Part 13

The material used for the folder is aluminium and, as I have said elsewhere, it is worth trying to obtain a known grade of this material - and preferably a grade with a good machinability rating.

One approach is to go for the general purpose grade 6082 (formerly H30 or HE30) which is a commonly available grade with good machining properties. 6062 is available in round, flat and hexagonal bar and has good machinability.

6262 is a free machining grade which costs roughly 50% more than 6082.

7075 has good machinability too, with high strength, but limited corrosion resistance.

As usual, you pay your money and take your choice. That popular pastime: scrounging cob ends, can give variable results unless you can get some idea of the grade of the metal, or can conduct a quick machining test.

The second thing that’s important is mentioned in the centre column of page 25 - the design of the cutter. This is absolutely crucial, and you should not expect to get anything like a decent result with a standard endmill. Instead, invest in an endmill or router cutter designed to do justice to aluminium.
The cutter needs two things:

first, and aggressive cutting angle,

and second, a highly polished finish to the insides of the flutes.

If the cutter has just one flute, it may be termed a router bit, but it is not a wood router cutter. Instead, it is a single flute cutter designed for machining aluminium. Cutter designs usually feature a high helix angle of 45 or 55 degrees or thereabouts. This is because the cutting takes place by slicing the aluminium at an angle. Most of these cutters are made of carbide and look quite different from cutters designed for machining steel. The weakest point of these cutters is the very tip of the cutting edge, where the high spiral flute meets the outer tip of the edge. 55 degree cutters have a very scary looking, and weak looking, sharp tip. 45 degree cutters are much the same, but a bit less pointed looking.

Some manufacturers of (more expensive) cutters use a variable flute angle, with a less steep flute angle at the bottom of the cutter, changing to a 55 degree flute further up behind the tip. This provides a stronger tip at the bottom of the cutter.

Expect to pay around £25 for a 6mm single flute cutter, perhaps £30 for a 10mm two flute cutter, and upwards of £100 for a 3 flute 55 degree cutter with a 40mm long cutting edge and a total length of 150mm. You won’t want to break too many of those.

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