The common problem of spring knife overcutting in processing
During processing, the knife often bounces at the corner and causes overcutting. If reasonable tools and processing methods are used, the chance of knife bounce can be reduced.
Knife position and overcut
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Problem Analysis and Countermeasures
As shown in the figure below, figure A is the state of the tool when it is processing a relatively flat position. When the machine reaches the emergency stop at position B and prepares for reverse machining, the tool will deform due to inertia, resulting in a relatively straight position at position B. The knife is overcut.
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bullet knife icon
The relational expression of tool deformation:
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From the above formula, we can see that there are three main factors affecting tool deformation:
L - Tool length
D - tool diameter
P - force acting on the tool
L - Tool length
It can be seen from the formula that the deformation of the tool is related to the third power of the length of the tool. For a tool with the same diameter, when the length of the tool is doubled, the deformation will increase by 3 times.
When processing, shorten the length of the knife as much as possible to reduce the risk of knife bounce.
D - tool diameter
It can be seen from the formula that the deformation of the tool is related to the 4th power of the tool diameter. For a tool of the same length, when the diameter of the tool is doubled, the deformation will increase by 4 times.
When processing, if possible, choose a large-diameter tool or use a stronger tool for processing to reduce the risk of knife bounce. (As shown in the right picture below: A uses hot wire and tapered neck cutter, B uses a tool with a stronger handle for processing)
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P - force acting on the tool
It can be seen from the formula that the deformation of the tool is directly proportional to the force it receives during machining. Reducing the force on the tool can reduce the chance of the knife springing. The following methods can be used to reduce the force on the tool during processing.
Reduced force analysis:
Cutting is a process of shear deformation, and each material has its own strength (σ). To separate the materials, the external strength must be greater than the strength of the material itself.
σ = F/S
σ : Strength of the material
F: force
S : contact area
It can be seen from the above formula that the force (F) on the tool is proportional to the contact area (S) with the workpiece. To reduce the force on the tool, it is necessary to reduce the contact area between the tool and the workpiece.
Reduced force example 1:
Use the corner function of the tool path or increase the R position to reduce the load on the tool at the corner, thereby reducing the probability of tool bounce.
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Reduced force example 2:
When machining a deeper position, a tool with a smaller feed rate and a fine R angle can be used to reduce the force on the tool during processing and reduce the risk of tool bounce.
The figure below shows the comparison between the D50R6 cutter and the D50R0.8 cutter when processing the same depth, and the contact position of the mold material. It can be seen that the cutting force can be reduced by using a thin R-angle cutter to process a deep workpiece than a large R-angle cutter.
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Summarize:
The comprehensive use of three related factors that affect tool deformation (tool length, tool diameter, and cutting force) can reduce the probability of tool bounce, increase processing time, and obtain better machining accuracy and surface roughness.
