The numerical control processing of honeycomb core has a history of more than ten years in China. However, due to the particularity of high-end application fields such as military industry, the processing technology and data are blocked by foreign countries, and many processes still remain at the level of experience operation. Honeycomb core, as a porous structural material with small transverse stiffness, its processing mechanism and clamping technology are different from those of traditional metal materials. With the development of aviation technology, China has mastered the processing technology and processing inspection of honeycomb core with relatively simple technology, but the processing technology and fixing method of honeycomb parts with complex profiles used in large and medium-sized passenger aircraft still need to be greatly improved.
Similar technologies at home and abroad
Due to the small lateral stiffness of the honeycomb core material, the traditional clamping method cannot be used to fix the honeycomb core. The following methods are used to fix the honeycomb core at home and abroad:
(1) Polyethylene glycol method
This method is to use polyethylene glycol heating (70 "C-90" C) to melt and then cool and solidify to fix the honeycomb core material. For paper based honeycomb core materials, this fixation method is stable and can be used to process parts from the perspective of fixation. The disadvantages are that, first, in order to keep the honeycomb core material close to the worktop during the PEG curing process, vacuum bags and autoclaves must be used. The autoclave equipment has huge investment and very expensive processing costs; Secondly, it is very difficult to remove the residual PEG on both the worktop and honeycomb core materials after processing; Third, for mechanical properties, paper based honeycomb core materials should not contact with water, oil and other substances, which also increases the difficulty of cleaning. Therefore, the operation process of this method is not very convenient, so it is rarely used in actual production.
(2) Double sided adhesive tape method
This method is widely used in many aircraft manufacturing companies. In this method, two kinds of adhesive tape materials are used, one is double-sided adhesive tape, the other is Velcro fastener. The main shortcomings of this method for fixing paper based honeycomb core parts are as follows: First, the fixing force provided by this method is small, and the honeycomb core parts are often pulled up by the milling cutter in the high-speed milling process, resulting in the over cutting phenomenon, which causes the parts to be scrapped; Secondly, the thickness of velcro velcro used in this method is about 0.8mm (in free state). In the process of CNC high-speed milling of honeycomb core parts, the milling force changes sharply with the change of tool swing angle, milling amount and height of honeycomb core materials, which will inevitably cause the change of the thickness of velcro layer, and ultimately affect the machining accuracy of honeycomb core parts
(3) Vacuum adsorption method
The plastic film or fiber reinforced plastic membrane is bonded to the honeycomb core material with double-sided adhesive tape, and then the platform and the membrane/membrane are vacuumed to achieve the purpose of fixing the honeycomb core material. Remove the diaphragm/film material after processing. Compared with the second method, this method overcomes the defect that the fuzz layer is easy to deform, but because the cross sectional area of the honeycomb core material is very small, it is difficult to bond with the adhesive tape, and it cannot obtain satisfactory fixation effect in the actual processing process.
Zhejiang University honeycomb core fixing system
At present, the clamping and fixing methods used in the high-speed milling process of honeycomb core parts use bonding materials to varying degrees. The practice shows that the existing fixing methods have low fixation, low machining accuracy of parts, and the subsequent cleaning of bonding materials is complex. Most importantly, the bonding materials used in the fixation process affect the health of workers. In view of the shortcomings of existing fixing methods, Zhejiang University proposed a fixing method for honeycomb core parts based on magnetic field and friction adsorption principle. The fixing system of paper-based honeycomb parts based on the principle of magnetic field and friction adsorption uses the magnetic field generated by the permanent magnet theory to generate suction on the iron powder poured into the honeycomb core. Under the combined effect of the gravity and magnetic field suction of the iron powder itself, the friction between the iron powder and the honeycomb wall and between the iron powder and the fixing platform is increased, so as to fix and position the honeycomb core material, The iron powder poured into the honeycomb core has four functions: (1) The iron powder generates tension under its own gravity, forming positive pressure on the honeycomb sidewall; (2) The honeycomb wall separates the iron powder into independent units, which generate mutual attraction under the action of magnetic field and form positive pressure on the honeycomb wall; (3) The iron powder poured into the honeycomb core increases the contact area between the honeycomb core and the fixed platform, and also increases its own gravity. The positive pressure between the honeycomb core and the fixed platform surface also increases. With the increase of the above three positive pressures, the friction between the iron powder and the honeycomb sidewall and between the honeycomb and the fixed platform will increase; (4) A single honeycomb core element is an axisymmetric element, which can be simplified as a cantilever beam. The increase of the height of iron powder in the honeycomb core reduces the hanging length of the honeycomb wall (the length of the cantilever beam), which can effectively reduce the bending deformation of the honeycomb wall.
