Giving due importance to can work wonders when it comes to milling titanium alloys. This potential is not fully exploited. Simple process can increase productivity imperatively.
Titanium alloys and aluminum alloys are alike – both types of metals are used structural components for aircraft, and the component require 90 per cent of the material needed to be milled away before the part is complete. Plenty of shops actually wish that the metals should have more in common than this. Aircraft-industry suppliers are comfortable with machining aluminum are finding themselves much more titanium because new aircraft designs make increased use of the latter metal.
Many shops have more titanium machining capacity than they realise. Many techniques for machining titanium are not difficult to work on, but few shops use all the techniques available for milling this metal productively. Titanium not at all have to be difficult – it is just that the entire machining process is to be considered because any one element could impact the overall process.
Stability is key, when the tool touches the workpiece, to close a circle. The tool, tool holder, spindle, column, table, fixturing and workpiece are all part of the circle, and part of the stability. Other important considerations include coolant pressure and volume including the method of coolant delivery and application. To realise the potential of machining titanium productively, ten simple steps that are requested to follow.
One crucial challenges of titanium is heat dissipation. In this case, relatively little amount of the heat generated during machining operation which is ejected with the chip. Compared to machining other metals, a larger amount of the heat in a titanium machining goes into the tool. Because of this effect, the choice of radial engagement dictates the choice of surface speed in this metal. Full slotting – meaning 180° engagement– demands a relatively low surface speed. But bringing down the radial engagement reduces the time the cutting edge generates heat, and allows the cutting edge to cool before entering the material during the next rotation. Thus, as radial engagement is reduced, the surface speed can be increased maintaining the temperature at the cut point. For finishing, a milling process very small arc of contact with honed cutting edge, a high surface speed, and minimal feed per tooth can realise exceptional results.
Commonly used endmills have four or six Flutes but titanium may have too few., and effective number of flutes could be ten or more. Increasing the number of flutes compensates the need of a low feed pertooth. Spacing of a ten-flute tool is too tight for chip clearance in many applications. Productive milling of titanium should have a low radial depth. The small chip provides the freedom to use a high-flute count to increase productivity. ‘Climb milling’ is a term for this idea. Do not feed the milling cutter so that the edge moves the same direction that the tool is feeding. Known as ‘conventional milling,’ the chip start thin and become thicker. As the tool impacts the material, friction forces create heat
to shear away from the parent metal. A thin chip is cannot absorb and eject this generated heat, which ultimately goes into the cutting tool. Then, at the exit point where the chip is thick, increased cutting pressure makes chip adhesion a danger.
Thick-to-thin chip formation starts with the cutting edge entering the material on the finished surface. On side milling, the tool creating a thick chip on entry for maximum heat absorption and a thin chip on exit to prevent chip adhesion.
Surface milling demands close examination to ensure that the tool continues to enter and exit on the finished surface. Achieving this is not always as simple as merely keeping the material to the right.
In case of titanium, tool life is lost in moments of change in force. The worst of these moments often occur when the tool enters the material. Directly feeding into the stock give an effect similar to hitting the cutting edge with a hammer. It is essential to glide softly. This can be done by a tool path that arcs the material instead of entering it in a straight line. In thick-to-thin milling, the arc should follow the same direction (clockwise or counter-clockwise) as the rotation of the tool. The arcing should allow for a gradual increase in cutting force, preventing tool instability. Heat generation also increases gradually until the tool is fully engaged in the cut.
Changes in force can also occur at the tool exit. Thick-to-thin cutting is, the problem with that of the thick-to-thin formation that suddenly stops the tool reaches the end of the pass and starts to clear the metal. The abrupt change produces a force, and perhaps marring the part surface. To prevent the transition so sudden, precaution needs to be taken by a 45° chamfer at the end of the pass, for a gradual decline in its radial depth of cut.
Sharp cutting edge minimises forces in titanium, but it also needs to be strong enough to resist cutting pressure. A secondary relief tool design positive area of the cutting edge resists forces, and second area falls away and accomplishes both these objectives. Secondary relief is common in tooling, but for titanium in particular, tools having different secondary relief might reveal surprising changes in performance of tool life.
At the depth of cut, oxidation and chemical reaction can affect the tool. Early damage can occur if the tool is used at the same depth time and again. When taking axial cuts, area of the tool can cause work hardening on part that is unacceptable for aerospace components, meaning this effect can necessitate an early tool change. To prevent this, it is necessary to safeguard the tool by changing the axial depth for each pass, distributing the problem area to different points of the flute. In turning, a similar result can be obtained by taper turning first pass and parallel turning the subsequent pass.
Ratio 8:1 is useful for milling thin walls and unsupported features in titanium. To avoid deflection of walls, these walls should be milled in axial stages of milling with one pass of an endmill. Specifically, the axial depth should not be greater than eight times the thickness of the wall left behind after these milling passes are performed. If the wall is 0.1 inch thick, for example, the axial depth of cut should be no more than 0.8 inch. Despite the depth limitwork this rule so that productive milling is complete. Thin walls need to be machined, remains around the wall, making the feature three to four times thicker than the final feature.
Because of the extent, the tool needs clearance to allow for cooling. When milling a small pocket, the diameter of the tool should be 70 per cent of the diameter of the pocket. Less clearance risks insulating the tool from coolant as trapping the chips that might otherwise carry heat away. The 70 per cent rule can also be applied to a tool for the top of a surface. In this case, the width of the feature should be 70 per cent of the tool diameter. The tool is offset 10 per cent to for thick-to-thin chip creation.
A tool concept in the die/mold industry – have been adopted in recent years for machining titanium. A high-feed millrequires a light axial depth, but when run at this light depth, the tool permits feed higher than milling cutters with conventional designs. Chip thinning, with a large radius curve of cutting edge, spreads the chip formation across a large contact area on the edge. Because of this, a 0.040-inch axial depth of cut might produce a chip thickness of only about 0.008 inch. In titanium, this thin chip overcomes the low feed typically required in this metal. The thinning of the chip opens the way in higher programmed feed rate than would otherwise be possible.
The challenges faced in milling titanium can be overcome if the right measures are taken. A simple tip suggested earlier will go a long way at the process level to machine titanium productively.