recommended to produce parts with a small surface roughness. The table below
suggests turning conditions that will generally yield good results.
Turning conditions
| Xantar PC | Stamylan UH | Other plastics |
|
cutting speed
|
40-120 m/min (130-400 ft/min)
| |
60-1000 m/min (200-3300 ft/min)
|
|
feed speed
|
0.1-0.3 mm/rev (0.004-0.012 in/rev)
| |
0.1-0.5 mm/rev (0.004-0.02 in/rev)
|
|
cutting depth
|
1.5-3 mm (60-120 mils)
| |
1.5-3 mm (60-120 mils)
|
To achieve a smooth surface, the finishing cut is done with a small cutting
depth and high cutting speed.
Punching, blanking, and die cutting
Punching, blanking, and die cutting are techniques that can be applied on
ductile plastics with a limited toughness. Filled plastics and
Xantar PC are therefore less suited. On the other hand these techniques
give good results when used on thermoplastic elastomers such as Arnitel TPE .
Cutting tools must be very sharp and preheating the part or sheet material to
soften the plastic can be considered. Sharp corners in cut out sections should
be avoided.
Laser cutting
Laser cutting is a technique that is rapidly gaining acceptance and its use is
growing. Round holes or holes with an irregular shape can be cut in plastics
with a laser beam, usually of the carbon-dioxide type, operating in the
infrared region. The laser beam may either be continuous or pulsed. Laser
cutting has the big advantage that the machined surfaces are free of machining
grooves, which when present might lead to stress concentrations. Furthermore,
laser cutting can often be applied in areas that are inaccessible for
conventional techniques or when firmly supporting the workpiece is problematic.
