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During the CNC machining process, what should we do if the workpiece hits the knife, bounces the knife, or overcuts?

Hits the knife

Collision tool refers to the cutting amount of the tool is too large, in addition to the cutting edge, the tool bar also hit the workpiece. The main reasons for knife collision are the unreasonable setting of safety height or no setting of safety height at all, the selection of improper processing methods, improper use of cutting tools and the margin of the second opening is smaller than that of the first opening.

  1. Eat too many knives

The solution:

Cut down on the amount of knives. The smaller the diameter of the tool, the smaller the amount of cutting it should take. Under normal circumstances, the cutting amount of each rough mold is not more than 0.5mm, and the cutting amount of semi-finishing and finishing is smaller.

  1. Choose improper processing methods

The solution:

The contour milling mode is changed to the cavity milling mode. When the machining allowance is larger than the tool diameter, the contour machining method cannot be selected.

  1. set the safety height incorrectly

Hit the fixture while lifting the knife

The solution:

(1) The safety height should be greater than the clamping height

(2) In most cases, the “direct” approach cannot be chosen, except for special workpieces

  1. Improper setting of second opening coarse allowance

The solution:

The margin should be a little larger than the margin of the first rough opening, generally 0.05mm. If the first rough allowance is 0.3mm, the second rough allowance should be 0.35mm. Otherwise, the tool bar is easy to hit the upper side wall.


In addition to the above reasons will cause knife collision, repairing scissors road will sometimes cause knife collision, so try not to repair scissors road, the most direct result of knife collision is to damage the tool and the workpiece, more serious may damage the machine tool spindle.

bounces the knife

bounces the knife refers to the tool due to the force is too large to produce relatively large amplitude vibration. The harm caused by the knife is to cause the workpiece to cut and damage the tool, when the knife diameter is small and the tool rod is too long or the force is too large to produce the phenomenon of the knife.

  1. The tool diameter is small and the tool rod is too long

The knife is too long and the diameter is too small

The solution:

Use a larger ball knife to clear corners or EDM deep corners

  1. Too much force (i.e. too much knife)

The solution:

Reduce the amount of cutting (that is, the global depth of each cutting tool). When the machining depth is greater than 120mm, it is necessary to install the tool twice, that is, first install the short cutting rod and process to the depth of 100mm, and then install the extended cutting rod and process the part below 100mm, and set the small cutting amount.


bounces the knife phenomenon is the most easy to be ignored by programming beginners, so it should be paid enough attention to. Programming, should be based on the performance of cutting materials and cutting tool diameter, length to determine the amount of cutting and maximum machining depth, as well as too deep place whether the need for EDM.

Over cut

Overcutting means that the tool handle can not cut the parts of the cutting, so that the workpiece is damaged. There are many reasons for workpiece overcutting, mainly the machine precision is not high, bump knife, knife, programming to choose a small tool but the actual processing of the misuse of large tools. In addition, if the operator is not accurate to the knife, it may also cause over cutting.

When programming, we must be careful and meticulous. After completing the program, we also need to check the tool path in detail to avoid over-cutting and other phenomena, otherwise the mold scrap and even the machine tool damage.

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