A question of the right (milling) strategy
In a modern aircraft the proportion of composite materials can be up to 50 percent. Even wings, tail units and fuselage segments are made of light, composite materials, today. Accordingly, the highest requirements for manufacturing quality apply: the result must always be perfect – at the least possible cost. This, in turn, has implications for the precision tools and moulds specially manufactured for this purpose. Their production must meet highest quality standards, and must, of course, be cost effective. Cooperating with LMT UK, Brinksway Tool, a British supplier of tools and moulds for the aerospace industry, has examined the milling strategies it applies to the production of moulds for making components out of composite materials. As a result, machining times have been reduced to five or six hours. In the past, a cycle of this sort took up to five days!
Following the milling tests, which were carried out together with experts from LMT, the company is now able to produce three complete moulds in the same time that was previously needed to make just one. At the same time, the service life of the tools, which now use LMT indexable inserts in a MultiEdge 4Feed miller, has been increased by almost 25 percent. How was such an enormous improvement possible?
- "Blueprint for the new milling strategy: sketch of the mould with the pockets to be cut."
Simple task – long machining times
Carl Nickerson, Factory Manager at the company in Audenshaw, near Manchester, first describes the position they started from: "The task we face is relatively simple – the removal of material at the bottom of the moulds, which have an overall size of 1.5 by 0.5 meters. Altogether we must cut six pockets, with a width of 170 millimeters, a length of 180 millimeters and a depth of 75 millimeters. The production time previously required for this phase of the job was simply unacceptable, on top of which the milling process described here is of course only the preliminary to subsequent machining stages." In the past, machining the moulds on a Correa 3-axis Prisma 20 CNC machining centre could cause a serious tailback in the flow of production, explained Nickerson. "The whole process was altogether too drawn-out."
Together with CNC Manager Simon Cornell, he decided to ask for advice from one of the company's tool suppliers: LMT UK.
The right tool for the right strategy
Cornell describes the next part of the process: "LMT's engineers immediately examined the machining centre's performance and torque curves. By the end of the analysis, there were two key proposals, and these are what led to the breakthrough. They affected both the tool itself and the milling strategy we were using."
The new milling strategy was based on a thorough analysis of the existing process, which quickly revealed the critical steps: formerly the cutter had been brought relatively deep into the material. This had a crucial disadvantage, in that the indexable inserts used became "loaded" with too many chips in the course of milling. The result was a reduction in the total torque. Together with LMT, Cornell developed a new machining strategy.
The decision was to use a LMT MultiEdge 4Feed milling cutter with a diameter of 32 mm. This is a cutter with indexable inserts and four effective cutting edges. This tool was particularly developed for roughing in mould and dies applications and requires 25 per cent less energy than comparable tools. It has, furthermore, an optimized cutting angle that reduces the vibrations caused by machining – this in turn benefits the quality with which the workpiece is machined. In this case, the tool is fitted with indexable inserts with an AlCrN coating. This makes them highly resistant to heat, which permits high cutting speeds without the need for coolant. In addition, an air blast was installed to keep the milling zone free from the rapidly growing quantity of chips.
25 percent higher performance
Only through this choice of tool was it then possible to reprogram the milling process: the tool speed has been greater since then. It removes material at a rate of 10 meters per minute, the cutting depth used, however, being just 1 mm. In this way, it is an hour before the milling inserts need to be replaced - an improvement of 25 percent in comparison with the tool from another manufacturer. Carl Nickerson is pleased with the result, explaining that "Without doubt, this is an optimum result. We are not just able to reduce machining times this way. We also have a stable production process, with machining times that remain constant without long waiting times."
Information: Brinksway Tool
Brinksway Tool is the largest tool manufacturer in the Hyde Group Tooling Division. It belongs to the Hyde Group aerospace concern, and has an annual sales figure of 200 million pounds sterling. The factory at Audenshaw employs a staff of 58, of whom 20 are highly trained mechanical engineers. In addition, there are eight apprentices and 30 specialists in CNC milling technology.