For this post, we will review tips on designing punches and dies for small diameter hole punching as well as the causes for chaff rising and how to prevent them.
Design for small diameter hole punching
Punching small diameter holes assumes a size of roughly less than 1.0 mm in diameter. Also, it is assumed that holes of about φ1.0 mm are punched in materials with a plate thickness of around 1.0 mm.
Design of the punch
For the sake of preservation, the tip of a punch for hole punching is guided (punch guide) by a stripper as is shown in Figure 1 thereby preventing the punch from breaking. A guide for the relationship between the dimensions P and B of the punch is a maximum length B less than or equal to 10P (B ≤ 10P). In the case of small diameter punches, dimension B becomes short, and punch guiding becomes difficult.
As a preventative measure for this, a two-stage punch is used as is shown in Fig. 2. The dimension d above P will be d ≤ P + 2t. The reason for this is to avoid causing any deformations in the hole when the punch is withdrawing from the material. The length of the guide at the tip of the punch is – guide length ≥ punch diameter. Also, the gap between the punch and the stripper should be 0.003 or more at each side. The reason for this is that 0.003 is the minimum gap that does not cause the oil film to break. It may also be good to wrap the side surface of the punch and make it clean.
Design of the die
The key point in die design is to make sure that no chaff gets clogged in the die hole. The reason for this is that the force for pushing down the chaff is quite large and the force for punching the hole becomes considerably large.
Chaff Rising Countermeasures in Punching Work
The chaff rising is a defect phenomenon in which the chaff does not remain in the die but rises along the surface of the die. Unfortunately, the fact that countermeasures against burrs have improved and the punching conditions have become better has decreased the frictional force between the punching chaff and the side surface of the die, therefore, making it easy for punching chaff to rise along the surface of the die.
Cause of chaff rising
The main causes of chaff rising are oil adhesion and suction. There are other causes such as magnetism or springing back, etc., but their effects are minor.
Oil Adhesion
The machining oil applied to the surface of the material makes the bottom surface of the punch come into close contact with the material, prevents the inflow of air from the periphery, and raises the material.
- The supply of oil to the material is made largely on the side surface of the die and made small on the punch side.
- A relief is prepared on the bottom surface of the punch (the surface that comes into contact with the material) thereby making the contact area smaller.
- An air hole blowing weak air is prepared. (It is better to use this along with the above relief.)
Suction
During the process in which the formation is complete and the punch is returning, the material inside the die becomes a base and the punch becomes a piston, the pressure decreases in the space between the material and the bottom surface of the punch, and hence the material is lifted up.
- Since the pressure reduction is proportional to the punch speed, reduce the spm of the operation.
- Open an air inlet hole
Placing a kicker pin and suppressing the rise of chaff forcibly is a countermeasure that is effective for all chaff rising irrespective of the cause.
If you have a custom die tooling design, MISUMI would be happy to have a chance to provide it to you. Simply upload your design at this link and receive a quote.
