Application and selection of measuring head and fixture in typical NC milling parts

Application and selection of measuring head and fixture in typical NC milling parts

As one of the numerical control milling projects of the World Skills Competition, precision machining of the verticality of the round hole has been difficult, with the characteristics of hollow cavity, difficult clamping and easy deformation. During several training sessions, participants found that the verticality of the round holes of the parts could not meet the requirements of the drawings. After understanding the relevant processing process of the player, the process is further optimized. By using plugs, special claws, screw jacks and craft tables, ensure that the tool rotation axis has the correct mutual position with respect to the part positioning surface to ensure the shape and position accuracy of the dimensions.

1. Part drawing analysis

As can be seen from Figure 1, the shape of the parts is complex and the processing technology is many. Clamping position is less, the contact surface is small, the cavity is long, the rigidity is poor. The verticality of the circular hole (F11) on Phi cannot meet the requirement. After analysis, the principle of multi-position clamping and mutual reference can be adopted. After rough machining on both sides, finish machining, priority machining datum and remachining hole, to ensure the verticality of the hole and the datum coincidence.

2. Analysis of parts processing technology

This part requires high surface quality and precision. Considering the accuracy, how to control the deformation of parts within the precision range is the biggest difficulty in machining parts. By analyzing the precision requirements of parts, materials and equipment, there are mainly the following aspects:

(1) Deformation cutting force caused by internal stress of blank itself.

(2) Deformation caused by clamping force.

(3) Deformation caused by cutting force and cutting heat.

(4) The deformation caused by clamping force has great influence. In order to resolve the issue of appeal, the following solutions are proposed.

2.1. Improve the traditional part clamping method.

For hollow thin-wall parts, the following clamping methods are used to reduce deformation: ① stuffing method. The parts are filled with a seat ring, so that the parts are not affected by the clamping force when clamping. ② Set aside a reasonable process table. The process table is left at the hanging position and clamping position of the part to enhance machining stiffness and reduce clamping deformation.

2. Arrange work procedures reasonably

In dynamic milling, the whole process will produce impact, affecting the accuracy and surface roughness of the parts. Therefore, CNC dynamic milling process can be generally divided into: rough machining, bottom clearance and rounded clearance, semi-finish machining, finish machining and other processes.

2.3 Processing method of thin wall and empty parts

Thin-walled and hollow parts are easy to be deformed by clamping force in the process of machining. If the clamping force is too small, the parts are easy to loosen due to processing; If the clamping force is too large, the parts will deform. In order to control the small deformation of the parts, when preparing the finishing step after semi-finishing, slightly loosen the pliers to let the parts spring back naturally, release part of the stress, and then gently clamp, to ensure that the parts are properly clamped, in order to obtain better processing effect. The principle of clamping is that the action point of the force should be on the support surface, and the direction of the clamping force should act on the direction of the workpiece stiffness. The smaller the clamping force, the better.

2.4 Selection of fixtures

The selection of fixtures mainly includes the following points:

(1) clamping mechanism will not interfere with the parts.

(2) Small clamping deformation of parts.

(3) Fast and convenient clamping.

(4) The fixture structure is simple.

(5) The fixture is convenient for positioning components.

3. Processing route

Parts shall be machined by flat pliers with force measuring devices, special soft clamps, platen and auxiliary plugs.

(1) Use a flat clamp with a wide end face of 100mm to grip the blank, take this as the rough basis, rough machining the front, leaving a 98mm process table.

(2) The 98mm end face is clamped on the opposite side for rough finishing.

(3) Fill the plug into the reverse slot and clamp it, correct the flatness of the parts (less than 0.01mm) when clamping, clean the front clamping platform, and finish the front, as shown in Figure 2.

(4) Install the special soft pliers for milling on the flat pliers, clamp the 94mm and 78mm end faces, clean the root process table (not including the cantilever process table), process the front side, then press the front cantilever part with the press plate, and support the bottom with the screw jack, as shown in Figure 3.

(5) Transverse installation of flat pliers, clamping parts processing surface.

4. Closing remarks

Through the machining design of the part, using the reasonable machining technology, successfully overcome the related soft claw, screw jack and press plate Phi 24(F11 position) hole verticality difficult to meet the requirements.