Machining Nylon and Acetal Parts from Stock Shapes

Highly precise nylon and acetal parts and large components can be manufactured economically by machining. No special machines or processes are required for machining nylon and acetal parts. The machines that are normally used in the woodworking and metal industries with HSS tools (high performance superspeed steel) or hard metal tools can be used.

The only thing to consider is that when a circular saw is used to cut plastic, hard metallic saw blades must be used.

Nylon and acetal have lower thermal conductance properties than metals, as well as a lower modulus of elasticity. If not handled properly, the workpiece can become extremely warm and thermal expansion can occur. Satisfactory results are easily achieved if these guidelines are followed.

Guidelines for Machining Nylon & Acetal Components

• The highest possible cutting speed should be chosen.
• Optimum chip removal must be ensured so that the chips are not drawn in by the tool.
• The tools that are used must be very sharp. Blunt tools can cause extreme heat, which results in deformation and thermal expansion.
• The clamping pressures must not be too high as this would result in deformation of the workpiece and the clamping tool would leave marks in the workpiece.
• Because of the low degree of stiffness, the workpiece must be adequately supported on the machine table and should lie as flat as possible.
• Perfect, high-quality surfaces can only be obtained when the machines operate with low vibration.

Conventional drilling emulsions can also be used for cooling and are especially recommended for deep bores and cutting threads. In addition, it is possible to achieve higher rates of forward feed and, consequently, shorter running times.

However, if using drilling emulsions, attention should be paid that these are completely removed after machining. This prevents oily components causing problems in subsequent processes such as bonding or painting, especially in the case of polyamides where the water in the emulsion can cause changes in the components through absorption.

Sawing

You can saw both nylon and acetal using a band saw or a circular saw. The choice depends on the shape of the semi-finished product. The use of a band saw is particularly recommended when a “support groove” (prism) is used to cut rods and tubes and also has the advantage that the built up heat is dissipated via the long saw blade. However, the teeth of the blade must be set adequately so that the blade cannot jam.

Circular saws, on the other hand, are mainly used for cutting sheets and blocks with straight edges. Here, attention should be paid that the feed rate is adequate so that chips are removed, that the saw blade does not jam and that the plastic does not overheat at the point where it is being cut. Table 1 contains guiding values for the cutting geometry of the saw blades.

Milling

With a high cutting speed and medium feed rate, it is possible to achieve high levels of machining performance with good surface quality and accuracy. In regard to the cutter geometry, we recommend the values given in Table 2.

Turning on a Lathe

Since most plastics produce unbroken chips, it is important to ensure that the chips are removed, as they would otherwise catch and revolve with the part being turned on the lathe. In addition, because of the low degree of stiffness of plastics, there is a great danger of longer parts sagging, and it is thus advisable to use a steady rest. The values given in Table 3 apply to the cutter geometry. The point radius should be at least 0.5 mm.

Drilling

Drill holes can be made with a conventional HSS drill. If deep holes are being drilled, it must be ensured that the chips are removed, otherwise the plastic on the walls of the hole will heat to the point of melting and the drill will “clog”. This especially applies to deep holes. For drilled holes in thin-walled workpieces, it is advisable to choose a high drilling speed and, if applicable, a neutral (0°) effective cutting angle. This prevents the drill from sticking in the workpiece and hinders the associated stripping of the hole or the workpiece being drawn up by the drill. Table 4 contains the recommended values for cutting edge geometry.
The angle of twist of the drill should be at least 12 – 16°.

If these machining guidelines are followed, complex parts made of engineering plastics can be finish-machined to the highest quality standards.

Written by Greg Waack, President of ZL– East which supplies cast and extruded nylon, homopolymer and copolymer acetal, PET, Polycarbonate, PEEK, PEI and PVDF stock shapes.