The process of milling small blind holes in parts

When developing the spindle for the SD-95 electromagnetic speed odometer, we encountered a special problem of dealing with small blind square holes. When the speed odometer is working, the instrument and the sensor are connected by a flexible shaft. In order to transmit torque, the spindle is machined with square blind holes with a side length of only 2.75 mm (7/64 inches) and a working length of 8 mm. In addition, the deep round hole at the rear of the square hole is larger than the diameter of the circumscribed circle of the square hole, and must accommodate the axial turbulence and radial vibration of the soft shaft head and store lubricating oil. …Because of the large batch of shafts, special processing methods cannot be used, which is a limiting factor in product production and affects economic benefits.

Because the hole is a blind hole and has a small diameter, undercuts cannot be formed in advance and chip removal is difficult, so traditional chip control methods such as cutting and punching cannot be used.
Therefore, consider using the bottom hole first, and then apply radial pressure to the shaft. Plastic deformation is carried out through round and square tipless forming processes. Based on the above ideas, we have designed and implemented various pressure forming methods. Here are some ways to form square holes.

Stamping forming method

The process steps are as follows:

The rough cut of the f6mm section of the rough turning is f7.2mm, the depth of the drill bit’s inner hole is f4mm, and the depth is 20mm. Remove the workpiece and insert a 2.75 mm square mandrel into the hole. After finding the correct direction, put the V shape with upper and lower molds on the punching machine at 90 degrees, square inside and outside, 8 mm long, and then take out the mandrel.

The method has the advantages of simple process steps, convenient mold making, short production cycle and the like. The disadvantage is that the spindle is easy to bend and deform under pressure, because the axial direction of the workpiece is not restricted during processing. The coaxiality between the square hole and the outer circle is difficult to guarantee. At the same time, the mandrel is easy to break, difficult to disassemble, and the labor intensity of workers is high. After the outer circle of the workpiece is deformed, it is also very inconvenient for the outer circle of the finished machine.
The practical square mandrel is made of white steel knife material directly by wire cutting, so it has excellent strength.
A vertical slope of 1:100 was created to facilitate the rod pulling the core. At the same time, always pay attention to lubrication when stamping, and use special tools to pull out the mandrel. The authenticity rate of this method is less than 85%, which is only suitable for small batch production.

Roll Forming

Processing procedures: rough turning f6mm section outer circle maximum f7.2mm, F4mm drilling depth 20mm, drilling depth f5mm, 3mm. Without removing the workpiece, fix the 2.75mm square mandrel on the special movable top of the tailstock of the lathe, and insert it into the hole to match the f5mm hole. Then start the lathe, use the roller to push the square core section to deform the shrinkage hole, and form a square hole under the constraint of the square core. After rolling out the mandrel, add a parking lot.

The advantage of this method is that the workpiece does not need to be unloaded, and all steps are completed on the lathe, so the formed square hole and the outer circle are kept highly coaxial, and the quality is easy to guarantee. …The equipment requirements are also low, and it is an ordinary lathe, especially suitable for small and medium-sized enterprises. The disadvantage is that during the rolling process, due to the large radial force, the workpiece generates heat and vibrates. In practice, in order to overcome the problem of large radial force, the method of simultaneously supplying oil to the front and rear double rollers is adopted, and hydraulic transmission is adopted. The radial forces basically cancel each other out, the rolling speed is also accelerated, and the effect is good. However, this method complicates the feed mechanism of the tool holder and is usually used in large batches. The key to the success of this process is to master the rolling and feed rate. Not only does it roll in place, but it also fills the shape of the square hole. Don’t overdo it, so as not to lose the finishing allowance of the outer circle. Specific parameters such as the outer diameter of the blank before rolling, rolling speed, and rolling feed rate can be obtained through process tests.

Mold forming method

In order to meet the needs of mass production, we have developed a special mold that greatly improves productivity.
The process steps are as follows:
After rough turning of the lathe, the outer circle of f6mm section is finely ground, leaving a grinding allowance. The hole depths inside the drill bit are f4 mm and 20 mm, and the hole depths are f5 mm and 3 mm, which are used as a positioning reference. The stamping equipment then uses customized stamping dies. In order to transfer the material into the hole, four grooves are pressed into the diameter corresponding to the cross section of the square hole. A square hole is formed under the combined action of the mandrel.

The operating state of the mold is as follows. The workpiece 9 is arranged in the concave main body 6 through the outer circumference of the complete vehicle. When the punch presses down on the upper template 8, the workpiece is first pushed on the positioning surface by the upper jack 12. The wedge 17 pushes the slider 18 to the left along the horizontal guide 19, and the moving cam 15 moves to the left under the drive of the pin 16. The cam light is matched with the roller 2, and the square core 3 is pressed upwards into the inner hole of the workpiece, and the hole is centered with a hole of f5mm. The top template continues to move downward, but at this time the wedge column is in an empty stroke, and the top thimble is kept tight under the action of the spring 10 to ensure the positioning of the workpiece. Then the ring wedge 14 starts to enter the working stroke, prompting the four forming punches 4 to move in the center direction together, the groove is squeezed into the workpiece, and a convex shape is formed in the hole. The square hole is formed by the interaction of the square core. When returning, the upper template advances, the ring wedge 14 first removes the forming punch, and the forming punch returns to the original position under the action of the return spring 5. Then the wedge 17 pushes the slider 18 to the right, correspondingly moves the cam to the right, and the square core leaves the inner hole through the inclined surface and the roller. Finally, the upper ejector 12 releases the workpiece and allows it to be ejected, completing the work cycle.

The advantage of this method is that after the outer circle of the workpiece passes through the fine grinding wheel, the positioning accuracy in the mold is high, the actual wall thickness is reduced, and it is easy to deform, so as to obtain high quality. Yes. The embossing amount is small (0.6mm per side), and it is performed in a local area, so the pressure on the workpiece is relatively small. Therefore, the deformation of the stamped workpiece is small, and it can directly enter the next grinding process, which greatly improves the production efficiency. In addition, this method is very convenient and labor-saving, and is very popular among workers. The disadvantage is that after forming, 4 grooves with a width of 2mm and a depth of 0.6mm remain on the outer surface of the workpiece, which cannot be removed after grinding. Obviously, this does not meet the requirements of the drawings. In this regard, we believe that the groove is theoretically unfavorable to the subsequent grinding process, so the effect is not large, and the effect in use is also small, because the groove is actually local. Since the oil tank is only used to store a certain amount of lubricating oil, it has proved to be very useful for lubrication in practice. Therefore, we made appropriate changes to the drawing so that these grooves can exist. If you need to remove these grooves to fully meet the requirements of the drawing, you can increase the outer diameter of the blank and rotate it after forming, but it is not recommended.

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