Aluminium not only offers many advantages due to it’s material properties. Aluminium is also extensively adaptable to fabrication and machining processes. Generally tooling costs are lower than with many other metals and the high speed at which certain processes can be completed, offer even greater labour cost savings.
Many additional fabrication processes that would have to be carried out on other materials, can be designed in to the extrusion process of aluminium. You can read about these in the extrusion design guide section. For all other fabrication processes there are some guide lines below.
- Tapping (threading)
- Shearing (pressing/punching)
- Insulation (thermal break)
- Plastic forming (bending/stretching)
Higher sawing speeds can be achieved with aluminium than with steel. The majority of aluminium alloys allow far greater sawing speeds and in most cases the method is an economic and very advantageous solution. Aluminium extrusions can be sawn accurately without the formation of burrs.
The appearance of the cut, the alloy used and the extrusion’s strength determine the size of the teeth, the number of revolutions per minute, the number of teeth, the diameter of the blade and the feed. The number of teeth should be sufficiently large to give a clean cut effectively. When sawing thin extrusions, several teeth should always cut in the material and cutting lubricant should always be used.
Deburring is a process for removing small chips and any remaining burrs on the extrusion cut. The most common method is mechanical using a brush or a grinding machine.
Abrasive tumbling, where fragments are removed by friction using circulating stones, is a suitable method for deburring smaller and medium large parts.
Milling machines for fabrication of aluminium have larger teeth pitches than equivalent tools for steel and therefore a more spacious groove for chips. As with other sawing, a high cutting speed is required for a good result.
A high quality surface demands high power and stability in the tool and feed mechanism.
There is a difference between end and peripheral milling machines depending on where the surface to be milled is situated in relation to the milling spindle’s central line.
The milling diameter should be at least 20% larger than the width of the surface being treated when surface milling with an end mill. 2/3 of the surface should be moved against and 1/3 with the cutting direction during milling. The milling teeth should move in the line of feed (down-feed milling) when milling peripherally (i.e. slab milling cutter, shank-end mill, side-milling cutter or spindle moulding cutter).
As with most machining, drilling should be carried out at a high speed. When using standard bits, they should be sharpened so as to reduce the pressure required and obtain a better result.
Special bits for aluminium are only required for deep holes or soft alloys. It is important to note that the hole will be considerably larger than the bit diameter when drilling in aluminium, especially when drilling in soft alloys.
A considerable amount of heat is generated when drilling deep holes, especially if the diameter is large. Cooling is therefore essential to avoid the hole contracting.
Aluminium can be turned in standard, special and automatic lathes and should be carried out at high speeds of rotation. Parts to be turned should therefore be fitted securely to avoid vibration. Spacers between the part and the mounting prevent marks on the metal and deformation.
Internal and external threads can be made using all available machining methods as well as through plastic deformation. Heat treatable alloys give especially high quality results. Taps for steel can be used for threads under 6 mm but special taps should be used for larger diameters.
Internal threads can either be made with taps in series or with a single tap. The groove for chips should be large and wide, well rounded and polished as well as have a large cutting edge angle. The back surface should run radially or be undercut so that the chips do not fasten between the tool and the thread when the tap is drawn out.
Special threading taps are normally divided into three types. The first is hole polished with the pitch against the cutting line so that the chips are pushed forward in front of the tap during threading. Another type is designed so that the thread is interrupted from groove to groove.
Finally there is one that has a spiral chips groove for lighter cutting with better pressure during threading. External threads are made using ordinary threading tools or screw cutting dies. The threads can also be formed plastically by rolling without any chips being formed. This creates a very strong thread. The external diameter of the part to be threaded should be 0.2 to 0.3 times the size of the screw pitch compared to the nominal thread diameter. It is very important that the centre lines of the metal part and the tool are aligned.
Press work is normally carried out in eccentric presses with a cutting (shearing) tool. The press tools for aluminium are slightly different from those designed for other metals. Punch and die of hardened tool steel are recommended.
Burrs are avoided by regularly sharpening the punch and die. Furthermore, the cutting force required can be reduced considerably if the punch’s surface is ground at an angle (shear). The angle ground part should at the most be equivalent to the thickness of the part of the material that is to be cut out. In certain cases, especially when punching holes, it can be an advantage to grind the punch at an angle while keeping the die flat.
The punch should be left flat irrespective of the shape of the die if the part cut out is to be used. It is important to maintain the correct clearance between the punch and the die during the actual cutting process. The clearance is determined by the material’s composition and the thickness of the cut material.
Insulation (thermal break)
Aluminium’s high coefficient of thermal conductivity is not so desirable in applications where low heat transfer is wanted such as in windows. There are many ways of insulating.
Two techniques that greatly reduce the ability to conduct are commonly used. In the first the extrusion is pressed in one piece and a closed space in the extrusion is filled with polyurethane. When the polyurethane has set, the extrusion is divided into two parts held together by the polyurethane. In this way the thermal bridge is interrupted (fill and mill).
In the other method two extrusions are joined using polypropylene or polyamide strips. These are rolled into position. This way of insulating makes it possible to use different colours on the inside and outside of the window.
Plastic forming (bending/stretching)
Aluminium extrusions can be bent using the same equipment as for other metals. Bending can take place with the hardened metal for larger radii but smaller ones usually require soft annealed or T4 (half-hardened) metal. It is possible to harden to full strength after bending. Bending should be carried out before anodising if a complete anodised layer without cracks is required.
The need for bending should be taken into consideration at the design stage. Large batches should not be produced in the T4 condition as there is a risk that the material will be left standing and will self-harden. The material in the bend can be harder than the rest in the event of high stresses, for example with very small radii. This is important if the original material is in the T4 condition and is to be hardened to T6. In such cases the bend can be annealed.
Draw bending is the most commonly used bending method. It is suitable for tight radii and has a high degree of repeatability. Using an adjustable clamping jaw, the work piece is fixed against a rotating die. The clamping jaw and the tool are shaped to reproduce the profile’s cross section.
The work piece rotates with the die. This stretches the material on the outside of the profile and compresses that on the inside. To prevent scratches and clamping marks on the profile, the tools are usually made of plastic.
Anodised profiles: Being hard and brittle, the oxide layer forms many fine cracks during bending. If a high quality surface is required, it is recommended that anodising is left until after bending.
Roller bending is used for forming large radii in the work piece. The work piece is rolled between two drive rollers and a pressure roller. The shape presented by the rollers corresponds to the profile’s cross section. Vertical adjustment of the upper roller (the pressure roller) alters the radius of the bend. Thus, in CNC machines, a number of different radii can easily be pressed into a single work piece.
As rollers are most usually made of steel, lubrication is often required to prevent cutting and scratching of the profile.
Stretch bending gives very high three-dimensional shape accuracy. The work piece is fixed between two clamping jaws and then gradually stretched over a shaping block. The shape presented by the block corresponds to the profile’s cross-section. The metal is stretched to its upper elastic limit and spring-back is thus negligible.
As the tooling investment is relatively high, stretch bending is best suited to large series production.
Press bending (point bending) is suitable for simple bending of large series. The work piece is formed using compressive force. An upper and a lower die are contoured to give the work piece the desired shape. Pressure is applied by some form of excentric or hydraulic press. Depending on the exterior of the part to be pressed, dies can be steel or plastic.
Hydroforming allows us to shape an aluminium profile three-dimensionally in a single operation. The process offers as yet unexplored possibilities.
All, or parts, of a profile’s cross section can be tailored using hydroforming. In a single operation, complex parts can be created with very good dimensional accuracy. In a single hydroforming operation, it is also possible to make local changes such as domes or holes. By eliminating several machining operations, lead times can be shortened.
The principle – The profile is placed in a die that has an inner geometry exactly replicating the shape of the finished component. The die is locked securely in position and hydrostatic pressure is then set up in the pipe (profile). As the profile is pressed against the die, it takes up the shape of the die.
It has become clear that hydroforming opens the way to unique solutions for a wide range of design problems.