Turning riveting process and fixture design of push block assembly

Time：2023-04-21 09:19:12 / Popularity： / Source：

1 Promote block component structure analysis and process plan determination

1.1 Structural analysis of push block components

Figure 1 shows a contactor push block assembly. Two semi-hollow rivets are used to riveted two push blocks together. Push block 1 and push block 3 are made of plastic. During riveting process, it is necessary to consider their capabilities. Once pressure is exceeded, cracks will occur and cause scrapping. Material of two semi-hollow rivets is H62, which has good plasticity and is suitable for turning riveting. After riveting, it is required that push block 1 and push block 3 are completely attached, and there is no crack around rivet point of push block.

Figure 1 Push block assembly
1. Pushing block 2. Semi-hollow rivet 3. Pushing block 4. Moving reed 5. Long transmission rod 6. Compression spring 7. Short transmission rod

1.2 Determination of turning riveting process plan and change rule of rivet turning riveting material

1.2.1 Determination of riveting process plan

Before turning rivet, it is necessary to assemble various parts of push block assembly and insert rivet. Since rivet and push block hole are clearance fit, improper operation will easily cause rivet to fall. Traditional turning riveting process adopts turning rivet head on the top and supporting rivet structure on the bottom, rivet is turned from top to bottom, and traditional turning riveting fixture structure of push block assembly is shown in Figure 2.

Figure 2 Structure of traditional riveting fixture
Although traditional flip riveting fixture can realize flip riveting of two semi-hollow rivets, and quality of push block assembly can also meet technical requirements after flip riveting, flip riveting process of fixture is complicated. In order to prevent rivet from falling, each part of push block assembly must be turned over to install rivets, then lower die of fixture must be fastened to position it, finally turned over and put into turning riveting equipment, as shown in Figure 3.

Figure 3 Turning riveting process
(a) Assemble parts (b) Install rivets (c) Reverse lower die of fixture
Based on problems of low production efficiency, only one rivet can be riveted at a time, and poor operability in traditional turning riveting process, combined with analysis of operation mode used by human body, it is finally determined that new turning riveting process of push block assembly is: turning riveting head is designed at the bottom, parts are placed in fixture from top to bottom for positioning. Upper die of fixture provides pressure on large end face of rivet through riveting equipment, and realizes riveting of two rivets at a time.

1.2.2 Change law of rivet turning riveting material

Rivet material currently studied is H62, Cu content is 62%, and the rest is Zn. Structure and properties of ordinary brass are related to Zn content. Strength and plasticity of brass will increase with increase of Zn mass fraction.
For riveting forming, material change can be studied with reference to stress-strain curve during stretching. Metal material will go through elastic stage → yield stage → strengthening stage → necking stage under action of external force. When turning riveting, material in hollow part of rivet is stretched after being stressed, outer layer material is compressed, and material is only allowed to deform from elastic stage to strengthening stage. Comparison between before and after change of rivet material under ideal conditions is shown in Figure 4.

Figure 4 Comparison of rivets before and after riveting

2 Fixture structure design

2.1 Analysis of fixture structure

Structure of newly refurbished riveting fixture is shown in Figure 5. It consists of a die handle, an upper die assembly and a lower die assembly. Working process is as follows: parts of push block assembly are loaded into cavity of positioning block 9 (A× B), then put rivet into corresponding hole of push block, as shown in Figure 6; Upper die assembly uses positioning pins 14, 15 to mold with lower die assembly. As shown in Figure 7, upper die assembly is pressed by die handle 1, so that pressing nail 5 is pressed against large end face of rivet, then positioning block 9 is pushed down along guide post 10 until rivet is turned by turning rivet head 18; After riveting is completed, die handle 1 moves upward, and positioning block 9 returns to original position under reaction force of spring 11 to complete a riveting action.

Figure 5 Structure of newly refurbished riveting fixture
1. Die handle 2. Backing plate 3. Screw 4. Fixing plate 5. Pressing nail 6. Screw 7. Gasket 8. Bushing 9. Positioning block 10. Guide post 11. Spring 12. Fixing plate 13. Base 14. Locating pin 15. Locating pin 16. Locating post 17. Screw 18. Turning rivet head

Figure 6 Schematic diagram of assembly

Figure 7 Fixture clamping

2.2 Key points of fixture design

2.2.1 Design points of fixture upper die assembly

Upper mold assembly of fixture is shown in Figure 8. It consists of a backing plate 2, a screw 3, a fixing plate 4, and a pressing nail 5. Large end faces of two pressing nails 5 should be flush with C surface of fixing plate 4, small end faces of two pressure nails 5 should be on same horizontal plane, so as to ensure that two rivets are evenly stressed during riveting process. Pressure nail 5 is made of Cr12MoV, and heat treatment hardness is 60~62 HRC to ensure that it has sufficient strength.

Figure 8 Fixture upper die assembly
Fixing plate 4 is shown in Fig. 9. Positioning holes 1, 2 and positioning pins 14, 15 respectively adopt a clearance fit of (0.02-0.04) mm to ensure accurate clamping of upper and lower mold components of fixture and facilitate mold opening.

Figure 9 Fixing plate

2.2.2 Design points of lower die assembly of fixture

Lower die assembly of fixture is shown in Figure 10, consisting of screws 6, washers 7, bushings 8, positioning blocks 9, guide posts 10, springs 11, fixing plates 12, bases 13, positioning pins 14, positioning pins 15, positioning column 16, screw 17 and riveting head 18. Small end faces of 2 rivet heads 18 are at same height. Cavity of positioning block 9 needs to be matched with outer dimension of push block assembly with a clearance of (0.05~0.1) mm. This gap can not only ensure position of rivet and turning rivet head does not have a large deviation, prevent riveting phenomenon, but also facilitate removal of push block assembly after riveting. Riveting head 18 needs to use Cr12MoV, and heat treatment hardness is 60~62 HRC to ensure that it has sufficient strength and service life.

Figure 10 Fixture lower die assembly

2.3 Design principle of rivet head

Design of turning rivet head affects rivet turning effect. Main dimensions of forming end of turning rivet head are turning riveting angle and arc radius R. If turning riveting angle is too large, rivet is easy to be riveted and cracked. If turning riveting angle is too small, phenomenon of upsetting riveting will occur, and arc radius R will affect fluidity of material. Design principle of turning rivet head is shown in Figure 11. Taking bending point as center point, draw a virtual circle I with arc radius R equal to rivet wall thickness d, and flexibly select turning riveting angle β (β≤90) according to plasticity of material °), angle β is tangent to virtual circle I, and diameter of intersection circle is obtained at this time as φ, as shown in Figure 12.

Figure 11 Design of rivet head

Figure 12 Dimensions of formed end of rivet head
In addition to following above-mentioned design principles, flip rivet head has made some improvements in combination with actual production process. α angle is added to forming end, and α angle is also tangent to virtual circle I, as shown in Figure 13. Main reason is that push block is made of plastic, excessive pressure cannot be used when turning riveting, and it is easier to rive material when material is squeezed outward along α angle.

Figure 13 Design of rivet head

3 Precautions for fixture manufacturing process

For upper mold assembly of fixture, after assembly, it is necessary to ensure that large end faces of two pressing nails are flush with C surface of fixing plate, and small end faces of two pressing nails are on the horizontal plane, that is, large and small end faces must be separated when processing pressing nails. Increase adjustment allowance of 0.5 mm, grind it with C surface of fixed plate first, then turn it over and grind small end surface together to meet assembly size requirements of drawing.
For the lower die assembly of fixture, it is necessary to ensure that heights of the two rivet heads are same after assembly. Since rivet heads are formed by a CNC lathe at one time, they will be slightly deformed after heat treatment, and large end face of rivet head needs to be increased by more than 0.5 mm. After heat treatment, fitter first polishes surface, then uses small end face as benchmark to flatten large end face so that heights of two turning riveting heads are same, that is, manufacturing process of turning riveting heads is: material preparation → CNC lathe processing →Heat treatment →Fitter polishing →Work grinding to adjust the height.

4 Debugging of riveting equipment and fixtures

Riveting equipment is controlled by a servo system and can move precisely on X, Y and Z axes, and riveting pressure provided is 10 kN. Riveting equipment is divided into upper and lower mechanisms. Upper mechanism can move along X-axis and Z-axis, lower mechanism can only move forward and backward along Y-axis. Upper and lower mold components of fixture are respectively installed on upper and lower mechanisms of equipment and locked. Fixture installation is shown in Figure 14.

Figure 14 Fixture installation
Fixture belongs to combined mold structure, debugging is simple and convenient. First, lower die assembly of fixture moves along Y-axis direction until it coincides with position of die handle. At this time, position of die handle in X-axis direction should be distributed as far as possible in the center of lower die assembly of fixture to ensure that pressure is evenly distributed during riveting process, then finely adjust displacement of die handle in Z-axis direction to ensure that rivet is turned in place, effect of pushing rivet of block assembly to turn rivet is shown in Figure 15.

Figure 15 Effect of rivet turning over riveting

Gud Mould Industry Co., Ltd