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You are here  PRODUCTS Plastics Processing
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Drilling
The drilling process is characterized in two movements: the rotating main motion and the fixing motion. The drilling also includes countersinking for stepped holes and reaming of cylindrical holes with a finer fit. Drilling is a roughworking method, and the manufacturing tolerance is rarely finer than IT12. |
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Tools
The quality strongly depends on the grinding of the bit as well as the machine and setup that is being used. The figure shows the details of the spiral drill. Pay special attention to the chisel edge as the speeds gets close to zero toward the centre of the edge. Therefore, the chisel edge does not make a cut, but an abrasive and crushing process. This develops heat and tensions in the processed item.
At plastics this is very unfortunate, and therefore the chisel edge should be as small as possible. When drilling larger holes it is necessary to predrill.
The quality of the major cutting edges is very important for the result of the processing. Therefore, do never use a drill bit that has worked in other materials than plastics.
During the drilling process it can be necessary to lift the drill bit various time to get rid of the chips. A brass drilling bit is suitable due to the small helix angle easily letting the chips disappear from the hole.
Always predrill at holes larger than Ø12 mm. |
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Follow the below rules when drilling in plastics:
- avoid unnecessary heat development
- use sharp drill bits that are only used in plastics
- lift the drill bit often to avoid chips burning into the hole
- use water or cooling agent for cooling, dending on the type of plastic
- do not make punch marks in crisp materials
- in some of the part crystallinge plastic materials such as PA and PE the hole get smaller than the drill bit
Geometry of the drill bit
The helix angle (pitch) is important for disposal of the chips and normally creates the chip angle g (gamma). The pointing angle f (Phi) forms the shape of the chip. At a very sharp angle the chip becom thin and wide.
The clearance angle a (alfa) is sharpened as for processing in metal.
The cutting edge angle b (beta) is the total of:
a + b + g = 90°
Cutting speed
The cutting speed of the drill bit is calculated at its periphery:
V = p x d x n / 1000 (m/min)
n = revolutions per minute
d = diameter of drill bit
From this you can determin number of revolutions, when the speed V is chosen, and d is known from drawing or plan.
n = V x 1000 / p x d (revolutions per minute)
Reaming
When reworking a hole with a reamer to a better bit you use a normal reamer. The hole has a tendency to be smaller than the tolerances of the reamer indicates. The reason is that reamer not only cuts, but also presses the hole.
Screw cutting
Be careful when making screw cutting in the notch sensitive plastic types. Taps with rounded edges must be preferred. At materials, squeezing around the tap, it could be an advantage to use taps having had every second tap removed on the cylindrical part of the tap. This reduces the friction and makes the lubrication more efficient. The disadvantage at this kind of taps, is that it requires forced motion.
Follow the below rulse when screw cutting in plastics:
- predrill mustbe 0.1 mm larger than at metal
- the taps must be sharps and can only be used for plastics
- do not use cutting oil without having controlled that the plastics tolerates it
- when return running of the tap be very careful, as the screw thread could otherwise be ripped off
Threaded bushings to be embossed are suitable for the more soft plastic types. They spread the tensions over a larger surface and thus enables transmision of larger forces. Furthermore, the risk of ruining the screw thread at unbolting and bolting is smaller.
Table drilling data
Please find directive cutting angles, speeds and fixing values in the table.
Download table for drilling data (PDF 42 KB) |
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Vink Holding B.V. · Oude Utrechtseweg 26-28 · NL-3743 KN Baarn · Phone +31 35 53 99 888 · Fax +31 35 53 99 870 · info@vink.com
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