How to reasonably determine the clearance value for your CNC Machining Parts?

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As one of the CNC manufacturing companies of hardware industry, Worthy pursue the high quality of parts, pay attention to the tolerance of parts, but also pay attention to the clearance of punching.What is blanking clearance?

Blanking clearance refers to the size difference between punch and die cutting edge.Unilateral clearance is denoted by C and bilateral clearance is denoted by Z.Z normal - upper and lower microcracks coincide.There are unilateral and bilateral gaps.

What effect will blanking clearance have on stamping?

1) Influence of clearance on section quality

Normal: upper and lower cracks coincide, large bright zone, collapse Angle, burr, small taper, smooth surface;

Too large: upper and lower cracks do not coincide, tear and pull, rough section, small bright zone, collapse Angle, large taper;

Too small: the upper and lower cracks do not coincide, secondary shear occurs, forming a second bright zone, large burr;

Non-uniformity: the side with small gap appears the feature of small mass section, while the side with large gap appears the feature of large mass section.

2) Influence of clearance on dimensional accuracy.

Due to the existence of elastic deformation, blanking after the end of the elastic recovery, so that the size and the size of the convex and concave die cutting edge size dimensional deviation, and the size of the elastic deformation and blanking clearance has a direct relationship.

When the clearance increases, the metal tensile degree increases and the elastic recovery makes the work piece size increase, the metal tensile degree increases and the elastic recovery work piece size decreases.

When the gap changes, the metal compression degree increases, the hole size decreases, the metal internal pressure degree increases, the work piece size increases.

3) Impact of clearance hedging force.

The impact law of blanking clearance against blanking force is that the smaller the clearance, the larger the compressive stress component in the deformation zone, the smaller the tensile stress component, the greater the material deformation resistance and blanking force.On the contrary, the larger the clearance, the larger the tensile stress in the deformation zone, the lower the deformation resistance and the smaller the punching force.When the clearance reaches 5%-20% of the material thickness, the blanking force decreases not obviously.When the unilateral clearance Z increases to 15%-20% of the material thickness, the unloading force is 0. 

4) Impact of clearance on mold life.

Due to the existence of friction between the workpiece and the side wall of the convex and concave die, the gap is small, the friction is large and the die life is short.During blanking, there is friction between punch and punch, between die and drop valve, and the smaller the gap, the more serious the friction.Therefore, too small clearance is very detrimental to the life of the die, while a large clearance can reduce the friction between the sides of the punch and the die and reduce the impact of uneven clearance, thus increasing the life of the die.

How to reasonably determine the clearance value?

The selection of clearance should make blanking achieve better section quality, higher dimensional accuracy, smaller blanking force and higher die life.

Reasonable clearance refers to a range of values, the maximum reasonable clearance, the minimum reasonable clearance.The determination of clearance is to choose an appropriate clearance range as a reasonable clearance considering the influence of the above factors.The upper limit is the maximum reasonable clearance and the lower limit is the minimum reasonable clearance, that is, the reasonable clearance refers to a range value.In its body design mold, according to the specific requirements of the workpiece and production

Selection can be made according to the following principles:

(1) When the section quality of the workpiece is not strictly required, in order to improve the life of the die and reduce the punching force, a larger clearance value can be selected.

(2) When the workpiece section quality and manufacturing tolerance requirements are high, should choose a small clearance value.

(3) When calculating the daily edge size of blanking die, it should be calculated according to Z min value considering that the wear of the die in the process of using will increase the daily edge clearance.

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