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What are the economical materials for moving, loaded machine parts?

1. PA 6 G or POM C ?

The two best known engineering plastics in general use are moulded polyamide 6 and POM C.

Quattroplast Ltd. produces an extremely tough, strategically important moulded polyamide 6 product, DOCAMID 6G H, which has become an indispensable product for the mechanical engineering industry. The other material grade, POM C, also has some of the properties that have justified its uptake in some typical applications.

Table 1. Comparison of key characteristics:

Features	Cast polyamide 6 (DOCAMID 6G-H)		POM C (DOCACETAL C)
Strength properties		Nearly identical
Thermal characteristics		Nearly identical
Electrical characteristics		Nearly identical
Toughness	better		weaker
Fatigue resistance	better		weaker
Size retention in wet media	weaker		better
Wear resistance	better		weaker
Market price	more favourable		more expensive
Rotatability	?	?	?

The conclusion is simple: if it's not a wet, underwater application or other precision, medical, food processing machine component:

• of adequate strength
• good wear resistance
• economical

component can be manufactured from moulded polyamide (DOCAMID 6G H).

What is the truth about turning the two materials?

In the following, we will compare the turning of the two materials and show how to turn moulded polyamide productively and well in order to:

• technically
• economically
• lifetime

to achieve the best for our partners.

2. Chipping with generally recommended parameters.

Anyone who has become familiar with turning engineering plastics after metal turning can, after clarifying the initial problems and difficulties (e.g. cutting blade material, cutting angles, cutting speed, cooling, lubrication), and with the help of literature recommendations based on practical experience, achieve the desired uniform surface quality and tolerance field within a short time.

After turning several types of engineering plastics, following the machining recommendations in the literature, many turners conclude that POM C is easy and good to machine, other materials are more problematic and less productive.

Is this statement true or false?

The answer is simple: partly true, partly not!

• True, because POM C has a somewhat similar machining behaviour to steels and is relatively insensitive to changes in turning parameters.
• False, because similar machining performance and surface quality can be achieved with tough moulded polyamide, but it does not matter what technological parameters are used.

One of the most important considerations when judging a lathe turning operation is the chipping and removability of the chips. From this point of view, the chip removal of the more rigid POM C is preferable, because the chip of the tougher moulded polyamide 6 is more fluid and tough, with a higher risk of sticking to the knife and workpiece.

It follows that the main task in PA turning:

• the removal of chips,
• to keep the chips away from the blade and the workpiece.

A large number of preliminary experiments have shown that this is most easily achieved by using a chip breaker and increasing the feed per revolution. (Considering the simplest case, when no chip extraction and cooling lubrication is used.)

3. Optimisation of turning of moulded polyamide 6, experimental demonstration.

We present a series of turning sample experiments, where neither DOCAMID 6G-H nor DOCACETAL C were turned without cooling and chip extraction, so that the regularity of chip removal can be better demonstrated. The cutting angles of the blades are in accordance with the literature tables, the cutting speed was reduced to 280 m/min due to the absence of cooling, and the depth of cut was chosen to be uniform at 2.5 mm.

Features used for slab turning, side-edge lathe knife:

a - back angle [ ° ] γ - front angle [ ° ] v - cutting speed [m/min] s - feed rate [mm/rev.] χ - main edge placement angle [ ° ]	CONTACT	POLIAMIDE 6	POM C
	α	8	6-8
	γ	5	5
	χ	90	90
	v	280	280

Three typical cutting knives are used in the experiments (Fig. 1):

1. Lathe with carbide flap side edge (Figure 2)

2. Side-edge high-speed steel knife with normal cutting angle and ground normal chipping point (Figure 3)

3. Side-edge high-speed steel knife with increased cutting angle and raised cutting edge (Figure 4)

Figure 1. Side turning slots (left to right 3-2-1 in order)

Figure 2. Knife with carbide blade. Angle of deflection λ = 4°. Soft-arc "chip breaker", chip ejection.

Figure 3. High-speed steel knife. λ deflection angle = 4°. Ground normal chip breaker, chip ejection.

Figure 4. High-speed steel knife. λ deflection angle = 6°. Exposed chip breaker, impacting chip ejection

Figures 2, 3 and 4 illustrate one important aspect, the "λ-angle", which is essential for the turning of DOCAMID 6G-H. Another important technological parameter is the feed per revolution.

Preliminary experiments have shown that varying the values given in the literature for the cutting edges and other machining parameters within the recommended range did not lead to radical changes in either chip removal or surface flatness.

Tables 2 and 3 summarize the effect of "λ-angle of incidence" and feed per revolution in relation to POM C and DOCAMID 6G-H.

Experiments have also shown that when turning DOCAMID 6 G-H, lubrication is a major factor in achieving uniform chip removal from the knife and workpiece environment, as well as a smooth machined surface. A small application of an oily emulsion applied by brush to the polyamide surface also gave a more favourable result.

This lubricating effect did not cause a significant difference in the case of POM C, and did not result in a spectacular advantage.

4. Conclusions

1. The machining parameter values given in the literature generally provide reliable, good results when machining polymers. Within the given value ranges, individual cases can be refined.

2. A sharp, well-ground tool is an extremely important basic requirement.

3. By adhering to the values in the table, chip removal of POM C is easier due to the material's more brittle behaviour, including its poorer wear resistance.

4. POM C is less sensitive to changes in machining parameters.

5. The more brittle behaviour of POM C is a limitation for the turning feed. Above 0.3 mm/rev. the surface quality deteriorates significantly without cooling and lubrication. (However, according to literature recommendations, the cutting speed can be 60% higher than for polyamide 6.)

6. The DOCAMID 6 G-H's tough river chip can be adjusted relative to itself by grinding a special chip breaker into the high-speed steel knife.

7. The tough and wear resistant properties of DOCAMID 6 G-H allow it to be turned at a much higher feed rate than POM C, with a nice, even surface finish at feed rates above 0.3 mm/rev.

8. The removal of DOCAMID 6 G-H chips from the blade and workpiece environment can be significantly improved by increasing the "λ- deflection angle".

9. At a minimum λ = 4º, the knife is already able to continuously lower the DOCAMID 6 G-H flowing chip past the knife.

10. Minimal lubrication on the workpiece surface can significantly improve the chip clearance of DOCAMID 6 G-H (less shear stress retention in the chip flow).

5. Proposals

DOCAMID 6 G-H has the strength characteristics of POM C, but is tougher, more wear resistant and has a more economical market price.

As described above, the machining performance and surface quality of DOCAMID 6 G-H can reach the favourable properties of POM C if the following technological refinements are observed:

1. Use a perfectly ground high-speed steel knife!

2. Grind a chipping tool into the knife!

3. Let's take advantage of the possibilities offered by the bias angle. Minimum λ = 4º already gives advantageous chip clearance.

4. You can turn at high tool feed rates ( values above 0.3 mm/rev can be used), making turning more productive.

5. Smooth surfaces can be produced productively with a high feed rate, just increase the tip radius of the knife. With a minimum tip radius of R = 0.6 mm, a smooth surface can be obtained with a high feed rate.

6. The minimal lubricant on the polyamide surface further improves chip removal, increases cutting speed and feed rate.

With all this in mind, let's answer the question posed in point 1:

By refining the turning properties of DOCAMID 6 G-H as described above, a good machinist can achieve the same cutting performance as with POM C in general.

Unless there is a predominant effect requiring the use of POM C, such as precision dimensional stability in wet and aqueous environments, special requirements in the food industry or in medicine, DOCAMID 6 G-H remains a strategic raw material for the general engineering industry.

The described turning proposals represent a minimal extra cost in the overall technology, however:

• tougher
• more wear-resistant
• more economically competitive

machine components can be produced for general engineering purposes, mainly for applications subject to moving - friction, rolling - and dynamic effects.